CHAPTER VII.

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.