that an iron hut or a complete suit of armor is at hand, and you will
naturally ask me what you ought to do under ordinary circumstances.
First, let me tell you what you ought not to do. You ought not to take
shelter under a tree, or under a haystack, or under the lee of a house;
you ought not to stand on the bank of a river, or close to a large
sheet of water. If indoors, you ought not to stay near the fireplace,
or near any of the flues or chimneys; you ought not to stand under a
gasalier hanging from the ceiling; you ought not to remain close to the
gas pipes or water-pipes, or any large masses of metal, whether used in
the construction of the building, or lying loosely about.
The necessity for these precautions is sufficiently evident from the
principles I have already put before you. You want to prevent your body
from becoming a link in that broken chain of conductors which, as we
have seen, the electric discharge between earth and cloud is likely to
follow. Now a tree is a better conductor than the air; and your body is
a better conductor than a tree. Hence, the lightning, in choosing the
path of least resistance, would leave the air to pass through the tree,
and would leave the tree to pass through you. A like danger would await
you if you stood under the lee of a haystack or of a house.
The number of people who lose their lives by taking refuge under trees
in thunderstorms is very remarkable. As one instance out of many, I
may cite the following case which was reported in the _Times_, July
14, 1887: “Yesterday the funeral of a negress was being conducted in a
graveyard at Mount Pleasant, sixty miles north of Nashville, Tennessee,
when a storm came on, and the crowd ran for shelter under the trees.
Nine persons stood under a large oak, which the lightning struck,
killing everyone, including three clergymen, and the mother and two
sisters of the girl who had been buried.”
Again, every large sheet of water constitutes practically a great
conductor, which offers a very perfect medium of discharge between the
earth round about and the cloud. Therefore, when a thundercloud is
overhead, the sheet of water is likely to become one end of the line of
the lightning discharge; and if you be standing near it, the line of
discharge may pass through your body.
When lightning strikes a building, it is very apt to use the stack
of chimneys in making its way to earth, partly because the stack of
chimneys is generally the most prominent part of the building, and
partly because, on account of the heated air and the soot within the
chimney, it is usually a moderately good conductor. Therefore, if
you be indoors, you must keep well away from the chimneys; and for a
similar reason, you must keep as far as you can from large masses of
metal of every kind.
Having pointed out the sources of danger which you must try to avoid
in a thunderstorm, I have nearly exhausted all the practical advice
that I have at my command. But there are some occasions on which it may
be possible, not only to avoid evident sources of danger, but to make
special provision for your own security. Thus, for example, in the open
country, if you stand a short distance from a wood, you may consider
yourself as practically protected by a lightning conductor. For a
wood, by its numerous branches and leaves, favors very much a quiet
discharge of electricity, thus tending to suppress altogether the flash
of lightning; and if the flash of lightning does come, it is much more
likely to strike the wood than to strike you, because the wood is a far
more prominent body, and offers, on the whole, an easier path to earth.
In like manner, if you place yourself near a tall solitary tree, some
twenty or thirty yards outside its longest branches, you will be in a
position of comparative safety. If the storm overtake you in the open
plain, far away from trees and buildings, you will be safer lying flat
on the ground than standing erect.
In an ordinary dwelling house, the best situation is probably the
middle story, and the best position in the room is in the middle of
the floor; provided, of course, that there is no gasalier hanging from
the ceiling above or below you. Strictly speaking, the _middle of the
room_ would be a still safer position than the middle of the floor;
and nothing could be more perfect than the plan suggested by Franklin,
to get into “a hammock, or swinging bed, suspended by silk cords, and
equally distant from the walls on every side, as well as from the
ceiling and floor, above and below.” An interesting case has been
recently recorded, by a resident of Venezuela, which illustrates in a
remarkable way the excellence of this advice. “The lightning,” he says,
“struck a _rancho_--a small country house, built of wood and mud, and
thatched with straw or large leaves--where one man slept in a hammock,
another lay under the hammock on the ground, and three women were busy
about the floor; there were also several hens and a pig. The man in
the hammock did not receive any injury whatever, while the other four
persons and the animals were killed.”[37]
But, as I can hardly hope that many of you when the thunderstorm
actually comes will find yourselves provided with a hammock, I would
recommend, as more generally useful, another plan of Franklin’s, which
is simply to sit on one chair in the middle of the floor and put your
feet up on another. This arrangement will approach very nearly to
absolute security if you take the further precaution, also mentioned by
Franklin, of putting a feather bed or a couple of hair mattresses under
the chairs.[38]
=Security Afforded by Lightning Rods.=--You might, perhaps, be inclined
to infer hastily, from the examples I have set before you, in the
course of this lecture, of buildings which were struck and severely
injured by lightning though provided with lightning conductors, that a
lightning rod affords a very imperfect protection to life and property.
