INQUIRIES INTO LIGHTNING PROTECTION.
From our present ignorance of the actual nature of electricity,
admitted alike by all scientific men, it has often been argued that no
claim can be set up for a perfect protection against the effects of
the electric force called lightning, since we do not know ‘whence it
comes, nor whither it goes.’ That this argument is entirely fallacious,
may be easily shown. The human mind does not understand, any more
than it does electricity, the great forces called centripetal and
centrifugal, which keep millions of suns and of planets in their path
through the boundless universe; yet there is no educated man who doubts
that astronomers are able to calculate, with the greatest mathematical
precision, the time when two particular stars will come near each
other, when the moon will obscure Orion, and Venus make her transit
across the sun. Again, no explanation can be given of the actual
nature, of the Why and the Wherefore, of the force called gravity,
simply in its operation on our globe. Still men can calculate, with
the greatest nicety, the result of any given weight, falling, from any
given height, on the surface of the earth or below it.
François Arago, reasoning on the disputed efficiency of lightning
conductors, puts another indisputably practical case. ‘If,’ says he,
‘we take the dimensions to be given to conductors from experience,
and if those which we adopt have been found to resist the strongest
lightning recorded for over a century, what more can reasonably be
asked for?’ When the engineer decides on the height and width of
the arches of a bridge, the vault of an aqueduct, the section of a
drain, and similar constructions, what does he concern himself with?
He examines all the facts and records on the matter as extensively as
he can, and, in making his plan, keeps somewhat beyond the dimensions
dictated by the greatest floods and the heaviest rains which have ever
been observed. He thus goes as far back in his research as the evidence
within his reach will enable him to do, but without confusing himself
either with searching for the hidden causes of floods and rains, or
with investigating the character of the physical revolutions, or the
cataclysms which occurred in prehistoric times, and of which geologists
only have been able to discover the traces and estimate the magnitude.
So with the engineer. Greater precaution or foresight than his cannot
be demanded from the constructor of lightning conductors, nor is any
needed.’
It may be laid down as an absolute fact, that a well-made lightning
conductor, properly placed, and kept in an efficient state, can never,
under any circumstances, fail in its action. Undoubtedly it has
happened that buildings to which conductors were attached have, in many
instances--of which some will be enumerated in another chapter--been
struck by lightning, and even damaged; but these cases, so far from
going against the truth that good lightning conductors are infallible,
only serve to prove it. A close investigation of all known instances
where the electric force has struck buildings, nominally protected
against lightning, shows most conclusively that the conductors placed
on them were either inefficient, in some way or the other, or did not
lead properly into moist ground--that is, had not the all-indispensable
‘earth connection.’ There is no case on record in which a really
efficient lightning conductor, properly placed, and with its terminal
in technically so-called ‘good earth,’ did not do its duty; and without
being dogmatic on the subject, it may well be asserted can no more
fail to give protection than an efficient drain-pipe can fail to carry
off the water upon the roof. Although the electric force is neither a
‘current’ nor a ‘fluid,’ often as it is so described, still the analogy
holds good so far as the one here given between the drain-pipe and the
conductor. And the reason is clear enough. The water, in running down
a hollow tube, obeys simply the law of gravity, but no less immutable
than this is that which governs the movement of the electric force. As
the water has no choice but to follow the channel made for it, under
the guidance of experience and mathematical calculation, so has the
emanation of the electric energy no option but to pursue the path which
scientific investigation has shown it always to take. Men may speak of
‘erratic’ lightning; but it is certain that the course of the electric
force is as subject to cosmic laws and as immutable as that of the
stars.
Most of the experiments and investigations for ascertaining the best
form of lightning conductors, and their application to buildings
so as to be invariably efficient, have been carried on by private
activity; still, the subject has also, at various times, undergone
the examination of official authorities, as well as of learned
societies. Little has been done in this respect in England, but very
much in France, where, ever since the publication of Franklin’s great
discovery, the question of protection against lightning has uniformly
interested the public, as well as the learned world, leading to
the production of more treatises on the subject than in any other
country, except perhaps Germany, the world’s centre of book-making.
