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LIGHTNING ROD CONFERENCE.
REPORT
OF THE
DELEGATES FROM THE FOLLOWING SOCIETIES, VIZ.:
METEOROLOGICAL SOCIETY.
C. BROOKE, F.R.S., _Past President_ [THE LATE].
E. E. DYMOND, F.M.S., _Vice-President_.
G. J. SYMONS, F.R.S., _President_.
ROYAL INSTITUTE OF BRITISH ARCHITECTS.
PROF. LEWIS, F.S.A., _Vice-President_.
J. WHICHCORD, F.S.A., _Past President_.
SOCIETY OF TELEGRAPH ENGINEERS AND OF ELECTRICIANS.
LATIMER CLARK, M. INST. C.E., _Past President_.
W. H. PREECE, F.R.S., M. INST. C.E., _Past President_.
PHYSICAL SOCIETY.
PROF. W. G. ADAMS, F.R.S., _Past President_.
PROF. G. CAREY FOSTER, F.R.S., _Past President_.
CO-OPTED MEMBERS.
PROF. W. E. AYRTON, F.R.S.
PROF. D. E. HUGHES, F.R.S.
_With a Code of Rules for the Erection of Lightning Conductors; and
various Appendices._
EDITED BY THE SECRETARY, G. J. SYMONS, F.R.S.
LONDON: E. & F. N. SPON, 16, CHARING CROSS.
NEW YORK: 446, BROOME STREET.
1882.
KENNY & CO., PRINTERS, 25, CAMDEN ROAD, N.W.
------------------------------------------------------------------------
CONTENTS.
PAGE
PREFACE v
REPORT 1
Section 1.—The purpose which a lightning conductor is intended
to serve 1
Section 2.—A statement of those features in the Construction
and Erection of Lightning Conductors, respecting which there
has been, or is, a difference of opinion, and the final
decision of the Conference thereupon 3
Points 3
Material for Conductor 5
Size of Rod 6
Shape of Rod (Rod, Tube, Tape, Rope, Plait) 7
Joints 10
Protection of Rod 10
Painting 11
Attachment to Buildings 11
Earth Plates 11
Space Protected 12
Height of Upper Terminal 14
Testing Conductors 14
Internal Masses of Metal 15
External Masses of Metal 16
Section 3.—Code of Rules for the Erection of Lightning
Conductors 16
APPENDICES—
A.—Circular and Questions issued to Manufacturers, and their
Replies (1)
B.—Analysis of, and remarks upon, the views of the
Manufacturers (17)
C.—Reply from Manufacturers received after the completion of
Appendix B. (23)
D.—Report of the Representatives of the Royal Institute of
British Architects (27)
E.—Particulars of Accidents by Lightning, collected 1857–59, by
Mr. Symons, and Report upon the same (43)
F.—Abstracts of Printed Documents (51)
G.—Catalogue of Works upon Lightning Conductors (143)
H.—Application to, and Replies from, the Local Hon. Secretaries
of the Society of Telegraph Engineers and other distinguished
Foreign Authorities (175)
I.—General Correspondence (183)
J.—Data respecting the Sectional Area of Metal requisite for
Lightning Conductors (223)
K.—Notes respecting Lightning Conductors, collected in Paris in
May, 1881, by Messrs. Preece and Symons (225)
L.—On the Lightning Conductors at the Paris International
Electrical Exhibition, by Messrs. Dymond and Symons (229)
M.—Miscellaneous (233)
Index to Appendices (245)
ILLUSTRATIONS.
PAGE
Sketch illustrative of terms employed face x
Sketch illustrative of area of protection 13
Plans and Elevation of Nottingham Castle (25)
(26)
Tower of Church of Week St. Mary, Cornwall (32)
Plan and Elevation of Twyford Moors, Winchester (34)
Plan and Elevation of St. James’ Church, West-End, Hants. (35)
Plan and Elevation of houses at Lewisham, Wandsworth, and Forest (38)
Hill
View of Tower of Holborn Union Infirmary, Holloway (40)
Plan and Elevation of Laundry at Gravesend (41)
Regnier’s System of Lightning Conductors (54)
Joints and Earth Terminals recommended in France in 1807 (55)
Powder Magazine, with oblique as well as vertical rods (57)
Mode of attaching Conductor to Upper Terminal (59)
View and Plan of Bruntcliffe Gunpowder Store (75)
View of Board House, at Purfleet (78)
View and Plan of Heckingham Poorhouse (87)
(88)
Diagrams illustrative of space protected (135)
(136)
Plan and Elevation of Church of Ste. Croix, at Ixelles (141)
Sketch of arrangements for Public Buildings in Denmark (177)
Plan and Elevations of Systems for Powder Magazines in Denmark (178)
Attachment and Earth Terminal used in Italy (180)
Sections of Rods used by the Trinity Board (183)
Eddystone and Spurn Point Lighthouses (184)
South Foreland High Light (185)
Eddystone Lighthouse (191)
Plan and Elevation of house at Trolley Bottom, St. Albans (197)
Plan and Elevation of washhouse of Middlesboro’ Fever Hospital (203)
Plan and Elevation of Indian Pagoda (207)
Plan and Elevation of Upwood Gorse, Caterham (211)
Sections of Rod at Upwood Gorse, Caterham (214)
(215)
Sections of Munson’s Rods (216)
View and Section of Cutting’s Conductor Coupling (217)
Plan and Elevation of Christ Church, Carmarthen (218)
Plan and View of house, and of Bootham Bar, York (220)
(221)
PREFACE.
Although France and other nations have taken active steps to give
official sanction to the best known means of protection from the ill
effects of atmospheric electricity, nothing in this way has ever been
done in England for the public generally.
The enquiries by householders and public bodies for advice and
instruction were so numerous, the absence of authorized or well-matured
directions was so marked, the practice in vogue so varied and anomalous,
that it occurred to the Meteorological Society to take some action in
the matter.
Accordingly, at a Meeting of the Council of the Meteorological Society,
held on 15th of May, 1878, it was resolved—
‘That the House Committee be instructed to address the following
Societies:—
THE ROYAL INSTITUTE OF BRITISH ARCHITECTS,
THE PHYSICAL SOCIETY,
THE SOCIETY OF TELEGRAPH ENGINEERS,
asking them to name delegates to co-operate in considering the
desirability or otherwise of issuing a code of rules for the
erection of lightning conductors, and to proceed in preparing a code
if it is thought desirable.’
In accordance with this resolution the following letter was addressed to
the Secretaries of the above Societies:—
THE METEOROLOGICAL SOCIETY,
30, GREAT GEORGE STREET, WESTMINSTER,
_June 14, 1878_.
SIR,
The Council of the Meteorological Society have had under their
consideration for some time the possibility of formulating the
existing knowledge on the subject of the protection of property from
damage by electricity, and the advisability of preparing and issuing
a general code of rules for the erection of lightning conductors.
They are of opinion that this would best be done by a joint
committee of representative members of those Societies before which
such subjects most naturally come; and they have, therefore, decided
upon inviting the co-operation of your Society by the nomination of
one or more delegates to join a Committee by whom the whole question
should be considered, and to whom also any written communications
would be submitted.
The Council trust that your Society may be represented by delegates;
but if that course be impossible, they invite any written
suggestions which you may have to offer.
A meeting of the delegates will be called for an early date after
the receipt from the Societies consulted, of the names of the
gentlemen nominated by each.
We are, Sir,
Your obedient servants,
G. J. SYMONS, }
JOHN W. TRIPE,} _Hon. Secretaries_.
In reply to this circular all the societies invited nominated delegates,
and the Conference was constituted as follows:—
Meteorological Society. │C. BROOKE, F.R.S., _Past
│ President_.
│E. E. DYMOND, F.M.S.
│G. J. SYMONS, F.R.S., _Secretary_.
Royal Institute of British │PROF. LEWIS, F.S.A.
Architects. │
│J. WHICHCORD, F.S.A., _Vice
│ President_.
Society of Telegraph Engineers and │LATIMER CLARK, M. Inst. C.E., _Past
of Electricians. │ President_.
│W. H. PREECE, F.R.S., M. Inst.
│ C.E., _Vice President_.
Physical Society. │PROF. W. G. ADAMS, F.R.S.,
│ _President_.
│PROF. G. CAREY FOSTER, F.R.S.,
│ _Past President_.
The steps taken by the delegates will be best explained by a short
narrative chiefly formed of extracts from the minute book of the
Conference.
The first meeting was held at the rooms of the Meteorological Society,
on November 14th, 1878, when all the delegates were present. Mr. C.
Brooke, F.R.S., was appointed _President_ of the Conference, and Mr. G.
J. Symons, F.R.S., _Secretary_.
Professor W. E. Ayrton was elected a member.
A circular, which will be found in Appendix A, was drafted for issue to
manufacturers of lightning conductors. This was sent to sixty-five
firms, but only eight replied, and their answers are printed verbatim in
the same Appendix. An analysis of the replies forms Appendix B. Appendix
C is a reply received too late for insertion in Appendix A, and after
Mr. Preece had compiled Appendix B. Another reply from an American firm
will be found in Appendix I, p. (192), making ten in all.
At a subsequent meeting, the delegates from the Royal Institute of
British Architects were requested to ask the Council of that body to
issue a circular to their members inviting them to furnish information
respecting buildings injured by lightning. This circular, together with
abstracts of the replies, and a brief Introductory Summary, by Messrs.
Lewis and Whichcord, will be found in Appendix D.
Mr. Symons submitted to the meeting a mass of statistics respecting
accidents by lightning which he had collected in the years 1857–59; they
were referred to Professor Ayrton, and his note upon them constitutes
Appendix E.
At the meeting on August 5th, 1879, the Secretary announced the death of
the President of the Conference, Mr. C. Brooke, F.R.S., a vote of
condolence was passed unanimously, and ordered to be forwarded to Mrs.
Brooke. The Conference then proceeded to elect a new Chairman, and it
was unanimously resolved that Professor W. G. Adams, F.R.S., be
requested to accept the office.
The following circular was approved and ordered to be forwarded to a
large number of the most important newspapers and periodicals throughout
the United Kingdom.
LIGHTNING CONDUCTORS.
_To the Editor of_ ——
SIR,—
In the summer of 1878 delegates were nominated by the following
Societies, viz., the Royal Institute of British Architects, the
Society of Telegraph Engineers, the Physical Society, and the
Meteorological Society, for the following purpose:—
“To consider the possibility of formulating the existing knowledge
on the subject of the protection of property from damage by
electricity, and the advisability of preparing and issuing a
general code of rules for the erection of Lightning Conductors.”
The delegates have held several meetings, and have already
collected, firstly, from the manufacturers of Lightning Conductors,
and secondly, from the Members of the Royal Institute of British
Architects, a large amount of thoroughly practical information.
Several of their number are also engaged in forming abstracts of the
salient features of the literature of the subject.
The Members of the Conference are, however, most anxious that their
Report should be as trustworthy and as exhaustive as possible, and
they have, therefore, instructed me to ask you to assist them by
publishing this epitome of their proceedings, and allowing them to
invite correspondence upon the points mentioned below.
I am, Sir,
Your obedient servant,
G. J. SYMONS, F.R.S.,
_Secretary to the Conference_.
LIGHTNING ROD CONFERENCE,
30, GREAT GEORGE STREET, S.W.
CLASS OF FACTS MOST REQUIRED.
Full details of accidents by lightning, stating especially whether
the building struck had a conductor or not. If there was a
conductor, state its dimensions—construction—mode of attachment to
building—whether its top was pointed—distance of its upper terminal
from the place struck—nature and extent of the connection between
the conductor and the earth, and whether the earth was dry or
moist—whether the conductor was itself injured—and whether the
conductor or the point struck was the most salient object in the
vicinity. Information is also desired, either verbally or by
sketches, as to the position of metal spouting and lead roofing
relatively to the point struck, and to the conductor.
Details of the thickest piece of metal melted by a flash of
lightning are much needed.
Unimpeachable evidence of the failure of conductors is much desired,
as such failures would be extremely instructive.
The replies were by no means as numerous as was expected: the most
important will be found in Appendix I.
At the meeting, October 27th, 1879, it was resolved “That the members of
the Conference will undertake to prepare abstracts of the principal
English and Foreign books upon Lightning Conductors.” This work became
extremely heavy, and occupied much time, as will be seen from Appendix
F, which contains abstracts of sixty separate treatises, of which 26 are
from English, 17 from French, 6 from Belgian, 5 from American, and 5
from German authors, and one is from the Norwegian.
In order to guard against omitting important works, it was resolved
“That application be made to the Society of Telegraph Engineers for
advance sheets of the Ronalds Catalogue.” From it, supplemented by Mr.
Latimer Clark’s and other lists, the Secretary compiled Appendix G.,
which contains the full titles of no fewer than 704 separate works upon
lightning conductors, or on subjects intimately connected therewith.
At the same meeting it was resolved that efforts be made to obtain a set
of the official instructions issued in all foreign countries. The
circular issued, and an abstract of the information collected, including
replies from America, Belgium, Denmark, Germany, Holland, India, Italy,
and Norway, will be found in Appendix H. Full details respecting the
practice in France will be found in Appendices F, K, and L, and a notice
of Zenger’s Austrian system, on p. (104).
At the meeting, Nov. 20th, 1879, the Secretary was unanimously requested
to act as Editor of the Report.
At the meeting, Jan. 22nd, 1880, a letter was received from Mr. R. H.
Scott, F.R.S., _Secretary to the Meteorological Council_, enclosing a
report respecting the injury to the “Southern Queen,” it was resolved,
“That some of the delegates visit the ship.” The report and a note of
the results of the visit will be found in Appendix I page (205).
At the meeting, April 15th, 1880, Prof. D. E. Hughes was unanimously
elected member of the Conference.
At the meeting, July 6th, 1880, the Secretary handed in a sketch of a
house with various parts of the lightning conductor marked upon it, and
obtained from the delegates definite names for each portion, in order
that in framing the report there might be no uncertainty as to what was
meant by any special term, great confusion in this respect having
previously existed.
The terms adopted have been: _Conductor._—The whole arrangement for the
protection of a building. _Point._—The upper termination of the
conductor, whether blunt or sharp, single or bifurcated. _Upper
terminal._—That portion of the conductor which is between the top of the
edifice and the point. _Joint._—Any connection between any two parts of
the conductor. _Rod._—The main portion of the conductor, whether it
consist of rope, tape, tube or solid rod. _Circuit des Faîtes._—A rod
running round the eaves of a house, the battlements of a tower, &c.
_Earth plate._—The termination of the conductor in the ground, the
pattern being indicated by special terms.
The accompanying lithograph will, it is hoped, supply all additional
necessary particulars.
It is desirable to state that the illustrations in this Report have been
prepared by Mr. E. White Wallis, F.M.S., so as to bring out the various
features distinctly, and as nearly as possible in true proportion, but
without any attempt at artistic finish.
The meetings during the latter part of 1880, and the early part of 1881,
were devoted chiefly to the discussion of various questions as bases for
the report. Much time was also occupied in perfecting the various
appendices, and in compiling an exhaustive index to them.
In May, 1881, Messrs. Preece and Symons, being in Paris, made careful
enquiries as to the existing practice in France respecting lightning
conductors. Their notes form Appendix K.
At the meeting held on May 27th, 1881, the Secretary was instructed to
draw up a draft report, and this having been put in type was sent to all
the delegates; carefully considered, revised, and amended at various
subsequent meetings, and finally adopted.
[Illustration:
INDEX SKETCH OF LIGHTNING CONDUCTOR, ILLUSTRATING THE TERMS EMPLOYED
IN THE REPORT.
]
TERMS APPLIED TO THE VARIOUS
PARTS OF A CONDUCTOR.
_Ⴤ_ _Crutch_
⊞⬜⊞ _Strap_
⋂ _Staple_
⚲ _Wall Eye_
A _Point_
B _Upper Terminal_
c _Joint_
D _Rod_
E _Ridge Rod_
F _Circuit des faîtes_
G _Earth Plates_
G^1 _Earth Plates Sanderson_
G^2 _Earth Plates Borrel_
G^3 _Earth Plates Spang_
REPORT.
The Delegates are of opinion that it will conduce to clearness of
statement if their Report be divided into three sections—
(1) The purpose which a lightning conductor is intended to serve.
(2) A statement of those features in the construction and erection of
lightning conductors respecting which there has been, or is, a
difference of opinion, and the final decision of the Conference
thereupon.
(3) Code of rules for the erection of lightning conductors.
SECTION I.—_The purpose which a Lightning Conductor is intended to
serve._
A flash of lightning is the passage of an electric spark between two
bodies oppositely or unequally electrified, and between which the
difference of electric pressure or potential is sufficiently strong to
break across the air space which separates them, and to produce what is
known as a disruptive discharge. A flash may pass either between one
cloud and another, or between a cloud and the earth. In the former case
damage is not likely to be done, in the latter damage is or is not done,
according to the point at or from which the lightning strikes. The more
any object projects above the general level, the less is the distance
between it and the cloud, and as the less the distance the less the
resistance offered to the discharge, high objects are, _cœteris
paribus_, most frequently struck. Some substances, such as copper or
iron, can conduct a large quantity of electricity with facility, and are
called good conductors. Other substances, such as living vegetable or
animal matter, offer much obstruction, and form only partial conductors;
while dry earth, stone, and wood almost entirely prevent the passage of
electricity, and are very bad conductors—in fact, insulators.
For instance, a man may with perfect impunity clasp a copper rod an inch
in diameter, the bottom of which is well connected with moist earth,
while the top of it receives a violent flash of lightning. But if the
electricity does not find a path prepared for it, it will utilise such
partial conductors as may be reasonably near, for example—the heated air
from a kitchen chimney, the soot inside, and then the metal range at the
bottom; here, however, stone or dry material is generally found, which
will not conduct it, and then it dashes across the kitchen at some gas
or water pipe, or some pump or drain leading to damp earth, doing
serious damage on the way: or it may meet some tree in its course and
rend it from top to bottom, and if the human body intervene life may be
destroyed. Mechanical injury is inflicted only where the conduction for
the discharge is imperfect.
A lightning conductor fulfils two functions: it facilitates the
discharge of the electricity to the earth, so as to carry it off
harmlessly, and it tends to prevent disruptive discharge by silently
neutralising the conditions which determine such discharge in the
neighbourhood of the conductor.
To effect the first object a lightning conductor should offer a line of
discharge more nearly perfect, and more accessible, than any other
offered by the materials or contents of the edifice we wish to protect.
To effect the second object the conductor should be surmounted by a
point or points. Fine points and flames have the property of slowly and
silently dissipating the electrical charges; they, in fact, act as
safety valves.
If all these conditions be fulfilled; if the points be high enough to be
the most salient features of the building no matter from what direction
the storm cloud may come, be of ample dimensions and in thoroughly
perfect electrical connection with the earth, the edifice with all that
it contains will be safe, and the conductor might even be surrounded by
gunpowder in the heaviest storm without risk or danger.
All accidents may be said to be due to a neglect of these simple
elementary principles. The most frequent sources of failure are
conductors deficient either in number, height, or conductivity, bad
joints, or bad earth connections. There is no authentic case on record
where a properly-constructed conductor failed to do its duty.
SECTION II.—_A Statement of those features in the construction and
erection of Lightning Conductors, respecting which there has been, or
is, a difference of opinion, and the final decision of the Conference
thereupon._
Points.
Material for Conductor.
Size of Rod.
Shape of Rod.
(Rods, Tubes, Tape, Rope, Plait.)
Joints.
Protection of Rod.
Attachment to Building.
Earth Plates.
Space Protected.
Height of Upper Terminal.
Testing Conductors.
Internal Masses of Metal.
External Masses of Metal.
=POINTS.=—Starting with the extreme top, we have first to deal with the
question of points. The utility of points was hotly contested rather
more than a century since, and an abstract of the discussion will be
found in Appendix F, page (79), and difference of opinion still exists
as to their precise functions and value. The decision as to the best
form of points is complicated by two opposing requirements (1), the
sharper the point the more rapid the silent discharge of electricity,
and, therefore, the more effective the conductor; but (2) the sharper
the point the more easily is it destroyed by oxidation, or fused, should
a heavy disruptive discharge fall upon it.
Attempts have been made by the use of gold, silver, and platinum, to
obtain a sharp point which should not only be durable, but, owing to its
high melting point, resist fusion by a disruptive discharge. But such
metals are very expensive, and the statements in Appendix F, pages (67,
69, 73, 103, 123, 128, and 139) prove that even platinum points are
often damaged. Copper points whose sectional area is less than ·05 of a
square inch are very liable to be melted. Lightning has even fused a
copper rod ·10 sq. in. in sectional area, _i.e._, 0·35 in. in diameter,
and there are many rods still standing of which the extremity has been
melted into a button or knob.
For these reasons it seems best to separate the double functions of the
point, prolonging the upper terminal to the very summit, and merely
bevelling it off, so that, if a disruptive discharge does take place,
the full conducting power of the rod may be ready to receive it, and,
therefore, that there may be no risk of melted particles of metal
setting fire to the building, as has occurred. [Appendix F, p. (93).]
At the same time, having regard to the importance of silent discharge
from sharp points, we suggest that at one foot below the extreme top of
the upper terminal there be firmly attached, by screws and solder, a
copper ring, bearing three or four copper needles, each 6 inches long
and tapering from ¼ inch diameter to as fine a point as can be made; and
with the object of rendering the sharpness as permanent as possible, we
advise that they be platinized, gilded, or nickel plated.
Vanes, finials, and ornamental ironwork so frequently form the upper
portion of edifices, that it is essential to consider their relation to
the conductor. They should always be in perfect metallic connection with
the conductor. The possibility of such metal work inducing the charge to
desert the conductor for some other path is sometimes suggested, but it
could not happen unless the conductor were out of order, _e.g._, of
inadequate conducting power, or had an imperfect earth-contact.
With respect to factory chimneys, a different practice prevails in
England from that which is nearly universal on the Continent. In this
country one straight rod is usually carried up on one side of the
chimney to a height above the top about equal to the diameter of the
chimney. On the Continent two arches of iron are put crosswise over the
aperture of the chimney, and a vertical rod is carried up from the
intersection. In both systems the upper terminal suffers from the
corroding effect of the fumes from the chimney. Dr. Mann thought,
Appendix F, p. (132), that considering the ready path for lightning
afforded by the heated smoke discharged from chimneys, a coronal
conductor should be placed upon them, as well as a multiple point.
Messrs. Gray say, p. (9): “For high chimney shafts we fit a copper band
round the top, and four points thereon connected to main down rod.” The
Edinburgh Gas Works chimney, 341 feet high and 14 feet across at the
top, was fitted with a conductor under the advice of Faraday, Appendix
F, p. (89). It had an iron plate on the top; Faraday directed that the
rod should be connected with this plate, and the upper terminal should
rise vertically 6 feet above it.
We are of opinion that a coronal or copper band, with stout copper
points, each about 1 ft. long, at intervals of 2 or 3 ft. throughout the
circumference, will make the most durable and generally useful protector
for a factory chimney, but these points should be gilded or otherwise
protected against corrosion.
=MATERIAL FOR CONDUCTOR.=—Iron and copper are practically the only two
metals which need consideration; brass, which has sometimes been used is
so perishable that its employment is a self-evident error. We will
assume the conductivity of equal lengths and weights of iron to be, in
the case of steady currents of electricity, ⅙th that of copper, and the
cost of iron to be ⅑th that of copper, this would make the cost of
copper for equal conducting power ^9⁄_{6}ths, or 50 per cent. dearer
than iron. But there are other matters to be considered: (1) the great
weight and bulk of iron rods; (2) their deterioration by rust; (3) the
serious obstruction offered by a rusty joint; (4) the suddenness of
lightning discharge which modifies the conductivity; and lastly, that
iron is so much more rigid than copper that (except in the form of iron
wire rope, of which we shall speak hereafter) it can rarely be used in
greater lengths than 20 feet, and thus numerous joints become necessary,
whereas every needless joint should be avoided.
As regards galvanizing, we think it scarcely judicious to trust entirely
to it for protection against oxidation, for many instances of imperfect
galvanizing have come to our knowledge.
On the other hand copper becomes brittle, not only when exposed to the
air, but also by the passage through it of powerful charges of
atmospheric electricity. Franklin used iron, and it is employed in
America and on the Continent much more generally than copper, and it is
less tempting to the thief.
Nevertheless, as the cost of erection bears a considerable ratio to the
cost of the rod itself, and as iron possesses the disadvantages above
stated, we think that in all ordinary cases a copper rod will in the end
prove the cheapest, as it will certainly be the most durable.
=SIZE OF ROD.=—This is perhaps the most difficult subject which has to
be determined. We greatly regret the shortness of Table I. in Appendix
K; but we think that it must be assumed from it that lightning has fused
a copper rod ·10 in. (⅒th) in area, _i.e._, weighing 6 ounces to the
foot. We have also the Caterham case, Appendix I, p. (214), where a
copper tube weighing 5¾ ounces per foot was heated to redness.
The saving of cost which might be effected by using, for very low
buildings, rather slighter rods than for ordinary edifices is not worth
considering. In a 30 feet rod it could hardly amount to 10s. We
therefore recommend as the _minimum_ to be used:—
Material. Pattern. Diameter. Sectional Area of Metal. Weight per foot.
in. sq. in.
Copper Rope ½ ·10 6 oz.
Copper Round Rod ⅜ ·11 7 oz.
Copper Tape ¾ × ⅛ ·09 6 oz.
Iron Round Rod 9/10 ·64 35 oz.
=SHAPE OF ROD.=—This depends upon a subject which until lately was
warmly discussed, viz., upon the relative importance of the sectional
area, and of the superficial area of a conductor; a matter which has
been the subject of active discussion among electrical authorities.
Faraday and Sir W. Snow Harris, for example, held diametrically opposite
views respecting it. [Appendix F, p. (89), and I, p. (195).]
There is abundant and conclusive evidence that in the case of steady
electric currents, conductivity depends upon sectional area alone, and
not at all upon extent of surface, and experiments by Mr. Preece and Dr.
Warren De la Rue tend to show that, in the case of sudden discharges
from condensers, to which lightning discharges are probably analogous,
the influence of form is not considerable. On the other hand, there is
equally conclusive evidence that the facility with which currents of
short duration pass through conductors is affected by the form and
arrangement, as well as by the sectional area of the conductors. Upon
the whole we agree with the opinion quoted below, from a writer
recognized in the United States as a high authority on lightning
conductors, who, after describing and engraving more than fifty patterns
of rods, says[1]:—
Footnote 1:
Spang, “A Practical Treatise on Lightning Protection,” p. 121.
“The alleged improvements in the said conductors are, in nearly all
cases, worthless, or of a trifling and unimportant character. The
fact is, the said conductors are quite inferior, and contain no
essential improvement upon the ordinary round iron rod used during
the days of Franklin.”
In Europe the only forms at all generally employed are:—
Rods (round or square); Tubes; Tape; Ropes (wire, or wire with hemp
centres); Plait.
=Rods= (round or square).—The advantages and disadvantages of rods are
easily stated. The advantages are their durability and their rigidity,
the latter being of importance for long upper terminals. The
disadvantages are the necessity for numerous joints, and the difficulty
of avoiding serious disfigurement to the building to which they are
attached.
=Tubes= have much the same merits and demerits, with the additional
objection that they are necessarily of larger diameter than solid rods,
and therefore more conspicuous. They have also an additional
disadvantage in that they are generally joined together by screw
collars. The cutting of the thread in the tube seriously diminishes the
sectional area, and the joint so made is electrically defective. If
tubes are used, the joints should be made as directed in the code of
rules under the head of joints.
=Tape= is a form of rod which is of comparatively recent introduction,
and possesses many advantages. Foremost among these is the length which
can be supplied in a single piece. Where, as at the junction with an
upper terminal, a joint is needed, it is easily made by clamping or
rivetting the two surfaces together and then imbedding the whole in a
mass of solder. No kind of coupling known to us is, in our opinion,
equal to this very simple one. Owing to the flexibility of the tape it
can be made to follow closely the outlines of a building, or may be
countersunk in it, and painted over, but, as stated further on, abrupt
bends should be avoided, and the precautions and instruction set forth
on page 18 should be followed. The objections to tape, Appendix A, pages
(5) and (16) will be found to be objections, not to tape _per se_, but
to bad practice on the part of some persons who have fitted it up and
availed themselves unduly of its flexibility.
=Ropes.=—For many years past rope constructed of twisted strands of
copper or of iron wires has been largely employed for lightning rods.
There is on record a very remarkable case of the complete destruction of
a brass wire rope, an event which, if it had been repeated, might justly
have been regarded as a serious objection to the use of ropes. This case
is fully reported in Appendix F, pages (62–63); and from it some French
electricians have concluded that lightning may single out some wires
from a rope and travel along them in preference to the rest, even when
the whole of them are hardly sufficient to give it a free passage.
Whatever may have been the explanation, this accident seems to be
unique, and even if we accept the explanation given, the only extra
precaution which it calls for, is the soldering of each extremity of the
several wires forming the rod, and at every joint, into a single mass.
We agree with M. Borrel in thinking that serious evil arises from using
wire of too small diameter, which involves an additional number of
interstices for the lodgement of dirt, smoke, and water, and at the same
time renders the wires too thin effectually to resist oxidation. We have
had before us rope ⅜ in. in diameter, composed of 49 strands of a copper
wire about No. 19 B.W.G., say 0·04 in. in diameter. On the contrary, one
firm speaks of employing No. 10 B.W.G., _i.e._ 0·14 in. diameter, and in
special cases Nos. 8 and even 7, which would be about 0·17 in. and 0·19
in. diameter respectively: these would not be open to the objection we
have raised.
The objection to thin wires is necessarily greater with iron ropes, even
if galvanized, than with copper, for irrespective of the doubt as to the
perfect galvanizing of every part, there is the greater brittleness, and
consequent risk of damage from defective continuity.
=Ropes with Hemp Centres.=—One English firm sent us a specimen of
6–strand copper rope with a hemp core, and we understand that the same
pattern is occasionally used both in iron and copper in France. We do
not know the precise object aimed at—probably flexibility—but
considering the perishableness of such a core, its variation in length
with the hygrometric state of the air, and its invariability when the
copper is varying with temperature, we cannot regard it as a wise
construction.
=Plait.=—This form of rod was probably designed in the belief that the
essential element in a lightning rod was plenty of surface. It is made
in two sizes, with copper wire, about No. 16 B.W.G., plaited into a sort
of ribbon. It invites oxidation as much as is possible, and is in our
opinion neither durable nor trustworthy. The original form of this rod
was ridiculously bad; for it consisted of 13 copper wires and 1 zinc
one. Every time that it became wet, feeble electric action was set up,
and the zinc wire was gradually destroyed, without the slightest benefit
to anybody.
=JOINTS.=—The most fruitful sources of danger in rods are _bad joints_,
not necessarily those that are mechanically bad, but those that are so
electrically. A joint is said to be electrically bad when it offers
resistance to the passage of electricity through it. _There should be no
resistance whatever._ A careful inspection by Capt. Bucknill, R.E.