But such an idea would be entirely at variance with the evidence at
hand on the subject. In all the cases to which I have referred, and in
many others which might easily have been cited, the damage was done
simply because the lightning rods were deficient in one or more of the
conditions on which I have so much insisted. Where these conditions are
fulfilled, the lightning flash will either not come down at all upon
the building, or, if it do come, it will be carried harmless to the
earth.
Perhaps there is no one fact that so forcibly brings home to the
mind the complete protection afforded by lightning conductors as the
change which followed their introduction into the Royal Navy. I have
already told you that in former times the damage done by lightning to
ships of the Royal Navy was a regular source of expenditure, amounting
every year to several thousand pounds sterling. But, after the general
adoption of lightning conductors about forty years ago, through the
indefatigable exertions of Sir William Snow Harris, this source of
expenditure absolutely disappeared, and injury to life and property has
long been practically unknown in Her Majesty’s Fleet.
I should say, however, that the trial of lightning conductors in the
Navy, though it lasted long enough to prove their perfect efficiency,
has almost come to an end in our own days. The great iron monsters
which in recent times have taken the place of the wooden ships of
Old England are quite independent of lightning rods in the common
sense of the word. Their ponderous masts are virtually lightning rods
of colossal dimensions, and their unsightly hulls are, so to speak,
earth-plates of enormous size in perfect electrical contact with the
ocean. To add to such structures lightning conductors of the common
kind would be nothing better than “wasteful and ridiculous excess.”
As regards buildings on land, I may refer to the little province
of Schleswig-Holstein, of which I have already spoken to you. From
some cause or other this small peninsula is singularly exposed to
thunderstorms, and of late years it has been more abundantly provided
with lightning conductors than, perhaps, any other district of equal
extent in Europe. Now, as a simple illustration of the protection
afforded by these lightning conductors, I may mention that, on the
26th of May, 1878, a violent thunderstorm burst over the little town
of Utersen. Five several flashes of lightning fell in different parts
of the town, but not the slightest harm was done, each flash being
safely carried to earth by a lightning conductor. Further, it appears
from the records of the fire insurance company that, out of 552
buildings injured by lightning during a period of eight years--from
1870 to 1878--only four had lightning conductors; and in these four
cases it was found, on examination, that the lightning conductors were
defective.[39]
It would be easy to multiply evidence on this subject. But as I have
already trespassed, I fear, too far on your patience, I will content
myself with saying, in conclusion, that according to all the highest
authorities, both practical and theoretical, any structure provided
with a lightning conductor properly fitted up in conformity with the
principles I have set before you to-day is perfectly secure against
lightning. The lightning, indeed, may fall upon it, but it will pass
harmless to the earth; and the experience of more than a hundred years
has fully justified the simple and modest words of the great inventor
of lightning conductors: “It has pleased God, in His goodness to
mankind, at length to discover to them the means of securing their
habitations and other buildings from mischief by thunder and lightning.”
NOTE I.
