LIGHTNING PROTECTION IN FRANCE AND AMERICA.
In this chapter it is proposed to give a brief _résumé_ of the
different systems of constructing, erecting, and repairing lightning
conductors in France and America. The laws of electricity being the
same all the world over, the methods employed in these countries are
necessarily similar in their essential principles; nevertheless they
vary somewhat in detail, both from each other and from the work of the
best firms in England.
Until a very few years ago the lightning conductors throughout France,
although many in number, were in a very neglected state. Badly
constructed in many cases, their original faults had grown worse
from want of attention. The connection of the terminal rod with the
conductor was generally made by means of a strap or iron collar, which,
after a short time, rusted to such an extent that the continuity was
practically reduced to nothing, and the conductor, so far from being a
protection to the building, was a positive danger to it.
Latterly, however, a reaction has taken place, and a more careful
method of connecting the various joints of the conductor has been
contrived, or rather, revived, and a better system of periodical
inspection and testing is carried out.
[Illustration: Fig. 10.]
[Illustration: Fig. 11.]
Under the French system, what is called in England a lightning
conductor, and to which the French give the name _Paratonnerre_, is
nominally divided into three parts: the terminal rod, the conductor,
and the _racine_, or root, i.e. the earth connection. With regard to
the terminal rod, the ‘area of protection’ theory is, in France at any
rate, still believed in by a great many people. In that country, as
a rule, it is made of wrought iron in a single length, and polygonal
or slightly conical; its height depends upon the size and area of
the building it protects, the general presumption being that, under
ordinary circumstances, a terminal rod will protect effectually a cone
of revolution, of which the apex is the point of the rod, and the
radius of the base a distance equal to the height of the said rod above
the ridge, multiplied by 1·75. Thus a rod rising eight yards above the
ridge of a building would effectually protect a cone-shaped space, the
base of which, at the level of the ridge, has a radius of 8 × 1·75 =
14 yards. In actual practice somewhat wider limits are allowed. The
height of the terminal rod having been determined according to the
circumstances under which it is erected, it is then galvanised with
zinc in order to prevent oxidation, and the connection between the
terminal rod and the conductor is formed by means of the following
arrangement. A little above the base of the terminal rod, say about
eight inches from the roof of the building, a flange A (see fig. 10) is
welded with a hole pierced through it. Through this hole the conductor,
previously filed down to the proper dimensions, must be tightly passed.
After scraping the iron around the hole, a washer of lead is placed at
P and P´ (see fig. 11), and the button B, by means of a strong layer of
solder, thoroughly binds everything together. In this way an excellent
joint is obtained; the contact surface is considerable, and, if the
work is carefully done, the joint is completely preserved from rust.
[Illustration: Fig. 12.]
The point of the terminal rod, although sometimes made of platinum,
generally consists of either pure red copper, or, what is considered
still better, an alloy of 835 parts silver and 165 parts copper. It is
fastened to the terminal rod in the manner shown in fig. 12.
C is the trunk of a red copper cone, upon the top of which a point, P,
made either of platinum or of an alloy of silver and copper, as before
mentioned, is screwed, pinned, and strongly soldered with pewter solder
at _a_, the whole being screwed on to T at _b_. To ensure complete
contact and continuity, a washer of freshly-scraped lead is inserted
between C and T, and the whole of the joint thickly covered with a
layer of pewter solder. It may be added that the point forms an angle
of fifteen degrees with the vertical, consequently the point terminates
in an angle of thirty degrees.
[Illustration: Fig. 13.]
For the conductor of the ‘paratonnerre’ lengths of iron bars are
principally used; formerly these were jointed together by means of a
pyramidal bolt let into a notch of the same form, and connected by
a simple iron pin. This method, however, was discovered to be very
bad, as it failed to preserve the continuity of the conductor after
it had been erected a little time. The following plan, as represented
in fig. 13, is now used for the best work, as being more durable and
affording a better contact. On each side of the bars to be joined, two
flanges, about six inches long, and half the thickness of the bars,
are filed out. A thin piece of carefully-prepared lead is then placed
between them. The whole is then firmly fastened together by bolts at B
and B and completely covered with pewter solder, and thus furnishes a
solid, durable contact which possesses very small resistance.