(Appendix M, p. 243), has proved that bad joints in lightning rods are
very abundant, though they appear perfectly sound; and everyone who has
measured the electrical condition of conductors confirms this fact. Bad
joints have the same effect as lengthening the conductor; and, in one
case, one bad joint was found to have the same effect on a discharge of
electricity as a conductor 1,900 miles long. It is evident that such
rods may be worse than useless, for other parts of the building may
offer easier paths for the discharge to the earth. If the joint be
imperfect, and the rod convey a charge to earth, heat will be generated
at the joint, the rod may be fused, and the discharge be diverted to the
building.
Screwed, scarfed, and rivetted joints, however well they may be made
mechanically, are certain to rust and corrode in time, owing to the
expansions and contractions due to changes of temperature admitting
moisture, and thus causing corrosion and resistance. No joint can
possibly be electrically perfect that is not _metallically continuous_,
and careful soldering, in addition to screwing, scarfing, or rivetting,
is the only certain mode of securing this. Soldering is a method that
has borne the test of experience, and its success as a means of securing
perfect joints leaves no excuse for its omission. The fewer joints the
better, but where there are joints they can only be made electrically
secure by careful soldering.
=PROTECTION OF ROD.=—The lower part of copper rods is sometimes stolen
for the sake of the metal. This can be guarded against by putting it
inside a length of iron gas-barrel, extending from some distance below
ground to 10 ft. above it.
=PAINTING.=—Iron conductors, even if they are galvanized, should be
painted throughout, except at the points, which should be gilded or
nickel-plated.
In France and Belgium painting is resorted to to a considerable extent,
and the practice was recommended by the late Professor Joseph Henry, and
followed very largely in America. [Appendix F, pages (99) and (113).]
=ATTACHMENT TO BUILDINGS.=—The evidence against the use of glass or
other material in order to insulate the conductor, is overwhelming, and
insulation may be regarded as unnecessary and mischievous. The
essentials are (1) that the rod be attached to the building by
fastenings of the same metal as itself, (2) that the fastenings be of
adequate strength, (3) that they be of such form as not to compress or
distort the rod, (4) that they allow play for its expansion and
contraction, (5) that they hold it firmly enough to prevent all the
weight falling on any one bearing.
Where practicable it is well to take the rod down that face of the house
which is most exposed to rain.
=EARTH PLATES.=—This portion of the lightning conductor is of the utmost
importance, but has hitherto been the most neglected. The majority of
cases in which lightning has caused injury very near to or upon
conductors are traceable to those conductors having imperfect earth
terminals. We know of many cases in which the earth terminals have been
miserably imperfect, or entirely neglected, when the above-ground
portion has been perfectly satisfactory. In fact, though it may be
admitted that the case found by Dr. Mann,[2] of the lightning rod of a
church tower, the lower end of which was thrust into an empty glass
bottle, is an exceptionally bad one; yet there are sadly too many, of
which the Middlesboro’ case, Appendix I, page (217) is a perfectly fair
type.
Footnote 2:
Quarterly Journ. Met. Soc., Vol. II., p. 420.
A convenient earth connection is often afforded in towns by the iron
mains for gas and water—arguments both for and against the utilisation
of both water and gas mains will be found in the Appendix—we, therefore,
need only state our opinion in favour of connection with both. But no
connection should ever be made with soft metal pipes, because of the
risk of their fusion; and the conductor should be kept as far as
possible from internal gas pipes on account of the risk of lighting the
gas at an imperfect joint.
As a general rule we advise the soldering of a plate of metal, copper to
copper, iron to iron, to the lower end of the conductor. The earth plate
should always be of the same metal as the rod, otherwise destructive
galvanic action sets in. This plate, which may be flat or cylindrical,
must not have less surface than 18 square feet, _i.e._, 9 square feet on
each face; there is no advantage in notching or pointing it. A hole must
be dug, or well sunk, to receive this plate, and the hole must be so
deep that the earth surrounding the plate shall _never_ be dry. Any
available drain or other water should be allowed to soak into the earth,
over the site of the plate. After the hole has been dug, and the plate
lowered into position, it should be filled with cinders, or coke. In
extremely dry rocky localities, it is sometimes impossible to fulfil
these conditions: then the best thing to do is to bury three or four
hundredweight of iron at the foot of the conductor, still using the
earth plate and the coke, and taking especial care that the rain-water
and sink pipes discharge over it.
All drains, water-courses, in fact, everything which will assist in
distributing the charge over a large extent of moist earth should be
utilized by leading branches from the earth plate to them, or a long
length of the rod may be laid in a drain if it be one which will be
constantly wet.
=SPACE PROTECTED.=—The question as to the extent of the space which will
probably be protected by a lightning rod is one which is of very great
practical importance, because it governs the number and height of the
upper terminals which are required for the protection of any given
building. The index to the Appendix shows that “Protection, Area of,” is
discussed upon twenty-nine pages in different parts of the Appendix. It
has been laid down that the space protected was a cone, having the point
for its apex, and a base whose radius was equal to twice the height of
the point, while the latest French official instructions, Appendix F, p.
(67), state that a point will “effectively protect a cone having the
point for its apex, and a base whose radius is 1·75 of its height.” The
English War Department instructions considerably reduce this space by
asserting, Appendix F, p. (71), that “no precise limit can be fixed to
the protecting power of conductors. In England the base of the protected
cone is usually assumed to have a radius equal to the height from the
ground; but though this may be sufficiently correct for practical
purposes, it cannot always be relied upon.”[3]
Footnote 3:
On page (96) two instances are recorded in which, if the evidence can
be trusted, the stroke fell within a radius equal to the height, but
it is only right to say that the facts are not very clearly recorded.
[Illustration: Sketch illustrative of area of protection]
According to this rule, the church of Ste. Croix (see Appendix F, p.
(141)), would require four upper terminals, one on steeple, one on
chancel, and one in the middle of each half of the transept.
From theoretical considerations stated by Mr. Preece, Appendix F, p.
(137), he arrives at the conclusion that “A lightning rod protects a
conic space whose height is the length of the rod, whose base is a
circle having its radius equal to the height of the rod, and whose side
is the quadrant of a circle, whose radius is equal to the height of the
rod.”
At present we have not sufficient data to enable us theoretically to
calculate the space protected by a lightning rod, and therefore we are
compelled to draw up our rules upon the question entirely from
experience, and here we find, that with the doubtful exceptions already
mentioned, there is no recorded instance of a building being struck by
lightning within a conical space, the radius of whose base was equal to
its height, and we think that the adoption of this rule may reasonably
be expected to yield that security in the future, which as far as we
know, it has done in the past.
=HEIGHT OF UPPER TERMINAL.=—This matter is one which may be left
entirely to the option of individual architects and engineers, subject,
of course, to the opinions expressed under the heading “Space
Protected.” In France extremely long _tiges_, or upper terminals,
generally 33 feet long, are used; but it is obvious that they are
necessarily very strong and heavy, and both by their weight and by the
great leverage which they exert when there is any wind, they must
produce serious vibrations in the roof. In England hitherto the opposite
error is almost universal, and we seldom see a conductor carried high
enough to protect all the building to which it is attached. The question
of appearance comes in here, but concerning it we need only remark that
while in England care seems generally taken to conceal the conductors,
in France they are, to a certain extent, made features of the edifice.
With a proper exercise of taste, the terminals of the lightning
conductors can be made to assist the ornamentation of the building, as
has been done in many cases.
=TESTING CONDUCTORS.=—Periodical examination and careful testing of the
lightning conductor are requisite to maintain the system in efficient
order. Points will corrode from oxidation and fusion; joints will get
loose and bad through the action of weather and workmen; connections
will decay both above and below ground; imperfections will develope
themselves; alterations will be made by landlords and tenants; and, in
spite of every precaution during erection, the conductor will thus lose
its efficiency if it be not _maintained_ in thorough order. For this
purpose inspection should be both visual and electrical. In order to
facilitate the electrical examination of the conductor, some firms have
erected a double rod, connected with one upper terminal, one on each
side of a chimney or shaft; this is a very efficient arrangement, for it
provides a means for testing from the ground. It has also been proposed
to carry an insulated wire alongside or even within the rod, connected
to the terminal at the top, and to the testing apparatus at the bottom.
A testing apparatus has been devised by Mr. Anderson (_Lightning
Conductors_, p. 60). M. Borrell, Appendix K, p. (226), Captain Bucknill,
R.E., Appendix M, p. (244), and Mr. Vyle, Appendix M, p. (244), have
also introduced apparatus for the purpose. The system in use in Paris,
Appendix K, p. (225), and M, p. (245), is perhaps the simplest and
cheapest, and is effective as regards testing the efficiency of the
conductor, but not that of the earth connection.
The efficiency both of the conductor and of its earth terminal should be
annually tested. As this testing involves some skill and familiarity
with electrical apparatus it would be advantageous if some competent
person were officially appointed, either by the government or by some
recognised authority, to perform this duty.
=INTERNAL MASSES OF METAL.=—All large and long masses of metal, such as
beams, girders, pipes, hot water systems, and large ventilators fixed in
the interior of buildings, should be electrically connected with the
earth, or with the conductor; but the soft metal gas pipes should never
be used as conductors. The inlet and outlet pipes of large meters should
always be, independently of the meter, electrically connected with each
other, for two remarkable cases of the explosion of a meter have
occurred through the presence of a joint in the pipe electrically bad
owing to the use of India-rubber packing. Appendix M, p. (239).
=EXTERNAL MASSES OF METAL.=—Large constructive and decorative ironwork,
such as guttering, flashings, railings, finials, vanes, &c., and all
masses of metals used in building, should be connected to each other,
and to the earth direct, or to the conductor. In fact, the gutters and
water pipes are already frequently utilized as a partially protective
system. The ventilators of soil pipes may also be employed in this way,
and even made sightly by the addition of an ornamental finial fitted
with points, but care must be taken that the joints are metallic and not
made with red lead or putty; and it must not be forgotten that the
conductivity of lead is very small, so that undue reliance must not be
placed upon pipes made of that metal.
SECTION III.—_Code of Rules for the Erection of Lightning Conductors._
The following Code of Rules should be carefully attended to in drawing
out a specification for a Lightning Conductor, the reasons for each
being given in the previous Sections and in the Appendix:—
=Points.=—The point of the upper terminal should not be sharp, not
sharper than a cone of which the height is equal to the radius of
its base. But a foot lower down a copper ring should be screwed
and soldered on to the upper terminal, in which ring should be
fixed three or four sharp copper points, each about 6 in. long. It
is desirable that these points be so platinized, gilded, or nickel
plated as to resist oxidation.
=Upper Terminals.=—The number of conductors or points to be specified
will depend upon the size of the building, the material of which
it is constructed, and the comparative height of the several
parts. No general rule can be given for this; but the architect
must be guided by the directions given at pp. 12 to 14. He must,
however, bear in mind that even ordinary chimney stacks, when
exposed, should be protected by short terminals connected to the
nearest rod, inasmuch as accidents often occur owing to the good
conducting power of the heated air and soot in a chimney (p. 2).
=Insulators.=—The rod is not to be kept from the building by glass or
other insulators, but attached to it by metal fastenings. (See p.
11.)
=Fixing.=—Rods should preferentially be taken down the side of the
building which is most exposed to rain. They should be held
firmly, but the holdfasts should not be driven in so tightly as to
pinch the rod, or prevent the contraction and expansion produced
by changes of temperature.
=Factory Chimneys.=—These should have a copper band round the top, and
stout, sharp, copper points, each about 1 ft. long, at intervals
of two or three feet throughout the circumference, and the rod
should be connected with all bands and metallic masses in or near
the chimney. (See p. 5.) Oxidation of the points must be carefully
guarded against.
=Ornamental Ironwork.=—All vanes, finials, ridge ironwork, &c., should
be connected with the conductor, and it is not absolutely
necessary to use any other point than that afforded by such
ornamental ironwork, provided the connection be perfect and the
mass of ironwork considerable. As, however, there is risk of
derangement through repairs, it is safer to have an independent
upper terminal. (See p. 4.)
=Material for Rod.=—Copper, weighing not less than 6 oz. per foot run,
and the conductivity of which is not less than 90 per cent. of
that of pure copper, either in the form of tape or rope of stout
wires—no individual wire being less than No. 12 B. W. G. Iron may
be used, but should not weigh less than 2¼ lbs. per foot run. (See
pp. 5 to 10.)
=Joints.=—Although electricity of high tension will jump across bad
joints, they diminish the efficacy of the conductor; therefore
every joint, besides being well cleaned, screwed, scarfed, or
rivetted, should be thoroughly soldered. (See p. 10.)
=Protection.=—Copper rods to the height of 10 feet above the ground
should be protected from injury and theft, by being enclosed in an
iron pipe reaching some distance into the ground.
=Painting.=—Iron rods, whether galvanized or not, should be painted;
copper ones may be painted or not according to architectural
requirements.
=Curvature.=—The rod should not be bent abruptly round sharp corners.
In no case should the length of the rod between two points be more
than half as long again as the straight line joining them. Where a
string course or other projecting stone work will admit of it, the
rod may be carried straight through, instead of round the
projection. In such a case the hole should be large enough to
allow the conductor to pass freely, and allow for expansion, &c.
=Extensive Masses of Metal.=—As far as practicable it is desirable
that the conductor be connected to extensive masses of metal, such
as hot-water pipes, &c., both internal and external; but it should
be kept away from all soft metal pipes, and from internal
gas-pipes of every kind, respecting which see page 15. Church
Bells inside well protected spires need not be connected.
=Earth Connection.=—It is essential that the lower extremity of the
conductor be buried in permanently damp soil; hence proximity to
rain-water pipes, and to drains, is desirable. It is a very good
plan to make the conductor bifurcate close below the surface of
the ground, and adopt two of the following methods for securing
the escape of the lightning into the earth. A strip of copper tape
may be led from the bottom of the rod to the nearest gas or water
_main_—not merely to a lead pipe—and be soldered to it; or a tape
may be soldered to a sheet of copper 3 ft. × 3 ft. and 1/16 in.
thick, buried in permanently wet earth, and surrounded by cinders
or coke; or many yards of the tape may be laid in a trench filled
with coke, taking care that the surfaces of copper are, as in the
previous cases, not less than 18 square feet. Where iron is used
for the rod, a galvanized iron plate of similar dimensions should
be employed.
=Inspection.=—Before giving his final certificate, the architect
should have the conductor satisfactorily examined and tested by a
qualified person, as injury to it often occurs up to the latest
period of the works from accidental causes, and often from the
carelessness of workmen. (See p. 14.)
=Collieries.=—Undoubted evidence exists of the explosion of firedamp
in collieries through sparks from atmospheric electricity being
led into the mine by the wire ropes of the shaft and the iron
rails of the galleries. Hence the headgear of all shafts should be
protected by proper lightning conductors.
(Signed)
W. GRYLLS ADAMS.
W. E. AYRTON.
LATIMER CLARK.
E. E. DYMOND.
G. CAREY FOSTER.
D. E. HUGHES.
T. HAYTER LEWIS.
W. H. PREECE.
G. J. SYMONS.
JOHN WHICHCORD.
_December 14th, 1881._
APPENDIX A.
CIRCULAR AND QUESTIONS
ISSUED TO
Manufacturers of Lightning Conductors,
AND
THEIR REPLIES THERETO.
NOTE.—There are only two points requiring mention respecting the
following replies. First, that in order to avoid useless
repetition of the questions, the answers are numbered, and the
corresponding question will be found in the following circular.
Secondly, that the replies are verbatim, as received from the
manufacturers, except that frequent entries will be found in
square brackets, _e.g._ [A 0·11 in.] These represent approximately
the sectional area of the conductors, and are given to facilitate
the comparison of the conducting capacities of the very various
patterns submitted to the Conference.
CIRCULAR.
LIGHTNING ROD CONFERENCE.
30, GREAT GEORGE STREET, WESTMINSTER, S.W.
_November 14th, 1878._
At the invitation of the Meteorological Society delegates have been
nominated by the following Societies:
Royal Institute of British Architects,
Society of Telegraph Engineers,
Physical Society,
Meteorological Society,
to consider the present modes of erecting Lightning conductors, and
improvements therein.
At a largely attended meeting held this day I was instructed to
forward to you the questions stated below, and to request you to
forward with your replies any remarks which you may wish to lay
before the Conference.
If you desire any specimens to accompany your remarks, I shall be
glad if, whenever possible, they do not exceed five inches in
length.
I am,
Your obedient Servant,
G. J. SYMONS,
_Secretary to the Conference_.
QUESTIONS.
(_It is requested that the replies be written on foolscap paper, on one
side only, and that they be numbered in accordance with the questions._)
1. Form, dimensions, and material usually adopted by you for upper
terminals.
2. Material and dimensions of conductor.
3. Is any definite proportion between the length and sectional area of
the conductor observed, and if any, what?
4. Joints, how made.
5. Attachment to building, how made.
6. Ground connection, how formed, and of what extent.
7. Extent of area supposed to be protected.
8. If there is more than one terminal, is the size of the conductor
increased?
REPLIES.
39, WAPPING, LONDON, E.
1. The upper terminals are made of a copper tube ⅝ inches in diameter
and 1/16 inches thick [A. 0·11 in.] In the upper end of the tube is
fitted 15 inches of copper rod tapered to a point at the top, into which
is fixed 3 or more smaller rods about ¼ inch in diameter [A. 0·05 in.]
each tapered to a point, and brought into the parent rod in a curve (not
at an angle). The next part of the tube, down to about 9 inches from the
bottom, is filled with a stiff iron rod to strengthen it, the lower end
of the tube being left open to receive the rope. This constitutes what
is called “the point.” These points vary in length from 2 or 3 to 8 or
10 feet when used for buildings. A square-topped tower would require a
much higher point than would be necessary for the top of a spire.
Sometimes the points are tipped with platinum, which we consider to be
altogether superfluous.
2. The conductor is simply a wire rope, varying in size, and mostly
either ⅜, ½, or ⅝ inches in diameter [A. 0·11, 0·20, or 0·31 in.] These
ropes are made in two different forms: the one ⅜ inch diameter [A. 0·11
in.], most suitable for ships’ use, is composed of 49 No. 18 guage
copper wires, each wire having a circumferencial measurement of ·157
inches [A. 0·002 in.]; the circumferencial or surface measurement of the
whole of the 49 wires is equal to 5·693 inches [A. 0·11 in.], or say,
equal to the surface of a copper band 2·846 inches wide—_i.e._,
measuring both sides of the band.
The other make, say ½ inch diameter [A. 0·20 in.], much used for lofty
buildings, is composed of 7 No. 7 guage copper wires, each wire having a
circumferencial measurement of ·581 inches [A. 0·027 in.]; the
circumferencial or surface measurement of the 7 wires is equal to 4·067
inches [A. 0·19 in.], or say equal to the surface of a copper band 2·033
inches wide.
3. There is no definite proportion observed between the length and
sectional area of the conductor. We take it that the sectional area
should be the same, irrespective of length, as we do not trace that
lightning varies in intensity while passing through a conductor of
greater or less length. The rather prevalent idea that a smaller
conductor is sufficient for a low building is, we think, erroneous, as
we do not find any data to show that lightning in its descent loses any
portion of its force until it actually enters the earth.
4. The copper rope is joined to the upper terminal by passing the end of
the rope into the tube at the lower end of the terminal for the space of
about 9 inches, and fixing it with 3 copper rivets. There is no other
join in the conductor whatever—a feature of much greater importance than
is sometimes admitted.
5. The upper terminal is passed through two strong earthenware
insulators which are usually fixed to the building by two strong
galvanized iron staples. Other modes of fixing the terminal must
sometimes be resorted to, as some factory chimneys are capped with iron,
and buildings of varied forms must be treated with according to
circumstances. Having fixed the terminal, the rope may then be led down
the building on the most convenient side for the purpose, and fixed at
intervals of 6 or 8 ft., according to circumstances, with glass
insulators supported by copper brackets. The rope should be given the
straightest course practicable from the upper point down to the earth,
carefully avoiding all angles, specially an acute angle, as much as
possible, and in its passage it should be kept clear from any other
metal in the building.
There are three matters to which we would call special attention, viz.:
_Insulators, Angles and Joints, Metal in Building._
_Insulators._—When copper rope lightning conductors were first
introduced, about the year 1837, a circumstance occurred which at once
proved the efficiency of the conductor, and suggested the use of
insulators. The late Mr. Andrew Smith, C.E., had fitted a factory
chimney in the East of London with a rope conductor, which was fixed to
the chimney by iron staples. In a violent storm which occurred soon
afterwards, the lightning was seen to pass down the conductor, which
remained unaltered in any way; but on examining the chimney it was found
that the brickwork had received a concussion at most, if not all, of the
staples, showing that the lightning in passing had expended part of its
force on the iron staples. It is probable that, if the staples had been
made of thicker iron, and had been so placed as to lead off from the
conductor with easy curves inwards, instead of being driven into the
wall at right angles with the conductor, the concussions would have been
much more violent than was the case.
_Angles and Joints._—It must be obvious to any one that lightning, as
well as any other matter or thing which travels at high speed, would be
greatly obstructed in having to turn corners. It must also be borne in
mind that lightning is of intense heat, and while passing in a straight
line the effect of its heat is lost in the velocity of its motion; but
in passing an angle its momentary pause (much too brief to be
calculated) is sometimes enough to create sufficient heat to fuse the
conductor at the angle. For this reason all angles must be avoided, and
easy curves having a downward tendency substituted.
The angles in copper tube conductors are doubly objectionable, for,
having joins as well as angles, they are liable, by the effect of heat,
to become disjointed. It would be difficult, if not impossible, to fit a
tubular conductor, except in a straight line from end to end, without
this double objection. Similar objections apply also, in a greater or
less extent, to the copper band conductors, as they are made with joins,
and, when fixed up, are usually carried into and over as many angles as
come in their way. They do not so readily follow all the sinuosities of
a building as a rope does on the curve principle.
The flat band conductors, which are composed of a number of galvanized
iron and copper wires combined, are simply a frivolity.
_Metal in Buildings._—Taking the conductivity of copper as from 7 to 10
times greater than that of iron, it would probably follow, that if two
rods, the one of copper the other of iron, in these proportionate sizes,
were brought together in one common terminal or point, and led by the
same course to the earth, as much of the fluid would possibly pass down
the one as the other. On this principle, we avoid contiguity with any
metal in a building, especially if in large masses, such as machinery,
&c.
_Ships’ Conductors._—In fitting the rope conductor to a ship’s rigging
it is only necessary to pass it through a hole in the truck, so that the
end may stand about 6 inches above the truck. It may be held up by a pin
or key passed through the rope close over the truck, and then carried
down the topgallant backstay (to which it should be tied at intervals
with yarn) to the gunwale, where a sufficient length of the conductor
should be kept in a coil to reach well down into the sea in any position
of the ship. In stormy weather the coil may be untied, and by its own
weight the end will drop down into the sea as required. Sometimes the
rope is shackled at the gunwale to a strip of sheet copper about 3
inches wide, which is nailed down the ship’s side till it meets the
sheathing at the bottom. The strip of copper should _overlay_ the
sheathing for a few inches. It may be noticed that this kind of
conductor, fitted with a coil at the gunwale, is without any join
whatever, and that it takes almost a straight course direct from the
truck into the water.
The copper band conductors let into the mast and carried through the
hull of the ship are objectionable and unsafe, as, in passing from each
portion of the mast, they require moveable joints, so as to admit the
several parts of the mast being run up or down as required. These joints
present angular interruptions which may become out of order, and, in
passing through the hull, any rupture of the band in that part, or the
contiguity of other metals, may cause serious consequences. Certainly,
there can be no necessity for carrying the lightning through the ship
when, by a safer and much more simple method, it may be kept altogether
outside.
In the smaller vessels, where the mainmast is well above the other
masts, it may be sufficient for that mast only to be fitted with a
conductor, but in larger ships, particularly long steam-ships, where the
masts are a considerable distance apart, each mast should have a
conductor.
We do not, either in theory or in practice, know any necessity for
protecting the yards with conductors, though it is not altogether
improbable that, in the absence of conductors on the masts, the yards
might get damaged while the masts remain uninjured.
6. The end of the rope should be buried in moist earth, and carried in a
curve to 5 or 6 feet from the foundation. In clay ground and on the
shady side of a building about 3 feet below the surface would be deep
enough; but in lighter ground, and particularly on the sunny side, it
should be buried 6 or 7 feet deep, to ensure sufficient moisture at all
times of the year.
7. As the course by which lightning approaches the earth is very
devious, it would be difficult to determine with certainty the extent of
area protected; but, viewing the absence of damage to the most remote
parts of the roofs of buildings which have been properly fitted with
conductors within the last 40 years, we should think the area protected
may be taken as equal to from 3 to 5 or 6 times the height of the
conductor.
8. When two or more terminals are used, the main rope should be somewhat
enlarged; otherwise, the collective quantity of fluid received on the
several points may be too great for the common channel.
WILKINS & WEATHERBY.
* * * * *
DORA STREET, LIMEHOUSE, E.
We have to own receipt of your valued communication of the 14th ult.,
and with great pleasure to submit for the consideration of the
Conference the following replies to their questions. We have endeavoured
to make them as explicit as possible, but it is difficult adequately to
describe our system on paper, and we suggest for the consideration of
the Conference the advisability of showing any Committee they may
appoint one or two of the numerous public buildings fitted by us.
Any further particulars or drawings you may require we shall be glad to
send you; and it is with great pleasure that we add that any services we
can render you in your valuable investigations are at your disposal.
1. A five-pointed copper spindle, the sharp points of which are
silvered, and single points on high chimney shafts to the number of four
or five.
2. Copper solid bands, or tubes, “as samples sent,” being simple,
durable, cheap, and the most capacious form for the safe conduction of a
heavy stroke of lightning, the bands being from 1 inch to 3 inches in
width and ⅛ inch thick [A. 0·12 to 0·37 in.], and the tubes from ¾ to 1½
inches in diameter and ⅛ inch thick [A. 0·24 to 0·54 in.].
3. Yes; experience has proved that nothing less than 1½ inch bands [A.
0·18 in.] should be used for the main conductor to ordinary houses, with
¾ [A. 0·09 in.] to 1 inch [A. 0·12 in.] bands for branches, and from 2
to 3 inches [A. 0·24 to 0·37 in.] bands as main conductor to buildings
of large area, with 1 to 1½ inch [A. 0·12 to 0·18 in.] for branches; or,
in the case of chimney shafts, ¾ inch to 1½ inch tube [A. 0·24 to O·54
in.] for main conductor, and 2 to 3 inches flat band [A. 0·24 to 0·37
in.] for tops of same.
4. The bands are in long lengths, are lapped, closely rivetted and
soldered, to form a continuous band; while the tubes have patent
insertion joints, the upper end being turned and fitted into the lower
end, which is bored, and the tube then forms a continuous line
externally and internally.
5. Copper holdfasts to suit shape and size of conductor.
6. Not less than 30 feet of 1½ inch to 2 inch copper bands [A. O·18 to
0·24 in.] in two or three branches, with forks at end of each band, and,
if water is not near, the trenches half filled with carbonaceous
materials and well watered, as this material will readily absorb the
least moisture and retain it, while being in itself the best conductor.
But much will depend upon the nature of the ground; for if chalk or rock
foundation and water cannot be got at, the ground branches must be at
least doubled, and the trenches deeper and made up of carbonaceous
materials and earth.
7. Our experience is that no appreciable extent is protected by a single
rod conductor in the presence of other influences. The chimney-stacks,
lined with carbon in the shape of soot, with the heated gases, cause a
rarefaction in the atmosphere, and form an easier passage for the
electric fluid. Roofs and buildings having large masses of metals will
be more likely to influence lightning than the single line of copper rod
generally fitted. Many cases have occurred of chimney-stacks 4 feet to 9
feet across being struck opposite the conductor, and lead roofs,
gutters, lead ridges, &c., from 10 feet to 20 feet from the rod
conductor.
8. No; the system of conduction used by us does away with this, the
lines of conduction being ample.
REMARKS.
From our close connection with the late Sir William Snow Harris, adviser
to the Crown for upwards of twenty-five years in regard to lightning
conductors for the navy, and having made lightning conductors our
especial practical study for thirty-five years, we may be pardoned for
making a few remarks on the protection of buildings from lightning.
We would, firstly, say that the system of conductors now fitted by us is
based upon these past years of experience, and upon facts collected
during this period, of accidents to buildings having the ordinary single
line of conduction, as also from the practical success of the conductors
in the navy.
The form of conductors used by us has been adopted after considerable
experience, as being the most simple, solid, durable, and capacious form
of conductor for the safe conduction of heavy strokes of lightning.
In place of insulators as fastenings, we use copper holdfasts, as we
found the former dangerous and useless, as the glass, being
non-conductive, the expansion and heat of the electric fluid, being
confined, broke them, and caused an unsafe concussion; and it is also a
disadvantage for a conductor to be away from the building, as nearly
every material in nature assists, without detracting from, the safe
discharge of the electric fluid through a good copper conductor. We find
that the copper wire rope conductor, usually applied, is seldom more
than ⅜th of an inch in diameter; but we did once remove, from the tower
of St. Mary’s Church, Taunton, a copper wire rope conductor of ⅞th of an
inch in diameter [A. 0·60 in.], said to be especially made to
order—certainly the largest we ever came across; but it failed to give
the necessary protection in a lightning storm, which did much damage to
the tower and roof of the church. As capacity or weight of copper is the
most important for safe conduction, copper wire rope is very deceptive
in this respect, as will be seen by the following comparisons, viz.:—A
copper wire rope conductor of ⅜ inch diameter [A. 0·11 in.] weighs 2¾
ounces per foot, not equal to a plain solid band ⅜ inch wide and ⅛ inch
thick [A. 0·046 in.], which weighs 2·907 ounces per foot. A copper wire
rope conductor of ½ inch diameter [A. 0·20 in.] weighs 5 ounces per
foot, not equal to a solid band of ¾ inch wide and ⅛ inch thick [A.
0·092 in.], which weighs 5·814 ounces per foot. A copper wire rope
conductor of ⅝ inch [A. 0·31 in.] weighs 9½ ounces per foot, not equal
to a solid band of 1¼ inch and ⅛ inch thick [A. 0·153 in.], which weighs
9·690 ounces per foot. This is the largest size of wire rope conductor
made or used.
From the above will be seen what protection can be given by conductors
of such small capacities; and we may add that solid band conductors of
the same weight, and superior in every way, can be fixed at less than
half the cost of the wire rope, _foot_ for _foot_.
Copper chains and copper wire bands, as conductors, answer in so
uncertain a manner with the galvanometer, that they should never be
used.
Iron in any form should be avoided, from its lower conducting power, and
its utter uselessness when in a rusty and decayed state.