ON THE LIGHTNING CONDUCTOR AT BEREHAVEN.[40]
It is satisfactory to know that the lightning conductor referred to
in my lecture as attached to the lighthouse at Berehaven has been
put in good order under the best scientific guidance. The following
interesting letter from Professor Tyndall, which appeared in the
_Times_, August 31, 1887, gives the history of the matter very
clearly, and fully bears out the views put forward in my lecture:
“Your recent remarks on thunderstorms and their effects induce me to
submit to you the following facts and considerations. Some years ago
a rock lighthouse on the coast of Ireland was struck and damaged by
lightning. An engineer was sent down to report on the occurrence;
and, as I then held the honorable and responsible post of scientific
adviser to the Trinity House and Board of Trade, the report was
submitted to me. The lightning conductor had been carried down the
lighthouse tower, its lower extremity being carefully embedded in a
stone perforated to receive it. If the object had been to invite the
lightning to strike the tower, a better arrangement could hardly have
been adopted.
“I gave directions to have the conductor immediately prolonged, and
to have added to it a large terminal plate of copper, which was to be
completely submerged in the sea. The obvious convenience of a chain as
a prolongation of the conductor caused the authorities in Ireland to
propose it; but I was obliged to veto the adoption of the chain. The
contact of link with link is never perfect. I had, moreover, beside
me a portion of a chain cable through which a lightning discharge had
passed, the electricity in passing from link to link encountering
a resistance sufficient to enable it to partially fuse the chain.
The abolition of resistance is absolutely necessary in connecting
a lightning conductor with the earth, and this is done by closely
embedding in the earth a plate of good conducting material and of
large area. The largeness of area makes atonement for the imperfect
conductivity of earth. The plate, in fact, constitutes a wide door
through which the electricity passes freely into the earth, its
disruptive and damaging effects being thereby avoided.
“These truths are elementary, but they are often neglected. I watched
with interest some time ago the operation of setting up a lightning
conductor on the house of a neighbor of mine in the country. The
wire rope which formed part of the conductor was carried down the
wall and comfortably laid in the earth below without any terminal
plate whatever. I expostulated with the man who did the work, but
he obviously thought he knew more about the matter than I did. I am
credibly informed that this is a common way of dealing with lightning
conductors by ignorant practitioners, and the Bishop of Winchester’s
palace at Farnham has been mentioned to me as an edifice ‘protected’
in this fashion. If my informant be correct, the ‘protection’ is a
mockery, a delusion, and a snare.”
NOTE II.
BOOKS OF REFERENCE.
As some of my readers may wish to pursue the study of lightning and
lightning conductors beyond the limits to which a popular lecture
must, of necessity, be confined, I subjoin a list of the books which
I think they would be likely to find most useful for the purpose.
Among ordinary text-books on physics--Jamin, Cours de Physique,
vol. i., pp. 470-494; Mascart, Traité d’Electricité Statique, vol.
ii., pp. 555-579; De Larive, A Treatise on Electricity, in three
volumes, London, 1853-8, vol. iii., pp. 90-201; Daguin, Traité
de Physique, vol. iii., pp. 209-280; Riess, Die Lehre von der
Reibungs-Elektricität, vol. ii., pp. 494-564; Müller-Pouillet,
Lehrbuch der Physik, Braunschweig, 1881, vol. iii., pp. 210-225;
Scott, Elementary Meteorology, chap. x. Of the numerous special
treatises and detached papers on the subject, I would recommend
Instruction sur les Paratonnerres adopté par l’Académie des Sciences,
Part i., 1823, Part ii., 1854, Part iii., 1867, Paris, 1874; Arago,
Sur le Tonnerre, Paris, 1837; also his Meteorological Essays,
translated by Sabine, London, 1855; Sir William Snow Harris, On the
Nature of Thunderstorms, London, 1843; also by the same writer,
A Treatise on Frictional Electricity, London, 1867; and various
papers on lightning conductors, from 1822 to 1859; Tomlinson, The
Thunderstorm, London, 1877; Anderson, Lightning Conductors, London,
1880; Holtz, Ueber die Theorie, die Anlage, und die Prüfung der
Blitzableiter, Greifswald, 1878; Weber, Berichte über Blitzschläge
in der Provinz Schleswig-Holstein, Kiel, 1880-1; Tait, A Lecture
on Thunderstorms, delivered in the City Hall, Glasgow, in 1880,
Nature, vol. xxii.; Report of the Lightning Rod Conference, London,
1882. This last-mentioned volume comes to us with very high
authority, representing, as it does, the joint labors of several
eminent scientific men selected from the following societies: The
Meteorological Society, the Royal Institute of British Architects, the
Society of Telegraph Engineers and Electricians, the Physical Society.