One of the most important of the French works here referred to, and
which may be regarded as the standard work on lightning conductors,
is a semi-official publication, entitled ‘Instruction sur les
paratonnerres,’ issued in new editions from time to time, and widely
dispersed, not only in France, but all over Europe and America. It
consists of several reports about lightning conductors made, from
1823 to 1867, by committees comprising some of the most distinguished
men of science at the time, to the ‘Académie des Sciences’ of Paris.
The earliest of these reports originated from an application of
the French Government to the ‘Académie.’ In the year 1822, there
happened to be in France, and over the greater part of Continental
Europe, an extraordinary number of violent thunderstorms, accompanied
by earthquakes and simultaneous eruptions of Mount Vesuvius, the
latter on a scale not witnessed for centuries. In France, the almost
continuous thunderstorms caused great alarm among the population;
and the priests in many places held processions in and around the
churches, with special prayer-meetings, to ‘appease the wrath of
heaven.’ In consequence of all this excitement, the Minister of
the Interior, deeming that something also ought to be done besides
the walking in procession to stay the fatal effect of lightning,
ordered that all the public buildings in France should be protected
immediately by conductors, made on the most perfect model and placed
in the best manner. To get pre-eminent advice as to the efficiency of
lightning conductors, the Minister applied officially to the ‘Académie
des Sciences,’ which learned body thereupon nominated a committee
consisting of six of the most celebrated investigators of the phenomena
of electricity--MM. Poisson, Lefèvre-Gineau, Girard, Dulong, Fresnel,
and Gay-Lussac. The committee held many sittings, collecting a vast
amount of evidence on the subject, and on April 23, 1823, presented
through M. Gay-Lussac its report to the ‘Académie des Sciences,’
which was adopted and ordered to be printed, being declared a highly
important document. The French Government took the same view as the
‘Académie des Sciences,’ and not only acted upon the recommendations
of the report, but issued it to all public functionaries, to the
clergy, and others, with directions to make it generally known. In
this way hundreds of thousands of copies of the ‘Instruction sur
les paratonnerres’ found their way all over France, and from thence
in translations all over Europe, as the best existing guide for the
erection of lightning conductors.
The information thus spread by the French Government gave rise to
important results. It caused the setting-up of lightning conductors
throughout the country, on private as well as public buildings, and
it likewise led to an improved construction of them, in as far as the
‘Instruction’ recommended the rods to be made of stout pieces of metal,
well fastened to each other, and, above all, led into the ground deep
enough to reach moist earth or water. If this was well enough, and
useful enough, to meet with general acceptation, there were some points
in the advice of the learned men of the ‘Académie’ that gave rise
to much criticism, as being more founded upon theory than practical
experience. In the first place, they laid it down as a hard-and-fast
rule that the upper rod of a lightning conductor--that projecting over
the roof--‘will be an efficient protective against lightning within
the circular area of a radius double that of its height,’[1] and the
acquiescence in this supposed absolute formula had for one of its
results the erection of monstrously huge rods, made to tower high above
buildings, so as to increase the field of protection to the largest
possible extent. Another and worse fault was committed by the authors
of the ‘Instruction’ in not saying anything about the necessity of
regularly inspecting the actual condition of lightning conductors, and
testing them in respect to their efficiency. While giving minute advice
as to the mode of construction and the general design of conductors,
the contents of the ‘Instruction’ were such that, on the whole, its
readers would take it for granted that it was only necessary to
properly join the strips of metals and bring them down into the ground,
after which, thenceforth and for ever, the protection against lightning
would be complete. This grave omission, together with the erroneous
dogma as to absolute rule of protection within an area prescribed by
the height of the ‘tige,’ or upper part of the rod, had the inevitable
result of causing disasters, and before the ‘Instruction’ had been
issued many years, there came report after report to the
[1] The original, long taken as a scientific dogma, runs: ‘Une
tige de paratonnerre protège efficacement contre la foudre
autour d’elle un espace circulaire d’un rayon double de sa
hauteur.’