With regard to testing with the galvanometer, the mere testing of the
conductors is no proof of the security of the building itself. We not
only test the conductors, but also the building, to prove that it is
under safe conduction in lightning storms.
In conclusion, we beg to state that our patent system of protection is
the application of one or more main down and ground copper conductors
and sizes, according to the height and area of the building, the fitting
of the copper bands to each chimney-stack, and connecting the same, and
the connecting of all the metals on the roofs thereto and to the main
conductor, so that there shall be no circuit by which the lightning
fluid would be likely to attack without having its exit to the main
conductor.
For high working chimney-shafts we fit a copper band round the top, and
four points thereon connected to main down conductor.
For further information, we earnestly solicit the careful perusal of our
pamphlet and papers herewith.
J. W. GRAY & SON.
* * * * *
CHIPPENDALE MEWS, HARROW ROAD.
1. Upper terminals pointed with one or more points, according to the
nature of the building to be protected. Dimensions vary in like manner.
Material—copper or brass, with electro-gilded points.
2. Conductor composed of copper or galvanized rope, according to height,
&c., of building, &c., dimensions varying with resistance of the
circuit.
3. The sectional area varies with the length.
4. Joints made, as far as possible metallically; where solder cannot be
used, screw joints are made use of.
5. Attachment to building direct by metallic ties of requisite form.
6. Ground connection—When practicable, the end of conductor is
metallically connected with gas or water _main_, otherwise a hole is dug
deep enough to meet always moist earth. The end of conductor is either
attached to an earth plate, or coiled up in a bundle and surrounded by
coke.
7. The area protected is supposed to be a radius equal to the height of
conductor.
8. If more than one terminal is attached to one conductor, the size of
the latter is increased, except under certain conditions.
F. RUSSELL & CO.
* * * * *
137, PRINCESS STREET, MANCHESTER.
1. A copper tube 1¼ inch diameter or 1 inch diameter, finished at the
upper end, with a forged copper point or cone, connected with the tube
by a cast copper (or gun-metal) coupling, into which coupling are also
screwed three or more smaller points round the larger central one. At
the lower end the tube is screwed into a somewhat similar coupling, to
receive also the brazed and screwed end of the conductor. Or a solid
copper rod ½ inch diameter [A. 0·20 in.], or _wrought_ iron rod 1 inch
diameter [A. 0·79 in.] (where iron conductors are used) the rod in
either case forged to a blunt point, and screwed at the lower end, like
the tube first described, to fit the coupling.
2. (_a_). Copper wire rope of 7 strands each, No. 10 Birmingham wire
gauge, or in specified cases of No. 8 or 7 wire gauge, making, when
spun, a rope with a sectional area varying from 7/16 to 11/16.
(_b_). Solid copper rods ½ inch diameter [A. 0·20 in.].
Solid iron rods 1 inch diameter [A. 0·79 in.].
(_c_). Copper band or “tape” of sizes from ¾ × ⅛ to 2 or 3 × 3/16
inches [A. 0·09 to 0·38 or 0·56 in.].
(_d_). Copper tube ⅝ inch diameter outside, and ⅛ inch thick [A. 0·20
in.]
3. Although no definite rule exists for the proportional sizes of the
conductor, it is usual and prudent in a large building to employ for the
main conductors, which should come from the highest and most exposed
points to the earth in the most direct way, a larger conductor than
would be required for a small building, and the branches or connections
to this main conductor may be smaller in sectional area than the
principal one. Thus, a church tower with four angle pinnacles may be
protected by four finials or points, one to each pinnacle, and these
four parts fitted to rope of 7 wires No. 10 gauge [A. 0·10 in.], to be
united to a continuous band round the parapet, from whence a rope of 7
wires No. 8 gauge [A. 0·15 in.] should descend into the earth; or an
infirmary or workhouse built with wings would have, perhaps, three
direct rod conductors, one to each chimney stack, and connections with
the water spouts, or lead flashing made of small copper tape ¾ × ⅛ [A.
0·09 in.] _soldered_ to the lead and worked round the rods.
4. The fewer joints the safer, and for this reason—the copper _rope_ or
_tape_ is better than the rod or tube, as the former is made
conveniently any required length, and the danger of a fault or break in
the continuity is avoided. Of the necessary joints the rope requires one
at its junction with the top rod or tube; this is made by brazing a
small ring of brass (or copper) round the rope; the solid end thus
formed being chased with a deep male thread, which fits the prepared
base of the rod. The branch conductors or connections, with adjacent
constructive or decorative iron work—as beams, girders; cresting, vanes,
&c., are made by threading a bead with a similar ring to receive the
branch, as that already described. Where the branch reaches its object a
ring or solid coupling should be “tapped” into the girder or cresting,
to ensure thorough metallic connection, if the destination of the branch
be the lead flashing, the seven wires must be opened like a fan, and
_each wire_ strongly soldered with common plumbers’ solder to the lead—
(_b_). Copper or iron rods are made continuous by couplings of either
metal, as the case may be, which should exceed the diameter of
the rods by enough metal to allow of a good thread. These
couplings should be hexagonal or octagonal in plan, to allow the
workman a certain grip; and the thread should be of the kind
called right and left, so that while screwing one length he may
not unscrew the other. These conductors require very careful,
steady workmen, as a great element of danger exists in these
numerous joints.
5. The various natures of the buildings provided with conductors require
separate, and often different treatment: but the principle in all cases
is the same, viz., to attach the conductor closely to the fabric, and
the more the conductor is made an integral part, as it were, the more
efficacious it will be. Any attempts at so called isolation are opposed
to the theory of protection by conductors. The mechanical means of
fixing are best illustrated by diagrams, the chief objects to be
considered are—
(_e_). Permanence or strength and durability.
(_f_). Room for expansion of the conductor.
(_g_). Facility in fixing without cutting or breaking the conductor.
(_h_). Neatness in appearance.
These objects are gained by a careful consideration of the materials to
which the conductors are fixed by “holdfasts,” for stone, slate or
tiles, wood, and iron. It is important that _sharp_ bends be avoided. A
string course, for instance, should be drilled, and the rod or rope
passed straight through. Also, that any metal bodies in the line of the
conductor should be connected with it by staples screwed into such
bodies. It is most necessary that the ends of vane bolts or rods should
be joined to the conductor, or, where this is impossible, should be
fitted with an independent wire or rod to the earth.
6. The connection with the ground is of special importance, as the
object of the conductor is to provide a free passage between the two
currents, and if this be not done, a lateral discharge is pretty sure to
result. A building provided with suitable conductors, properly fixed,
should at all conditions of the atmosphere, allow a free course to the
electricity, and be in all its parts electrically equivalent, and with
this intention the several parts (as mentioned in answer to question 3)
are brought into connection with each other or with the ground. The
actual length of the ground conductor is fixed by the nature of the
subsoil, as it is obvious that dry sandy soil is unsuitable for a
termination. We therefore continue the rope or tape until a good damp
earth is reached, if possible, a spring or open water—generally
speaking, about 5 to 10 yards will be sufficient in most localities. The
conductor is then buried 5 to 10 feet, or upwards, in the damp earth or
water. If a rope, the several strands are unravelled and opened out: if
a rod or tape, a discharging fork is usually attached to the end to
promote the easy discharge, for which purpose it is also usual to fill
the trench with charcoal. The trench must be dug with a slight fall from
the building downward.
7. The extent of area supposed to be protected by the conductor is
estimated by many as included in a radius of double the height of the
conductor from the base line; but the immunity from accident enjoyed by
many buildings situated at a greater distance from a number of tall
factory chimneys; or to take an opposite example, in a city where there
are many lofty spires or towers, would go to show that a number of
conductors attached to tail objects, serve to obviate the dangers
arising from lightning by providing, at many different points, a direct
communication between the positive and negative currents which exist in
the clouds and earth. We have never known a church spire, when the
conductor was fixed in accordance with ordinary skill, injured by
lightning; and the tall factory chimneys of our manufacturing towns
afford strong corroborative evidence of the value of conductors, and
this in two ways—first, because those to which conductors are fixed, do
not get struck; and, second, because those unprovided with conductors,
_do_ get destroyed from time to time.
8. A reference to the answer to No. 3 question, will show that we
consider that when several terminals are used, an increased diameter is
advisable in the main or principal conductor; but it must be remembered
that either of the conductors referred to in the answer to question 2,
is greatly in excess of what many eminent electricians consider
necessary. A single wire being thought sufficient of 3/32 inch diameter
(A. 0·06 in.) for any ordinary current of electricity. But both the
English and French Governments have thought it prudent to specify a
copper body, with a sectional area of ½ inch in English, or 1 centimetre
in French (0·40 in.)—partly to provide against corrosion, which would
rapidly deteriorate a thin wire, and partly to obviate the danger of the
melting of the smaller conductor under the continued force of an
unusually strong shock of lightning. We, therefore, respectfully follow
the decision of such experts as have, by careful experiment and
considerable diligence, acquired the knowledge they possess—both as to
the substance, the form, and the treatment of this subject; and have
only to add the fact, that any small experience we have practically had,
goes to support the conclusions already arrived at by these authorities.
FREEMAN & COLLIER.
* * * * *
24 & 26, LEVER STREET, MANCHESTER.
1. Our upper terminals are made of copper or brass, plain spike or ball
with spike at top, and three radiating from it, or four or five spikes
radiating from the ball. Attached to the ball (screwed into it) is a
solid rod of copper, to which the conductor is fastened, as explained
below.
2. Conductor is made of good quality copper wire strand 7 ply: ⅜ inch
[A. 0·11 in.] to 7/16 inch [A. 0·15 in.] diameter.
4. Joints of the strand not usually permitted, as we spin it any
reasonable length.
The end of the conductor is knotted and drawn through a cup-shaped ring
of metal one end, the top of which is screwed into the bottom of the
solid rod of the terminal. This makes a good connection.
5. Copper holdfasts fasten the rod to the building.
6. Ground end is coiled loosely in damp earth or a well.
RICHARD JOHNSON, CLAPHAM, & MORRIS.
* * * * *
180, ROTTEMORE, GLASGOW.
We beg to reply to your queries on the material, system, and fitting of
lightning conductors, as practiced by us for over 25 years, during which
time we have never had a building injured in which we have been engaged,
and have fitted from 15,000 to 20,000 feet a year, without
advertisement.
1. Uniformly solid copper, consisting of 1 centre concave point, about
14 inches long, presenting 8 sharp angles = 3½ inch surface; this is
surrounded by 4 smaller points of same construction. These all terminate
or spring out of a hollow copper ball, which is screwed on a copper tube
¾ inch diameter inside, and from 4 inches to 5 inches long, according to
requirement. The copper cable is passed through this tube, is knotted
inside of the ball, and the points are all screwed against it, which
forms the point of contact, and thoroughly fixes the cable at the top;
but the fixing of the top or terminal rod is fashioned in accordance
with the requirements of the building or material to be fixed to.
2. Uniformly copper cable constructed of 49 strands, _hard drawn square_
copper wire Nos. 17, 18, or 19 w.g.
3. We never use less than 6–inch surface, _i.e._, measuring the
circumference of each wire, we contending that surface is the only power
of the conductor. Up to 150 ft. we use No. 19 (= ½ inch diam.) [A. 0·20
in.], ¾ inch for a longer length of cable (_i.e._, 17 or 18) [A. 0·44
in.].
4. Usually with a gun-metal screwed coupler.
5. With brass holdfasts, lined with porcelain, glass, or guttapercha.
6. Spread out end of strands of cable like a fan, and bury it in the
moist earth a few feet deep, in an oblique way tending from building.
7. 30 to 40 yards.
8. We invariably run one cable from each terminal or top rod: but in
spires we commonly take a connection from the _bottom_ of the vane rod,
and connect it to the main conductor, which goes to the highest point of
vane or final: if the former, we fix a copper bush or disc to the vane
rod at foot of vane, which is fast to cable, and a corresponding one on
vane, with cable at highest point, when the cable is fringed out,
presenting its 49 points, and by these discs the vane revolves with that
portion of the conductor attached, and the point of contact is given by
the discs.
C. H. PENNYCOOK & CO.
* * * * *
ALL SAINTS’ WORKS, DERBY.
1. Form for upper terminals:—A straight copper tube, ¾ inch diameter;
thickness of metal, 15 B.W.G. [A. 0·15 in.], with solid copper point (no
branches); the point is soldered and rivetted into the tube; or a solid
copper rod, ½ inch diameter [A. 0·20 in.], tapering towards the top.
2. Material and dimensions of conductor:—Either a copper band of 2½
inches wide and No. 16 B.W.G. thickness [A. 0·16 in.]; or a copper wire
rope, ½ inch diameter, of 6 strands, each strand containing 6 wires [A.
0·20 in.].
3. Proportion between length and sectional area of conductor:—The ½ inch
copper rope [A. 0·20 in.], or 2½ × 16 B.W.G. band [A. 0·16 in.], is used
for heights not over 120 feet; for higher buildings, a ¾ inch rope [A.
0·44 in.] or band, 2½ × 12 B.W.G. [A. 0·27 in.] should be used.
4. Joint, how made:—Joint is made between band and copper rod with a
brass screwed socket, the rod is soldered and rivetted into socket, and
the band is soldered round socket, then soldered and rivetted. When the
copper rope is used, a hole is drilled into socket, same diameter as
rope, at the lower end, and turned out conical shape; the rope is then
passed through the socket, the ends spread out, and the spaces filled up
with solder.
5. Attachment to building:—The conductor is fixed close to building
_without_ insulators, and is brought into close contact with the
spouting; is closely attached to chimney and walls by means of copper
straps and copper nails driven into the masonry.
6. Ground connection:—Should a good, permanent drain be near, the
conductor is brought to it and bound round and firmly fixed.
If there should be an open drain or brook, the conductor is brought
under it at _sufficient_ depth that if the stream be dry at any time
there will be sufficient moisture to carry away the charge without
disruption. Should there be neither drain pipes nor brook sufficiently
near, the conductor is taken from 12 to 20 feet below the surface to the
clay, where it is certain to be always damp, even in seasons of the
greatest drought ever known.
In no case should the earth connection be taken into a closed tank or
well.
If a band be used, it should be cut into strips about 18 inches long and
laid in different directions; rope should be unwrapped and spread in a
similar manner.
7. Supposed area protected:—It is impossible to determine exactly the
area the conductor protects. It is erroneously supposed that the rod
will protect buildings within its radius, but experience will not bear
out this axiom. Many instances may be related of buildings being struck
much within the radius of well-protected churches or chimneys.
The protection a conductor affords depends to a great extent on the
relative positions of the electric discharge and the objects that it may
meet in its course. As a general rule, a church with a high spire with a
proper conductor may be considered to protect the remainder of the
edifice; but a low, straggling building should have several conductors
at the _outside_ highest points.
8. If there is more than one terminal is the size of conductor
increased?—No; as sufficient material should always be used to carry off
without disruption the heaviest known charge, it is unnecessary to
increase the size of conductor. Should two or more upper terminals be
connected with the main conductor, the size of material need not be
increased; for if two or more terminals receive the charge
simultaneously it necessarily follows that it is sub-divided; therefore
the conductor will have no more work than if one point only had been
struck.
_Note._—We quite agree with Snow Harris regarding insulators, that if
there be anything in insulators they are a disadvantage, for if the
building be struck in any other part than the conductor, the current
cannot easily find its way to the conductor. The current will take the
line of least resistance; therefore it is reasonable to assume that the
building is more certain to escape the disruptive force of lighting when
the conductor is in close proximity with the building.
JOHN DAVIS & SON.
* * * * *
BIGG MARKET, NEWCASTLE-ON-TYNE.
1. For upper terminals I generally use ½ inch diameter solid copper rod
[A. 0·20 in.], or ¾ inch diameter tube [A. 0·24] with four points, and I
fix them 4 or 6 feet above the building they are intended to protect. I
always endeavour to get the upper terminal as near the size of the
conductor as is consistent with strength. I make my points of the best
copper tipped with platinum.
2. For the conductor I use ½ inch diameter copper wire rope [A. 0·20
in.], which is (in my opinion) the best and most applicable conductor
used, as it appears to be an open question, at present, whether it is
surface or mass which conducts. If it is mass, then a tube conductor is
insufficient. If it is surface, then a solid rod is superfluous. The
copper tape conductor I consider the worst form of any, as it bends too
easily round sharp corners, projections, &c., of buildings, which is a
thing to be avoided as much as possible. A conductor should be brought
to earth as direct as possible, and with no bends if they can be
avoided. The copper wire rope conductor has both surface and mass
conduction, and can be led about roofs and other difficult places better
than any other form of conductor that I know of.
3. None; I imagine it is not necessary.
4. I avoid joints as much as possible; but, when they must be made, I
scrape the ends of my wire bright, and then splice or interlace them
together, covering the whole with thin sheet lead—I object to solder, as
I think it must interfere with surface conduction; the wire is fastened
to the upper terminal, with a Matthew Walker knot let into a hollow cup,
and the terminal screwed down on it.
5. I attach my wire to the building with a brass or gun-metal holdfast 4
inches long, having a ⅝ hole, the inner edge being flush with the wall
of the building, so as to allow the conductor to touch the wall of the
building all the way up, and still allow plenty of room for the free
passage of the electric fluid. I do not approve of insulators, nor yet
of that kind of holdfast that is driven in tight on to the wire, for I
think that must interfere with the clear passage of the electric fluid.
6. I cut a trench some 15 or 20 feet long, gradually deepening from 1
foot at the commencement to 4 ft. at the termination, which I fill with
pounded charcoal and bury the wire in it. Earth-plates are not necessary
when this is done.
7. It is calculated that a conductor will protect a surface in the shape
of a cone, the diameter of the base of which is equal to the height of
the conductor. Thus, if a conductor were 100 feet high, the space
protected would be represented by a straight line drawn from a radius of
50 feet from the base of conductor, to a radius of 8 or 10 feet from its
highest point.
8. I consider, if there are two terminals, there should also be two
wires, or the wire should be of sufficient capacity to carry off a
double charge, in case both terminals should be struck at one time. I
think the conductors should certainly be of sufficient capacity to carry
off any charge that might be received by the terminals, be they few or
many.
T. MASSINGHAM.
APPENDIX B.
ANALYSIS OF, AND REMARKS UPON,
THE VIEWS OF MANUFACTURERS.
On Nov. 14th, 1878, a circular was issued to the principal lightning-rod
manufacturers in this country, inviting their replies to various
questions that were submitted to them, and also any remarks that they
might wish to lay before the Conference.
Replies have been received from—
Messrs. WILKINS & WEATHERBY, of London.
Messrs. GRAY & SON, of London.
Messrs. F. RUSSELL & CO., of London.
Messrs. JOHNSON, CLAPHAM, & MORRIS, of Manchester.
Messrs. FREEMAN & COLLIER, of Manchester.
Messrs. PENNYCOOK & CO., of Glasgow.
Messrs. DAVIS & SON, of Derby.
Messrs. MASSINGHAM, of Newcastle-on-Tyne.
All well-known firms, who have written fully and freely, and whose
experience is very extensive.
It is impossible to read these replies without feeling the absolute need
of such a Conference as that which has been formed, to collect facts, to
digest opinions, and to endeavour to formulate some guiding principles
for uniformity in practice—for here we have the most diverse modes of
execution detailed, the most opposite views expressed, and the most
varied experience narrated. In fact, some ideas enunciated are quite
opposed to the teachings of science. Where practice is so opposite,
error must abound: and, therefore, there must be great need for an
effort to reduce the system of constructing lightning conductors in this
country to some uniform basis. On no one single point, except in the use
of copper and the necessity for reaching damp earth, do any two
manufacturers agree in adopting similar measures.
I will take each question submitted _seriatim_.
1. _Form, dimensions, and material usually adopted for upper terminals._
There are single points and branching points, fine points and blunt
points, cones, spikes, balls with spikes on top, and balls with
radiating spikes.
The dimensions vary with each form, and they are made of solid copper
and copper tube, of brass, of iron, and of gun-metal. The ends are
sometimes silvered, sometimes gilt, and sometimes tipped with platinum.
But there is no rule or uniformity; and one manufacturer acknowledges
that, while he sometimes tips the points with platinum, he considers the
practice to be altogether superfluous.
Now it is clear that if there be any electrical efficacy in points _as
points_, they should be made in such a form, and of such a material, as
to maintain their efficiency permanently. The writer is very strongly of
opinion that the efficiency of lightning conductors is due principally
to the peculiar electrical action of their points. He sees no advantage
whatever in multiplying these points. In his opinion each conductor
should end in one fine platinum point. It would thus act as a dissipator
of the electric charge in its immediate neighbourhood, and would then
_prevent_, and not favour discharge. Moreover, points demand frequent
inspection, attention, and renewal. He thinks that one function of the
Conference should be to examine some of these points _in situ_, if
possible. At present they are erected and left to their fate.
2. _The Material and Dimensions of the Conductor._
The use of copper is almost universal, but two manufacturers
occasionally use iron. The form varies. The majority use wire rope, but
some use rods, others bands or tapes, others tubes. One firm uses a
cable “constructed of 49 strands of hard-drawn _square_ copper wire.”
Another firm uses a wire rope, simply because “it appears to be an open
question, at present, whether it is surface or mass which conducts.” The
dimensions are as varied as the form, from a wire rope ⅜th of an inch in
diameter to a copper band 3 inches wide and ⅛th thick.
The only point worthy of note is, that no one uses a smaller conductor
than a copper rope ⅜th in diameter (_i.e._ 4 oz. of copper per foot
run).
Leaving the dimensions as a question for future investigation, the
points submitted for the consideration of the Conference under this head
are—
1. Is conduction a question of surface or of mass?
2. Is copper alone to be used?
3. Is the conductor to be in the form of a rope, a rod, a tube, or a
band?
Now, on the first point the writer entertains no doubt whatever that the
conduction of atmospheric electricity is simply a question of mass, and
that the lightning protector acts simply as a conductor obeying the laws
of Ohm.
On the second point he sees no objection whatever to the use of iron,
when properly galvanized, in situations free from chemical impurities.
The reasons urged against its adoption are extremely weak. First, it is
said to decay rapidly; and, secondly, it is said to be a very much worse
conductor than copper.
The rusting of iron is almost entirely checked, in pure air, by
galvanising or coating with zinc. It is used for nearly every other
purpose in connection with building, and it is difficult to understand
why it should be discarded on account of its liability to decay for this
particular purpose, where it is always under supervision.
Again, pure copper conducts about six times better than pure iron: but
we never get pure copper in lightning conductors. Moreover, the
manufacture of iron wire for telegraphic purposes has increased so
enormously during the last two or three years, that the wire now
supplied conducts 50 per cent. better than it used to. Hence the
difference between the two in this respect is not so great as theory
indicates; and it would be well for the Conference to satisfy itself on
this point by having similar sized wires made of the two materials, and
having them measured electrically for their resistance.
But it has been pointed out by the late Mr. Brough (_Phil. Mag._, May,
1879), that by regarding (1) the influence of the rise of temperature,
(2) the difference between the specific heats, and (3) the relative
dimensions, iron conductors can be made much smaller than was formerly
supposed: and, that as iron is so much cheaper, iron rods can be made
equally efficient for a much less sum than copper. Moreover, the use of
iron enables the architect to use one kind of metal throughout his
structure, and thus avoid anywhere the contact of dissimilar metals,
which always results in decay.
On the third point, the writer is clearly of opinion that a galvanized
iron _rope_ is amply sufficient for country residences and buildings
free from chemical actions. In such places, and in towns, copper should
be used. A rope, whether of iron or copper, is easily handled, it can be
made of any size, it can be led in any direction without bends or
angles, it is neat and easily jointed, diverted, or lengthened.
The writer refrains from expressing any opinion on its dimensions here,
for this is a point that will require most careful examination by the
Conference.
3. _Is there any definite proportion between the length and sectional
area of the conductor?_
The majority of the manufacturers increase the size of the conductors
for high buildings—one making the limit 120 feet, another 150 feet,
while a third “varies the sectional area with the length.” One firm does
not consider any difference necessary, while another takes it that the
sectional area should be the same irrespective of length, for “lightning
does not vary in intensity while passing through a conductor of greater
or less length.”
Now, the laws of electricity clearly show that to maintain equal
efficiency we must vary the sectional area as we increase the length of
the conductor; but it is a question for the Conference to decide whether
we should not recommend a rope of uniform dimensions that would be
equally applicable for high and low buildings. Within ordinary limits
the necessity for increased thickness for increased height is scarcely
evident, but the remedy of an increased sectional area, with the number
of separate points erected, is very clear. Indeed, each point should be
the terminal of a conductor, whose sectional area should be uniform to
the earth. For if it be not so, and each conductor be fully charged with
electricity, then when the sectional area diminishes there will be
congestion, resulting in heat and discharge to the building. Hence the
thickness of the main conductor must increase with the number of
separate points erected.
4. _Joints, how made._
Some are rivetted, others are screwed, others are coupled by right and
left-handed screws. Tubes are socketed into each other. In one case “the
end of the conductor is knotted and drawn through a cup-shaped ring of
metal.”
There can be no doubt that joints are the greatest source of danger in
lightning conductors. If a joint be imperfect, and the conductor be
conveying a charge to earth, heat will be generated there, the conductor
may be fused and rendered useless, and the discharge will be diverted to
the building. Or the joint may be so bad—that is, its resistance may be
so great—that it renders the conductor practically useless, for other
parts of the building will offer easier paths to the earth. Though the
use of solder is pretty general, it is not universal. Indeed, one
manufacturer objects to it because “it must interfere with surface
conduction!” It certainly should be imperatively used. No joint can
possibly be perfect that is not metallically continuous. Careful
soldering is the only certain mode of securing this, and that this is
practicable is evident from the millions of perfect joints in telegraph
wires. To scrape the ends of wire bright, and cover the whole with thin
sheet lead, as is done by one firm, is simply to court danger. The
absence of joints in wire rope is one great element in its favour.
5. _Attachment to building, how made._
Some attach the conductor to the building by copper straps and nails;
some use holdfasts, either of copper wire or gun metal; others use
staples; one uses metallic ties. Several pass the conductor through
insulators of glass, porcelain, or earthenware. But the majority discard
insulators as useless.
In the opinion of the writer they are quite right, for it is difficult
to understand what useful function the insulator performs. One fact that
occurred in 1837 is given as a reason for their use, but the fact
militates against the efficiency of the conductor rather than the
absence of insulators. If the conductor were perfect there could have
been no concussion at the point of attachment. If it were imperfect
there may have been, for the discharge would seek other paths to earth.
Some manufacturers use holdfasts of a different metal to that of the
conductor. This is wrong, for where different metals are used galvanic
action sets in, tending to decay and rupture. The attachments for this
reason should always be of the same metal as the conductor.
6. _Ground connection, how formed, and of what extent._
The necessity for reaching moist ground is generally recognised, but
various curious ways for making earth connection are suggested. One firm
considers that a band cut into strips 18 inches long would suffice,
while another says that not less than 30 feet, in two or three branches,
with fork at the end of each band, should be used. One firm is very
brief: “Ground end is coiled loosely in damp earth or a well.” The use
of coke, powdered charcoal, or carbonaceous materials, is insisted upon
by others.
It is questionable whether the difficulty of fitting a good connection
with the earth is fully realized. None but telegraphists know the great
difficulty there is in doing this. The first object to be secured is a
good damp soil, and the next as large a conducting surface as possible.
Metal pumps, iron, gas, or water pipes, wells in which plates of metal 2
or 3 feet square are placed, or similar plates may be buried in
perpetually damp ground, or in holes well filled with powdered coke.
Moisture in some form is essential, and without it a lightning protector
is of small service.
7. _Extent of area supposed to be protected._
The majority of the firms consider that the area protected has a radius
equal to the height of the conductor; but one firm considers that this
should be multiplied by five or six times; while another asserts, “that
no appreciable extent is protected by a single rod conductor;” and
another, that “many instances may be related of buildings being struck
much within the radius of well protected churches or chimneys.”
We have no experience at present to enable us to form a definite opinion
on this point. The Committee of the French Academy, gave the radius as
equal to twice the height of the conductor from the ground, but
buildings have undoubtedly been injured within this limit. The writer
does not think that a greater radius than the height should be taken:
but thinks that this is one of the most important questions that the
Conference could determine. Calculation might, to a certain extent,
settle the point: but it is more a case for experience.
8. _If there is more than one terminal, is the size of the conductor
increased?_
This question has been partially considered. (See No. 3.)
* * * * *
Some firms do not consider any increase necessary: others think that
when two or more terminals are used, the main rope should be somewhat
enlarged; while others run one cable from each terminal, or make the
conductor of sufficient capacity to carry off the double charge.
The writer considers that every conductor should be complete in itself:
or, if this is inconvenient, then the size of the main conductor should
be enlarged in proportion. It does not at all follow, as one firm
implies, that if two or more terminals receive the charge
simultaneously, it is necessarily sub-divided. Each charge may be full
and complete in itself, and be sufficient to fill the wire; and,
therefore, if the main conductor be not increased, accident may result.
* * * * *
There is no doubt whatever that great consideration should always be
given to the lessons of experience, and the opinions of those who have
made the erection of lightning conductors for 35 years their especial
practical study, are much entitled to weight; but such practice may have
originally been based on error, and the teachings may not have been
guided by science. Where such variety of practice abounds, there must be
fallacy somewhere, and, therefore, danger; and not the least of the
beneficial labours of the Conference will be to point out to these
different practical men, where their faults and their departures from
truth exist.
W. H. PREECE.
_August 8th, 1879._
APPENDIX C.
REPLY FROM MANUFACTURERS, RECEIVED AFTER THE COMPLETION OF THE ANALYSIS
WHICH FORMS APPENDIX B.
FARADAY STEAM WORKS,
ST. JOHN’S ROAD, HUDDERSFIELD.
_11th November, 1879._
SIR,
Enclosed we have pleasure in handing you our replies to the eight
questions which you ask lightning rod manufacturers and erectors,
together with three tracings, showing our system of protection under
different conditions. In the case of Nottingham Castle we considered
it necessary, on account of the rock on which the castle was built,
to adopt an extensive system of lateral points in earth termination,
by running all the main conductors from the building down the
shrubbery into the moat, where we formed the flat copper band into
the form of gridirons, in which several hundred feet of the copper
band has been used, and the termini of the ribs pointed, and the
whole being sunk eight feet, and two cart-loads of gas carbon laid
over each grid.
Yours obediently,
SANDERSON & CO.
REPLIES TO THE QUESTIONS OF THE LIGHTNING-ROD CONFERENCE.
I.—Usually a length of copper tube 5 feet long 1 inch diameter × No. 8
B.W.G., which is termed the elevation rod, surmounted by a solid copper
point, forged from ⅞th of an inch round bar, wrought three parts of its
length to a square tapering point, the said elevation rod and point are
screwed together by a copper ball-shaped union, into which are screwed
four smaller points at an angle of 45 degrees. When fixing lightning
conductors on church spires and turrets we usually run the copper tape a
few inches above the vane or finial, having previously prepared and
pointed the tape; by this system all joints are avoided.