Formerly the conductors were, at regular intervals, rivetted to
cramps let into the wall for the purpose of retaining the conductor
in its place. As this plan left no room for the play of expansion and
contraction caused by variations in the temperature, it was found that
at times the conductor was very much strained and even bent by reason
of this expansion and contraction. To avoid this evil an apparatus,
which has been approved by the Paris Academy of Sciences, has been
substituted for the cramps and rivets. This apparatus consists of a
fork in which the conductor is held fast by a pin (see fig. 14). Being
able to move backwards and forwards in the fork with great facility,
the conductor is thereby permitted to expand or contract under the
influence of temperature without threatening its supports with
destruction.
[Illustration: Fig. 14.]
The question however arises, upon what part of the paratonnerre ought
the effect of such contraction or expansion to be borne? The Paris
Academy of Sciences has sanctioned and recommended the use of a
compensator, which is designed to bear this strain. This compensator,
which is now much used in France, may be seen in fig. 15. It is
composed of an elastic plate F, made of well-annealed red copper,
three-quarters of an inch wide, at least twenty-eight inches long, and
about a quarter of an inch thick. The two extremities of this plate
are firmly fastened to the two ends of two lengths of the conductor
by the bolts and counterpieces B B´, and afterwards covered with a
thick coating of pewter solder. When, in consequence of the heat, the
conductor expands, the curve of the copper plate F will become greater,
and in cold weather it will become less. As a rule, a single apparatus
is supposed to compensate for the effects produced by long straight
lengths, and it is therefore thought sufficient to place one at each
bend.
With the exception of the terminal rod, it is the rule in France to
cover the whole of the paratonnerre with some coating in order to
preserve it from contact with the air. This is attained by covering it
with either a strong coat of tar, or a painting of a metallic basis,
such as zinc or tin filings.
[Illustration: Fig. 15.]
In larger buildings what is termed a ‘ridge-circuit’ is often used. It
consists of an unbroken metallic connection running along the ridges
of the building to be protected, and connected with the conductors
and terminal rods, and consequently with the subterraneous sheet of
water which forms the common reservoir. It is made of lengths of
square iron bars or rods having a thickness of about three quarters
of an inch square, and fastened together by overlaying the ends,
bolting them together with two bolts, and covering them well with
solder in the manner shown in fig. 13. New branches are formed by
T-shaped connections, the cross-piece of the T overlaying the original
ridge-circuit, and the stem making the first length of the new branch.
In some cases the ridge-circuit rests directly on the ridge of the
roof; but in order to avoid injury during the repairs to the roof or in
other ways, the plan adopted in good work is to raise it some distance
above the ridge on supports at suitable distances, and thus prevent the
possibility of damaging the joints and solderings.
The form and arrangement of these supports depend on the nature of the
roof. Sometimes forked uprights are used--these allow for the expansion
and contraction due to changes of temperature; in other cases simple
cast-iron bearings, weighing from ten to twelve pounds each, are laid
upon the ridge, their upper surfaces being grooved to receive the bars
of the ridge-circuit.
All masses of metal used in the construction of the building are
metallically connected with the paratonnerre. As a rule, this is
done by pieces of iron about half an inch square, which are strongly
soldered to the metal surfaces, and then connected with some part of
the conductor or ridge-circuit.
Although in France, as elsewhere, all experts are agreed as to the
prime importance of the disposition and arrangement of the _racine_ or
earth-end of the paratonnerre, a difference of opinion prevails as to
the best means of insuring a good earth-contact, and many methods have
been tried, all of them similar in principle, but differing somewhat in
application. It is proposed to give here a brief outline of the best
contrivances employed for this purpose.
One main object, in arranging the earth terminal of a lightning
conductor, is to avoid the gradual destruction of the _racine_ by
the action of alternate dryness and moisture which, unless the iron
is protected in some way, corrodes, and eventually eats it entirely
through. There are several ways of remedying this evil. In France it
is common to find used for this purpose a vertical spout of tarred,
boucherised, or creosoted wood, rising a few inches above the soil.
Some authorities recommend the simple plan of covering this part of
the conductor with a strong coating of tar, others covering it with
a wrapper of sheet lead, and this last method is probably the best.
With regard to the extreme end of the conductor, the system approved
of by the Paris Academy of Sciences is generally used in good work.
This system is the use of a trough filled with broken charcoal, through
which the conductor runs; charcoal preventing the too rapid oxidation
of the iron. For charcoal, coke may be substituted. The trough (see
fig. 16) is made either of wood, gutter tiles, or ordinary bricks
without mortar, so as to allow the moisture of the soil to permeate
through. It is preferable, even at the expense of lengthening the
conductor, to carry it through the lowest and dampest plots of ground
around the building.
[Illustration: Fig. 16.]