Government that well-constructed lightning conductors had failed to
do their duty. For a length of time these reports were either not
believed in, or the failure ascribed to partial non-compliance with
the strict rules laid down by the ‘Académie des Sciences.’ However, in
the end, when thirty years had passed, the instances of buildings with
conductors being struck became so numerous, that it was impossible to
ignore them any longer and, flying once more for advice to the savants
of the ‘Académie des Sciences,’ the French Government desired them
to investigate anew the question as to the best means of protecting
buildings against lightning. Complying with the behest, the learned
body nominated again a committee of six, the names of those selected
comprising the most eminent men who had made electricity and its
phenomena their study. They were MM. Becquerel, Babinet, Duhamel,
Despretz, Cagnard de Latour, and Pouillet.
The ‘Instruction’ of the new committee, drawn up by Professor Pouillet,
was read before the ‘Académie des Sciences’ on December 18, 1854, and
having been unanimously approved, was, like the former one, taken up by
the Government and extensively circulated. The report began by modestly
excusing the short-coming of its predecessor. ‘For the last thirty
years,’ Professor Pouillet remarked, with no fear of being gainsaid,
‘the science of electricity has made great progress--in 1823 the
discovery of electro-magnetism had only just been made, and none could
foresee the immense results that would spring from its revelations.’
Based upon these grounds, the new ‘Instruction’ entirely reversed
many of the conclusions of the old one. First of all, it declared
inadmissible the theory of a fixed area of protection, to be calculated
by the length of the upper rod. ‘Such a rule,’ Professor Pouillet
justly remarked, ‘cannot be laid down with any pretence to accuracy,
since the extent of the area of protection is dependent from a mass of
circumstances--such as, among others, the shape of the building and the
materials entering into its construction. It is clear, for example,
that the radius within which the conductor gives protection cannot be
so great for an edifice the roof or upper part of which contains large
quantities of metals, as for one which has nothing but bricks, wood, or
tiles.’ Professor Pouillet then proceeded to give detailed instructions
in respect to the design and mode of manufacturing lightning
conductors. He insisted that the rods should be of greater capacity
than those recommended by Gay-Lussac in the report of 1823, and that
there should be as few joints as possible from the point to the earth.
He considered it of the greatest importance that all the joints should
be carefully tin soldered, otherwise the metallic continuity of the
conductor could not be assured. He also advised that the top of the
air-terminal should not taper to so fine a point as formerly, but
be rather blunt. A lightning conductor, said Professor Pouillet, is
destined to act in two ways. In the first place, it offers a peaceful
means of communication between the earth and the clouds, and by virtue
of the power of points the terrestrial electricity is led gently up
into the sky to combine with its opposite. In the second it acts as
a path by which a disruptive discharge may find its way to the earth
freely. In the latter case he considered there was a risk of a sharply
tapered point becoming fused, and recommended that the angle of the
cone at the top of the air-terminal should be enlarged. He also advised
that the point should be made of red copper instead of platinum, and
based his argument on the fact of copper being a better conductor of
electricity than platinum, and considerably cheaper. A copper point,
remarks M. Pouillet, subjected to a heavy stroke of lightning, would
be much less heated than a platinum point, and would scarcely in any
case be fused. While in the report of 1823, iron ropes were recommended
almost exclusively as the best material for conductors for ships, the
‘Instruction’ of 1854 declared strongly in favour of copper as the far
superior metal for the purpose. ‘Copper,’ affirmed Professor Pouillet,
‘is superior to iron as well as to brass for the purpose of lightning
conductors, it having the advantage not only of being less influenced
by atmospheric agencies, but the still more important one of allowing
a freer passage to the electric force of over three to one. Copper
should therefore be exclusively used in the construction of lightning
conductor cables for the protection of ships.