II.—We, the sole inventors, manufacture the solid copper tape lightning
conductors of the following sizes:—
Nos. │ 1 │ 2 │ 3 │ 4 │ 5 │ 21 │ 22 │ 23
│ in. │ in. │ in. │ in. │ in. │ in. │ in. │ in.
│⅝×1/12 │ ¾×⅛ │ 1×⅛ │ 1½×⅛ │ 2×⅛ │1×1/16 │1½×1/16│2×1/16
[A │ ·05 │ ·09 │ ·13 │ ·19 │ ·25 │ ·06 │ ·09 │ ·13]
and in continuous lengths up to 500 and 600 feet.
III.—Yes. For heights of say 50 feet we recommend our No. 2 (¾ in. × ⅛
in.) size; for 100 feet our No. 3 (1 in. × ⅛ in.) size; and for 200 feet
or over, our No. 4 (1½ in. × ⅛ in.) size; or our Nos. 21 (1 in. × 1/16
in.), 22 (1½ in. × 1/16 in.), or 23 (2 in. × 1/16 in.) according to
position and circumstances.
IV.—In the case of church spire or turret conductors we have no joints
whatever, as described in Answer I., but where elevation rods and points
are used we make a copper coupling, screwed at one end to receive the
elevation rod, and at the other end to receive the copper tape, which is
firmly rivetted into the coupling, and thence soldered or brazed. But
when a complete system is employed with branches or tributaries, running
from several points on to the main conductors, we make the joint by
means of copper rivets, and then solder.
V.—By means of gun metal clips, or holdfasts, let into the building,
which secures the copper tape in close contact with the face of the
building. Under no circumstances do we use glass, ebonite, or other
insulators.
VI.—In good moist earth 5 feet or 6 feet deep we simply run the copper
tape out from the building some 20 feet, and then rivet a copper earth
plate on to same, or sometimes employ a large gridiron made of copper
tape, using as much as 200 lineal feet in its construction. Wherever we
make earth terminations in rocky, dry, or gravelly soil, we always fill
in with a load of small coke, charcoal, or other carbonaceous matter,
and also divert the rain fall-pipe system over the termination so
formed; also, wherever possible, we connect the conductor with the gas
and water systems outside the building. In all cases of earth
terminations, the size of earth-plate is in proportion to the size of
conductor employed, and other circumstances.
VII.—We are unable to come to any conclusion as to any definite area
which one lightning conductor will effectively protect, and no two
writers appear to agree on the subject; but from actual practical
experience of 30 years, combined with the closest observance and
research, we are in the position to say, emphatically, that a conductor
on one prominent elevation—for example, a turret will not protect a
similar elevation, be it only 1 yard, or 50 yards distant; but that
providing that two prominent features be provided with a conductor point
on each, then on the same foundation, we say that both of them would be
effectively protected. But for the purpose of simplifying and
practically illustrating our views on this subject, we enclose you
tracings and particulars of several buildings for which we have designed
the system of lightning conductors, and which we believe to be perfect.
VIII.—Yes,—always, and in proportion to the number of extra terminals
adopted.
SANDERSON & CO.
DETAILS OF LIGHTNING CONDUCTORS APPLIED TO NOTTINGHAM CASTLE.
The following three engravings render very few verbal details necessary.
Fig. 1 gives the east elevation of the castle, it shows part of a
flag-staff 115 feet high, which has a conductor, also three of the
principal terminals, and twenty-six minor points upon the building, and
by two dotted lines the position of two of the main conductors to earth.
The principal terminals are tapered iron tubes, 13 feet long, carrying
copper tapes 1 inch × 1/16th and terminating with copper points tipped
with platinum; the minor points are of solid copper 9 inches long. The
main conductors to earth are copper tapes 2½ in. × 1/16th.
[Illustration:
FIG. 1.
]
Fig. 2 give a plan of the roof, much of which is of glass with wooden
rafters. The twelve principal terminals are shown by small rings, the
ninety-four minor points by round dots, the horizontal copper tape (2
inches × 1/16th), uniting all the upper terminals, by a pecked line, and
the position of the main conductors to earth by dotted crosses. All the
gutters are metallically connected with the conductors.
[Illustration:
FIG. 2.
]
Fig. 3 gives a general plan (for which we are indebted to the
architects, Messrs. T. C. Hine & Sons) of the castle and grounds, and
also a little section indicative of the precipitous eminence on which
the castle stands. From these it will be seen that two of the main
conductors to earth are carried underground at a depth of about 4 feet,
under the terrace and down the slope and terminate in trellis-work,
about 14 feet square, of 2½ inches × 1/16th copper tape rivetted at
every intersection. The other earth contact is obtained by bolting the
terminal on to the town water-main. The total length of tape used in the
earth connections was about 500 feet.
SANDERSON & CO.
[Illustration:
FIG. 3.
]
APPENDIX D.
REPORT OF THE REPRESENTATIVES
OF THE
ROYAL INSTITUTE OF BRITISH ARCHITECTS
TO THE
LIGHTNING ROD CONFERENCE.
The Council of the Royal Institute of British Architects sent out
upwards of 600 circulars (a copy of which follows this Report) to their
Architectural Members requesting information as to injury by lightning
to any buildings known to them.
The Council also requested the same information from their Honorary
Associates (upwards of 100 in number), who are chiefly men eminent in
the Scientific, Literary, and Artistic world.
The Council have received up to this date only 35 answers from
Architects, and 1 from the Honorary Associates.
Of these answers many are to the effect that no case of injury has
arisen to works under their direction. The remainder give 33 instances
of damage, and enter, in many cases, very fully into the details of
them.
The instances given may be roughly classified thus:—
There are 26 of buildings injured where there were no conductors.
In 9 of these the lightning did some injury to the chimneys and other
exposed parts, and was then conducted safely to the ground through the
metal gutters and rain water pipes.
In three other instances the lightning appears, from the statement of
persons in the building struck, to have dispersed and passed out by open
doors, &c.
We give no opinion as to this, but the facts are distinctly stated. In
several other cases the lightning passed off in several distinct
directions and currents.
There are 6 cases of buildings being injured although protected by
lightning conductors. In one of them (No. 14) the failure is clearly
explained by the fact that the lower part of the conductor had been
stolen, leaving only two or three feet of it in the ground. The
lightning in this case broke through a wall 4 feet 6 inches thick, at a
height of 6 feet from the floor to a gas pipe.
In another case (No. 3) a gable was struck, although close to a spire
and turret which had a lightning conductor.
In another (No. 7) the part struck (a chimney) was 64 feet away from a
tower, in the same building having a lightning conductor.
In No. 24 the conductor was sufficient protection until it passed at a
sharp bend round some mouldings; these it injured, but did no further
damage.
In two cases (Nos. 21 and 23) the discharge injured some gas tubing near
it, and set fire to the gas, and by its means to the building.
We beg finally to call attention to the drawings attached to No. 7 (Mr.
Colson, of Winchester), showing the injury to trees 130 feet away in a
direct line from a spire which was destroyed, it having no conductor.
T. HAYTER LEWIS, V.P.
JOHN WHICHCORD, V.P.
CIRCULAR.
LIGHTNING ROD CONFERENCE.
SIR,
I beg leave to inform you that the Council have appointed two of
their members to meet delegates from several scientific societies in
order to confer as to the best methods of protecting buildings from
lightning; and in accordance with a resolution of that conference I
have the honour to forward to you, by the desire of the Council, the
questions appended below.
I shall be much obliged if you will return me this paper, with any
answer you may be in a position to make to the questions, on or
before Monday the 20th of January, 1879; and
I remain, Sir,
Your faithful servant,
WILLIAM H. WHITE,
_Secretary_.
9, CONDUIT STREET, HANOVER STREET, W.
_19th December, 1878._
QUESTIONS.
1. Have any buildings, in the construction of which you have been
professionally engaged, or which are otherwise well known to you, been
struck by lightning?
2. If so, state briefly the damage done to them, describing their
general plan and construction by sketches or otherwise, particularly
noting the position of any metal work to roofs, pipes, &c.
3. Were the buildings furnished with lightning conductors? If so,
describe them in relation to the following heads:—
(_a_). Their materials and dimensions.
(_b_). Their attachment to building.
(_c_). Their connection with the ground.
(_d_). Their upper terminals.
(_e_). The height of conductor above chimney or other adjacent part of
the building.
(_f_). If there existed more than one conductor state the distance from
one another.
4. What was the distance of the point struck, horizontally and
vertically from the conductor?
5. Was any damage done, and if so how much, to the conductor, and in
what manner?
6. Give particulars as to any trees within a short distance of the
building struck.
* * * * *
The replies received to this Circular are too long to be printed in
full, they have therefore had to be epitomised in the following
list, and consequently cannot be given as separate answers to each
question.
The replies were all numbered consecutively, so that the numbers
omitted in the list refer to circulars returned by members who had
no information to give on the subject.
REPLIES TO CIRCULAR.
2. _St. Aubyn, J. P. Week St. Mary, North Cornwall._—The tower of this
church stands on very elevated ground, and has lofty pinnacles, three of
which have been struck at different times, on each occasion one of these
pinnacles was shattered, and had to be taken down and rebuilt. Some of
the stones are held by iron cramps, but no iron or other metal spindles.
The roof of the tower, as well as that of the church, is slate, without
spouts, and there are no lightning conductors to the building. There is
open country all round the church, and no tree of any size within a mile
of the tower.
The following detailed report was received direct from the Rev. G. H.
Hopkins, the rector of the parish:—
_An Account of the position of the Church of Week St. Mary, in the
County of Cornwall, and the effect of Lightning upon the Pinnacle
and Tower when struck for the fourth time this century on November
8th, 1878._
_Situation of the building._—The situation of the church is at the
northern angle of an extensive triangular plateau, which towards the
south is much broken by small valleys and low hills, while the high
land is for the most part moor, broken in places by cultivated
ground and small plantations. Within a quarter of a mile from the
church, on three sides, the ground commences to fall very rapidly to
a depth of 200 or 250 feet; it is three miles from, and nearly 500
feet above, the sea; to the N.W. lies Widemouth Bay, one of the very
few breaks in the cliff along the coast of North Cornwall; the
entire extent of this break is quite a mile-and-a-half; between the
Bay and the extremity of the plateau, at which the church is built,
the surface is broken by low hills, only one of which exceeds 250
feet above the sea level, and this exception is separated by one
valley from the church hill; half a mile south of the church is the
highest ground in the parish, but neither this nor any hill for
several miles exceeds in height the pinnacles of the tower. The
elevation of the building above the surrounding country can be
better understood from a local rumour that 28 churches are visible
from the battlements of the tower, and the average size of a parish
attached to each church is 6000 acres. The highest point of the
pinnacle is 90 feet above the ground.
_No mines or spring of water beneath it._—There is no evidence of
the existence of any metalliferous lode in the parish, and certainly
no such attractor of electricity lies beneath the church, nor is
there any spring of water near the foundations; but as the surface
soil is clay, the rain water has no means of flowing away, except
over the surface, and a few hours of moist weather make the soil
like a wet sponge.
_Circumstances._—The tower was struck at 6.45 a.m. on November 8th,
1878, the weather having previously been gusty, with sudden storms
of hail and rain as each heavy cloud came up from the sea: many
times during the night the downfall of hail was very violent, and it
was during one of these storms that the single electrical discharge
took place; the hailstones were considerable, both in number and
size, when the flash occurred, and they certainly commenced falling
before the shock took place.
_Brightness of the flash._—The brightness of the lightning was
intense, and I have been at some trouble to inquire into the effect
which it had upon those who saw it. I was awake, and the lightning
illuminated the room through double chintz curtains and dark-green
blinds, the windows looking away from the church, and being more
than a quarter of a mile from it; during the storm a farmer took
refuge in a closed cattle shed, 200 yards from the church, and he
spoke afterwards of his impression that he was surrounded by fire;
two farmers going to Camelford fair, were at the time waiting on the
road, a mile-and-a-half from the church, and their impression was
that they were enveloped in flame, and the flame came between them;
these experiences were given to me at different times, and were
independent evidences of individual opinion. At Holsworthy, eight
miles away, in a direct line, two ladies were attending their sick
mother, and the vividness of the lightning obscured the brightness
of the light of two candles and a paraffin lamp.
_The loudness of the thunder._—The loudness of the clap of thunder
was very great; of course it shook my house; and a neighbouring
rector, who lives three miles away, in an adjoining parish, felt the
effect of the clap to an extent which was very unusual; at
Camelford, lying W.S.W., and distant about twelve miles, with a
considerable range of hills between, the thunder was not heard; but
two miles nearer, and in the same line, it was just heard: this
latter station being on the summit of the range; at Holsworthy,
lying E.N.E., it was heard as an awful peal; at Kilkhampton, which
lies directly N., and separated by a broad broken valley, the
thunder was blamed with causing colts to break through a fence from
terror, and the distance is ten miles. I am unable to give any
further account either of the distance the thunder was heard, or of
the intensity of the light of the flash. As the wind was blowing
from the west, with a slight bearing towards north, the effect of
the wind upon the sound is evident.
_Effect upon the pinnacle._—The S.W. pinnacle (A) was struck, and
apparently the effect of the lightning was not felt upon the two
uppermost stones, namely a small cross and a truncated cone, which
supports it at the summit, both of granite. It may be remarked that
the entire facing of the tower is granite, the interior masonry
being made up of small stones of different kinds, which exceeds 3
feet in thickness, while the blocks of granite which face the tower
vary from 10 to 12 inches in thickness, and in some cases are of
immense size and weight. As soon as the current could reach that
part of the pinnacle which is made up of courses of separate stones,
the mischief commenced, and the effect was to force the stones out
all round the axis of the pinnacle, so that in the same course many
of the stones were separated by intervals of from 1 to 5 inches; one
great block, measuring 2 feet in length, was thrown right out, but
fortunately fell outside the tower walls, and left a gap in the
pinnacle opening towards that quarter from which the storm came. The
entire pinnacle was shattered, and all the courses of stones which
make it up dislocated, as well as the two courses of stones which
lie beneath it. The fierce rain storms had long ago washed away all
the mortar from between the stones which compose the outside of the
tower, and probably every shower wets the interior of the masonry;
and this was especially the case at one part where there is a
considerable leak of drainage from the roof of the tower.
[Illustration: Tower of Church of Week St. Mary, Cornwall]
_Effect upon the tower._—The course of action of the current was
from the pinnacle to this leakage, where a stream of water was
running down the wall and between the granite facing and interior
masonry; the downward course of the water was arrested by the belfry
light, and then has to fall to the masonry below the window; just
above the window a large block of granite C (outside measurement 2
feet by 14 inches), was thrown out in such a way that it hangs like
a halfopen door, the projecting edge being that which lies just
below the leakage, and standing out about 10 inches from the wall;
across the belfry window runs an horizontal iron bar, and at the
bottom of the window lies an old iron bar; the stonework beneath
this bar was much knocked about. From that spot the effect of the
lightning disappears, until it reached an immense carved granite
block D, which lies on the south side of the tower, and very near
its south-east corner; and a few feet below this the leaden gutter E
(through which part of the roof drainage is poured to the ground),
runs some feet down the wall towards the earth, but does not reach
the earth by 12 feet. The immense carved granite block is broken
into two almost equal parts by a line parallel to its vertical
edges, and the two parts are separated by quite half an inch; the
fracture of the stone is not quite straight nor clean, and the parts
of the stone do not project beyond the surface of the tower. I have
been unable to trace the course farther; it may have passed along 70
feet of leaden gutter, between the nave and south aisle (F) to the
east end of the church, or gone to ground at the base of the tower.
Three or four days later, during a very high wind, a second stone
fell from the pinnacle; this same stone had been partially thrust
out on a previous occasion in 1865. Upon examination the pinnacle
was found to be in such a precarious condition that a single blow
with a hammer on one small stone would have endangered the whole.
_Previous injuries._—On October 19th, 1843, at 10 p.m., the S.E.
pinnacle (B) was completely torn down, and two courses of stones
just beneath it were greatly damaged. The line of action of the
current was to the north-east edge of the tower, towards the leaden
gutter, between the nave and north aisle, over this it threw out a
great block of granite; from that point it passed along the leaden
gutter and across the roof at the north aisle, to a strong iron bar
running vertically down the third out of the four north windows;
this window was considerably damaged, and still bears marks of rough
usage; how it happened that two other windows near the tower, and
similarly fitted up with iron bars, were passed untouched is a
mystery; to some extent all the windows in the church were somewhat
damaged, the framework being of wood they were much shaken, and
partially separated from the masonry. This was probably caused by
the effect of the current upon the air in the building: the
direction of the damage being due to the outward pressure.
In 1812 the north-east pinnacle was struck, and also some little
time before 1688, as there is a stone engraved with that date upon
it, and the date of the tower is the close of the fifteenth century.
The dates of these misfortunes have been as follows:—
About 1688 N.E. pinnacle.
1812 N.E. pinnacle.
1843 S.E. pinnacle.
1865 S.W. pinnacle.
1878 S.W. pinnacle.
The north-west pinnacle appears to have escaped, and it stands just
over the tower stairs. The south-east pinnacle, which was struck in
1843, was at that time surmounted by a weathercock.
_There never has been a lightning conductor to any part of the
church._
One word further. I have been the holder of the benefice since the
autumn of 1876; last summer the specifications for the complete
restoration of the church, at a cost of £2000, were sent to me by
the architect; before forwarding the same to the Bishop of the
Diocese I supplied the omission of a lightning rod in the
specification.
_Meteorological Notes._—It is a noteworthy fact that on each
occasion during this century when a pinnacle has been struck, the
season has been between November and March, with _one_ electrical
discharge during the storm. It is also remarkable (an experience
founded certainly on only two summers, but during that time the rule
has been invariable) that all round the neighbourhood summer
thunderstorms may be passing in their usual fitful manner of storm
and sunshine, but immediately a summer thunderstorm passes over this
village there is a complete break in the weather for eight or ten
days.
_Rainfall._—1877: 49·11 in., 213 wet days; 1878: 48·03 in., 212 wet
days.
3. _Baker, A. J. Rosherville Church, near Gravesend._—The west gable
of the south aisle was struck by lightning, although close to the
tower and spire which were provided with a lightning conductor, and
received no injury.
5. _D. Brandon. St. Ann’s Hotel, Buxton._—In 1875 a chimney-stack
was shattered by lightning, the concussion in the flue drove fire
and smoke into the drawing-room, displaced the mantle-piece, and
broke many panes of glass. The hotel occupies half a crescent, the
stack being in the middle of the crescent. The building had no
lightning conductor, and there were no trees nearer than five or six
hundred feet.
7. _J. Colson. Twyford Moors, near Winchester._—Struck by lightning
in June, 1878. This building (of which a plan is given) was provided
with one lightning conductor fixed to the tower. The upper terminal
branched into five points, about four feet above tower roof; the
conductor, which was ⅜-inch copper wire-rope, was attached to the
upper part of the tower, with glass insulators, and in the middle
nailed to the wall through lead flashing, then carried down
rain-water pipe into cesspit. The point of the building struck by
the lightning was distant about sixty-four feet horizontally and
sixteen feet vertically from the upper terminal of the conductor.
Damage done was very slight, tiles and laths being knocked off, but
no sign of scorching. The conductor was not injured; there are no
trees near the building.
[Illustration: Plan and Elevation of Twyford Moors, Winchester]
C Conductor.
* Point struck.
P Rain-water pipes attached to iron gutters.
7_a_. _St. James’ Church, West End, Hants._—Struck by lightning at 5
p.m., on June 12th, 1875. The church stands on the top of a hill
with many trees near, it is built of brick with a lead ridge to
roof, iron and lead gutters, iron rain-water pipes P, and two iron
chimneys. The spire is of brick, with stone angles fixed by iron
cramps; the spire was finished by an iron bar at the top, but was
not provided with a lightning conductor. The damage done to the
spire was considerable, as shewn in the engraving, making it
necessary to pull it down, but the tower was not injured. Stones
from the spire were thrown through the trees at B, which are 126
feet distant from the church, cutting off some of the boughs. The
tree at A was untouched.
[Illustration:
St. James Church, West End, Hants.
]
12. _T. Hawksley._ Several Steam Chimneys not provided with
lightning conductors; upper portions knocked down, chimney split or
often skinned by the lightning, _i.e._, the four and a half inches
of brickwork taken off; details not given. Now uses Gray’s system of
lightning conductors for such buildings, which is found successful.
13. _A. Hill._ In South Africa houses are generally roofed with
corrugated iron, and protected from lightning by planting a circle
of high trees round them.
14. _G. J. Hine. All Saints’ Church, Nottingham._—Struck about
twelve years ago; tower and spire 150 feet high, with one conductor
of half-inch copper wire-rope, with platinum terminal, and secured
by insulated brackets, but earth contact only two feet long at time
of accident the rest having been stolen. The lightning passed down
the conductor till within six feet from the ground, where it passed
through a wall of solid masonry four feet six inches thick,
displacing some of the stones, to an inch-iron gas pipe inside the
church. In passing off along the gas pipes under the floor, it so
far disarranged them as to cause a considerable leakage of gas,
which was set fire to by a candle some hours after the accident and
exploded. There were no trees, only a few shrubs near.
16. _J. Jerman. Alphington Church, near Exeter._—Tower struck about
March, 1828; the church had no lightning conductor. The tower was
rent through the masonry vertically, damaging parapet and ungearing
and injuring bells, which were being rung at the time; one ringer
was killed, and some of the others had the heel-plates melted off
their boots. There are few trees of any size near the tower, which
surmounts all adjacent buildings; it had pinnacles and a weathercock
on the top, and a lead roof with spouts, no down pipe. Very few
casualties from lightning occur in Devonshire.
18. _E. J. Law._—The tower, surmounted by a cast iron vane, of a
house built under my superintendence, was struck; the slates
stripped from the roof, and the charge apparently escaped down the
rain-water pipe; it divided, however, and passed to an adjoining
ridge, chipped a piece off the iron cresting and hurled it some
twenty yards from the building. Lightning conductor ordered, but not
erected; cast iron ridges to all the roofs. Large infirmary within
two hundred yards and high church tower within three hundred yards,
and houses nearer, of equal height to the one struck, and with cast
iron crestings, none of these were injured.
18_a_. _St. Sepulchre’s Church, Northampton._—Vane on top of spire
struck by lightning, passed down the rod, then to frame of one of
the spire windows, and thence to clock face, from clock face it
passed down the gas pipe, leaving no further trace.
19. _T. Hayter Lewis. Lewisham, 1872._—Zinc chimney of house struck;
lightning went down flue A, thence to a gasalier (glass) B, broke it
to pieces and passed harmlessly to the other end of the house where
the pipe ended at C, broke through a partition there and the window
D, and passed down the rain-water pipe E to the earth.
[Illustration:
SECTION.
]
19_a_. _Wandsworth, 1875._—Chimney of house struck and damaged as
shown in sketch, lightning then passed along eaves gutter F, and
down the iron water-pipe G, doing no further injury.
[Illustration:
PLAN AND ELEVATION. PLAN.
]
19_b_. _Addiscombe, 1878._—Chimney struck above H, the lightning
passed down flue, slightly injured the chimney-pieces, and
apparently passed through the two open doors to the road, as the
tenant standing at J distinctly felt a shock.
19_c_. _Forest Hill._—Chimney (K) struck, lightning followed gutters
shown by dotted line in sketch, part no doubt escaped by pipe L, but
some passed along gutter to M doing slight injury to brickwork
there, the window N was broken, and the gilt bead under cornice in
rooms K and O was blackened.
[Illustration:
PLAN AND ELEVATION.
]
19_d_. _University College, London._—A chimney has been struck on
two occasions, but little damage done; the lightning passed off by
gutters and rain pipes which enter the drains; the top of the dome,
which is of stone, has escaped.
21. _J. Murgatroyd. St. Mary’s, Crumpsall, near Manchester._—A
lightning conductor from spire touched the eaves gutter, and a gas
pipe touched the end of this gutter. The lightning passed from the
conductor along the gutter to the gas pipe, melted it, and set the
church on fire by igniting the gas.
22. _T. Oliver._—Never had a building damaged during thirty years
practice; uses ½ inch copper rope for lightning conductors, in
contact with any iron work near, and buried 8 feet in ground in
ashes.
23. _Wyatt Papworth._—Tall spire struck. The church stands in an
open position with no large trees near. It was provided with an iron
lightning conductor ¾ in. diam., fixed with iron holdfasts, and
carried down inside the spire and tower into ground; the top of it
was said to be attached to a bold copper finial on the spire about
150 feet from the ground, and 50 feet above ridge of roof; the
lightning is supposed to have first struck the finial, it slightly
deranged some beds of masonry in upper part of spire, then descended
by iron rod to belfry, melted a gas tube in the floor, and set fire
to the belfry by igniting the gas.
23_a_. House in country road. The lightning struck chimney-pot,
descended flue to fire-grate and there divided, one part passed to
fire-grate below and damaged the gasalier, another part destroyed a
box of clothes near grate, then passed out of door into another
room, struck the grate and passed into room below doing no further
damage.
23_b_. Another house situated at the corner of country road with
high trees near, lightning followed bell wires, stripping paper, &c.
23_c_. At a third house, chimney pot struck, shaft and eaves gutters
damaged.
24. _J. L. Pearson._—Weathercock of a tall spire in an exposed
situation struck.—There was a wire rope conductor attached to the
bar carrying the vane and passing down inside the spire and out at
the belfry window, the bells being connected with it; it was
attached to the tower by ordinary metal hooks, and was carried 6 or
8 feet into the ground, and about 10 feet from the base of the
tower, the strands being spread out. The conductor was bent about
very awkwardly under copings, and in some places, at right angles,
the damage was very slight, and was limited to projections of
mouldings close to a bend in the conductor about 20 feet above
ground. The conductor itself was uninjured. Some insignificant trees
100 yards distant.
26. _E. C. Robins. St. Matthias’s Church, Brixton._—No conductor,
although the church had previously been struck. I have now put one
up, leading its lower end into a cistern of water. The portland
stone terminal cross was shattered, and the stones of the cornice of
the two topmost stages were displaced.
28. _H. S. Snell. The Holborn Union Infirmary, Upper Holloway, in
course of erection._—Conductor not fixed. Apex of tower roof, 160
feet from ground, having only roof timbers, some lead-work A at
apex, and vane (gilded iron) fixed. The damage commenced just below
lead-work on apex, and three out of four hips were much torn and
shattered, necessitating taking down and rebuilding; the hips were
each framed in three sections, bolted together with iron bolts, and
in nearly every case the bolts seem to have specially attracted the
fluid causing slight charring. One of the dormer windows B was also
separated from the spire. The fluid appeared eventually to have been
attracted by the water-pipes, which rise to top story of building,
and so passed away. It will be noted as peculiar that the iron vane
was not touched, and that the damage commenced immediately below it.
[The damage evidently occurred _only_ where the conducting materials
were absent, the iron vane and the lead would naturally bear no
trace of injury.—ED.] No trees nearer than 150 feet, and these much
below the top of tower.
[Illustration: View of Tower of Holborn Union Infirmary, Holloway]
32. _J. B. M. Withers._ Detached house, near Sheffield, in course of
construction.—No conductor; the top of a chimney fifty-two feet six
inches above the ground was struck and deranged but not thrown down.
The nearest ironwork was an ordinary cast gutter, twenty feet from
the top of the chimney. No trees within sixty yards of the building.
34. _G. Wrottesley (Col. R.E.)._ Chimney shaft of a laundry at the
Barracks at Gravesend.—No conductor. The chimney shaft, forty feet
high, was entirely destroyed by a heavy charge of electricity as low
down as the eaves of the building—at this point iron gutters went
round the building and outside the chimney shaft, and the charge
passed harmlessly away to the earth by the rain water pipes P. Not a
brick was left in place above and not one disturbed below the
gutter; the shaft appeared as if cut off by a knife at this point.
No trees within 100 or 150 yards. The disruptive force was so great
that the bricks were scattered over a radius of 200 feet, and the
slate roof was riddled like a colander by the brickbats.
[Illustration:
ELEVATION AND PLAN.
]
36. _E. N. Clifton. Bethnal Green_—A four-roomed house, one of a
row, with a V shaped roof, was cut in two by lightning; a fissure
was made in the front and back walls, and also in the middle plaster
partition. The fluid entered the house between the front windows and
passed through the partition and back wall, rather to the side of an
iron pipe at the back which was the only metal near. No trees in the
neighbourhood.
APPENDIX E.
PARTICULARS OF ACCIDENTS BY LIGHTNING COLLECTED IN THE YEARS 1857, 1858
AND 1859 BY MR. SYMONS, AND REPORT UPON THE SAME BY PROF. W. E. AYRTON.
_Selected accidents._
I.
About a quarter past ten p.m. on Aug. 14, 1857, an occurrence took place
at the Brick-lane station of the Chartered Gas Company, St. Luke’s,
which caused some alarm. It appears that the lightning struck one of the
iron columns which supported one side of a gasometer, or gas holder,
situate on the right hand side of the yard. Owing to the column having
been thus struck by lightning, the gas, comprising many hundred thousand
feet, became ignited. Fortunately, the services of the firemen were not
required, for, owing to the admirable directions given by Mr. Upward,
the superintendent of the works, and the exertions of the men under him,
the flames were subdued in a comparatively short period. Fortunately no
person was injured, and no damage was done to any of the surrounding
property.
II.
At half-past eleven on Aug. 14, 1857, there was a terrific discharge of
lightning, by which the south-east pinnacle of St. Michael’s Church,
Stamford, was instantaneously struck down. The Church of St. Michael is
a modern structure, erected in 1832. It is situate in the centre of the
town. The south-east pinnacle, which received the electric fluid, was
composed of a mass of masonry, weighing about fifteen hundred weight;
the iron clamps or ties by which the work was bound together served as
partial conductors. At every break in their arrangement a series of
disruptive discharges of the electric fluid took place in lateral
directions, driving out large masses of the stonework, spreading them
over the roof of the nave and churchyard, doing considerable damage to
the roofing and tombstones. The effect of the fluid when it reached the
base of the pinnacle, from not meeting with a ready conducting medium,
was to uplift the whole mass imparting to it at the same time a kind of
circular motion to the southward, the apex of the pinnacle falling in a
line with its original base; and the base having traversed about the
eighth part of the circle, fell into the roof of the tower. Immediately
at the base of the pinnacle there is a three-inch iron spout or tube
erected to convey the water from the tower roof. This iron tube the
electric fluid entered, and, finding through it an unopposed channel,
passed down the tower, and finally into the earth, without doing more
damage. The iron tube or spouting in this instance, and by mere
accident, acted the part of a lightning conductor, and served to protect
the other parts of the tower from most serious injury, if not entire
destruction.
III.
At Walthamstow, at 7.30 a.m., on June 5th, 1858, the flag staff of the
church was shivered, the gutters were torn up, the robing room and
various parts of the exterior injured, and the gas pipes torn open.