To obtain a perfect contact between the end of the conductor and the
earth, or common reservoir, the French use several methods. One of
the earliest ones was the multiplication of the iron bars attached to
the end of the conductor, and inserting them for some distance into
well-water. Theoretically this arrangement is good, but it has been
found that the decay of these terminals by the action of rust was
so rapid that, unless they were carefully watched and periodically
repaired, they soon became insufficient, if not useless. In addition
to this, it is the opinion of many French savants that a mere water
contact is not enough, a soil that is always moist being in their
judgment far preferable. The simplest plan adopted for attaining this
end is that of inserting into the moist ground to a certain depth,
regulated by the nature of the soil, one or several metallic branching
stems, which are connected with the conductor. By another arrangement,
invented by M. Callard, the conductor is terminated by a kind of
galvanised iron grapnel, placed in a wicker basket filled with pieces
of coke. Where the soil is dry, and moist ground cannot easily be got
at, the harrow or grating shown in fig. 17 is often used. It is placed
between two layers of horn embers, or charcoal, and sunk as deeply
as it conveniently can be, the end of the conductor being carefully
connected with it by soldering or by a quantity of melted zinc.
In towns, the water-pipes and gas-mains, possessing as they do,
large metallic surfaces, are generally utilised for making the
‘earth-contact.’
[Illustration: Fig. 17.]
Sometimes, instead of iron bars, galvanised iron cables of about an
inch in diameter are used for the conductors of paratonnerre, and
occasionally red copper cables of half-an-inch only in diameter, but
the use of these latter is uncommon.
[Illustration: Fig. 18.]
Fig. 18 exhibits a modification of the point of the terminal rod which
is advocated by M. R. F. Michel. The arrangement is based on the
principle that, on the approach of a tempest cloud, the more points
there are, the greater will be the neutralising effect. M. Michel
considers that when a terminal rod has only one point, it acts only
in one direction; but if there is a large number of points branching
in all directions, the preventive action is materially increased; he
therefore proposes the use of this contrivance, which is carried out
by having the ordinary conical trunk copper point on the top of the
terminal rod melted down, and moulded so that it presents in its middle
a circular swelling. Into this swelling arrows are fixed, inclined at
each side of the horizontal plane to an angle of 45 degrees, as shown
in the engraving. These arrows radiating in all directions are supposed
to ‘hasten the neutralisation of the electrified cloud; and in the
event of a discharge, the discharge, by dividing amongst them, will
prevent their fusion.’
Before quitting the French system, mention should be made of a novel
form of lightning conductor devised by M. Jarriant. This gentleman
proceeds on the hypothesis that the most essential requisites of a
lightning conductor are:--a terminal rod metallically homogeneous,
which should rise to a good height; that it be sufficiently light
to avoid damage to the roof, and yet be strong enough to resist the
violence of the wind. To attain these requirements, M. Jarriant secures
his conductor with three or four stays, which are firmly fixed to the
roof and converge to the top of the terminal rod, to which is fastened
the ordinary copper point, recommended in the ‘Instruction’ of the
Academy of Sciences. Iron supports are placed at different heights in
order to ensure the perfect solidity of the system. Galvanised iron
is employed, and all the various stays and supports are metallically
connected with each other. The angles of the irons are all acute,
and placed so as to offer the least resistance to the wind. The
advantages claimed for this method are that the upper part of the
conductor bristles with spikes and aigrettes, which he considers a
great advantage in regard to the preventive effect produced by the
conductor; it allows of the conductor being raised much higher above
the building; it presents a large surface to the electrified cloud; the
joints are so arranged that they cannot be dislocated by the expansion
and contraction caused by variations of temperature; and, lastly, it is
affirmed that these conductors cost thirty per cent. less than those
erected under the ordinary system.
America stands pre-eminent above other countries in the numerous and
extraordinary schemes that have there been promulgated in regard to the
protection of buildings from the effects of lightning, and probably
no other nation has been so systematically victimised and swindled in
this matter. The tramping ‘lightning-rod men’ of the United States are
notorious for extortion and ignorance: they use all kinds of fantastic
and peculiar shaped terminal rods and conductors, the main object
being to make as great a show with as little metal as possible. Their
work is almost entirely confined to the upper portion of the conductor,
to the neglect of the most important part--the earth terminal. In
consequence, the majority of the lightning conductors in America are
untrustworthy; very often they are practically insulated from the
‘common reservoir’ or subterraneous water, and are therefore more often
a source of danger than a protection. Unhappily, these peripatetic
mechanics are by no means extinct, although increased knowledge is
gradually driving them from the field.
