Lightning Conductors: Their History, Nature, and Mode of Application

by F.C.S. Richard Anderson

CHAPTER IX.
THE BEST MATERIAL FOR CONDUCTORS.

‘The art of protection against lightning,’ says a recent German writer, in a book on conductors, ‘is precisely the same now as it was a hundred years ago: still, it has made immense progress since that time.’ Though apparently involving a paradox, the words nevertheless are literally true. The art, or rather science, of guarding objects against the destructive effects of lightning is theoretically the same as it was in the days of Benjamin Franklin; nevertheless, the practical execution of the appliances necessary to attain this aim has undergone extraordinary improvements since that time. This has been due simply to the astounding progress of the metallurgical arts for the last forty or fifty years. With the help of machinery on a colossal scale, such as was never dreamt of before, our factories have come to produce metallic masses of dimensions and shapes such as make all former achievements of the kind appear utterly insignificant. We build huge iron ships, armed with cannon of ponderous weight; we throw iron bridges across rivers and arms of the sea; we lay metallic cables through the ocean and over the earth, encircling the globe. All these wonderful achievements, in which the development of engineering science went hand in hand with that of tool-making and the ever-growing employment of the power of steam, have gone to the constant improvement of lightning conductors. They have benefited, indirectly, in the result of great inventions, and of immense toil and labour, originally directed to other ends.

There is something half touching, half comical, in reading of the troubles which Benjamin Franklin had to undergo before he was able to set up his first lightning conductor. He could meet with no assistance but that of the blacksmith of little Philadelphia; and the ability of the latter in the art of forging iron rods more than a few feet in length was of the most limited kind. The ingenuity of Franklin overcame this difficulty by a variety of clever contrivances, such as connecting a number of small rods by caps and joints, fitting closely; but others were not so successful as he in the matter. Even in Paris there were no artisans to be found, for many years after lightning conductors were first recommended, able to make them, and foreigners, chiefly English, had to be brought there for the purpose. The difficulties arising from this backward state of the industrial arts were greatly increased by the belief, prevalent for a long time, that lightning conductors, to be efficient, ought to be of very great height, their so-called ‘area of protection’ being in proportion to their height. The supposition, originating in France, was carried to extremes in that country, chiefly through the teachings of M. J. B. Le Roy, a very able but eccentric man. Guided by vague analogies in electrical phenomena, M. Le Roy, who enjoyed in his time—the latter part of the eighteenth century—the reputation of being an authority on the subject of lightning conductors, laid it down as an indisputable fact that the ‘Franklin rods’ only protected buildings if rising high above them. He recommended the length of the rods above the chimney, or summit of any edifice, to be not less than fifteen feet, guaranteeing that, if of this height, they would offer absolute protection against lightning over an area of four times the same diameter—that is, sixty feet. Modern experience has proved this to be an absurdity; still, in the infancy of all knowledge about lightning conductors it was, perhaps, not unnatural that even learned men should believe in such fancies. Lightning was looked upon, not only in name but in reality, as an electric ‘fluid’ and the conductor was supposed to draw this ‘fluid’ from the clouds. Therefore it was but cogent reasoning to raise conductors as high above the roofs, and as near to the storm-clouds, as could possibly be done. If possessed of modern means for manufacturing pieces of metal of almost any length, M. Le Roy would not improbably have recommended to elevate lightning conductors a couple of hundred feet, instead of only fifteen, above the summit of buildings.

It was owing chiefly to the difficulty of forging long iron pieces, and of welding them together in a satisfactory manner, that, for many years after lightning conductors had been introduced into Europe, there were constant attempts made to find substitutes for the rods devised by Franklin. Chains were largely used towards the end of the last and the beginning of the present century, both in France and Germany, their employment having been suggested by the example of the English navy, where they were introduced, as already mentioned, upon the recommendation of Dr. Watson. The Continental mode of using iron chains for the protection of buildings against lightning was to hang them between the upper part of the conductor, surmounting the roof, which continued to be a straight piece of metal or rod, and the lower portion buried in the ground, sometimes, but not always, likewise a chain, but thicker than the rest. The characteristic of this method, and showing its long existence, is that it gave rise to a nomenclature existing to this day in France and Germany, where in all books on lightning conductors they are described as consisting of three distinct parts. The French call the upper part of the rod, over the roof, ‘la tige,’ the stem or stalk; and the Germans, ‘die Auffangstange’ literally the reception-rod. In both languages the middle part, from the roof downwards to the earth’s surface, is described as the conductor proper, ‘le conducteur’ and ‘der Leiter.’ Again, the lowest underground part of the conductor is designated, by the French, ‘la racine,’ the root, and by the Germans as ‘der Bodenleiter,’ or the ground-conductor. It has often been said that, as language springs from ideas, so it reacts upon them, and if the proposition be true, as most will admit, the French and German designations of the parts of lightning conductors—also to be found in Italian, and adopted in a few of the older English treatises on the subject, mostly translations—have a strongly misleading tendency. Nothing could be further from the truth than the assertion that a conductor ought to consist of three distinct parts. On the contrary, the more it is ‘one and undivided,’ the better it will be as a lightning protector.