IV.
_Effects of lightning on a chimney stalk 240 feet high._—Facts collected
by Alexander Cruickshank, 28th June, 1859:
During the thunder storm at Aberdeen, between 8 and 9 a.m., 26th June,
1859, the lightning struck Messrs. Richard & Co’s chimney, 240 feet
high, at Rubislaw, Bleachfield, one mile west of the city. At the height
of 120 to 140 feet three patches of surface bricks were torn off. By the
aid of a telescope and knowing the size of the bricks and the thickness
of the mortar between them, the two largest patches of denuded bricks
were 7 feet by 3 feet and 4½ feet by 3 feet—the longest measurements are
vertical. These patches were visible to the naked eye at least two miles
off. The parts denuded were 4½ inches thick, or the breadth of a brick
when placed with its largest surface horizontal and its sides external
and internal. Every fourth layer, however, of the bricks have their ends
placed external and internal with respect to the axis of the chimney,
and these bricks are broken across at the depth of 4½ inches, or midway
between their internal and external ends, the latter being at the
surface of the chimney. Thus three-fourths of the bricks of the denuded
patches were torn off through the lime seam parallel to the surface of
the chimney, while a fourth of the number has been broken across in the
same vertical plane. Another portion of the surface bricks, 10 feet
(vertical) by 3 feet, has not been entirely detached from the side of
the chimney but forms a bulging of 1 foot at its greatest projection,
and is visible in profile half a mile off. The lightning on striking the
chimney appeared like a cricket ball, of the brightness of iron at a
white heat. This instantaneously passed into a bluish flame a little
darker than that of common salt when thrown on the fire. A momentary
flicker and a hard crack were perceived. The lightning seems to have
struck the chimney 20 feet above the uttermost denuded patch at a small
abraded spot occupying a few bricks, and reddish when seen from the
ground. The chimney has no lightning conductor and the damage done has
not affected its stability and draught.
_Additional remarks_, by Alexander D. Milne, chemist, of Rubislaw Works.
6th December, 1859.—Half the lower bulging part, where the force of the
electric fluid seems to have become diffused or spent, fell during the
gale of 3rd and 4th December. The 3rd inst. had been frosty, followed by
thaw, rain, and wind from S.W. The part newly exposed is 10 feet in
vertical height and 2 feet across, and the first mortar joint forms also
the plane of separation, the radial bricks being cut right across. The
lower edge of the patch is 100 feet from the ground, and the four
patches extend upwards in an irregular line for 40 feet, not vertically,
but in a spiral of about one-third round the circumference of the
chimney. The abraded spot through which the fluid seems to have
penetrated is 20 feet farther up in the same oblique direction. It seems
to be about 6 inches in diameter, and the part appears as if broken by a
hammer from the outside, instead of being forced out from within, as in
the denuded parts below. We may form a conception of the immense
disruptive force exercised, thus: 105 bricks are torn off, area of each
14⅞ths square inches; total area, 1562 square inches. Force or dead
weight required to tear asunder: 1 square inch of brick has been found
to be 300 lbs. Total disruptive force 468,600 lbs., or 209 tons, and
this on the bricks cut across alone. In addition we have a mortar joint
three times the above area, which at a moderate estimate of one-third
the strength, or 100 lbs. per square inch, gives 209 tons more, or 418
tons in all, the approximate dead weight required to tear off what has
fallen. Allowing for what is damaged but has not fallen, the electric
fluid must have had a momentary disruptive force of 500 tons.
V.
_Gloucester, July 2nd, 1859._—Two clumps of objects were struck, two elm
trees in the Spa walks and Rycroft Chapel—with the adjoining elm. This
shows the lightning to have been forked, as they were both struck at the
same time, and there was a double explosion of thunder; the extremities
of the fork were 1480 feet from each other. The trees in the Spa
standing close to each other were stripped from a great height, of six
or seven inches width of bark, which, with the branches, was strewed to
the distance of several yards. The elm at California had a large bough
struck off; the lightning then ran along another branch, struck the
stone edging of the roof of the chapel, scorching the end of the bough
and chipping great pieces off the stone; it then ran along the metallic
gutter to the end of the roof near the schoolroom, where it descended
the iron spouting to the ground, bursting the spouting at the joints,
where it was a quarter of an inch thick, and in one place knocked a hole
in the wall ten inches deep, as if some superior conductor had attracted
it inside.
VI.
I delayed answering your note until I could give you a correct
description of the damage done to the chimney by examination from the
scaffolding (which we were erecting at the time of its receipt.)
The chimney is a portion of some additions made to my manure works only
last November. It was struck during a fearful thunderstorm on Tuesday
the 19th of July, about three o’clock in the afternoon. The electric
fluid detached about one-third of the topmost stonework, which fell with
great violence through the roof of the buildings below; it then
displaced and passed through the joints of the remainder of the
stonework to the brick shaft. This octagonal brick shaft it split and
shattered in all directions on three of its sides, for a space of about
twenty-five feet, completely detaching portions of the brickwork several
feet in length, both inside and out; after which it split the remainder
of the shaft asunder in a straight line through a further space of about
fifteen feet to the stone base. This stone base it also displaced
(passing through the joinings,) and through seven feet more solid
brickwork, to an open ventilator placed under the roof of a building at
the foot of the chimney.
Through the ventilator a portion of the electric fluid seems to have
escaped from the chimney into the interior of a large warehouse, some of
the main timbers of the roof of which it has split and shattered very
much. A portion only of the fluid seems to have escaped in this way, as
the chimney is split below the ventilator for a further space of about
ten feet.
Several persons were at work in the warehouse at the time, none of whom
were injured in any way (although they felt benumbed.) Two strong horses
standing in a cart were, however, struck down by the lightning on its
escape from the warehouse.
The greater portion of the chimney must come down; in fact, we are now
taking it down.
JOHN STERRIKER.
Driffield, _August 8th, 1859_.
P.S.—In the construction of the chimney, hoop iron was imbedded in the
body of the brickwork every five or six courses, to bind it together;
and this, I think, prevented the whole of the upper shaft from being
thrown down, although in many places the iron has been completely fused.
The total height of the chimney was 85 feet.
_Extract from Mr. Symons’s report on Thunderstorms in 1857–58 and 1859._
[Read at the Oxford Meeting of the British Association, 1860.]
_Lightning Conductors._—No building provided with a conductor is
recorded to have been injured during the three years; in a few cases
bars or pipes of metal acted as such, so far as they extended. The first
instance was at Wibsey School, where the charge, which killed one boy
and injured eight others, had passed safely down an iron pendant from
the roof—in fact, an iron rod of, I believe, small diameter.
In the case of a house in Camden Square, the charge which overturned one
end of a stack of chimneys, passed safely down the iron water-pipe at
the back of the house.
The flash which injured Ryecroft Chapel, Gloucester, first struck an
elm-tree close to the chapel and broke off a large bough, it then darted
to the roof, ran along the metallic gutter to the end of the roof, where
it descended the iron spouting to the ground, bursting the spouting at
the joints, where it was a quarter of an inch thick, and in one place
knocked a hole in the wall ten inches deep, as if some superior
conductor had attracted it inside.
I presume few persons will now oppose the results obtained by the
elaborate investigations of Sir W. Snow Harris, either as to the
_utility_ of conductors, or their best form and distribution. These
points being admitted, it remains to ascertain why they are not more
generally used—why, in short, the accidents I have enumerated (with,
perhaps, as many more of which I have not heard) are allowed to
occur—that they are preventible there is no reasonable doubt. I believe
that the reason that conductors are so comparatively seldom used may be
expressed by one word—expense; a remark made by Professor W. Thomson, at
the Aberdeen meeting, was a strong illustration of this point, “If I
urge our manufacturers to put up lightning conductors they say, ‘It is
cheaper to insure than to put up conductors.’”
But as no insurance nor ought else can compensate for loss of life, it
becomes important to consider if any cheap and effectual substitute for
a regular conductor can be found.
One plan for effecting this, as far as private dwellings are concerned,
is that of connecting the lead gutters of the roof with the rain water
pipe, and with a rod projecting a few feet above the chimneys; it is
obvious that both gutters and pipe would derive additional conducting
power from the water which (at such times as the conductor is required)
is usually flowing along them.
I am not sufficiently acquainted with the laws of electric action to
offer an opinion on this plan; as far as my own limited experience goes,
I think it would be decidedly better than the entire neglect which now
so largely prevails, for it would probably induce the shock to pass down
the outside of the house instead of down the chimney inside, which has
hitherto been its most frequent course.
I much wish that those who have turned their attention to electric
action would express a decided opinion on the matter. In one of the
foregoing cases the iron pipe was perfectly competent and effectual in
conveying the charge; and in the other the damage (limited, be it
remembered, to bursting the joints) doubtless arose from the
intervention of the _lead_ between the two lengths of pipe—considering
the somewhat low conducting power of the lead, such a result might
almost have been anticipated.
_Kind of Trees Struck._—In sixteen cases the class of tree struck has
been mentioned; of these one-third were elms. The next in order of this
unenviable distinction are the oak, ash, and poplar; instances also
occurred of the crab, the lime, and the willow being injured by
lightning.
It is satisfactory to find that as far as so short a series is
competent, it corroborates previous opinions on the subject. I may
perhaps be permitted to quote one of the earliest with which I am
acquainted. In the year 1787 Mr. Hugh Maxwell wrote to the American
Academy that he thought he might state from his own experience that the
elm, chestnut, oak, and pine, are _often_; ash _rarely_; and beech,
birch, and maple _never_ struck.
A communication with which I have been favoured by Mr. Ingram, of
Belvoir Castle, bears closely on this subject, and is, I think, worthy
of consideration. He says, “I filed your letter, resolving to keep a
sharp look out in my rides about the neighbourhood for all the
thunder-blasted trees. It is of course difficult to obtain perfectly
accurate information, because trees are taken away after their
destruction; but I have ascertained that within the area of Croxton
Park, twenty per cent. of the trees (oaks) have been struck by
lightning. The park is situated on high ground; the substratum is rock
(limestone), which has more or less iron in it. The oaks, where the soil
is strongly ferruginous, are useless as timber trees; the wood, when
sawn, splits and rives in every direction, possibly from the quantity of
iron.”
* * * * *
The accounts given in the notes supplied to me by Mr. Symons are of
great interest, but as the majority of the buildings struck had no
lightning rods the details of the destructions do not bear
immediately on the object of our Conference. There are, however,
some few facts which may probably be of interest.
1.—_Damp air although not a conductor for ordinary electricity (see the
writings of Sir Wm. Thomson) may be a conductor for lightning_:—
For there are many instances of sheep and horses being killed in open
fields. This may have been due to the sheep collecting together in a
flock, and the air above them becoming moist from the perspiration
arising from the flock.
2.—_Certain coincidences of earthquake waves and atmospheric electrical
storms have been observed._—The following may, perhaps, be one:—
June 5th, 1858.—During thunderstorm at Pegwell Bay the water in the Bay,
the tide being then about two hours past flood, suddenly receded about
200 yards, and returned to its former position within the space of about
twenty minutes.
3.—_Open doors allow lightning to pass through._
August 12th, 1858, Bedford.—The lightning passed through five open doors
in its way from a chimney, which was originally struck, to an open
window, by which it went out, all the doors being on the ground floor.
4.—_Difficulty of making lightning conductors to protect buildings._
August 18th, 1858. Neighbourhood of Norwich—A boy riding on a pony
escaped unhurt, while the pony was killed by lightning.
St. Peter’s Church, Brighton.—The tower was provided with a lightning
conductor, but it was only carried up one of the pinnacles, hence one of
the other pinnacles of the tower was struck—the distance between the
pinnacles being scarcely ten feet.
Sometimes trees are struck in the middle, and not at the top.
New Kent Road.—While a man was sawing wood, the lightning entered by the
window, struck the blade of the saw, burnt the handle, but did not
injure the man.
5. _A small body perfectly insulated from the ground is not safe from
lightning._
October 11th, 1858. Kilham, Yorkshire.—Two sea gulls, while flying, were
killed by lightning.
6. _Advantages of lightning conductors._
During 1857, –58, –59 almost the whole of the buildings reported as
damaged by lightning were unprovided with lightning conductors. Among
those struck but not damaged were buildings on which metal bars or pipes
acted as conductors as far as they went, proved by the lightning having
burst the metallic spouting at the joints.
7. _Expense of Conductors._
Sir W. Snow Harris’ rule:—
Copper solid 0·5 in. in diameter
tube 1·5 in. in diameter ¼ in. thick.
Iron solid 0·75 in. in diameter
tube 2·00 in. in diameter ¼ in. thick.
Minimum cost one shilling per foot, not including cost of carriage and
fixing.
* * * * *
Sir William Thomson, at the meeting of the British Association at
Aberdeen, said “If I urge on Glasgow manufacturers to put up lightning
conductors they say it is cheaper to insure than to do so.”
This shows the importance of economy in the construction of conductors,
and consequently of the determination of the least expensive conductor,
which will be safe for any special building. One of the most important
points to determine, it appears to me, is whether an electric current,
when the electro motive force is very high, passes along the surface or
through the body of a conductor, since on the result of this must depend
whether we give a lightning conductor large surface, or large sectional
area—in fact, whether a tube of large diameter, but with comparatively
thin walls, is better than a solid rod of much smaller diameter.
In the May number for this year of the Philosophical Magazine, there
appeared an interesting article, by the late Mr. Brough, “On the proper
Relative Sectional Areas for Copper and Iron Lightning Rods,” in which
Mr. Brough arrived at the result that the sectional area of an iron rod
conductor should be to the sectional area of a copper rod in the ratio
of 8 to 3; from which he concludes that an iron rod will be the cheaper
conductor. But this result is obtained on the assumption that the
resistance of rods of the same length, and of the same material for
lightning, are inversely as their sectional areas, a result about which
I think there may well be doubt.
W. E. AYRTON.
APPENDIX F.
ABSTRACTS OF PRINTED DOCUMENTS.
FRENCH OFFICIAL PUBLICATIONS.
_Preliminary Note._
In 1784, the attention of the French Government having been directed to
the desirability of protecting the powder magazines of the kingdom from
damage by lightning by the employment of conductors, a system of
construction was proposed by two officers of the Engineers and
Artillery. This system was referred by the Minister of War to the
Academy of Sciences for consideration and report. From time to time
subsequently other proposals of a like nature, and other inventions and
improvements in the construction of lightning rods were considered by
the Academy, and reported upon by various Committees.
At the request of the Conference, I have endeavoured, in the following
pages, to give in as condensed a form as possible an accurate abstract
of their contents, and to avoid, in all cases, the expression of any
opinion, either adverse or concurrent, upon the principles or
suggestions contained in them.
E. E. DYMOND.
REPORT _made to the_ ACADEMY OF SCIENCES, _by_ FRANKLIN, LEROY, COULOMB,
DE LA PLACE, _and_ ROCHON.
_24th April, 1784._
Certain proposals for erecting lightning rods for protecting the powder
magazines at Marseilles having been submitted to the Academy for their
opinion, a committee, consisting of the above-named, was appointed to
examine and report.
They begin by enunciating the theory which should regulate the erection
of conductors, and they lay down the following rules:—
1. The extent of the building should first be ascertained to decide
whether one or more conductors should be used. Electrical experiments
had not yet made known anything of the extent to which the action of the
point of the conductor reached. But since buildings had been supplied
with conductors many observations had shown that those parts of them
which were more than 45 feet French (48 English) from the point of the
conductor had been struck by lightning.
2. When there are many points or arrows on the building they should be
connected together and also connected with all parts of the roof which
are covered with lead, and also connected with the weathercocks or
ornamental metal points so as to form one metallic system with the
conducting bars.
3. It is not less important that these bars should be thoroughly joined
together; for a solution of continuity in them produces a resistance to
the passage of the electricity according to the extent of their
separation.
4. It is necessary that the bars should communicate thoroughly with
moist earth or, better still, with water.
As to the height of the points they should be at least 12 or 15 feet (13
to 16 feet English), or even more if the building is a large one. It is
certain that the higher they are the wider the extent of their action.
They should be 2 inches (2·2 English) square at the base and greater in
proportion as their height exceeds 15 feet (16 English). If the
conducting bars are 8 or 10 lines (or, say 1 inch) square, it will be
more than enough. No case had occurred in which iron bars of this size
had been in any way damaged or altered by the passage of lightning.
The reporters then proceed to examine the two proposals for protecting
the powder magazines at Marseilles, sent in by M. Ravel de Puy Contal
and M. Pierron. They were both for the same building which was 31 toises
long and 8 toises wide (about 198 by 51 English feet). The first
provided for the erection of three points on the ridge of the roof, and
of four others, one at each angle of the building; the second had also
three points on the ridge, but the other four were alternated on the two
sides of the roof, and iron bars were carried all along and connected
with all the points. The manner in which the terminals were fastened to
the roof and the conducting bars fastened together and led to the water
was the same in each proposal.
The reporters remark concerning the second that the conducting bars laid
horizontally along the roof would involve a great and unnecessary
expense, but the points should be retained, only instead of placing them
alternately they should be set up so that each of them was half way
between the middle and the end of the roof, and instead of connecting
these points by bars along the length of the roof, they should be
connected with the one connecting the three points on the ridge by bars
joining it perpendicularly.
As to the method proposed for joining the several parts together the
reporters cannot help thinking that in their desire to make thoroughly
good connections MM. Pierron and de Ravel had proposed a plan involving
too much difficulty and superfluous expense [It seems to have been
proposed to screw the bars into each other], and they recommend instead
of this to make at the base of each point, immediately above its
insertion into the roof, a circular flange about 2 inches in diameter
and 2 lines thick, with a hole half an inch in diameter in the middle
and at the ends of each conducting bar to make a similar flange and to
bolt the flanges together with a sheet of lead between them. Crutches
should be fixed on the roof to carry the conducting bars. The points
should be fixed three on the ridge and two on each side of the roof half
way between the point in the middle and that at each end. These four
should be connected with the conducting rods running along the ridge and
should overtop the ridge by at least 6 feet (6 feet 5 inches English).
By this arrangement all parts of the roof would be well protected.
The reporters highly approve of the way in which the conducting bars are
connected with water by being led into the sea, but if at the other end
of the building there is sufficient earth on the surface, and the soil
is not entirely rock the conductor from the point placed at that end
might be led into it. It is recommended that the points of copper should
be screwed to the terminals for convenience of removal when necessary.
REPORT _made to the_ NATIONAL INSTITUTE, _by_ LEROY, LA PLACE, _and_
COULOMB, _on a Lightning Rod for Powder Magazines proposed by_ REGNIER.
_6 Nivose, Year 8_ (_23rd December, 1789._)
The reporters think it desirable to make some general observations on
lightning rods, the rather that it appears that some persons have had
fears as to the certainty of their effect.
It is impossible to reject the theory upon which Franklin had proceeded
in providing lightning rods for the purpose of protecting buildings from
damage by lightning. Still, as the theory needed to be confirmed by
facts, it might at first have been doubted whether the lightning rods
were really effectual; but now that observation, and experiment had
proved the truth of the theory there was no longer any room to question
their utility. It may even be remarked that observations had not only
proved that they were effective when well constructed, but that they
conducted the lightning down without accidents, even when they had some
defects, which might have caused one to doubt their efficacy. The
defects alluded to were a blunted point and a break in the continuity of
the conductor. With reference to these two cases observations have
shown—1st. That although the points have been blunted, they still
attract the lightning from the clouds to themselves in preference to the
surrounding objects. 2nd. That although the several parts of the
conductor are not thoroughly joined together, the lightning will still,
if the break be not too considerable, pass along the conductor without
accident.
In support of the first proposition they quote the observations of
Doctor Rittenhouse, of Philadelphia, who had examined several of the
points in that city, and had found them melted, showing clearly that
they had been struck by lightning, and probably more than once, as it
had been shown by many observations that where, from local circumstances
(not then fully ascertained,) lightning had struck in certain places or
on certain buildings, it was not uncommon to see it strike again; and a
number of observations of a different sort had shown that lightning was
attracted by metals on buildings even when they were but slightly
pointed, such as tin weathercocks, or iron crosses, and even plain
sheets of iron.
One of the most striking examples in support of the second proposition,
was the case of an American ship, reported in the Phil. Trans. for 1770.
During the night, in the midst of a storm, the crew reported that there
was a stream of fire in the rigging, just above the middle of the
lightning conductor. The captain saw a stream of fire, sometimes in
sparks, and sometimes only a steady light; and on examining the
conductor next morning, found that one of the links of its chain was
broken. Fortunately the two pieces, being kept in place by the fastening
to the shrouds, were only about three quarters of an inch apart. These
two broken ends formed a sort of points, and on its passage between them
the lightning had become visible. But this was all; no shock was felt,
nor anything which caused any suspicion that the fracture of the
conductor had in any way hindered the passage of the lightning. Franklin
also had shown by experiment that in a lightning rod where the upper end
was only connected with the part entering the ground by a very fine
brass wire, although the wire was melted by the passage of the
lightning, it still was conducted from top to bottom without any damage
to the house; and in other instances metallic wires, though partly
melted by the lightning, had still served as conductors. But it is not
contended from these examples that a very exact and continuous
connection of all the different parts should be dispensed with.
The lightning rod proposed by Regnier consisted of a piece of wood,
coated with resin, rising 2 metres (6 feet 7 inches) above the roof, and
having fixed on its top a sort of inverted funnel of copper, at the
upper end of which was fixed the point. To the lower edge of the funnel
were fastened ropes formed of twenty-seven annealed iron wires well
bound together, which were, at a suitable distance, connected with iron
bars, fastened to masts, and leading to moist earth. The point had a
small piece of platinum at its upper end.
[Illustration: Regnier’s System of Lightning Conductors]
The reporters observe that the wooden support may be employed by way of
extra precaution, though there was no known instance of lightning
leaving metal for wood; but it should be strong enough to resist the
wind. They approve of the method proposed for connecting the point with
the metal bars, metallic ropes being very suitable for this purpose, and
keeping them well away from the building was quite right; but they add
that the metallic bars should not only communicate with moist earth, but
also with water in wells or otherwise.
INSTRUCTIONS FOR ERECTING LIGHTNING RODS FOR POWDER MAGAZINES, _adopted
by the_ FORTIFICATIONS COMMITTEE.
_25th August, 1807._
A lightning rod is an electrical conductor terminating in a point and
carried down to the common receiver. It may be regarded as a metallic
tree, and divided into (1) the upper terminal, (2) the trunk, and (3)
the roots.
1. The upper terminal is a very pointed, conical or pyramidal spike of
metal having a base 3 or 4 centimetres (1½ inches) in radius. The point
is of gold or platinum, soldered to a copper rod 1 or 2 metres long (3
feet 3 inches to 6 feet 7 inches). This rod is joined to the rest of the
upper terminal, which is of iron, either by solder, a screw, or a pin.
It is important that all the parts of the upper terminal should be
joined with care so as to prevent fracture; at the bottom of the
terminal are several feet by which it can be leaded to the vault or
bolted to the framing of the roof. Several devices for giving some play
to the terminal so as to diminish the effect of vibration have been
proposed, but it is better to make the terminal strong enough to resist.
At the bottom of the terminal is joined the piece connecting with the
conductor; this ought to be very complete and continuous, especially at
the point of junction with the terminal. Frequently the terminal is
enlarged at this point to facilitate the passage of the lightning. To
preserve the terminal from rust it is sometimes gilded—it has been
proposed to tin it—more frequently it is merely painted; experience
shows that this is sufficient. Instead of making the whole terminal
conical or pyramidal, a square bar of iron, finished with a point of
copper tipped with gold or platinum, is sometimes used. This plan may
usually be adopted without danger, but they are more liable to be broken
or bent by vibration.
[Illustration:
FIG: 1.
]
[Illustration:
FIG: 2.
]
[Illustration:
FIG: 3.
]
[Illustration:
FIG: 4.
]
2. The trunk or conductor is made of iron bars 13 to 20 millimetres
square (½ to ¾ inch) notched at the ends and bolted together with a
plate of lead between the two (Fig. 1). For powder magazines a bar of 27
millimetres (1 inch) square is recommended. They follow the outline of
the roof, cornice, and wall, and each bar is fixed by a half collar
(Fig. 2) or cramp placed in the middle of the bar or as far as possible
from the junction of two bars. Instead of the iron bars ropes of copper
or iron wire, or even of hemp, may be used; these last may be used
provisionally, but for permanent conductors they have no advantage
either in economy or conductivity. The copper rope conducts the
lightning better, but its smaller size and cylindrical form, by
diminishing its absolute and relative surface, counterbalances its
superior conductivity. The great and real advantage of metallic, and
especially of copper ropes, is in their continuity and their
flexibility. The conductor is led down to the surface of the ground
where it is bent and led parallel to the surface towards a pit full of
water, or deep enough to allow the end of the conductor to rest in damp
earth; from 2 metres (6 feet 6 inches) above the ground to the pits the
conductor is enclosed in a channel or trough like the fuse of a mine,
the object of this is to protect the conductor from the dampness of the
soil and from contacts. These would be unimportant so long as there is a
perfect connection between the point of the conductor and the common
reservoir, but this continuity may be destroyed by degradation of the
conductor, and it is chiefly at the joints that this discontinuity is to
be feared. When the conductor has to be buried it should be in an oaken
trough, well put together and tarred or charred or surrounded by
powdered charcoal so that the metal cannot be rusted by infiltrations or
humidity; in some soils it is better to make the subterranean part of
the conductor of lead, taking care by increasing the surface to make up
for its inferior conductivity. Sometimes water pipes may be made use of,
but only when they serve to lead water away and when they terminate in
an isolated reservoir. It is important to lead the conductor far away
from water pipes carrying water to public fountains or into the interior
of houses.
3. If the conductor leads to a well full of water the roots (Fig. 3)
need not be more than a few spindles terminated in points and long
enough to be always immersed. When the conductor only leads to a bed of
earth it is supplied with a system of roots (Fig. 4), having for its
object the multiplication of the points for the escape of the lightning,
and these are increased in number according as the soil is a less good
conductor. The pits should be some distance from the foundations of the
building, so that the lightning may not damage them, and it is
important, by all possible means, to increase the natural humidity of
the soil. When the wells cannot be closed it is necessary that the
conductor should be insulated and plunged deeply in the water for fear
that the communication of the electricity to the well-chains or
pump-rods might cause accidents or alarm. After some other instructions
it is added that the dispersal of the electricity in the common
reservoir is, next to the continuity of the conductor, that which most
deserves the attention of the physicist and the engineer.
It has been remarked that a point extended its sphere of activity as far
as 10 metres (32 feet 9 inches), that beyond this distance its effect
became less sensible, and that when the points were too near together
they neutralised one another. So upon a building of a given size it is
necessary to set up so many that all parts shall be covered by their
spheres of attraction, which should meet and not overlap each other.
Lightning in passing from a cloud to the earth does not always take a
vertical direction, it sometimes follows the path of the rain drops,
which is inclined by the wind, so when a magazine is very lofty, or on
an elevated spot it is not useless to fix horizontal or inclined points
on the gables or angles. In some places the magazines are dominated by
other buildings; in these cases the neighbouring buildings should be
protected, or the magazines should have horizontal points towards them.
If the ramparts dominate the magazine it will be prudent to set upon
them a lightning rod on a mast. Trees are only struck by lightning
because their tops serve as points but their trunks are bad conductors,
hence it is prudent not to have plantations, especially of lofty trees
near magazines. However many points may be set up on a magazine they
should all be connected together, and all joined to the principal
conductor, and it would be well to have more than one principal
conductor so that if one loses its continuity the lightning may have a
path by the other. Stone, wood, and gunpowder are bad conductors, and
pieces of metal may without danger be used in the inside of magazines,
provided they are connected with the principal conductor by branch
conductors of suitable size: still it is prudent to keep the metal
outside.
[Illustration: Powder Magazine, with oblique as well as vertical rods]
Reference is then made to “Regnier’s System of Lightning Rods,” Appendix
F., p. 53, which is thought to be much too expensive.
REPORT _on the foregoing Instructions made by_ LA PLACE, ROCHON,
CHARLES, MONTGOLFIER, _and_ GAY LUSSAC _to the_ NATIONAL INSTITUTE.
_2nd November, 1807._
The reporters say that experience has taught that the point of a
lightning rod 4 or 5 metres (13 to 16⅓ feet) does not effectually
protect a space round it greater than one having a radius of 10 to 12
metres (32¾ to 39¼ feet). That when there are points or considerable
masses of metal on a building having a lightning rod it is absolutely
necessary to connect them by branches with the principal conductor. That
it is not less important that the metallic bars should be thoroughly
well connected together so that the electricity may find no resistance
in its path from the point to the common receiver. And lastly, that it
is necessary that the conductor should have a perfect communication with
moist earth, or better, with water. They then proceed to discuss the
instructions, or that part of them which relates to the construction of
the lightning rods. They recommend the use of gilded copper points,
notwithstanding the doubt concerning them which had been raised in
consequence of their deterioration by oxidation, and their being blunted
by lightning. They say that experience has shown that an iron rod 20
millimetres (·8 inch) square is more than sufficient to carry the most
violent discharge of lightning, and that it is consequently needless to
make them larger, as recommended in the Instructions; that it is only at
the joints that there is any cause for fear because, in spite of the
insertion of the piece of lead, the contact is not perfect; that it
would be easy by enlarging the bars at their junctions to increase the
number of points of contact, and by lengthening the bars to make fewer
joints. That in this respect the use of iron wire ropes would be very
advantageous, but they fear that the ropes would be easily destroyed,
and that the use of copper wire rope instead of iron would be too
expensive.
When the conductor reaches the ground too much care cannot be exercised
in making a free communication between it and the soil. It is upon this
that its good effect principally depends, for houses have been struck
although provided with a conductor, because it only communicated with a
very dry soil. M. Patterson, of Philadelphia, in the fourth volume of
the American Phil. Trans., has published a means of making a good
contact which seems useful. He proposes to lay the conductor in a bed of
galena worked into a paste with melted sulphur. The galena is a good
conductor, and would have the advantage of protecting the iron from the
damp. He has also proposed a simple means of providing for the easy
dispersion of the electric fluid in cases where the soil is not very
damp, which consists in making a hole in the ground and filling it with
charcoal, into which the conductor is plunged. But M. Guyton used the
conducting power of charcoal for this purpose more than thirty years
ago, and it has been applied in many ways. Charcoal, like galena, is a
good conductor, and this property renders its employment desirable in
cases where the soil is dry.
Upon the proposal to fix inclined or horizontal points they think that
vertical points will suffice; and with reference to the Regnier system,
they remark that it would certainly be very expensive, and that it would
not be necessary to adopt it until the usual system had been found
insufficient.
INSTRUCTIONS _about_ LIGHTNING RODS _adopted by the_ ACADEMY OF
SCIENCES.
_First Part, 23rd April, 1823._
PREPARED BY A COMMITTEE CONSISTING OF MM. POISSON, LEFEVRE-GINEAU,
GIRARD, DULONG, FRESNEL, AND GAY LUSSAC.