In America, a strong point is made of utilising, as far as possible,
all the existing natural conductors that are to be found upon a
building, such as gutters, rain-pipes, and other metal surfaces. During
a tempest, the opposite electricities of the earth and the air often
select, by their inductive influence, a rain-pipe, gutter, metal roof,
&c., for the passage of an electric discharge between them, and unless
these metallic surfaces are connected with the earth, they are apt to
be dangerous. But if they are properly connected together, and provided
with a good earth-contact, they materially assist to diminish the
intensity of a discharge.
In the case of a building with a roof of slate, wood, or other material
of low conductivity, a conductor made of either bar iron or stranded
cable is placed along the ridge and gable ends, and carefully connected
with the gutters and rain-pipes; where the rain-pipes are less than
three inches in diameter, the bar or cable conductor is often extended
from the roof down the side of the building, and connected with the
earth terminal. When this is done, the bar or cable conductor is placed
between the rain-pipe and the wall of the building, or at any rate
close to the rain-pipe, and connected with it by solder or bolts.
All metallic chimney caps, cornices and railings on the tops of
buildings, as well as the water-pipes, gas-pipes, hot water-pipes,
and other large or long pieces of metal, whether they occur inside or
outside the building, are connected with each other by a conductor
composed of light stranded wires, each about three-sixteenths of an
inch in diameter; they are also connected with the main conductor
at its nearest point. Where several adjacent buildings have each
a metallic roof, these roofs are connected together by means of a
horizontal conductor.
The terminal rod of the conductor generally projects about four feet
above the chimney or other highest point of the building. It consists
of a round iron rod seven-sixteenths of an inch in diameter, the lower
extremity being hammered out for the purpose of fastening it to the
conductor by soldering and screws or by bolts. A small building, not
exceeding twenty-five feet in length or breadth, is generally fitted
with either one terminal rod placed on the centre of the ridge of the
roof, or with two rods, one at each end of the ridge, the latter method
being the preferable one. In larger buildings terminal rods are placed
at intervals of about twenty feet along the roof. The upper end of
the rod is sometimes pointed, but not always, the argument being that
although the ordinary end of a rod is blunt when used in connection
with a Leyden jar, but that when applied to a thunder-cloud, which
extends over thousands of acres, it becomes pointed, and bears the
same proportion to a thunder-cloud as the sharp point of a needle does
to the hand of a man. Occasionally the point is tipped with platinum,
gold, silver, or pure copper, in order to prevent oxidation, but this
is not considered essential, it being presumed that, practically no
amount of rust on the top would impair the efficacy of the terminal rod.
In the case of a building having a flag-staff upon it, a galvanised
iron wire is fastened along it and projects about six inches above the
top, the lower end of the wire being of course carefully soldered or
otherwise connected with the main conductor.
Steeples and spires, in addition to the ordinary vertical conductor,
are fitted with horizontal conductors placed around them at intervals
of about twenty feet, and connected with the vertical conductor. This
is to provide against the occasional discharges that take place in
the centre of steeples, and which are caused by the deflection of the
discharge in the air by the rain.
Chimneys and air shafts, from which heated air or smoke escapes,
are fitted with metallic caps which are connected with the general
conductor. In order to protect this metallic cap from the effects
of the sulphurous fumes arising from the chimney, a terra-cotta cap
is contrived to fit inside the metallic cap. An analogous method is
adopted with regard to the ventilators of barns and ice-houses. If
these buildings have the ordinary ventilators in the form of dormer
windows upon the roof, an iron rod seven-sixteenths of an inch in
diameter is placed vertically across, and above the centre of the
opening of each ventilator, and connected with the conductor. Should
the barn or ice-house have openings or doors through which warm vapour
can escape, a conductor is fixed to the roof at the gable ends above
the centre of each opening or door, and extended outwards about five
feet, at an angle of forty-five degrees from the roof, so as to be in
line with any ascending vapour, or any descending charge of electricity
following the course of the vapour. All these auxiliary conductors and
terminal rods are metallically connected with the main conductor of the
building.
The conductors are simply fastened to the building by iron staples or
by straps of sheet iron, pierced with two holes for nails or screws.
In America, as elsewhere, the earth-terminal is regarded as of prime
importance, and in all properly constructed lightning conductors
receives the greatest care and attention. In the first place, such
metal pipes as lead from the building to the water-mains, gas-mains,
and sewers are carefully connected with the principal lightning
conductor, in order that they may act as auxiliary earth-terminals.