The use of iron chains as conductors gave rise to very many fatal accidents, and for a time resulted in an outcry that the system itself could not be depended upon, as it was known to be not always efficacious. Lists were published of numerous instances in which buildings with what were supposed to be the best conductors were struck by lightning, from which it was argued that Franklin’s great discovery of the electric force always seeking a metallic path to the earth was a myth. It was not till some painstaking scientific men, deeply interested in the subject, had set to work to discover the causes of the failure, that the whole became plain enough. The chains, in some of the instances in which they had proved inefficient lightning conductors, were found to be corroded to such an extent as barely to hang together. Of course this corrosion would impair the efficiency of the conductor by reducing the quantity of metal; but the chief objection to the use of chains lies in the fact long ago pointed out by Mr. Newall, that even supposing a chain were formed of links of half-inch copper rods, and were perfectly bright and clean, the area of the conductor is reduced to a mere point where the links touch each other, and the resistance becomes so great in such a small conductor that instances have been recorded of the fusion of the links. In other cases, as in that of H.M.S. ‘Ætna’ in 1830, the chain was boomed out, and did not touch the water!

Simultaneously with the chains, there was trial made, in several Continental states, and also in England, of several other metallic conductors besides iron. Tin and lead had both their advocates, but the latter more than the former, on account of its far lower price. As regards tin, it had really no advantages whatever over iron, except pliability and non-oxidation. Against this was to be set that it was much more expensive than iron, with only about the same conducting power, according to Becquerel, Ohm, and other investigators. Professor Lenz, it is true, ranked tin very much higher, asserting, from experiments of his own, that its power of conductivity was nearly twice that of iron; and it was partly owing to his great influence that the metal obtained a trial in several countries, more particularly in Russia and in the United States of America. Still, the result was not satisfactory on many accounts, and its price alone brought tin to be soon abandoned as a conductor. Lead had a far longer trial. Its cheapness recommended it strongly, and equally so its extreme pliability. One of the greatest difficulties of the constructors of ‘Franklin rods,’ when first they came into demand, was to make the iron pieces fit properly around sharp corners of buildings, either by bending them in fire, or, as was more commonly done, soldering them together, or employing screws and other joints. But it was early discovered that these junctions, when occurring at acute angles, were bad conductors, occasioning sometimes the electric force to leave its traced course, and fly off in some other direction. It is probable that, in several well-authenticated instances in which this really did happen, the joints were eaten away by oxidation, as in the case of the chains; still, the effect of such occurrences was all the same. The joining of strips of lead together was a far easier task than that of handling iron in the same way, particularly for inexperienced workmen, and thus the employment of the metal continued for some time. However, it had to be abandoned gradually, on account of its manifest disadvantages. Its extreme softness, which made it liable to be broken by any accident, was one of them, and, still more so, its want of conducting power—only about one-half that of iron. Thus leaden conductors slowly went out of use, except in the form in which they still act often to great advantage, that of water-pipes.

Among all the experiments made for producing the most perfect lightning conductors, the one which created the greatest attention, some fifty years ago, both on the Continent and in England, was the employment of ropes made of brass wire. They were first recommended about the year 1815 by a professor at the University of Munich, J. C. von Yelin, distinguished for his researches into the nature of thunderstorms. Through his influence most of the public edifices of Bavaria, more particularly the churches, were provided with conductors of brass ropes; and within a few years their employment became so popular, owing to the ease with which they could be attached to all buildings, that even the Roman Catholic clergy changed their attitude, and, from being opposed to ‘heretical rods,’ advocated their extension in every direction. But it was not long before the trust in brass ropes as protectors against lightning was rudely shaken. Several instances occurred in which buildings so protected were struck and damaged by lightning, and at last there came a case which attracted the widest attention, leading, on account of its supposed importance, to the institution of a Royal Commission to report thereon. The little town of Rosstall, in Franconia, Bavaria, had a church the steeple of which was 156 feet high; and, standing on the brow of a hill, it overlooked the country far and wide, visible for many miles. Necessarily much exposed to the influence of lightning clouds, it had been provided with one of the best brass-wire conductors, designed by Professor von Yelin himself, and made of unusual thickness, being over an inch in diameter. Nevertheless, on the evening of April 30, 1822, while a dark storm-cloud, of extraordinary thickness, was passing over Rosstall, a heavy flash of lightning was seen to fall vertically upon the church steeple, followed by a terrible peal of thunder, which seemed to shake the earth. When people looked up they beheld the church clock thrown from its place, and part of a lower wall of the edifice thrown to the ground. It was clear that the electric discharge from the atmosphere had been one of unusual energy, but equally clear that the trusted conductor had not done its work.