[Illustration: Mode of attaching Conductor to Upper Terminal]
After some theoretical remarks the Committee describe the conductor they
recommend, giving the name of _tige_ (upper terminal) to the part rising
into the air above the roof, and that of conductor to that part
extending from the upper terminal to the ground. The upper terminal is a
square or round bar of iron tapering from base to summit. If from 7 to 9
metres (23 feet to 29 feet 6 inches) high, which is the smallest height
to be used on large buildings, it should be 54 to 60 millimetres (2·1 to
2·3 inches) square or diameter at the base, if 10 metres (32 feet 9
inches) high, it should be 63 millimetres (2·5 inches). About fifty-five
centimetres (1 foot 9½ inches) of the upper end is cut off and replaced
by a point of copper either gilded at the end or tipped with a little
piece of platinum. At the lower end of the terminal (A), 8 centimetres
(3·15 inches) above the roof, is fixed a base (B) to throw off the rain
which would run down the terminal, and above this base the terminal is
clasped by a collar (C), as shown in the drawing, to which is bolted the
conductor (D). The engraving shows the modification of the arrangement
as adapted to both round and square terminals. The conductor is a bar of
iron 15 to 20 millimetres (·59 to ·79 inches) square, joined firmly to
the upper terminal by bolting it tightly between the two ears of the
collar. The best way of joining the bars together is shown in figure 1,
p. 55. It is to be held up at a distance of 12 to 15 centimetres (4·7 to
5·9 inches) from the roof by crutches, and to be kept at a like distance
from the walls of the building. At 50 or 55 centimetres (19·6 to 21·6
inches) below the surface it is turned away perpendicularly from the
wall for a distance of 4 or 5 metres (13 feet 1 inch to 16 feet 5
inches) if it does not sooner meet with water. To avoid rusting the rod
is carried in a trench filled with charcoal, and then turned down a well
so as to have at least 65 centimetres (25·7 inches) in the water when at
its lowest level, where it terminates in three or four branches to
facilitate the exit of the electricity from the conductor.
If there is no well convenient, a pit should be made 13 to 16
centimetres (5·1 to 6·3 inches) in diameter, and 3 to 5 metres (9 feet
10 inches to 16 feet 4 inches) deep, down the middle of which the
conductor should be led and the hole filled with charcoal tightly
rammed. As the iron bars forming the conductor are not easily bent to
follow the lines of the building a metallic rope may be used. It is made
of four strands, each composed of 15 iron wires, and forming a rope of
16 or 18 millimetres (·62 to ·7 inches) in diameter. Each strand is
tarred separately, and the whole also well tarred when put together. It
is attached to the upper terminal in the same way as the bars by
pinching between the ears of the collar (c). At 2 metres (6 feet 7
inches) above the ground it is joined to the bars which form the earth
connection by being pinned into a socket formed at the end of the first
bar. Ropes of copper or brass wire may be used, and they need not be
more than 16 millimetres (·62 inches) in diameter.
It is necessary to connect any considerable metallic masses (lead roofs,
metal gutters, or tie rods) with the conductor, because if this be not
done, and the conductor be broken, or have a bad earth connection, the
lightning may leave the conductor for the metallic mass.
Modifications of this form of conductor for use on churches, ships, and
powder magazines (for the latter carrying the conductors on masts is
recommended) are then described.
The report says that the terminal of a conductor protects efficiently a
circular space round its base, having a radius equal to twice its
height; but that it is prudent to estimate that a conductor on a church
spire only protects a circle having a radius equal to the height of the
conductor.
The conductor should go the shortest way to earth. It should be on the
side most exposed to the weather, especially on spires.
_Second Part, 18th December, 1854._
PREPARED BY A COMMITTEE CONSISTING OF MM. BECQUEREL, BABINET, DUHAMEL,
DESPRETZ, CAGNARD DE LATOUR, AND POUILLET.
Notwithstanding the considerable advance in knowledge since 1823, the
instructions of that date have no need to be altered, at least in their
essential principles; but the methods of construction of buildings
having materially altered, and metal having largely replaced wood and
stone, buildings had, so to speak, become metallic masses, which would
have incomparably greater attraction for thunder clouds. The Palais
d’industrie in the Champs Elysees, for example, nearly 3 hectares (7·4
acres) in extent, and 40 metres (131 feet) in height had everywhere
enormous masses of iron, brass, and zinc.
The company undertaking the building had sought the advice of the
Academy as to the means to be employed to protect it from lightning, and
it had been found necessary to revise the instructions of 1823, in order
to introduce such modifications as were necessary.
Quoting the passage referring to the connecting of metallic masses with
the conductor, the Committee think that the time had come to enter into
fuller details on this point.
Formerly the use of metal was almost restricted to ridges, gutters, and
tie rods; now metal was used everywhere, and what is important, in large
surfaces and great masses; and this new system realised on a large scale
the first objection to lightning rods—it attracts the lightning.
When this objection was applied to lightning rods, it had only the
appearance of truth, but when applied to the masses of metal then used
in buildings, it was not only specious, but true, and founded upon well
established laws; these buildings do attract the lightning, and render
its effects more disastrous.
In the case of two buildings alike in size and shape, situated on the
same soil, one made of wood and stone as formerly, the other with much
metal as now, and both without lightning rods—if the conditions are such
that the lightning must discharge itself, it will always strike the
latter, and never the former; in the same way as on bringing to the
conductor of an electrical machine a ball of wood or stone, and one of
metal, it is always the latter which will receive the spark. Lightning
rods, therefore, are so much the more indispensable as the buildings
contain greater surfaces and greater masses of metal.
The nature of the soil must be taken into account, as well as the
buildings and other objects upon it. A dry soil, with a subsoil of dry
sand, chalk, or granite, does not attract the lightning, because it is a
bad conductor. Unless when accidentally wetted the buildings on it
participate to some extent in this immunity, at least if they are not
built in the modern style, and are not very large. But if there are at a
moderate depth underneath this dry ground, large metallic veins, vast
caverns, sheets of water, or only abundant springs—these will attract
the lightning, which will destroy everything in its path unless
protected. If the wet or metallic strata are very deep, the danger of an
explosion is diminished by the difficulty of passing the intervening
envelope, and by the weakening of the action of the cloud by the
increase of distance.
On the 19th April, 1827, the packet boat _New York_ was twice struck by
lightning. On the first occasion, having no conductor, it received
considerable damage; on the second, the conductor was fixed; it was made
of a pointed bar of iron, 1·2 metre (about 4 feet) long, and 11
millimetres (·43 inch) diameter at the base, and a surveyor’s chain
about 40 metres (131 feet) long, forming a connection between the foot
of the rod and the sea; the chain was made of iron wire 6 millimetres
(·24 inch) in diameter; the links were 45 centimetres (17·7 inches)
long, ending in loops, and joined together by two round rings. When
struck the chain was dispersed in burning fragments and globules, which
set the deck on fire in many places, notwithstanding the hail upon it
and the rain which fell heavily; the bar at the top was melted for a
length of 30 centimetres (11·8 inches) from the point, and down to a
diameter of 6 millimetres (·2 inches). The rest of the rod remained with
about 8 centimetres (3·1 inches) of the chain attached to it, the
longest piece of chain found was less than 1 metre (3 feet 3 inches)
long, and was blistered as by fire.
On the 13th June, 1854, the _Jupiter_ was struck by lightning. The
conductors were in place; that of the mainmast which was struck went 2
metres (6 feet 6 inches) into the sea, and had at its end a ball 2 kilos
in weight. After being struck the conductor had disappeared and the
pieces of it were scattered everywhere. The conductor, about 70 metres
(230 feet) long, was a cable of three strands formed of sixty brass
wires, each one half or two-thirds of a millimetre (·019 or ·026 inches)
thick. The cable was mostly in bits no bigger than pins, but there were
some pieces a few decimetres long, these had been turned violet colour
as by fire, and those first touched were still burning hot.
These two examples show that a conductor may be destroyed, but they also
show that it is not useless even then, since it will have received the
discharge and directed it, and so prevented greater mischief. The
_Jupiter_ received no damage; whilst not far off, a Turkish vessel,
which also had a conductor (but the chain of which did not reach the
water) having been struck by lightning in the same storm, had a hole
more than 30 centimetres (11·8 inches) deep, and almost such as would
have been made by a cannon ball, in her side just above the copper, and
near the water line.
The question is, are such accidents to conductors inevitable, or are
they the result of faulty construction? All the facts established in the
accounts of lightning and its phenomena, leave no doubt on this point.
All the lightning rods which have been destroyed were of bad materials,
insufficient, badly constructed, not in accordance with the principles
which theory has deduced from experience.
The conductor of the _New York_ had several faults; its upper terminal
was too small, and too much drawn out; its conductor had much too small
a sectional area; and the use of a chain in such cases should be
strictly excluded.
There is no example known in which lightning has been able to melt iron
rods 2 centimetres (·78 inch) in diameter, or 3 square centimetres (1·18
inch) in section; and copper may be used in still smaller sizes.
The conductor of the _Jupiter_, although better than the former, had
also a radical defect. The fragments of the conductor which were
examined bore but few traces of fusion, and none of these traces
extended to the entire thickness of the cable; they were also limited to
a group of some of the sixty wires of which it was composed. This seemed
to show that the discharge was not carried equally by all the wires, and
that those wires which it followed being insufficient to carry it, were
the ones melted, and the others were broken or volatilised with
explosion. Hence the breaking of the cable and dispersion of fragments
of some decimetres in length, which, though too hot to be touched, were
not hot enough to set wood on fire. This explanation, however, raises a
singular question, whether, in a cable of similar wires twisted and
bound together, the lightning can choose some wires in preference to the
rest, even when the whole of them are hardly sufficient to give it a
free passage.
Undoubtedly, yes; at any rate under certain conditions. No doubt if at
both ends of the cable, for the length of a decimetre, the wires first
tinned separately are afterwards soldered together, so as to make a sort
of metallic cylinder, electricity, whether natural or artificial, having
to pass along the cable, will not show a preference for one wire over
another; but where this is not done—if at the two ends, or, more
generally, at the two points of junction with other conductors, the
wires are isolated by layers of dust or oxide—if, in addition, the cable
only touches the terminals by its outside wires, then things happen very
differently. The electricity takes those wires that are in contact with
the terminal; these reduced to few in number become incapable of
carrying it; and the whole cable broken by the explosion exhibits the
phenomena shown in the case of the _Jupiter_.
The deficiency in each case was due to one cause—insufficiency of
sectional area. In the first case the insufficiency is apparent, the
iron wires 6 millimetres (·24 inch) thick were nine or ten times too
small; in the second, the insufficiency is more hidden, it results from
badly made junctions.
The two most fundamental rules for the construction of the rod and
conductors are—1st. That they shall have a sufficient sectional area.
2nd. That they shall be continuous and without a break from the point of
the upper terminal to the common receiver (the earth). But this
continuity may in strictness be interpreted in two ways: it may be said
that two pieces of metal in contact form a sufficiently continuous
connection; and it may be said, on the other hand, that most frequently
this simple contact is no more than a break in consequence of oxidation
and the interposition of foreign bodies.
The instruction of 1823, without adopting the first interpretation, does
not appear to have sufficiently recommended the second, which should
exclusively regulate all construction of lightning rods. No doubt it is
possible, by taking great care, to join and bolt together two pieces of
iron or copper closely enough to make a practically continuous
conductor, but when there are many joints we fear that evil might arise
from the negligence of workmen, and still more from the chemical
alteration of the surfaces, the deposition of foreign matter, and the
mechanical dislocation produced by time and repeated shocks.
Hence, the three following practical rules should always be observed:—1.
To reduce as much as possible the number of the joints. 2. To make all
the joints with hard solder, and they should be upon surfaces of at
least 10 centimetres (3·9 inches) square, and further strengthened by
straps and bolts. 3. Not to make the upper terminal so gradually pointed
as usual. The upper terminal of iron should be not less than 2
centimetres (·78 inch) diameter, the end should be filed down and a
screw tapped 1 centimetre (·39 inch) high and 1 centimetre diameter, and
to this a cone of platinum 2 centimetres diameter and 4 centimetres (1·5
inch) high, and consequently having an angle at the point of 28° or 30°
should be fitted, screwed, and carefully soldered.
In other respects the instructions of 1823 should be followed; no fact
which leads to a modification of the general rules there proposed 1, for
the sectional area of the conductors; 2, for the method of fastening to
buildings; 3, for the method of making the earth connection, has since
come to light.
The subject, however, is not exhausted, there still remains the
important and difficult question: what is the circle of protection
afforded by a well constructed lightning rod? The opinion generally
received at the end of the last century was that the circle of
protection had a radius of twice the height of the terminal, and the
instruction of 1823 adopted this opinion, but with some restrictions as
in the case of spires. It is important to remember that these rules rest
upon a more or less arbitrary basis, and this is said not to condemn
them, but only to prevent there being attached to them a value which
they do not possess.
More observations are required, and it is only with reserve that these
rules are admitted. They are neither general nor absolute, they depend
upon a variety of circumstances, and especially on the materials of the
buildings. For example, the radius of the circle of protection, which
would be sufficient for a building having only wood tiles or slate on
its upper portion, would not be sufficient for a building in which the
covering or the framing of the roof was of metal. In the former case the
active portion of the thunder cloud, although further from the lightning
rod than from the roof, would exert a greater action on the rod, whilst
in the latter the action on the rod and on the roof would be almost
equal at an equal distance.
A special note upon ships, and another on the Palais de l’Exposition
close the report.
SPECIAL REPORT FOR THE NEW BUILDINGS OF THE LOUVRE, 18 DECEMBER, 1854,
BY THE SAME COMMITTEE.
Referring to the subject of the earth connection the Committee say; in
the earliest instructions, it is said that the conductors should
communicate with the water in a river, a pond, or wells, or at least
with moist earth. This rule, although quite correct in itself,
frequently leads to erroneous practice. It is sometimes thought that
lightning is extinguished by water, as fire is; and when water is scarce
the conductors are plunged into a well-cemented cistern. This is a most
dangerous mistake; the conductor should be in connection with the common
receiver, that is, the great water-bearing strata (nappes d’eau,) of
much greater extent than the thunder cloud. At other times where wells
are possible but costly, advantage is taken of the alternative allowed
by the instructions. Instead of wells the conductors are put in
connection with the earth, without being careful to see that it
preserves sufficient moisture in times of drought when storms are most
to be expected, and without being careful to see that the moist
connection is sufficiently large. They specially note this latter error,
as it appears to be still more common than the former. They do not
hesitate to say that recourse should never be had to this method of
connection with the common receiver. They recommend that in default of
rivers or very large ponds, the conductor should always be connected by
large surfaces with the inexhaustible subterranean water-bearing strata.
Secondly, where these strata are at a moderate depth below the surface,
the Committee consider it necessary to make use of a conductor with two
branches, the principal to descend to the subterranean water; the
secondary, leaving it at the ground level, is put in connection with the
surface. And for this reason; after great droughts thunder clouds exert
but a feeble influence upon a dry, badly conducting soil. All their
energy is felt by the subterranean waters; and the electricity will be
carried by the principal branch. On the other hand, after a summer
shower, when the surface soil gets moist, it is at once made a good
conductor. It is that which is affected by the thunder cloud: while, at
the same time, it screens the subterranean water from electrical
influence. In such a case it is indispensable that the surface of the
ground should be in direct connection with the conductor; and this the
secondary branch supplies.
There is a final question how the conductors should be connected with
the various metallic portions of the building. The ridges are throughout
of iron; but the interior arrangements require that, in some portions of
the building, there should be, properly speaking, only one floor, whilst
in other parts there are six. Each floor may be regarded as a great
metallic network, composed of several strong plate girders, crossed by
numerous joists analagous to rails, while these are, in their turn,
crossed by a multitude of smaller iron rods; and the meshes of this
network are filled with tiles. In enquiring into the effect of a thunder
storm upon those portions where there are six such floors one above the
other, it is easy to see that if the roof were a great continuous sheet
of metal, it would take up the whole electrical energy of the cloud, at
any rate, as far as the floors underneath it are concerned. In this case
it would be amply sufficient if the covering were well connected with
the lightning rods. But in this case the roof is metallic, only in a
very small portion; it may be said that the ridges only form a network
with very large meshes, and, consequently, is an insufficient shield,
through which the upper floor may still receive a considerable shock.
Therefore the Committee propose the following arrangements:—1st. The
principal pieces of each floor should be put in connection with the
conductor. 2nd. It is very desirable that all the joists of the upper
floors should be connected together by a rod bolted, and, if possible,
soldered to each, which rod should be connected with the conductors.
3rd. It seems probable that, in general, the roof frames are in good
connection with each other, and, consequently, it would suffice if all
the upper terminals are connected with them. If, however, it happens
either by changes of level in the gutters, or from other causes that the
connections become doubtful special iron connections must be made. 4th.
The zinc gutters and ridges should be connected with the lightning rods.
REPORT _on the points of upper terminals made by Messrs. Delieul, by a
Committee consisting of_ MM. BECQUEREL, BABINET, DUHAMEL, DESPRETZ,
CAGNIARD DE LATOUR, REGNAULT, DE SENARMONT, _and_ POUILLET.
_5th March, 1855._
The committee examined the points presented by Messrs. Delieul, one of
platinum, made exactly as described in the report of the previous 18th
December; the other, a cone similar in form, size, and external
appearance, but rather less costly, being made of a cap of platinum,
fixed with hard solder upon the conical end of the iron rod. It was
thought that this second arrangement would not practically be inferior
to the other; but it must be made by a skilful workman, who knows how to
insure that the solder should take to the whole of the surfaces brought
together. They see no objection to the substitution of palladium, or
gold or silver of a standard of ·950 for the platinum. But all these
metals are costly; few workmen know how to work in them, or at least to
employ that precision, and take that minute care, which are
indispensable to success. These reasons have raised again a proposition
that was discussed in the former commission, which consists in making
the points of copper. The copper point is 2 centimetres (·78 inch) in
diameter, like the upper part of the iron rod, to which it is screwed
and brazed; its length is about 20 centimetres (7·87 inches), and it
terminates in a cone 3 or 4 centimetres (1·1 or 1·5 inches) high.
They see no reason why this should not be used with almost the same
confidence as the preceding forms. If there is ground to fear that it
may undergo changes from atmospheric influences, this is counterbalanced
by certain advantages. 1st, copper is with palladium, gold, and silver,
among the best conductors of heat and electricity; and the point of the
cone will be much less heated than the platinum point; and 2nd, the
terminal, with a copper point, is much less expensive, and can be made
everywhere.
On the report being put to the vote M. Despretz could not approve the
proposal to employ copper points, fearing that the deposition of
carbonate or some other badly conducting matter would diminish the
efficacy of the lightning rod.
INSTRUCTIONS _upon_ LIGHTNING RODS _for_ POWDER MAGAZINES, _by a
Committee consisting of_ MM. BECQUEREL, BABINET, DUHAMEL, FIZEAU, EDM
BECQUEREL, REGNAULT, _le Maréchal_ VAILLANT, _and_ POUILLET.
_14th January, 1867._
After referring to some general principles, and to the construction of
lightning rods recommended in the reports of the earlier Committees: the
Committee recommend, that the upper terminal including the copper point
should be from 3 to 5 metres (9 feet 10 inches to 16 feet 5 inches)
high; that the junction of the conductor and the upper terminal, and
also the several joints of the conductor, should be covered with solder,
and insist very strongly upon the necessity of communication with the
_nappe d’eau souterraine_, which they define as “the water level in
neighbouring wells which never dry up, and which retain at least 50
centimetres (19·68 inches) in depth of water in the most unfavourable
seasons.”
The special arrangements to be adopted in setting up lightning rods for
powder magazines are: not to fix them on the building itself but outside
the surrounding walls. For each large sized magazine (27·89 metres, by
20 metres, and 11 metres high, equal to 91 feet 6 inches by 65 feet 7
inches, and 36 feet high) there should be three conductors—two near the
ends of the long side of the enclosing wall most exposed to storms, and
the third in the middle of the opposite side. The upper terminals should
be only 5 metres (16 feet 5 inches) high, and should be raised on a
pier, a mast, or other support 15 metres (49 feet 2 inches) high, down
which the conductor should be led to the ground. There should be a
circuit which the Committee call _circuit de ceinture_ carried entirely
round the enclosing wall to which each conductor should be joined, and a
conductor should be carried from the most convenient point of this
circuit to the underground water. For middle sized magazines two
terminals and supports, and for small magazines one terminal and support
will suffice; but in all cases there should be a _circuit de ceinture_.
This need not be deep below the surface, nor covered over; it may even
be in an open gutter, but a conductor must be led from it to the
underground water, even if in order to do this it is necessary to carry
the conductor several hundred metres or several kilometres. It need not,
however, be made of bars and carried all the way in a trench, but it may
be made of six wires 6 or 7 millimetres (about ·25 inches) in diameter,
and carried on posts like telegraph wires, except that they need not be
insulated.
INSTRUCTIONS _by the Committee consisting of_ MM. ALPHAND, BELGRAND,
FIZEAU, _Comte du_ MONCEL, ED. BECQUEREL, DESAINS, CH. SAINTE
CLAIRE-DEVILLE, DUC, BALLU, MAGNE, DAVIOUD, FELIX LUCAS, _and_ R.
FRANCISQUE MICHEL, _appointed to inspect the_ LIGHTNING RODS _on the_
MUNICIPAL BUILDINGS _of_ PARIS.
_20th May, 1875._
The Committee find that platinum tips are useless, and recommend instead
that the point of the terminal should be made of pure copper, 50
centimetres (19·7 inches) long, and terminating in a cone, forming an
angle of 30°. This should be scarfed, pinned, and soldered to the end of
the terminal. The terminal should be of wrought iron in one length, and
where possible galvanized; but on no account painted. The connection
with the conductor should be by a piece fitted and bolted; and, lastly,
the whole joint should be well covered with solder.
The Committee consider that on an ordinary building a terminal will
effectively protect a cone, having the point for its apex, and a base
whose radius is 1·75 of its height. But in practice the terminals may be
much farther apart, if there is a _circuit des faites_. This is defined
as a metallic conductor, which extends without break over the ridges of
all the buildings which it is intended to protect, and which is joined
by metallic contact to all the upper terminals and to the conductor, and
consequently to the underground water which alone forms the common
reservoir. All pieces of metal of any considerable size should be
connected with the conductor.
If the conductor is made of iron bars, they should be galvanized if
possible, and the joints should be fitted, bolted, and finally covered
with solder. If the bars cannot be galvanized, they should be well
painted. The Committee recommend the employment, especially in the
_circuit des faites_, of an arrangement for compensating for the
lengthening and shortening of the bars by the variations of temperature.
This is made by inserting in the circuit a curved band of copper which
will yield to the movement of the rods. If the conductor is made of
galvanized iron wire rope, each wire should be 2·5 or 3 millimetres (·09
to ·11 inch) in diameter, and there should be such a number of them that
the sum of their sectional areas shall be equal to one-fifth more than
that of a bar of iron 20 millimetres (·78 inch) square. The rope should
be all in one piece, and the joints with the terminal and earth
connection should be covered with solder.
The supports should not be insulated, and there should be as few as
possible of them. At the underground end of the conductor should be
fixed a large sheet or hollow cylinder of metal, and this should be
always, even in the greatest droughts, plunged at least 1 metre (3 feet
3 inches) into the subterranean water. If from any cause this water
cannot be reached, the conductor may be joined to one of the main
water-pipes of the city; but if the conductor cannot be led either to
the subterranean water or to a main water-pipe, no lightning-rod should
be erected. It would do more harm than good.
In the case of buildings of any importance, two or more conductors
leading to the subterranean water should be employed. It should be so
arranged that the underground part and the earth connection may be
easily inspected and cleaned from rust, and the whole should be
inspected and cleaned at least once a year, at the end of the autumn.
The Committee is of opinion that it would be better to put all the
lightning-rod work in the hands of special workmen, under the control of
an agent appointed by the administration, and not to trust it to the
blacksmiths and locksmiths usually employed. The Committee lastly
recommend that they should be permanently appointed, and meet every year
after the inspection, to report and decide upon the steps to be taken to
remedy any defects that may be discovered.
REPORT _by the joint Secretary_ (FRANCISQUE MICHEL) _of the Lightning
Rod Committee to the Prefect of the Department of the Seine_.
This report gives a detailed description of the state of the lightning
rods attached to the public buildings of Paris.
In most cases the upper terminals were of great length, some of them as
much as 9 metres (nearly 30 feet) in height; the conductors were in
almost all cases of iron, either in bars or wire rope; the earth
connections were of various kinds and extent.
The report frequently states that the points were blunted; that the
upper terminals and the conducting rods were deeply rusted; that
especially at the joints the conductors were seriously deficient; and
that the underground portion was greatly deteriorated by rust.
A description is given of an accident from lightning to the church of
St. Sulpice; but this building had no lightning rod. In the case of the
church of St. Clotilde there are five upper terminals, two on the two
spires, the remaining three along the ridge of the main roof. The
building was amply protected as far as its length was concerned, but the
transept was not so thoroughly protected. The five terminals were joined
to a conductor which went round the building, and was connected with the
ground. A second conductor led from one of the terminals to the ground,
where it terminated in a second pit. The conductors were made of iron
rods 18 millimetres (·71 inch) in diameter, joined by collars and pinned
and the whole covered with paint. They terminated in distributors
plunged in the underground water in walled pits. They were supported by
insulated collars. The building has an iron roof. The church had been
struck by lightning at least four times since the lightning rods had
been erected. The first time, twelve years ago, the lightning struck the
rod placed on the transept, and carried away the platinum tip of the
copper point. Since then the rod has received another discharge, and the
copper point is bent to the S.W. In January, 1872 or 1873, the lightning
struck the western tower, and shattered one of the stones above one of
the windows of the staircase.
“One of the platinum tips is gone, and many are blunted. The
conductivity of the conductor is very bad, and the joints are very much
damaged: hence the accident to the tower. The greater number of the
glass insulators are broken, or gone altogether.”
In the case of the church of St. Eloi, which had one terminal on the
spire, one conductor, formed of iron wire rope 2 centimetres (·78 inch)
in diameter, joined at 3 metres (9 feet 10 inches) above the ground to
an iron rod 25 millimetres (·97 inch) in diameter, which entered the
ground and ended without branches in a pit filled with charcoal. The
soil was dry and calcareous. The conductor was made up of many lengths
of rope, old pieces apparently having been used; the joints were in bad
condition, and needed soldering. The underground part was deeply rusted.
“In September, 1874, lightning struck the spire, twisted the conductor,
broke the terminal, threw down the part above the cross, and made great
cracks in the apse.”
During the building of the Mairie of the 20th Arrondissement, the
lightning struck a fir-pole in the scaffolding. It did not do any
damage, being carried away by the chain attached to the pole, from which
it took all the rust, and being thence conducted by some pieces of iron
roof framing lying on the ground.
There are several other accounts of accidents, but they are mostly
represented by the foregoing examples.
INSTRUCTIONS _as to the application of_ LIGHTNING CONDUCTORS _for
protection of_ POWDER MAGAZINES, ETC.
Issued with Army Circulars, dated May 1st, 1875.
(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)
1. The principles adopted by Sir W. S. Harris, as shown in the
Appendices A and B, to this paper, still held to be sound.
2. The terminating plane of action of lightning is sometimes beneath the
surface of earth, which, if moist, forms good medium for diffusion of
electricity.
3. Dry soil is to be regarded as non-conducting matter.
4. Therefore conductor to be taken into soil permanently damp.
5–6. Underground magazines are usually in dry soil, and should
therefore be fitted with conductors as in the case of similar magazines
above ground.
8–9. Casemated batteries of modern construction, with magazines in
basement should have conductors on the parapet or terreplein from end to
end of battery, attached to vertical conductor into earth. Flagstaff
should have conductor. In large works there should be several points 5
feet above top of building. Iron verandahs and railings are good
conductors when with good earth connections.
10. Iron buildings are good conductors. But if covered with asphalte,
concrete, &c., rods or points must be provided projecting above
asphalte, &c., and with good earth connections. Iron shields should be
connected with conductors.
11. Copper is recommended as best conductor; it is not liable to
corrosion, and very durable.
12. But if exposed to injury, or likely to be stolen or corroded, copper
may be replaced by iron, provision being made for its smaller
conductivity—viz., ⅕th that of copper.
13. Copper rods to be ½ inch diameter; copper tubes to be ⅝ × ⅛ inch
thick; copper bands to be 1½ × ⅛ inch thick.
14. If the conductor be of iron, solid rods to be 1 inch diameter; solid
bands to be 2 inches wide × ⅜ inch.
15–16. The fusing temperature of copper is 1994° Faht.; whereas that of
iron is 2786° Faht. So far there is a marked advantage over copper. But
it rusts easily, and then the electrical resistance is immensely
increased. Roughly speaking, an equal conducting power may be obtained
either in iron or copper for the same cost, the number of iron
conductors being greater in proportion to the less cost, and the more
conductors being the better.
17–19. Expansion and contraction are to be carefully provided against;
_e.g._ by suitable bends at intervals in long lines of horizontal
conductors and by bearing collars, allowing of slip in vertical lines.
20. Soldered or welded joints are desirable, but not absolutely
necessary.
21. Gives engravings of connections recommended by Sir W. S. Harris,
where soldered joints cannot be used, and which fulfil the conditions
specified in sections 17–19.
22. Soldered or welded joints to be used where discharge is possible
with unsoldered joints, and likely to ignite dust or inflammable
substances near.
23. Iron may be connected by similar joints as for copper, or by screw
joints as for gas pipes. No white lead to be used, it being a bad
conductor.
24. Iron flat bands may be connected by rivets or screws, working in
slots, to provide for expansion, each surface in contact being at least
six times the sectional area of band.
25. Copper bands to be similarly connected. Joints between different
metals may be soldered, screwed, or rivetted, the extent of surface in
contact being regulated by the dimensions of the metal of the least
conducting power. Access of moisture to surfaces in contact must be
prevented, on account of local galvanic action and decomposition.
26. No precise limit can be fixed to protecting power of conductors. In
England the limit is usually assumed as being the radius of the height
from ground. It may be sufficiently correct for practical purposes, but
cannot always be relied upon.
27. Conductors do not attract lightning; they only diminish the
resistance due to the air. Even a change in the nature of the soil -over
which a cloud passes may produce a discharge.
28. One angle of a building may receive a discharge, though another
angle have a conductor. So every prominent part of a building containing
explosive material should have a conductor.
29. In buildings of uniform height, provide a solid rod 5 feet above it
at each end, and at each 45 feet in length; if the conductor be of iron
the top should be gilt.
30. Buildings not over 20 feet long to have one vertical conductor at
end, and a horizontal conductor on ridge.
31. If 20 to 40 feet long to have one vertical conductor in centre, and
one along ridge, as last.