Since the above was in print, two lectures given before the Society
of Arts by Professor Oliver Lodge, F. R. S., have appeared in the
_Electrician_, June and July, 1888, in which some new views are put
forward respecting lightning conductors, that seem deserving of
careful consideration.
FOOTNOTES:
[17] The Thunderstorm, by Charles Tomlinson, F. R. S., Third Edition,
pp. 153-4.
[18] Two Lectures on Atmospheric Electricity and Protection from
Lightning, published at the end of his Treatise on Frictional
Electricity, p. 273.
[19] See Report of Lightning Rod Conference, p. 119.
[20] _Loco citato._
[21] Sir William Snow Harris, _loco citato_, p. 274.
[22] _Id._, p. 275.
[23] The Thunderstorm, by Charles Tomlinson, F.R.S., Third Edition, p.
172.
[24] See for these facts, Anderson, Lightning Conductors, p. 197;
Tomlinson, The Thunderstorm, pp. 167-9; Harris, _loco citato_, pp.
273-4.
[25] See Anderson, Lightning Conductors, pp. 170-5.
[26] The Thunderstorm, pp. 158-9. See also an account of four persons
who were struck on the Matterhorn, in July, 1869, all of whom were
hurt, and none killed: Whymper’s Scrambles Among the Alps, pp. 414, 415.
[27] See Philosophical Transactions of the Royal Society, 1773, p. 42,
and 1778, part i., p. 232; Anderson’s Lightning Conductors, pp. 40-2;
Lighting Rod Conference, pp. 76-9.
[28] See A Lecture on Thunderstorms, by Professor Tait of Edinburgh,
published in Nature, vol. xxii., p. 365.
[29] Report of the Lightning Rod Conference, p. 4.
[30] The dimensions here set forth are greater in some respects than
those “recommended as a minimum” in the report of the Lightning Rod
Conference, page 6. But it will be observed by those who consult the
report that the minimum recommended is just the size which, in the
preceding paragraph of the report, is said to have been actually melted
by a flash of lightning; and, therefore, it seems not to be a very
safe minimum. It will be also seen that there is some confusion in the
figures given, and that they contradict one another. For the dimensions
of iron rods, see the instructions adopted by the Academy of Science,
Paris, May 20, 1875; Lightning Rod Conference, pp. 67-8.
[31] See letter of Mr. R. S. Newall, F. R. S., in the _Times_, May 30,
1879.
[32] See Nature, June 12, 1879, vol. xx., p. 146.
[33] See letter of Mr. Tomes in Nature, vol. xx., p. 145; also
Lightning Rod Conference, pp. 210-15.
[34] See Anderson, Lightning Conductors, pp. 208-10.
[35] See Lightning Rod Conference, pp. 208-10; see also the note at the
end of this Lecture, p. 52.
[36] Lecture on Thunderstorms, Nature, vol. xxii., pp. 365, 437. See,
also, a very interesting paper by the late Professor J. Clerk Maxwell,
read before the British Association at Glasgow in 1876, and reprinted
in the report of the Lightning Rod Conference, pp. 109, 110.
[37] Nature, vol. xxxi., p. 459.
[38] See further information on this interesting subject in the Report
of the Lightning Rod Conference, pp. 233-5.
[39] See “Die Theorie, die Anlage, und die Prüfung der Blitzableiter,”
von Doctor W. Holtz, Griefswald, 1878.
[40] See page 44.
APPENDIX.
RECENT CONTROVERSY ON LIGHTNING CONDUCTORS.
The lecture on lightning conductors contained in this volume fairly
represents, I think, the theory hitherto received on the subject. It
is, moreover, entirely in accord with the report of the Lightning Rod
Conference, brought out in 1883, by a committee of most eminent men,
representing several branches of science, who were specially chosen to
consider this question some ten years ago.