The inquiries into lightning protection instituted by the ‘Académie
des Sciences,’ and resulting in two reports, the second valuable in
the highest degree, had the good effect, not only of drawing public
attention to the necessity of providing such safeguards, but of
bringing the whole matter under due scientific control. Henceforth the
ground was cut away under ‘lightning-rod men,’ perambulating towns
and villages, and offering their trumpery ware--mostly bits of wire
tied together, with perhaps a lacquered piece of wood on the top--to
credulous persons, as a substitute for good conductors. The French
Government set a laudable example in appealing for the future always to
scientific aid. A few months after the publication of the ‘Instruction
sur les paratonnerres,’ drawn up by Professor Pouillet, a decision was
come to for protecting the new wings of the Louvre, at Paris, with the
most perfect lightning conductor that could be made, and thereupon
appeal for counsel was once more made to the ‘Académie des Sciences.’
The case was one of special interest. The palace of the Louvre, with
its inestimable treasures of art, had been the first public building
in France provided with a lightning conductor. It was due to the
initiative of an enthusiastic admirer of Benjamin Franklin, David Le
Roy, that this was accomplished, he having excited the public feeling
as to the dangers from lightning to which the Louvre was exposed to
such a degree as to compel the Government, in 1782, to carry out his
plans, under his own superintendence. The conductors erected by Le
Roy had stood the test of experience from 1782 until the year 1854,
many a thunderstorm having passed over the extensive buildings of the
Louvre without causing the least damage. But, in the last month of
1854, one more lightning cloud swept along the banks of the river
Seine, and the electric fire, falling on one of the chimneys of the
palace, knocked off a few bricks. The damage was very trifling, but
the alarm nevertheless was great, and very naturally so. If there was
one building in France, it was said, which ought to be beyond the risk
of being struck by lightning, it was the Louvre, and, if this could
not be accomplished, the art of constructing protective conductors was
altogether vain and ineffectual. It was under these circumstances,
incited by the public outcry, that the Government hastened to submit
the new case to the ‘Académie des Sciences.’
Once more the ‘Académie’ nominated a committee on lightning conductors,
composed of the same members who had signed the ‘Instruction’ of 1854,
and drawn up by Professor Pouillet. He again drew up the report, which
was adopted by the ‘Académie’ on February 19, 1855, and contained some
notable additions to the directions previously given. They related,
as was desired, in the first instance to the Louvre alone, but were
made applicable to all large public buildings. For their efficient
protection, the professor insisted, two things should be kept in view
above all others--namely, first, that the point, always of copper,
should be of greater thickness; and, secondly, that it should have a
never-failing connection with either water or very moist earth. To
ensure the latter, it was recommended, as had been done before, that
the underground part of the conductor should be divided.
The necessity for such a division, and for forming at least two
subterranean arms--the first of it, described as ‘the principal
branch,’ going very deep into ground, into perennial water, and the
second, ‘the secondary branch,’ running nearer the surface--was
explained by Professor Pouillet very clearly in this last report.
‘After a long continuance of dry weather,’ he observed, ‘it often
happens that the lightning-bearing clouds exert their influence only
in a very feeble manner on a dry soil, which is a bad conductor; the
whole energy of their action is reserved for the mass of water which
by percolation has formed below it. It is here that the dispersion of
the electric force (_la décomposition électrique_) takes place; it
will follow the principal branch of the conductor underground, and
leave the secondary branch untouched. The case is entirely different
when, instead of dry weather, there have been heavy rains, moistening
the earth thoroughly, up to the surface. It is the latter now that
is the best, because the nearest, conductor of the electric force,
which will not go to the more permanent sheet of water, lying more or
less deep in the ground, if there is moisture above it. Under these
circumstances, it is indispensable that there should be a direct
connection between the surface soil and the lightning conductor, and
this is what is accomplished by the secondary branch. It is a power in
aid of the principal branch, and one often of the highest importance.’
The suggestion here made was one so evidently good, that it was at
once accepted by the French Government, and the Louvre not only, but
other public buildings, received lightning conductors ending in two
subterranean branches, as proposed by Professor Pouillet.