32. If 40 feet long to have two vertical conductors; if 100 feet long
three conductors; in both cases with conductor along ridge.
33. Similar principles to be adopted in larger and more complicated
buildings.
34. Each prominent part should have a conductor. The value of three or
four points to terminals is not apparent unless the points are widely
separated.
35. Conductors are to be connected horizontally, _e.g._, by ridge or
eaves, which, when of metal, should be invariably connected with
conductor. All metal surfaces whatever to be also so connected.
36. Sir W. S. Harris considers the relative conductivity of the several
metals as being—of lead 1, tin 2, iron 2½, zinc 4, copper 12. So lead
cannot be altogether depended on.
37. Avoid long lengths of horizontal conductors without earth contacts,
as the currents might leave the conductor, and pass to earth, causing
danger. Avoid sharp angles.
38. Good earth connections most important. Conductors are to be led into
springs or wells or earth permanently wet. Not into watertight tanks.
Shingle, dry sand, or dry mould are not sufficient. Provide several
earth connections in all large systems of conductors as a precaution.
39. Lead conductors into ground in trenches 18 inches deep. Not less
than 30 feet of metal to be in contact with moist earth.
40. Lead a flow of water over trenches if possible, _e.g._, from rain
water pipes.
41. Trenches in rocky or dry soil to be 30 to 120 feet long, so as to
obtain all moisture possible.
42. Connections in trenches may be of old iron, forming continuous
metallic surface, the trenches to be filled with cinders or coal ashes.
Water pipes form excellent earth connections, but gas pipes are
dangerous.
43–44. Frequently inspect conductors, especially as to joints connecting
different metals and defects in iron from rust.
45–46. Galvanize iron, care being taken that the coating is good.
47. Great care to be taken in case of contact of zinc coating with other
metals, especially copper.
_April 8th, 1875._
FRED. E. CHAPMAN.
_Inspector General of Fortifications._
APPENDIX A.—BY THE LATE SIR W. SNOW HARRIS, F.R.S.
1. The earth’s surface and clouds are the terminating surfaces of
electric actions, and buildings, &c., are only points, as it were, of
earth’s surface in which the whole action vanishes.
2. Electricity when confined to substances resisting its progress, as
air, glass, dry wood, stones, &c., exerts a terribly explosive power.
3. But when confined to bodies, such as metals, offering small
resistance, its violent expansion or disruptive action is greatly
reduced or avoided altogether, and becomes a continuous current
comparatively quiescent. But if body be small, as wire, it may be heated
or fused. Resistance is so small that a shock has traversed copper wire
at the rate of 576,000 miles a second; resistance increases with length
and diminishes with area of section of conductor.
4. So a building metallic in all its parts, or a man in armour is safe.
5. So endeavour to bring buildings into the same passive or
non-resisting state as if of metal.
6. So conducting channels of copper should be systematically applied to
walls, either in plates united in series one over another, not less than
3½ inches wide and 1/16th and ⅛th of an inch thick, or of stout copper
pipe not less than 3/16ths of an inch thick, and 1½ to 2 inches
diameter, fixed to building by braces or copper nails or clamps.
Terminals to be solid metal rods, projecting above to a moderate and
convenient height. Earth connections to be by one or two branches,
leading out about a foot below ground—if possible into moist ground, but
if dry, use old iron or other metallic chains so as to expose a large
metallic surface.
7. All metals in roof, &c., of building to be connected with main
conductors; any prominent chimney to have a pointed conductor taken
along it to metals of roof.
8. An electrical discharge never leaves a perfect conductor to pass to a
very bad one, so the apprehension of lateral discharge is absurd.
Furious discharges have fallen on the conductors to the masts of H.M.
ships, and passed through copper bolts in bottom without injury even to
persons leaning against the conductors.
9. Metallic bodies have no specific attraction for electricity more than
wood or stone have; all matter is indifferent so far as regards a
specific attraction. Lightning falls indiscriminately upon trees, rocks
and buildings, whether with metals about them or not; _e.g._, at
Plymouth Dockyard in May, 1841, a granite chimney, 120 feet high,
without any metal in it, was struck, and yet it was within 300 feet of a
clock-tower of equal height, having metal weathercock, a dome covered
with metal and large conductor along it to ground. The damage ceased
where the chimney passed through a massive metallic roof, having a
conductor from it to the ground. Here the lightning fell on a building,
which, according to the popular idea, held no “invitation” in preference
to a structure which _did_ hold such “invitation.”
10. If efficient conductors provide free and uninterrupted course for
electrical discharge, it will follow that course without danger to
general structure; if _not_, then this irresistible agency will find a
course for itself and shake all imperfect conducting matter in pieces in
doing so. The great object is to provide a line or lines of small
resistance in given directions, less than the resistance in any other
line of the building. The conductor no more attracts lightning than a
gutter or water pipe attracts a flow of water.
11. It follows that a magazine if of metal would be safer than if built
in the usual way. Metallic gutters and ridges, with continuous metallic
communications to earth, are unobjectionable.
NOTE.—It is as wrong to isolate conductors from buildings by glass or
resin, as it would be to place rain water pipes 10 feet from the
building from which they should carry off the water.
An instance is given of an iron conductor which was placed 10 feet from
a house, the latter being, notwithstanding, struck at the point nearest
to the conductor, which was untouched.
12. Pointed terminations tend to break the force of lightning when it
falls on them. Before explosion a large amount of discharge passes off
through pointed conductors.
Pointed conductors should be solid copper rods, about ¾ inch diameter,
and a foot long, united by brazing to the conducting tube. It is not
necessary to gild the points, or form them of platinum. Sometimes even,
this would be detrimental, as platinum has only half the conducting
power of copper. The oxidations of the surface of conductor is of little
moment; and in case of copper very trifling. In any case the conducting
surface is better than the bad—or non-conducting air. The electric
telegraph wires work well, though enclosed by gutta percha or other
non-conducting matter. It is sufficient if the terminal solid rod be
even roughly pointed. But even a ball, a foot diameter, would be a point
as opposed to 1,000 acres of charged clouds.
NOTE.—Experience contradicts the idea that the conductor protects a
certain area. The foremast of a ship has been struck, though the
mainmast has been protected by conductor.
13. Copper linings to doors and windows of magazines, are not
objectionable, but useless for keeping out lightning. They should be
connected with the general system of conductors.
APPENDIX B.—AS TO SOLID OR HOLLOW CONDUCTORS. BY SIR W. SNOW HARRIS,
F.R.S.
1. A given quantity of electricity melts the same quantity of metal,
whether in a solid or hollow form. So far it is immaterial which form
the conductor has. But supposing the mass of metal to be so large that
the heating effect may be neglected. It is proved that the greater the
surface, the less is its intensity or power at any point, the intensity
approaching the second power or square of the surface inversely. It is
important to give the charge free room of expansion by increasing the
surface of conductor, so as to reduce the mechanical activity of shock
to the least possible. Rectangular flat bars may be employed.
2. A rain water pipe communicating with main conductor, should have
earth connection. All imperfect substances, as masonry, and ship masts,
transmit a certain portion of electricity without explosive action. One
great use of the conductor is to relieve the wood or masonry of the
quantity it cannot discharge without explosion.
3. Conductors of small iron rod or wire are very objectionable. They
commonly rust at the joints, and have fallen to pieces, and often been
knocked to pieces by lightning. Iron may, certainly, be employed with
advantage, but should be galvanized. Zinc is an even better conductor
than iron; and being spread over the surface is not open to the
objection of making a conductor of two metals of unequal conducting
power. A good and efficient conductor might be formed of galvanized
iron. It should be of wrought iron, galvanized, of 2 inches diameter,
with screwed joints of _extra_ thickness. Copper tubing is, however,
always to be preferred.
4. In dry or rocky soil, complete the conductor by leading old iron
chains out from the walls in several directions, or by leading a flow of
water over them. Fortunately a thunder storm is usually attended by
heavy rains. The iron chains should extend 30 to 50 feet, and be a foot
or 18 inches under ground. The termination in a large surface of moist
earth would be preferable to that in a well, as the action is a
superficial one of expansion in all directions. In the _tin_ leaf
coatings of the electrical jar, the charge is not influenced by the
thickness of metal.
W. SNOW HARRIS.
REPORT ON THE DESTRUCTION BY LIGHTNING OF A GUNPOWDER STORE AT
BRUNTCLIFFE, YORKSHIRE. BY MAJOR Y. D. MAJENDIE, R.A.
(_Abstracted by G. J. Symons, F.R.S._)
The Gunpowder exploded at 4.30 p.m. on August 6th, 1878, during the
greatest intensity of a violent thunderstorm. The building, was brick,
with brick arched roof, length 9 feet, width 5 feet, height 6 feet
(internal dimensions). The store had a uniform thickness of three
bricks, and was furnished at the one end with an iron door, at the other
end with a lightning conductor. The conductor consisted of a copper wire
rope, 10 gauge copper wire, the rope being 7/16 inch thick, having four
points at the top (one large one in the centre, and three smaller ones
round it), it extended to about 13 feet above the top of the building,
and about the same length was carried into the ground and terminated in
a drain. The conductor had been erected in 1876, by Mr. John Bisby, of
Leeds, and was fixed to a pole distant about 2 inches from the end of
the building opposite to that in which the iron door was fixed, it was
not connected with the iron door in any way. No one was near the store
when the powder exploded, and it seems probable that the earth
connection of the conductor was bad, that the mass of iron in the door
offered at least an equally good path—and that the gunpowder was ignited
by a flash passing between, the two imperfect conductors.
[Illustration: View and Plan of Bruntcliffe Gunpowder Store]
“The only structural damage effected was produced by the impingement of
bricks, which striking with great force, had in a few instances,
partially penetrated or displaced brick work in the dwelling-houses and
buildings, and a portion of the iron of an iron church was broken by a
piece of projected _débris_. A brick was driven through a window in one
of the houses at three hundred yards, and broke a bedstead. As far as I
have been able to discover no other structural injury was occasioned.”
This accident appears to suggest several conclusions:—
“In the first place it appears to me to afford a striking confirmation
of the principle which has been repeatedly and emphatically enunciated
by Sir William Snow Harris and other authorities on the subject of
lightning conductors, that in order to secure an efficient protection
for a given building, all the metal of the building, and as far as
possible the whole of the structure itself, should be brought into
actual connection with the system of conduction; in other words, that
the general conducting power of the mass of the edifice should be
completed, and all attractive and prominent parts allied in one
protective combination, so as to “bring the whole” (as it has been
expressed by Sir William Snow Harris) “as nearly as may be into that
passive or non-resisting state which it would assume, supposing the
whole were a mass of metal.” In the present case, assuming the conductor
itself to have been efficient, a point which there seems no sufficient
reason for doubting, the system of conduction was obviously defective.
Not only was the whole length of the building left unprotected, the
conductor having been on a pole at one end, and carefully insulated from
the building, but the iron door which was at the opposite end, was
absolutely unconnected therewith, and was not itself supplied with any
earth connection.”
“It appears clear, therefore, that even what may be deemed _per se_ an
efficient lightning conductor, _i.e._ a conductor, which considered
alone, offers a path of little or no resistance even to a powerful
electric current, does not afford a reliable protection to a building
unless it be scientifically applied, and with due regard to those
principles upon which the more eminent authorities on electrical science
are agreed. To a disregard of these principles, especially in respect of
the iron door being left out of the system of conduction, and
unconnected therewith, I believe the present accident may be
attributed.”
REPORTS OF COMMITTEES ON THE POWDER MAGAZINES AT PURFLEET.
(Phil. Trans., 1773, p. 42, and 1778, Part I., p. 232.)
(_Abstracted by Prof. W. G. Adams, F.R.S._)
Report of a Committee consisting of the Hon. Henry Cavendish, Dr.
Watson, Dr. Franklin, Mr. J. Robertson, Mr. Wilson, and Mr. Delaval,
appointed by the Royal Society, “to consider of a method for securing
the powder magazine at Purfleet.”
A powder mill at Brescia having blown up in consequence of being struck
by lightning, the Board of Ordnance applied to Mr. B. Wilson to know in
what way the powder magazine could be protected. He recommended that a
blunt conductor should be employed, whereas Dr. Franklin recommended a
pointed conductor. The Committee met and Dr. Franklin read a paper on
the subject, and the report of the Committee was in conformity with Dr.
Franklin’s views.
The Committee went to Purfleet and examined the buildings. They found
that the barrels of powder, when the magazines were full, lay piled on
each other up to the spring of the arches; on each barrel were four
copper hoops, which with vertical iron bars formed broken conductors
within the building. These iron bars were ordered to be removed.
The Committee advised that at _each end_ of each magazine a well should
be dug in or through the chalk, so deep as to have in it at least four
feet of standing water. From the bottom of this water should arise a
piece of leaden pipe to or near to the surface of the ground, where it
should be strongly joined to the end of an upright iron bar, an _inch
and a half_ in diameter, fastened to the wall by leaden straps, and
extending ten feet above the ridge of the building, tapering from the
ridge upwards to a sharp point, the upper twelve inches of copper, the
iron to be painted.
Lead was mentioned for the underground part as less liable to rust, in
the form of a pipe as giving greater stiffness for the substance, and
iron for the part above ground as stronger, and less likely to be cut
away. The pieces of which the bar may be composed should be screwed
strongly into each other by a close joint with a thin plate of lead
between the shoulders. Each rod in passing above the ridge should be
strongly and closely connected by iron or lead, _or both_, with the
leaden coping of the roof, so making metallic communication between the
two bars of each building.
It was also advised that two wells be dug within twelve feet of the
doors, one to the north of the north building and the other to the south
of the south building, and that metallic communications be made between
the water in them and the leaden coping of the roof.
The Board house stood 150 yards from the magazines, on elevated ground,
and was a “lofty building with a pointed hip-roof, the copings of lead
down to the gutters, from which leaden pipes descend at each end of the
building into the water of wells of forty feet deep, for the purpose of
conveying water forced up by engines to a cistern in the roof.”
As to the Board-house, they thought it already well furnished with
conductors by the several leaden communications above-mentioned from the
point of the roof down into the water, and that by its height and
proximity it may be some security to the building below it; they
therefore proposed no other conductor for that building, and only
advised erecting _a pointed iron rod_ on the summit, similar to those
before described and communicating with those conductors.
Mr. Wilson dissented from that part of the Report which recommended that
each conductor should be pointed, because, _he says_, “by points we
solicit the lightning, and may promote the mischief by drawing the
charges from charged clouds, which would not discharge at all on the
building if there were no points on the conductors.” By experiments made
and appealed to at the Committee the difference in the effects between
pointed and blunted conductors is as twelve to one. Mr. Wilson states
that, “A thunder cloud, therefore, if it acted at 1200 yards distance
upon a point, would require a blunted end to be brought within the
distance of 100 yards, and beyond those limits would pass over it
without affecting it at all.” He also says, “The _longer_ the conductors
are above the building, the more danger is to be apprehended from them.
I have always considered pointed conductors as being unsafe by their
great readiness to collect the lightning in too powerful a manner.”
Mr. Wilson adds an account of an accident to St. Paul’s Church, and some
curious reasoning on it in support of his own views. (See Phil. Trans.
1773, p. 59–61.)
* * * * *
On the 15th of May, 1777, the Board House at Purfleet was struck by
lightning, and some of the brickwork damaged (See Phil. Tran., 1778, Pt.
I., p. 232). About 6 p.m., after heavy rain through the day, a heavy
cloud hung over the house for some time, and Mr. Nickson, who watched it
from the house and gives the account, says he suspected that some of the
conductors might find employment from it. He had not been long at the
window before a violent flash of lightning and clap of thunder came
together. The lightning struck one of the iron cramps that hold the
coping, and made a dent in the lead of the cramp and the stone adjoining
it, throwing some stone down and slightly disturbing about a cubic foot
of brickwork at A. The iron cramp was situated over a plate of lead, and
the ends of it, inserted in the stone, came within 7 inches of that
plate, which communicated with the gutter, and served as a fillet to it;
this gutter was part of the main conductor of the building. The
lightning struck through the stone, &c., to the corner of the plate,
fusing a very small portion of it. From this point no farther effect of
the lightning could be traced. At the distance of seven feet and a-half
from the place struck, a large leaden pipe went down from the gutter to
a cistern of water in the yard. It is remarkable that the surface of one
of the hip-rafters, four inches and a-half in diameter, covered with
lead (communicating with the gutter), and _reaching within twenty-eight
inches_ of the place struck, seems not to have been at all affected. The
distance from the point of the conductor on the house to the part struck
was forty-six feet.
[Illustration: View of Board House, at Purfleet]
A fresh Committee of the Royal Society, consisting of Mr. Henly, Mr.
Lane, Mr. Nairne, and Mr. Planta, recommended a channel to be made from
cramp to cramp round the parapet, filled with lead, and connected in
four places with the main conductor on the roof of the building.
Mr. Wilson again dissented from their report, and attributed the hanging
of a heavy cloud over the house (it being calm at the time) to the
presence of the pointed lightning conductor.
An account of Mr. Wilson’s elaborate series of experiments at the
Pantheon on a long cylinder to illustrate the effects of pointed and
rounded conductors occupies seventy pages of the Philosophical
Transactions; and another Committee of the Royal Society, consisting of
Sir John Pringle, Dr. Watson, Henry Cavendish, W. Henly, Bishop Horsley,
T. Lane, Lord Mahon, E. Nairne, and Dr. Priestley, report in favour of
having additional conductors ten feet high, with copper eighteen inches
long, finely tapered and acutely pointed placed upon the magazines. They
conclude that “elevated rods are preferable to low conductors terminated
in rounded ends, knobs, or balls of metal,” conceiving that, the
experiments and reasons, made and alleged to the contrary by Mr. Wilson,
are inconclusive.
Mr. Wilson’s objections are again urged by Dr. Musgrave, but called in
question by Mr. Nairne (see Phil. Trans., 1778, Pt. 2, p. 823), who
makes a series of experiments to illustrate the advantage of pointed
conductors.
Both Mr. Wilson’s and Mr. Nairne’s experiments agree in showing that
“pointed conductors draw off the electricity from a cloud at a much
greater distance than those which are blunted.” Mr. Wilson objecting
that this draws the charged cloud from a greater distance; and Mr.
Nairne concluding that “a charged body is exhausted of more of the fluid
by a pointed than by a blunted conductor,” and so is not likely to cause
so much damage since it discharges itself more gradually.
EXPERIMENTS AND OBSERVATIONS ON ELECTRICITY.
BY BENJAMIN FRANKLIN. Fifth edition. London, 1774.
(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)
The author shows that pointed bodies draw off electricity much more
effectually than blunt ones.
When the land is hot, “the lower air is rarified and rises; the cooler,
denser air above, descends.”
The clouds meet over the heated place, “and if some are electrified and
others not, lightning and thunder succeed, and showers fall.”
“As electrified clouds pass over a country, high hills and trees,
towers, spires, masts, chimneys, &c., as so many points, draw the
electrical fire and the whole cloud discharges there.”
Therefore it is dangerous to take shelter under a tree. It is safer to
be in the open fields, especially if the clothes are wet.
Metals are fused, possibly without heat; the lightning creating a
violent repulsion of the particles of the metal it passes through.
[He afterwards admits this opinion to be erroneous.]
Describes experiments with sharp-pointed metallic bodies, and says: “May
not the knowledge of this power of points be of use to mankind in
preserving houses, churches, ships, &c., from the stroke of lightning,
by fixing on their highest parts upright rods of iron, made sharp as a
needle, and gilt, to prevent rusting; and from the feet of these rods
lead iron wire down the outside of the building into the ground; or down
one of the shrouds of a ship and her side till it reaches the water.”
“Would not pointed rods probably draw the electrical fire silently out
of a cloud before it came nigh enough to strike, and thereby secure us
from that most sudden and terrible mischief?”
He mentions the case of the topmast heads of a ship being struck, but
having flames upon them like very large torches before the stroke.
He thinks that if there had been a good wire conductor from the heads to
the sea there would have been no stroke or damage.
He records the experiments on the 10th of May, 1752, at Marly, of M.
D’Alibard, who placed upon an electrical body a pointed bar of iron 40
feet high. In a thunder storm sparks of fire were attracted from it.
Again, at Paris, on the 18th of May, with the same result, by M. de Lor,
with a bar of iron 99 feet high upon a cake of resin 3 inches thick and
2 feet square.
Similarly in London in July, 1752, by Mr. Canton.
He refers to other experiments.
He experimented in 1752 with a kite of thin silk (as being able to bear
the wet), having a very sharp-pointed wire fixed to its top, above which
it rose about a foot. The kite was raised by twine, the part in the hand
being made of silk and kept quite dry.
The pointed wire will draw the electric fire from thunder clouds, and
when the rain has wet (_sic_) the kite and twine, so that it conducts
the electric fire freely, they will be electrified, and the electric
fire will stream out plentifully on the approach of the knuckle.
“Spirits may be kindled, &c., as with a rubbed glass or tube, and
thereby the sameness of the electric matter with that of the lightning
be completely demonstrated.”
September, 1752. He erected “an iron rod to draw the lightning into his
house in order to experiment on it.”
After many experiments, he concluded that “the clouds of a thunderstorm
are most commonly in a negative state of electricity, but sometimes in a
positive state.” The latter, he believed, rare.
“So that, for the most part, in thunderstrokes, it is the earth that
strikes into the clouds, and not the clouds into the earth.”
In the contrary (rare) case the cloud was, “I conjecture, compressed by
the driving winds or some other means, so that part of what it had
absorbed was forced out, and formed an electric atmosphere round it in
its denser state, so communicated positive electricity to my rod.”
“The electric fluid, moving to restore the equilibrium between the cloud
and the earth, takes, in its way, all the conductors it can find (_v._
page 132 of Franklin’s book)—as metals, damp walls, moist wood, &c.—and
will go considerably out of a direct course for the sake of the
assistance of a good conductor.”
“Explosions only happen when the conductors cannot discharge it as fast
as they receive it, by reason of their being incomplete, disunited, or
too small, or not of the best materials for conducting.”
He supposes that a wire ¼ inch diameter will conduct the electricity of
any one stroke of lightning ever known.
Iron is the best material, as least liable to fuse.
“Pointed rods erected on buildings and communicating with moist earth
would either prevent a stroke, or, if not prevented, would conduct it so
that the building should suffer no damage.
He gives instances of a small wire acting as conductor and saving the
building, though the wire, being too small, was utterly destroyed.
His theory as to the crooked course of lightning is as follows:
“Who knows but that there may be, as the ancients thought, a region of
this fire (electric) above our atmosphere, prevented by our air and its
own too great distance of attraction from joining our earth. Yet some of
it be low enough to attach itself to our highest clouds,” which thence
become electrified, &c.
“I am still at a loss about the manner in which clouds become charged
with electricity, no hypothesis I have yet formed perfectly satisfying
me.”
He describes how he and others have been struck down by electric shocks
without feeling pain or sustaining permanent injury.
For protecting powder magazines, erect a mast not far from it, and 15 or
20 feet above the top of it, with a thick iron rod fastened to it,
reaching down till it comes to water.
“In buildings the rod may be fastened to the walls, chimneys, &c., with
staples of iron. The lightning will not leave the rod (a good conductor)
to pass into the wall (a bad conductor) through these staples. It would
rather, if anywhere in the wall, pass out of it into the rod to get more
readily into the earth.”
If the building be very extensive, two or more rods may be placed at
different parts for greater security.
It is well not to sit near the chimney, or gilt objects, during a
thunderstorm.
AN ESSAY ON THE CAUSE OF LIGHTNING, _and the manner by which the thunder
clouds become possessed of their electricity, deduced from known facts
and properties of that matter, to which are added plain directions for
constructing and erecting safe conductors. By_ JOHN SIMMONS. 8vo. 1775.
(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)
“As on the earth the operation necessary for the excitation and
collection of the electric fluid is attrition.” ... “So we may
rationally conclude that attrition is the means of excitation and
collection of electric matter in the clouds as well as on the earth.”
By metallic conductors buildings may be preserved from the effects of
lightning
Electricity ascends from the earth to the clouds by means of moist air.
“A conductor is a continuation of metal from a certain height above the
highest part of a building to moist earth or water” ... “for easy and
safe passage of lightning.”
Metal is the best of all conductors.
The author quotes from Franklin “buildings that have their roofs covered
with lead and spouts of lead continued from roof into ground to carry
off the water, are never hurt by lightning when it falls on such a
building.”
The conductor may be made of any metal, and flat or round.
But nowhere less than ¾ inch diameter except at terminal.
But iron rusts, so copper or lead should be used. Lead is best, used in
strips 4 inches wide and ⅒th inch thick.
Good earth contact required in moist earth (going therein at least 5
feet) or water.
The several lengths of the conductor must be well in contact by being
screwed, if of iron; soldered, if of lead.
The upper terminal to be iron or copper rod 9 or 10 feet long, ¾ inch
diameter, and 2 to 5 feet above top of highest chimney or other part of
building.
It should be pointed as this attracts electricity better.
Lead roofs to be connected with conductor. (Examples given of house and
ship struck.)
No building or object is known to have been struck by lightning within
50 feet of a proper conductor. But a tree has been shivered within 52
feet, so we may conclude that protecting influence extends to 50 feet
horizontally in every direction from the point of conductor.
In gunpowder stores, conductors are not to be fixed to the buildings,
but at (say) 12 feet away, fastened to a standard, the top being as high
above the building as it can be conveniently.
No metal on sides or roof of the building is to be exposed to the
lightning so as to attract it.
A TREATISE on ATMOSPHERIC ELECTRICITY.
BY JOHN MURRAY. 1830.
(_Abstracted by Prof. W. G. Adams, F.R.S._)
In Chapter V., on lightning identified with electricity, the author
speaks of fire-balls and the Aurora Borealis, and ascribes the formation
of shooting stars to electrical action. He does not believe they come
from distant space into our atmosphere, but regards them as concretions
formed by a flash of lightning darting through gaseous media and
atmospheric air _expanded by heat_, carrying metallic dust and earthy
particles ejected from volcanoes, or carried up by _evaporation_ or
other causes, and diffused over an immense surface in the upper regions
of the air. “The lightning carries, like a _ploughshare_, the
accumulated matter in its progress, and, by the powerful electrical
attraction thus excited, these particles will be drawn into the vortex
of the lightning instantaneously; for, the lightning finally
encountering an electricity of an opposite kind, an explosion ensues,
and the collected mass is instantaneously fused and agglutinated, while
the meteorite thus formed tumbles to the ground.... We therefore do not
see the necessity of considering meteoric stones _extra atmospheric_.”
In this way John Murray goes on page after page, but the above will
probably be sufficient notice of his work.
The following are the conditions he lays down for a good conductor:—
1. A finely pointed summit to offer an unresisting entrance.
2. A sufficient length to anticipate, as it were, the descending
electricity, and receive it on its summit before it could reach any part
of the building.
3. A superior conducting power in the material of the rod to facilitate
its passage to the earth.
4. A sufficient thickness to prevent its fusion, which, however, will
greatly depend on the resistance it has encountered in entering the
conductor. And, finally
5. A safe conduction to a well or moist surface below ground.
He says: “Let the wires below ground in contact with moisture pass
through a cylinder of zinc before they diverge to form the root, the
copper wires will in this case always remain free from any oxidation.”
HARRIS’S LIGHTNING CONDUCTORS. REPORT _to the Committee upon_ MR. SNOW
HARRIS's _and other_ LIGHTNING CONDUCTORS.
(February 11th, 1840. Parliamentary Paper. Fcap. folio).
(_Abstracted by Professor W. E. Ayrton_).
Instances are given of ships not provided with lightning conductors
being struck and damaged, whilst others lying near, and provided with
conductors, were not injured. The question of lightning conductors
attracting lightning considered, and evidence shown to the contrary.
Lateral discharge from a lightning conductor considered. Evidence
against it, if only the conductor were continuous and of sufficient
size. Faraday considered that a man leaning against one of Harris’s
conductors when the electricity descended would not be hurt. Proposition
to place a globe of glass on the head of the mast in place of a
lightning conductor considered, and the conclusion arrived at that it
would do harm.
Wheatstone stated that “in the Report of the Committee of the Academy of
Sciences of Paris, appointed to investigate the utility of lightning
conductors, there is no instance on record of an iron rod of ½ inch in
diameter being fused or even made red-hot by a flash.”
Mechanical objections to lightning conductors on ships considered and
discussed. Decided that the application of Mr. Harris’s conductor tended
rather to strengthen than weaken the mast and spars. Then follows a
large number of letters, giving accounts of accidents from lightning to
ships, &c.
Decision arrived at that on the whole Mr. Harris’s conductor is the best
of those examined.
THE DIFFERENCE BETWEEN LEYDEN DISCHARGES AND LIGHTNING FLASHES. BY C. V.
WALKER, Hon. Sec. Lon. Electrical Soc. London. 1842.
(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)
The author alludes to the experiments of Franklin, &c.
The distance to the lower surface of clouds, observed by Le Gentil and
others, shows an average of 1000 to 2000 feet, whereas the greatest
length of spark with a large machine is 3 to 4 feet.
The inductive action bears some inverse ratio to the distance.
Leaves of trees have a remarkable property of silently drawing off
electricity.
He gives the particulars of a large number of experiments, with
arguments thereon, to prove the theory of the difference between Leyden
discharges and lightning.
Quotes examples of lightning on conductors and buildings to show that
the conductor takes part only of the charge, the remainder taking other
paths. Contiguous semi-insulated bodies must not be left unconnected
with the lightning rod.
He quotes, with approval, the advice of Faraday, viz., to tie together
with a metallic connection all contiguous readily-conducting bodies.
Cites numerous other opinions to the same effect, viz., that all
metallic parts of a building should be connected with the conductor.
He sums up by stating “that the Leyden charge differs considerably not
so much in _nature_ as in _degree_ from that of the cloud, inasmuch as
the proximity of the coatings in the one case is infinitely small
compared with the distance in the other,” &c.
He expresses great confidence in Sir W. S. Harris’s system for
protecting ships.
THE EFFECT OF A LIGHTNING FLASH ON THE STEEPLE OF BRIXTON CHURCH, AND
OBSERVATIONS ON LIGHTNING CONDUCTORS GENERALLY. BY C. V. WALKER. London.
1842.
(_Abstracted by Prof. T. Hayter Lewis, F.S.A._)
The author refers to Faraday’s experiments, as shewing instances of
lateral discharge, and says, “unless precautions are taken to prevent
its proceeding from a lightning conductor, that instrument literally
invites the enemy within doors.”
He gives detail of the accident at Brixton, there being no lightning
conductor.
The stroke did much damage to the steeple and then passed off harmlessly
by the metal gutters and rain-water pipes.
One side of the steeple was drenched with wet and carried off part of
the stroke.
He quotes examples of the apparently protective action of high trees.
Lofty trees near lofty buildings would materially mitigate, if not
prevent, the violence of the stroke.
The accident at Brixton shows that the lightning takes not simply the
_shortest_, but, in addition, the _largest_ path.