=Lectures of Professor Lodge.=--But, in the month of March, 1888,
two lectures were given before the Society of Arts, in London, by
Professor Oliver Lodge, in which this theory was directly challenged,
and attacked with cogent arguments, supported by striking and original
experiments. These lectures gave rise to an animated controversy, which
culminated in a formal discussion at the recent meeting of the British
Association in Bath. The discussion was carried on with great spirit,
and most of the leading representatives of physical and mechanical
science took an active part in it. The greater portion of this volume
was printed off before the meeting of the British Association took
place. But the discussion on the theory of lightning conductors seemed
to me so interesting and important that I thought it right, in the form
of an Appendix, to give some account of the questions at issue, and of
the opinions expressed upon them.
Professor Lodge maintains[41] that the received theory of lightning
rods is open to two objections. First, it takes account only of the
conducting power of the lightning rod, and takes no account of the
phenomenon known as self-induction, or electrical inertia. Secondly,
it assumes that the whole substance of a lightning rod acts as a
conductor, in all cases of lightning discharge; whereas there is reason
to believe that, in many cases, it is only a thin outer shell that
really comes into action. I will deal with these two points separately.
=The Effect of Self-Induction.=--When an electric discharge begins
to pass through a conductor, a momentary back electro-motive force
is developed in the conductor, which obstructs its passage. This
phenomenon is called by some self-induction, by others electrical
inertia; but its existence is admitted by all. Now, when a flash
of lightning, so to say, falls on a lightning rod, the back
electro-motive force developed is very considerable; and it may
offer so great an obstruction that the discharge will find an easier
passage by some other route, such as the stone walls and woodwork, and
furniture of the building.
According to this view, the obstruction which a flash of lightning
encounters in a conductor consists partly of the resistance of the
conductor, in the ordinary sense of the word resistance, and partly of
the back electro-motive force due to self-induction. The sum of these
two Professor Lodge calls the _impedance_ of the lightning rod; and he
considers that the impedance may be enormously great, even when the
resistance, in the ordinary sense, is comparatively small.
In support of this view he has devised the following extremely
ingenious and remarkable experiment. A large Leyden jar, L, was
arranged in such a manner that, while it received a steady charge from
an electrical machine, it discharged itself, at intervals, across the
air space at A, between two brass balls. The discharge had then two
alternative paths before it; one through a conducting wire, C, the
other across a second air space, between two brass balls at B. During
the experiment, the two balls at A were kept at a fixed distance of one
inch apart; but the distance between the two balls at B was varied.
The conductor, C, used in the first instance, was a stout copper wire,
about forty feet long, and having a resistance of only one-fortieth of
an ohm.
[Illustration: INDUCTION EFFECT OF LEYDEN JAR DISCHARGE.
M Electrical Machine.
L Leyden Jar.
A B Air Spaces between Brass Knobs.
C Conducting Wire.]
It was found that, so long as the distance between the B knobs was less
than 1.43 inches, all the discharges passed across between the knobs,
in the form of a spark. When the distance exceeded 1.43 inches, all
the discharges passed through the conductor, C, and no spark appeared
between the balls at B. And when the distance was exactly 1.43 inches,
the discharge sometimes took place between the knobs, and sometimes
followed the conductor, C. The interpretation given to these facts
is that the obstruction offered by the conductor C was about equal
to the resistance of 1.43 inches of air; and it is proposed to call
this distance, under the conditions of the experiment, the _critical
distance_.
Coming now to the application of these results, Professor Lodge argues
that the conductor C, in his experiment, represents a lightning rod
of unimpeachable excellence; and yet, in certain cases, the discharge
refuses to follow the conductor, and prefers to leap across a
considerable space of air, notwithstanding the enormous resistance it
there encounters. In like manner, he says, a flash of lightning may,
in certain cases, leave a lightning rod fitted up in the most orthodox
manner, and force its way to earth through resisting masses of mason
work and such chance conductors as may come across its path.
This conclusion, he admits, is altogether at variance with the received
views on the subject; but he contends that it is perfectly in accord
with the scientific theory of an electrical discharge. The moment
the discharge begins to pass in the conductor, it encounters the
obstruction due to self-induction; and this obstruction is so great
that the bad conductors offer, on the whole, an easier path to earth.