For the principal earth-terminal many contrivances have been brought
forward, but very few possess any originality, and many are positively
useless. Some of the best are similar to those in use in England;
among others, perhaps the best method is that of placing a cast or
wrought-iron pipe of three inches inside diameter, and about ten feet
long, vertically in thoroughly moist earth and carefully connecting the
conductor, or conductors, with it. The chief objections to this plan
are the occasional difficulty of getting a moist earth at all, and the
possibility of earth that is generally moist getting dried up in hot
weather. To obviate these risks, the following arrangement is used:—
In a pipe of wrought or cast iron, at least ten feet long, and having
an inside diameter of two inches with a thickness of three-eighths of
an inch, are made a number of longitudinal openings or perforations,
about ten inches long and a quarter of an inch wide. These openings
or perforations are made at intervals of ten inches, and are placed
in one or two lines opposite to each other. If it is preferred, round
holes of from half an inch to one inch in diameter, and about six
inches distant from each other, may be substituted for the longitudinal
openings. This perforated pipe is placed in an upright position in the
earth, and is so situated that it receives at its top opening the waste
or rain water flowing along a channel or drain constructed for that
purpose. The water, after running into the top of the pipe, gradually
percolates down, and passing through the perforations or openings into
the earth around and underneath the pipe, moistens it to such an extent
and at such a depth as to render it but little affected by the heat of
the sun. The pipe is generally placed at some little distance from the
building, so as to give a sufficient area of earth to be kept moistened
and to prevent the walls of the building being affected by the damp.
Occasionally, the pipe is made triangular or square, and with
perforated branches and other metallic conductors. It is also sometimes
constructed with enlargements at the top or bottom, so as to hold more
water. Probably, however, the simplest plan is the best, as--if the
soil be suitable--a plain round wrought-iron pipe can be driven into
the earth. If a cast-iron pipe is used, a hole of a convenient size is
excavated for it. In this case, great care has to be taken that the
earth is thoroughly well rammed down all round the pipe.
Another arrangement is to employ, instead of a cast or wrought-iron
pipe, a number of round or flat-iron bars, fastened together at the top
and bottom by rivetting to metal hoops in such a manner that intervals
are left between each bar, through which the water can pass. Sometimes
a solid pipe without the openings is used, but it is not found to be
so satisfactory as the perforated pipe, because the latter allows a
greater amount of water to pass through it into the soil, thereby
furnishing a larger area of moist earth.
The French method of carrying the conductor to the bottom of a
neighbouring well is frequently adopted where it is practicable, and
the water of the well is not required for drinking or cooking purposes.
A few words may be added here on the method of protecting the large
mineral oil tanks which are to be found in the United States. Many of
these oil tanks are of very large capacity, some of them containing a
million gallons of oil. They are generally constructed of thick iron
plates rivetted together. The roofs are usually made of wood coated
with tar, but in some cases iron is adopted. As a rule, several of the
tanks are grouped together and connected with each other--and in some
instances with distilleries--by means of subterraneous iron pipes.
One method of protecting these tanks is to erect around them, at
a distance of some ten feet, wooden supports, on which are placed
upright metallic conductors which overlook the tank, and are connected
with each other near their tops by stout iron wires, thus forming a
network of conductor which is supposed to intercept any discharge of
electricity from a tempest-cloud, and prevent it from reaching the oil
tank. This method, however, has failed in several notorious instances,
and is not countenanced by the best authorities.
A better and less complex arrangement is now usually adopted by the
best firms. The chief object of this arrangement is to prevent the
temperature of the oil tank, and of the atmosphere above and around it,
being raised by means of an electric discharge. This is accomplished
by using large conductors, which are carried some distance above the
oil tank. These conductors, of which there should be at least four,
are formed of flat iron bars about one and a half inches wide and
half an inch thick; they are securely fastened to the sides of the
tank at equal distances from each other, and metallically connected
with it. About thirty feet above the roof of the tank they meet, and
are carefully and substantially joined together, and supported, if
necessary, by a wooden post extending from the centre of the roof of
the oil tank.
The earth terminals, of which there must be one to every two
conductors, consist of perforated iron pipes, as before described,
three inches in diameter and fifteen feet long. They are sunk into
thoroughly moist earth, and metallically connected with the lower part
of the tank. These perforated pipes are so arranged that they catch the
rain water from the roof of the tank; by this means the surrounding
earth is kept moist. It may be mentioned that by utilising the tank
as a portion of the system of conductors, the electric discharge is
distributed and much weakened.