Had the steeple been provided with a lightning conductor outside,
passing near the clock face or the bells, or water pipe, it is more than
probable that a flash would pass from it to these vicinal conductors.
If _outside_ the tower the danger would be greater. He recommends that
the _metal_ cross on the steeple be _replaced_ by a _stone_ one, and
that the present iron rain water-pipes be connected by copper rods or
plates, which are also to be connected with the lead work of roof.
The bells are also to be connected with each other and with the
conductor.
Every bolt-clamp or other piece of metal within “striking distance” of
the conductor, unless in direct communication with it, is liable to
cause lateral discharge.
The odour developed by lightning was, at Brixton, decidedly sulphurous,
as a piece of stone which was shattered by the stroke retained the odour
of sulphur distinctly for several hours.
ON THE NATURE OF THUNDERSTORMS; AND ON THE MEANS OF PROTECTING BUILDINGS
AND SHIPPING AGAINST THE DESTRUCTIVE EFFECTS OF LIGHTNING. BY W. SNOW
HARRIS, F.R.S. 1843.
(_Abstracted by Prof. Ayrton._)
_The backstroke may do injury_, that is, a person may be killed in
consequence of a flash of lightning passing between the clouds and the
earth at some distance from the person.
In the Phil. Trans. for 1787, Mr. Brydone writes to the President of the
Royal Society, and mentions the case of two men riding in two carts, the
front one drawn by two horses, these horses and the man driving them
were killed;, the man on the hinder cart and a shepherd at a distance,
saw the occurrence and heard a report but observed no lightning.
_A metallic screen appears to protect the interior from the action of a
current, as well as from static induction._
Dr. Franklin found he could not destroy a wet rat by artificial
electricity, although he could a dry one.
The first lightning conductor was erected in England at Payneshill, by
Dr. Watson, in 1762.
The lightning conductor should expose a large surface, and should be
united with all the great masses of metal in its vicinity. For
stationary elevations the conductor should consist of solid or tubular
rods or flat plates of metal. We must consider the _mechanical_ action
the lightning may produce on the conductor, as well as any possible
heating action. Sir W. Snow Harris mentions that there were no signs of
fusion in the fragments of the linked brass rod, at Charles Church,
Plymouth, torn to pieces in 1824, or in the small pieces of the
conductor at the Hotel des Invalides, at Paris, consisting of a strand
of twenty iron wires, and which was smashed in 1839.
He says the benefical effect of _superficial_ conductors appears to
depend on the removal of the electrical particles further out of the
sphere of each other’s influences.
“Thus we find,” says Sir W. Snow Harris, “in a variety of cases of
damage by lightning that the passing charge, in striking on large
expanded sheets of metal has become comparatively tranquil, and has been
traced no further, whilst in striking on large masses of metal exposing
but a small surface, it has assumed an intensely active state.”
He goes on to state that the resistance of the conductor must be kept as
low as possible, and as neither the resistance nor the heat developed is
increased by rolling the wire out into a flat surface, he argues that
“there is, consequently, no disadvantage in giving a lightning rod as
much superficial capacity as possible, as regards conducting power,
whilst, on the contrary, the diminished intensity attendant on it is
very advantageous: this effect of superficial conductors appears to
depend on the removal of the electrical particles further out of the
sphere of each other’s influence.”
_What quantity of metal is requisite for a lightning rod?_ He concludes
from the results of a number of accidents that “a copper rod ¾ inch
diameter, or an equal quantity of copper under any other form, would
withstand the heating effect of any discharge of lightning which has yet
come within the experience of mankind.”
_Practical deductions._—“From the various enquiries contained in the
first 123 pages of this book, we arrive at the following deductions:—
“1st. Copper is the best kind of metal for a conductor.
“2nd. The quantity of metal should not be less than that represented by
the section of a solid cylinder ½ inch diameter.
“3rd. The metal should be placed under as great an extent of surface as
is consistent with strength, and should be perfectly continuous.
“4th. The conductor should involve in its course the principal detached
masses of metal in the building.
“5th. It should be placed as close as possible to the walls which are to
be defended, and not at a distance from them, and be carried at once
directly into the ground.
“6th. It should be attached to the most prominent points of the
building, and if the length be very considerable its dimensions should
be increased.
“Lastly. In extensive ranges of buildings, all the most prominent parts
should have long pointed rods projecting freely into the air, and the
greater the range of building the higher they should be.
“In particular cases, in which expense must necessarily be considered,
wrought iron tubing may be employed; it should not, however, be less
than 2 inches in diameter, and 3/10ths of an inch in thickness.”
Insulating the lightning conductor from the building is quite valueless.
The method of fixing lightning conductors to ships is explained at
considerable length.
_Range over which the protecting power of the lightning rod
extends._—Great doubts exists as to the answer to this question, since
in many cases one portion of a building has been struck while a
lightning rod in good condition existed close by.
For example, the powder magazine at Bayonne was 56 feet long, 36 feet
wide, covered with thick vaulted masonry and a sloping roof with gable
ends, protected by plates of lead; the gutters were also of lead, and
there were the usual spouts for discharging the rain. The lightning rod
projected about 20 feet above the building, and was attached to the lead
of the roof by a metallic socket through which it passed, and which was
soldered to one of the lead coverings. Instead of being carried,
however, directly into the earth at the foot of the wall, it was turned
outward at about 2 feet from the ground, and being bent at right angles,
was continued on semi-insulating posts of wood into a trench filled with
charcoal, distant 33 feet from the wall.
On the 23rd of February, 1829, the building was struck, the point of the
conductor melted, and the leaden plates by which it was attached to the
wood posts at the foot of the wall, were more or less torn and
perforated by holes. No damage, however, ensued to the building in the
course of the conductor. At the south-west corner, a sheet of lead
covering the gable end was torn out immediately over a point where two
stones of the cornice were united by an iron cramp.
Sir W. Snow Harris considers the possibility of this damage having
arisen “from the conductor (in consequence of being continued at so
great a distance from the building) not offering a sufficiently easy
line of transit for the discharge to the earth,” but he rejects this
explanation and concludes that the damage arose from the lightning
striking the building in two points.
Again, the Heckingham poorhouse, although armed with eight pointed
lightning rods, was struck, in 1787, at a point _m_, 70 feet from the
nearest conductor _c_.
[Illustration: View and Plan of Heckingham Poorhouse]
[Illustration: View and Plan of Heckingham Poorhouse]
The squares at _a_, _b_, _c_, _d_, _e_, _f_, _g_, _h_, indicate chimneys
to which lightning conductors were attached. The centre range was 108
feet long, the flanks each about 160 feet long: the details of the
lightning conductors are not given. One portion of the lightning
discharge struck one of the conductors and was carried off by it without
damage to the building, one portion struck the building at the point _m_
and also the shed at _s_, doing some damage, and a third portion struck
the ground immediately in front of the building near a gate, G.
The ship _Ætna_ was struck in 1830 by several heavy electrical
discharges when at Corfu. These for the most part passed down a chain
conductor attached to the mainmast. One of the discharges, however,
struck the ship near the bow, and exploded about 12 feet above the
forecastle close to the foremast, knocking people down, &c.
The Board-house at Purfleet was a lofty building with a pointed roof,
well leaded and connected by lead gutters and pipes with the earth, and
with wells 40 feet deep for the purpose of conveying water forced up to
a cistern on the roof. It was, therefore, only thought necessary to add
an iron spike about 10 feet long to the middle of the highest part of
the roof. The building, however, in 1777, was struck and slightly
damaged at a point 46 feet from the conductor.
Several other examples illustrating how small an area a lightning rod
protects follow.
Sir W. Snow Harris further concludes that experience shows that
lightning will not leap from a lightning rod to a piece of insulated or
semi-insulated metal near it, although a discharge may take place
between the rod and a distant metallic mass in connection with the
earth, but not otherwise in connection with the rod.
He lastly considers the question, formerly much debated as to whether a
lightning rod attached to a house will attract to the house a discharge
that otherwise would not have struck it, and he concludes that there is
no foundation for the erroneous impression that the existence of a
lightning conductor can ever cause damage.
AN ACCOUNT OF THE CHIMNEY OF THE EDINBURGH GAS WORKS. BY G. BUCHANAN,
C.E., F.R.S.E.
[Proceedings of the Royal Scottish Society of Arts, 1850–51.]
(_Abstracted by G. J. Symons, F.R.S._)
This chimney has a total height of 341½ feet (329 feet above ground), it
is circular; at the top the internal diameter is 11 feet 4 inches, and
the external 13 feet 10 inches; and at the bottom, internal diameter 20
feet, external 26 feet 3 inches.
Respecting the conductor Faraday was consulted, and replied as follows:—
“The conductor should be of ½ _inch copper rod_, and should rise above
the top of the chimney by a quantity equal to the width of the chimney
at the top. The lengths of rod should be well joined _metallically_ to
each other, and this is perhaps best done by screwing the ends into a
copper socket. The connection at the bottom should be good; if there are
any pump pipes at hand going into a well they would be useful in that
respect. As respects electrical conduction, no advantage is gained by
expanding the rod horizontally into a strap or tube—surface does
nothing, the solid section is the essential element.[4] There is no
occasion for insulation (of the conductor) for this reason. A flash of
lightning has an intensity that enables it to break through many hundred
yards (perhaps miles) of air, and therefore an insulation of six inches
or one foot in length could have no power in preventing its leap to the
brickwork, supposing that the conductor were not able to carry it away.
Again, six inches or one foot is so little that it is equivalent almost
to nothing. A very feeble electricity could break through that barrier,
and a flash that could not break through five or ten feet could do no
harm to the chimney.
Footnote 4:
The very reverse of what was formerly held by high authorities.—[Note
by Editor of Proc. Roy. Scot. Soc. of Arts.]
“A very great point is to have no insulated masses of metal. If,
therefore, hoops are put round the chimney, each should be connected
metallically with the conductor, otherwise a flash might strike a hoop
at a corner on the opposite side to the conductor, and then on the other
side on passing to the conductor, from the nearest part of the hoop
there might be an explosion, and the chimney injured there or even
broken through. Again, no rods or ties of metal should be wrought into
the chimney parallel to its length, and therefore to the conductor, and
then be left unconnected with it.”
In answer to some further inquiry, Professor Faraday again wrote:—
“The rod may be close along the brick or stone, it makes no difference.
There will be no need of rod on each side of the building, but let the
cast-iron hoop and the others you speak of be connected with the rod,
and it will be in those places at least, as if there were rods on every
side of the chimney.
“¾ rod is no doubt better than ½ inch, and except for expense I like it
better. But ½ inch has never yet failed. A rod at Coutt’s brewery has
been put up at 1½ inch diameter—but they did not mind expense. The
Nelson column in London has ½ inch rod, ¾ is better.
“I do not know of any case of harm from hoop-iron inclosed in the
building, but if not in connection with the conductor, I should not like
it; even then it might cause harm if the lightning took the end furthest
from the conductor.”
The following paragraph states what was done:—
“The electric conductor stands 6 feet above the iron top-plate, ⅝-inch
round copper, made fast to stone and brickwork with 7⅞-inch copper
holdfasts let 4 inches into the masonry or brickwork, with a head on the
inside and an eye on the outside to receive the rod as it was carried
up. By these holdfasts an ascent can easily be made to the top by a
small tackle suspended to the holdfasts. The conductor is metallically
connected to all the ironwork on the stalk—the plate on the top,
projecting cope, malleable iron hoops, bolts on the top of stone
pedestal, and also the ascending chain. The rod descends into a well
about 10 feet from the foundation, and is immersed about 8 feet deep in
water, and the end turned up 2 feet in a horizontal direction, and
flattened.”
PAPERS _relative to_ SHIPWRECKS BY LIGHTNING, _as prepared by_ SIR SNOW
HARRIS, _and presented by him to the Admiralty_.
(August 5th, 1854. Parliamentary Paper. Fcap. folio).
(_Abstracted by Professor W. E. Ayrton_).
Number of merchant ships destroyed by lightning, loss to the country.
Application of lightning conductors to ships in 1820. Mode of applying
them. Mechanical difficulties; how overcome. The saving to the Exchequer
which has resulted.
Long account of various ships in the Royal Navy not provided with
lightning conductors, struck by lightning and damaged. Loss of life and
injury that has resulted. Long account of ships provided with lightning
conductors, and so preserved.
Sir Snow Harris states that “although his system of lightning conductors
ought to guard against all those violent and regular shocks of lightning
falling within the ordinary experience of mankind, it is not to be
expected that the system could guard against every possible kind of
atmospheric electrical discharge, be the circumstances what they may,
such as thunderbolts, fire-balls; nor is it expected that it should
guard against meteorolites, or against sweeping electrical action mixed
up with convulsions of nature; nor can it quiet those minor electrical
effects producing electric glow; nor can it always obviate that
tremendous concussion and expansion of the atmosphere in cases in which
a thunder-cloud discharges its lightning in a dense explosion on the
masts, and which may rupture, or mechanically tear to pieces, frangible
matter.”
STATISTICS OF BUILDINGS AND SHIPS STRUCK BY LIGHTNING. BY F. DUPREZ,
MEMBER OF THE ACADEMY.
[Académie Royale de Belgique, Extrait du Tome 31 des Mémoires, 5th
December, 1857.]
(_Abstracted by Professor T. Hayter Lewis, F.S.A._)
M. Duprez refers to the Report of a Committee of the Institute of
France. (Vide _Comptes rendus_, 1852–6.)
He divides the subject into the following heads:—
1. The frequency with which lightning rods are struck.
2. Their terminal points and the effects of the stroke on them.
3. The conductors and their ground connections.
4. The protective power of the lightning rods.
1. _Concerning the frequency with which lightning conductors are struck
by lightning._
The author cites 144 cases of lightning rods having been struck. Of
these seventeen were struck two or three times, so that the total number
of electric discharges on them was 168, as far as recorded.
But very many cases are not recorded at all, _e.g._, from 1793 to 1813
only two cases were noted. The great number of lightning rods struck
would seem at first to support the idea that they attract lightning.
But we must compare the number of rods struck with those fixed, and we
find from a communication made in 1777 to the Academy of Berlin, that,
even then, a large number were fixed to the most important edifices of
N. Italy and England.
The same in 1784 to those in the ports of France and to the ships in the
said ports.
In 1794 the fortresses of Russia were ordered to be so protected.
In 1769 there were 166 edifices in Hamburg alone, and 104 in its
environs, with conductors.
If the number of conductors were so great in the last century, we must
conclude that the number of those struck must be very inconsiderable as
compared with those fixed.
In Hamburg, _e.g._, not one rod is recorded as having been struck.
In 1785, Ingen-Housz reports that of all the lightning rods placed by
his direction on the Austrian powder magazines and other buildings only
one had been struck.
In 1772, Franklin wrote, that during the twenty years in the course of
which lightning rods had been fixed in America he knew of five cases
only in which these rods had been struck.
Sir W. S. Harris reports in 1854, as the results of twenty-two years’
experience, that the number of vessels struck unprotected by lightning
rods, as compared with that of vessels protected by his plan, was as
three to two.
The above show that the idea of danger from lightning rods is not well
founded.
Besides which it must be remembered that they are frequently placed in
the most exposed positions, _e.g._, of the 144 rods struck, seventy-four
were on ships, and fifteen others on buildings which had been struck
before.
One would think that the number of terminals placed on a building would
diminish the chances of their being struck, but it does not seem to be
so; _e.g._, twelve buildings in the first list had many terminals
communicating with a common conductor or different conductors.
Yet the lightning struck, with explosive effect, one or other of the
rods of these buildings.
And in each of two cases the lightning struck at once the three rods
fixed to a building.
Of the 144 cases above cited:—
74 were to lightning rods fixed on ships
30 were to lightning rods fixed on towers
9 were to lightning rods fixed on powder magazines
31 were to lightning rods fixed on ordinary buildings.
———
144
In forty-four cases where one of Sir W. S. Harris’ conductors was fixed
to each mast of a ship, the mainmast was struck twenty-seven times; the
foremast was struck fourteen times; the mizen was struck twice; both the
main and foremast twice.
2. _As to the points of the lightning rods struck, and the effect
produced on them._
(Sir W. Snow Harris’s system as adopted in the British Royal Navy since
1830 is described. They are formed of bands of copper let into the
masts. They have no upper terminals or points, and fifty-five are
included in the list already quoted of 144 lightning rods struck.)
Of the eighty-nine cases remaining in the list, only fifty-one are
recorded as having their upper terminals ended with points.
Of these, thirty had their points melted to a greater or less extent;
six of them were of copper or brass; five were of copper gilt or iron
gilt; one was of brass silvered; and four were of platinum. The others
are not distinctly described, and the sizes seldom given.
One of brass was 25·4 centimetres (c. 10 inches) long, and 5 millimetres
(⅕th inch) diameter at its base, and was melted for ¼th of its length.
One of copper was 24 centimetres (c. 9½ inches) long, and 9 millimetres
(c. ⅓rd inch) diameter at base, and was almost all melted.
One of platinum was 8 centimetres (c. 3 inches) long, and 1 centimetre
(c. ⅓rd inch) diameter at base. This was melted for a length of 5 or 6
millimetres (c. ⅕th inch.)
It results from the above facts that the points of the lightning rods
have been much too slender.
The Institute of France recommends, therefore, for the points 2
centimetres diameter (c. ¼th inch) at base, and only 4 centimetres (c.
1½ inches) high, with an angle of opening of 28 to 30 degrees.
It has been urged, especially in Germany, against the employment of
pointed upper terminals that these points are fused by the lightning,
this fusion being regarded as dangerous on account of its action on
inflammable substances near.
As to this, the author cites three cases of buildings set on fire,
though protected by lightning rods. But the precise cause of the fire
was not ascertained.
Several observations show that the melted metal trickled down the side
of the lightning rod.
At Strasbourg the metal was pressed down on one side, and had bent like
wax softened by heat. At other times the lightning disperses the melted
metal in all directions. (Examples quoted.)
With these facts before us we cannot altogether deny that some danger
may arise from the fusion of the metal at the point of the terminal. But
this danger can be much lessened, if not removed, by adopting the size,
etc., of the lightning rods recommended by the Institute of France.
Besides fusion, the points sometimes show distinct traces of mechanical
action caused by lightning.
The author quotes six examples of this where the points had been curved.
This shows the necessity of strengthening the points of the upper
terminals. The curvature arises, probably, from the points being much
heated by the lightning, and acted on by the wind.
One case is noted of a point which had the appearance of having been
struck violently by a hammer.
Also of one in which the base of a point, where it was screwed to the
rest of the upper terminal, was split for a length of 11 millimetres (c.
½ inch).
Also of a platinum point screwed on the upper terminal (copper), and
retained by a pin, where the stroke tore away the pin, the point falling
intact at the foot of the lightning rod.
3. _Of Conductors of lightning rods struck, and their contact with the
ground._
The author refers to forty-one cases of lightning rods struck when not
on Harris’s principle.
Of these, 5 were of copper bands soldered together; 5 were of copper
wire either as rope or chain; 1 was made of bands of sheet iron; 11 were
of bars of iron joined by screws or by solder; 3 had pieces of lead
between the parts where they were screwed together; 3 were of simple
iron wire, or of rope or chain of iron wire; 3 were of iron joined
together by hooks; 12 are described as chains (metal not specified); 1
is described merely as a conductor.
The dimensions of the above are seldom given.
The largest bands reported are 16 centimetres (c. 6¼ inches) in width.
The largest bars reported are 55 centimetres (c. 2¼ inches) in width and
15 centimetres (c. ½ inch) in thickness.
The description of the earth connection is also imperfect.
Of eighty-nine lightning rods described as struck, only twelve are noted
as having their ends in running water or wells, and one in damp soil.
Fifteen simply entered the ground, it being noted expressly of six of
these that it was dry.
In three cases were the lightning rods were struck the author found that
the part at the base and in the damp earth had terminated in a plate of
lead, protected above the ground by a wooden enclosure.
Three conductors of ships did not communicate with the sea.
Twenty-three cases are noted of ordinary conductors (not on Sir W. S.
Harris’s principle).
The lightning melted, or reduced almost to powder, three.
The first was on a house, and was of copper wire, the diameter not
known, ending with a chain of iron buried in the earth.
The second was on a ship’s mainmast, and was of iron wire 6 millimetres
(c. ¼ inch), diameter, 46 centimetres (c. 18 inches) long, folded at
their extremities, and united by rings.
The third (also to a ship) was a rope of three strands formed in the
whole of 60 brass wires, each being one half to two-thirds of a
millimetre thick.
The two last conductors had their ends in the sea.
The parts of these conductors, in place of being soldered or screwed
together, were joined merely by hooks and rings like a surveyor’s chain.
Evidently a bad form as their contact is imperfect.
In three other conductors, whose different parts were screwed together
with lead between them, the stroke melted the lead.
This shows the danger of lead from its fusibility, in addition to its
less conducting power.
The author gives examples of this, wherein a leaden pipe, 8 centimetres
(c. 3¼ inches) external diameter, and 13 millimetres (c. ½ inch) thick,
was melted.
He quotes Arago as calling attention to the importance of the form of
the bends in conductors, abrupt bends being dangerous.
Two examples are quoted to prove this, the conductors having been broken
by the lightning stroke at a sudden bend.
To provide lest the lightning, after having struck the lightning rods,
should abandon them for larger masses of metal near them, these masses
should be made to communicate with the conductors.
Cases are cited where the lightning quitted the conductor and struck
metallic bodies near. Also, in respect of painting conductors, the
author quotes a case where part of a bell wire adjoined a lead pipe
which communicated with the conductor. Part of the wire was painted in
oil colour, the other part not. The latter was melted, the first not,
but the paint (though otherwise uninjured) had ceased to adhere to it.
Three examples are cited of danger from conductors ending in watertight
tanks.
In one case the stroke broke the conductor.
In another it left the conductor and injured the building.
In the third it merely melted the point of the upper terminal.
Nevertheless, it often happens that the lightning, in spite of imperfect
communication with the earth, disperses itself inoffensively.
Out of fifteen cases of lightning rods struck, in which the conductors
were simply buried, more or less, in the soil, they carried off the
stroke in eleven without the buildings being injured, or any trace being
left of it, except that the ground was upheaved where the latter was too
dry.
The French Institute, in their report on the protection of the Louvre,
considered it necessary to employ, under certain circumstances, a
conductor with two branches, the one descending into a subterranean
source of water, the other communicating simply with the surface of the
earth.
On the other hand, Arago thought that conductors need not enter the
ground, but communicate only with a metallic surface lying on the
ground.
This view is confirmed by the cases which the author mentions where the
surface of the earth being wetted by rain formed a conductor.
Nevertheless, the two branches are desirable, in case one should fail.
Fifty-five conductors on Sir W. S. Harris’s system are recorded as
having been struck, but the damage was quite trivial.
Two electrical phenomena are to be noted as sometimes occurring when a
lightning rod is struck.
First, when a conductor is formed of metallic plates a peculiar noise is
heard like water pouring on a fire.
Second (independently of the form of the conductor), electric sparks are
emitted from bodies near. The author cites example at Berne, 1815.
4. _Protective agency of lightning rods._
Out of 168 cases of lightning rods struck (_vide_ page 91) there are
only twenty-seven (c. ⅙th) in which the buildings or ships have not been
preserved, and of this sixth many of the conductors were imperfect;
_e.g._, four terminated in earth which was unusually dry, and two of
them were of insufficient size.
Another was formed of pieces having their ends hooked.
Two conductors ended in watertight tanks.
Another was in the form of a surveyor’s chain, the parts not being,
consequently, in close contact.
Others were badly jointed, or had imperfect communication with the
ground or with the sea.
In two cases the stroke broke the conductor at points where its
direction was abruptly changed.
In two other cases the lightning left the conductors struck, and fell
upon buildings near without causing damage to those on which the rods
were fixed.
In the instance of a lightning rod fixed to the mainmast of the
_Jupiter_ (1854), the conductor was made of sixty brass wires, one half
to two-thirds of a millimetre (0·02 inch) thick, and was broken by the
stroke into thousands of pieces. The Institute Committee concluded that
the lightning was not conducted by all the wires of the conductor. Those
which it followed were insufficient to transmit it; some were melted,
some broken. The Committee recommended, therefore, that each metallic
wire be tinned separately at the extremity of the conductor, and
soldered thereto for a length of about a decimeter (c. 0·4 inch), so as
to form a metallic cylinder.
In the last six cases the particulars of the lightning rods are not
given sufficiently to show the cause of their failure, but five are
described as being of chain or ropes of metal wire.
It results from the above facts that when the lightning rods have proved
insufficient protection, their failure has been owing to defects in
their construction; it is rather surprising to find how well buildings
and ships have been protected, even when the lightning rods have not
been well constructed.
In every one of the fifty-five cases where Sir W. S. Harris’s rods were
fixed they have protected the ships, except that not having points some
slight damage has sometimes occurred to the tops of the masts.
This shows their superiority over ropes or chains.
Arago thought that lightning rods were protection against ordinary
lightning, but not when it assumed the form of fire-balls. The author
cites several examples to show that this opinion was not well founded.
He considers a perfectly constructed lightning rod to be a perfect
safeguard.
But he adds that the lightning stroke produces electric disturbances in
its vicinity, although the building be intact.
He cites an example of this in respect of a prison whose inmates (300)
experienced a great enfeebling of their muscular power during some
seconds.
Very few records exist relating to the area of action of lightning rods,
and the elements for determining their protective power are slight. The
author gives a table showing the heights of points, horizontal
distances, &c., in certain cases, and cites four instances of ships
whose foremasts were struck although the mainmasts had lightning rods,
and one where the mizen was struck though the fore and mainmasts were
protected.
TABLE GIVEN BY M. DUPREZ.
┌─────────────────────┬───────────────────────┬───────────────────────┐
│ │ IN METRES. │ IN ENGLISH FEET. │
├─────────────────────┼─────┬─────┬─────┬─────┼─────┬─────┬─────┬─────┤
│ │ 1st │ 2nd │ 3rd │ 4th │ 1st │ 2nd │ 3rd │ 4th │
│ │Case.│Case.│Case.│Case.│Case.│Case.│Case.│Case.│
├─────────────────────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┼─────┤
│Length of upper │ 1·5│ 3·4│ 1·5│ 2·3│ 5│ 11│ 5│ 8│
│ terminal, or height│ │ │ │ │ │ │ │ │
│ of point above │ │ │ │ │ │ │ │ │
│ that portion of the│ │ │ │ │ │ │ │ │
│ building on which │ │ │ │ │ │ │ │ │
│ the upper terminal │ │ │ │ │ │ │ │ │
│ was fixed. │ │ │ │ │ │ │ │ │
│Vertical height of │ 1·5│ 7·6│ 6·7│ 71·2│ 5│ 25│ 22│ 232│
│ point above the │ │ │ │ │ │ │ │ │
│ place struck. │ │ │ │ │ │ │ │ │
│Horizontal distance │ 15·2│ 7·3│ 17·4│ 59·9│ 50│ 24│ 57│ 197│
│ of place struck │ │ │ │ │ │ │ │ │
│ from the base of │ │ │ │ │ │ │ │ │
│ upper terminal. │ │ │ │ │ │ │ │ │
└─────────────────────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┴─────┘
These instances show that we should be misled in considering, as being
protected, a circular space whose radius was double the height of the
lightning rod.
The protected radius appears to be only equal to double the simple
height of the upper terminal above any required point, and reckoned
horizontally from a point vertically under the conductor.
[It will be observed that M. Duprez here contradicts himself in two
consecutive sentences, and in a subsequent part of his work (p. 30) of
the Memoir, he again says: “Aucun des cas indiqués dans le numéro
précédent n’infirme la règle généralement admise, savoir que la sphère
d’action d’un paratonnerre s’étend, dans toutes les circonstances, à un
espace circulaire d’un rayon égal au double de la longeur de la tige,
c’est-a-dire de la hauteur de la pointe au-dessus de la partie du
bâtiment sur laquelle la tige est fixée.”
But the table given by M. Duprez gives two instances in which the stroke
fell within the radius of once the height.—Ed.]
RESUMÉ.
In the paragraphs which the author numbers 1, 2, 3, 4, and 6, he refers
to former statements as to the proportion of lightning rods struck, &c.
(_Vide_ page 91, &c.)
5. There being several terminals on an edifice does not seem to diminish
the chances of each being struck.
7. In vessels, when the three masts have lightning rods, the mainmast is
most frequently struck.
8. Refers to Sir W. S. Harris’s lightning rods as being without terminal
rods or points.
9. The points of ordinary lightning rods have been made too slight.
10. Out of fifty-one cases of lightning strokes, thirty points have been
more or less melted; and the fusion is not without danger to the
buildings.
11. The lightning often leaves traces of mechanical action more or less
decided.
12. Refers to defective constructions of ordinary lightning rods.
13. Lead plates in conductors composed of bars joined together are
dangerous.
14. So are abrupt bends.
15. Conductors should communicate with the masses of metal near.
16. And must not end in watertight tanks. But
17. Conductors often protect buildings, though the ground connections
are imperfect.
18. It is well for a conductor to have two branches, viz., one in water,
and the other on the surface of the ground.
19, 23. Refers to the complete efficiency of Sir W. S. Harris’s
conductors.
20. Mentions the noise, electric sparks, &c., given off during a stroke,
as before stated (page 95).
21. Mentions the efficacy of lightning rods generally.
22. Their failure being owing to defective construction.
24. There is no proof that the electricity being in the form of a ball
has been the cause of any conductor’s inefficiency.
25. The lightning rarely bursts on a building or ship without striking
the lightning rod placed on it. Exceptions have, however, occurred in
ten cases, as here described. But
26. None of these instances invalidate the rule generally admitted, that
the protective action of the lightning rod extends, under all
circumstances, to a circular space whose radius is equal to double the
length of the upper terminal, _i.e._, the height of the point above the
part of the building on which the upper terminal is fixed.
ON ATMOSPHERIC ELECTRICITY.
BY REUBEN PHILLIPS. LONDON, 1863.
(_Abstracted by W. H. Preece, C.E._)
This is a pamphlet of seven chapters, and fifty-seven pages, written to
ventilate the author’s own notions of the nature of electricity and its
production in the atmosphere. He considers electricity to be two fluids
of a species of substance, consisting of separated subsidiary atoms.
“Electricity is comparable to a flying bullet; the _vis vivâ_ of the
bullet is like electrical intensity, and the mass of the bullet answers
to the quantity of electricity.” What the subsidiary atoms are like he
does not say.
Chapter I. is a fair resumé of what is known of the electricity evolved
by the friction of wet steam against solids in the hydro-electric
machine. He agrees with Faraday that the cause of the evolution of
electricity by the liberation of confined steam, is not evaporation, but
the friction of the water particles against the sides of the jet-piece
or orifice. Pure gases do not excite electricity; but impure air, when
compressed does, from the friction against the orifice of those
particles of water which are suddenly condensed by the cooling influence
of the expanding air.