The report on the protection of the Louvre Palace did not contain
the last inquiry of the ‘Académie des Sciences’ on the subject of
lightning conductors. Twelve years after it had been issued, the
Government of France once again called upon that learned body for
advice as to the best mode of protecting powder magazines. Several
cases had happened--among others at Rocroy, on the borders of the
forest of Ardennes--of such buildings being struck, notwithstanding
that they had conductors placed upon them, and the Government,
naturally alarmed, made inquiry as to whether nothing could be done to
ensure protection against lightning, infallible under all atmospheric
conditions and every possible emergency, to these dangerous stores. The
demand was made in a letter of the Minister of War, Marshal Vaillant,
dated October 27, 1866, pressing the ‘Académie’ to give another
‘Instruction,’ without delay, the Government being ‘in fear that some
of the powder magazines are not as completely protected from lightning
as could be wished.’ Thereupon the ‘Académie des Sciences’ nominated
another commission, this time of eight members, including the Minister
of War himself--not complimentary, but as being an author, and with a
warm interest in electrical science; and, besides him, MM. Becquerel
Sen., Babinet, Duhamel, Fizeau, Edmond Becquerel, Regnault, and
Professor Pouillet. The list represented a galaxy of names unsurpassed
in the investigation of such a subject as lightning conductors, looked
upon in most countries of Europe, at least in recent years, as rather
plebeian, to be left to builders and lightning rod men. Many sittings
were held by the committee, all fully attended, so that, although
the Minister had desired to get the new report ‘_le plus promptement
possible_,’ it was not till nearly three months after the receipt of
his message that it was completed, Professor Pouillet again being the
author. It was a most remarkable paper, this one, read before and
approved of by the ‘Académie des Sciences’ on January 14, 1867.
Before entering upon the subject of the protection of powder magazines
against lightning, the new ‘Instruction’ signed by Professor
Pouillet and his colleagues laid down a few so-called ‘_propositions
générales_‘--that is, either hints, suggestions, or statements,
the French word ‘_proposition_’ being most serviceably vague for
use--on the subject of lightning and of thunderstorms. The first
thesis affirmed that ‘clouds which carry lightning with them are but
ordinary clouds (_ne sont autre chose que des nuages ordinaires_)
charged with a large quantity of electricity.’ The second thesis
boldly defined the nature of lightning. ‘The fire which flashes
from the skies is an immense electric spark, passing either from
one cloud to another, or from a cloud to the earth; it is caused
by a tendency for the restoration of the electric equilibrium (_la
recomposition des électricités contraires_).’ It was laid down in the
third ‘_proposition_’ that, when lightning falls from a cloud upon the
earth, it is but an effort of the electric force to return to its
grand reservoir. That it is similar to water, which, having risen in
the form of vapour from the earth-surrounding ocean high up into the
air, then falls down as rain upon hills and plains, and finally runs
down again in rivers to the ocean, Professor Pouillet did not say in
so many words; but there were vague hints to that effect in the new
‘Instruction.’ Its practical recommendation, offspring of the theories
thus enunciated, was that the best protection against lightning
would be afforded by the most substantial metal rods, made of iron,
surrounding a building on all sides, and passing deep into the ground.
The new declaration of the ‘Académie des Sciences,’ though merely a
repetition of former reports, was not without important consequences.
First in France, and then in other countries, the conviction became
general among scientific men, and others well informed on the subject,
that well-designed conductors, if properly made and kept in good order,
form an absolute, unconditional, and infallible protection against
lightning.
Professor Pouillet also laid it down that lightning conductors, to be
efficient, must be regularly inspected, he, with his colleagues on the
committee, having come to the conclusion that such examination should
take place at least once every year. So much stress was laid upon the
importance of an annual inspection, that a strong recommendation was
made to the Government to have a _procès-verbal_, or special report,
drawn up on each occasion in the case of all public buildings, so that
it might be known by the central authorities whether the examination
had taken place at the specified time, and what had been the
declaration of the examiners. The advice was judiciously followed, with
the result that at this moment the public buildings of France have the
most complete protection against lightning--greatly in contrast with
the public buildings in England.