It was partly through scientific controversies about the relative conducting value of metals, and partly through the action then taken by several German Governments of providing all buildings with lightning conductors, that the Rosstall case excited an extraordinary interest at the time. The Royal Commission appointed by the King of Bavaria, presided over by an eminent savant, Professor Kastner, went to Rosstall to inspect the effects of the lightning discharge, and Professor von Yelin did the same, as an independent, though not disinterested witness. Their reports as to actual facts were the same. The lightning, after striking the steeple of the church, had melted the top of the ‘Auffangstange,’ or highest part of the conductor, and further down had passed along the brass rope till coming to the clock, only a few inches distance from it. Here the electric force had evidently divided itself into several streams—the one exerting its disastrous effects upon the clock and brickwork, and several metallic objects underneath, and the other passing down the rope conductor, but not without bending it, and, in one or two places, tearing it to pieces. Such were the facts, visible to all eyes. But the conclusion drawn therefrom differed widely. The members of the Royal Commission made it public that the reason of the Rosstall lightning conductor not having been efficient had simply arisen from its nature. Brass-wire ropes, they declared, though perhaps useful against small discharges of electricity, formed no reliable safeguards against powerful ones; and they therefore strongly advised a return to the old-fashioned iron rods. The conclusion was vehemently disputed by Professor von Yelin. He admitted that it might be better, to provide for the proper discharge of extraordinary masses of the electric force, to make his brass ropes, when applied to high churches and other large edifices, even thicker than they had been at Rosstall; but at the same time he utterly denied that, even in this case, they had been the origin of the disaster. He showed that the real cause of it was that the conductor had not been laid deep enough into the ground, so as to touch moist earth. The church stood upon sandy soil, on an eminence, and to touch ‘good earth’ the brass rope ought to have been sunk down to a depth of at least fifty feet, whereas it did not reach one-third of that depth. The professor was undoubtedly right, but his antagonists nevertheless prevailed. A public prejudice, which no argument could overcome, set in against brass-wire conductors, and they were pulled down from nearly all buildings on which they had been laid, to be replaced by iron rods. Some time had to elapse before real justice was done to metallic ropes as lightning conductors.

With our present knowledge of electrical phenomena, and the practical art of making conductors, it may safely be affirmed that the Munich professor was right in recommending ropes, though not in approving of brass as the best metal. In its very nature, brass, a compound, can never be thoroughly reliable, because its conducting power varies according to its composition. The facility with which it allows the electric force to pass through it depends, in fact, entirely on the amount of copper which brass contains, and is greater or less accordingly, since the other metal entering into its composition, zinc, has less than one-third the same conductivity. Now brass is, for various purposes, made sometimes of 70 parts of copper and 30 parts of zinc, and again, only equal amounts of both metals, setting calculation as to its conducting power entirely at nought. But besides this, brass has the great fault of being excessively liable to destruction by atmospheric influences, and it was found, among others, in Germany, that while brass ropes were used as lightning conductors, they were frequently destroyed, in a comparatively short space of time, by the action of smoke alone. It is true, the Continental mode, existing both in France and Germany, of spanning conductors over the tops of chimneys—illustrated in the engraving here as a warning ‘how not to do it’—had much to answer for this atmospheric deterioration, since even tougher metals than brass could not be expected to stand the constant action of smoke, often containing sulphurous fumes. But even without such an evidently absurd arrangement as that of running any conductors, whether in the form of ropes or cords, across the orifices of chimneys, brass could never have answered all the requirements of a lightning conductor. It was with justice that brass-wire ropes were nearly altogether discarded some thirty or forty years ago, after having had a short-lived reputation.