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

by F.C.S. Richard Anderson

‘The excavation of the ditch at the foot of the lightning conductor could not be the direct effect of electricity, but would be the result of the sudden evaporation of the moisture of the ground, generating steam, and forming, as it were, a mine.

‘The breaking of the tube is most singular. It seems to me that it can with difficulty be attributed to the mechanical shock of the electricity itself. As the lead which united the broken tube to the one beyond was found melted, it is evident that, in spite of the water which flowed in this tube, it was raised to an enormous temperature in the place where it was struck, and probably it was the instantaneous evaporation of the water inside which caused the breaking of the tube.

‘But the most singular fact, in a certain respect, is what was observed in the tube which descends to Alatri—that is to say, the alteration in form of the leaden slab. The little interruption which necessarily exists in this tank between the conducting-pipe and the metallic receptacle evidently gave occasion for a discharge by a flash, and, in consequence, for an explosion of steam. But we see at the same time by that that the distance traversed in the tube from the building to the slab, a distance of more than 200 metres [650 feet], in which the pipe is buried underground, did not suffice for the charge to lose itself in the ground, although during the passage it had to cross the reservoir, and might there have distributed itself. Our surprise is still greater when we reflect that it was only part of the discharge, since the greater portion had to flow by the water-pipe of Ferentino, which was the first struck in a direct manner, and that these pipes are joined together with lead. The quantity of electricity must have been enormous, in order to be able to have so much force and to run another 300 metres [975 feet] to reach the public fountain, and leave its traces there. A circumstance which deserves attention is, that this storm took place after a long and constant drought; and consequently the earth was less moist, and could offer little facility for dispersion.

‘These cases are not so rare among us as one might suppose. Not very long ago, at Lavinia, a flash of lightning destroyed a great part of the belfry, passed to the bell, broke and melted it in its passage in such a manner that the metal had run away like wax. I do not believe this breakage of the bell to have been a mechanical effect of the lightning in a rigorous sense, for the bell could have been broken by the instantaneous expansion produced by the heat at the point of the passage, an expansion which had had no time to disperse, as a glass vase breaks when touched with a red-hot iron.

‘Let these facts come about how they may, they enable us to see that it is necessary to devote great attention in the erection of lightning conductors, that we must allow them a large surface for discharge, and that there can never be too much of it. The surface of the foot of our lightning conductor was certainly superior to what has been judged sufficient by Matteucci for the discharges of telegraphic conductors, and yet it has not sufficed. Further, it is a confirmation of the necessity of making the neighbouring metallic masses communicate, and especially with water and gas-pipes.’

From out of the almost endless number of cases in which lightning conductors failed for want of a good earth connection, another one or two may be given, illustrated as having happened quite recently in England, and as such showing, in a very striking manner, in what a neglected state the knowledge of the subject still is at this moment. A thunderstorm passed over the town of Clevedon, Somersetshire, in the afternoon of March 15, 1876, and a flash of lightning fell upon the steeple of Christchurch, provided, as was generally thought, with a most efficient conductor of recent construction, made of good copper rope. What happened is graphically and minutely told in a letter addressed to the ‘Journal of the Society of Telegraph Engineers,’ by Mr. Eustace Buttor, of Lewesfell, near Clevedon. ‘There was but a single flash,’ Mr. Buttor relates, ‘which appeared to many observers to travel horizontally through the air. However, the lightning passed down the lightning conductor of Christchurch. The flag-staff, about 100 feet high, and the four pinnacles, about 90 feet high, have each a conductor, the flag-staff having the usual conical point, the pinnacles having the copper rope attached to their vanes. The five copper ropes unite inside the tower in the neighbourhood of the clock. Lower down the conductor passes through a slanting hole to the outside, and for the lowest 12 feet is encased in a pipe. On reaching the ground it passes into a dry freestone channel for about a dozen feet, and then dips down into the drain which carries rain-water from the roof. As no rain preceded or accompanied the flash, it may be presumed that the drain was dry.

‘The protector is copper throughout, and, with the exception of the termination, seems to have been carefully and efficiently placed. The diameter I estimate to be half-inch, or it may be a trifle more. Just at the point where it leaves the pipe and enters the ground, the electric charge left it, dashed through three feet or more of solid wall supporting the tower, in order to reach the gas-meter inside, then it passed safely along the gas-pipe. The cavity made was considerable, but very irregular. I was unable to ascertain when the workmen were engaged in repairs, and therefore cannot give their estimate of the weight of stone displaced, but it must have been many hundredweights, though only a few pounds were actually thrown out on to the path, or inside into the vault. A large quantity of stone was pulverised, and the whole gave one the idea of the explosion of a charge of gunpowder under great compression. In a house about 100 yards from the church, the inmates felt the shock intensely, but did not know that the house had been touched. Some hours after, however, on going to turn on the gas, a hissing noise was heard, and a hole was found in the composition gas-pipe, about five-eighth inch diameter, just where the pipe passed within an inch of a water-pipe. The lightning must have come along the main from the church gas-pipe to this house, and then passed to the water-pipe as the readiest way to moist earth. The whole soil in the neighbourhood is mountain limestone, very dry. There is not the slightest evidence of displaced plaster, or any other sign of the passage of an electrical discharge through the house.’ There need be little comment on the facts stated in this letter, notable though they are. It is the old delusion that a lightning conductor need be brought down underground only, and that then all is right. In this case, those who protected Christchurch, Clevedon, thought it quite sufficient to bring the conductor down into a drain-pipe carrying rain-water from the roof, without reflecting for a moment that an earthenware drain-pipe would insulate the conductor from ‘earth.’ A similar instance came under the writer’s notice about a year ago. One of the pinnacles of Cromer Church, in Norfolk, was struck by lightning, although fitted with a conductor on one of the pinnacles. On examination it was discovered that the earth terminal had been inserted into an earthenware drain.

It is not very easy to give exact prescriptions as to the best manner in which the underground connection should be effected. The means vary entirely with the circumstances, and the matter should in all cases be intrusted to an expert. Simple as is the whole theory of lightning protection, consisting in nothing else but laying a good metallic path from the top of a building down into moist earth, as an unfailing path for the electric force, the practical execution of it is not the less often very complicated. It is especially so as regards the most important of points, that of the underground connection. Of course, wherever there is running water at hand, a river, or even a tiny stream that never dries, the matter is easy enough, but as in the great majority of buildings to be protected such water does not exist, the solution of the question becomes more difficult, and frequently one of the greatest perplexity. It tends even to be more and more so in consequence of the progress of sanitary arrangements under which towns and villages are ‘drained’ until the soil has been made as dry as a rock. Immense as the benefit is to public health, it is, like all benefits, attended by certain drawbacks. One of these certainly is a greater danger from lightning. It is often proposed by builders to use the drain-pipes themselves in making ‘good earth’ for lightning conductors, but the fallacy of this recommendation need scarcely be exposed, seeing that these conduits are generally made of earthenware, as happened when Christchurch, Clevedon, was struck by lightning.

While broad rules cannot be laid down, still it may be affirmed that a good earth connection, sufficient to carry off the heaven’s electric discharges, may always be obtained by either of two means. The first, and in all cases most preferable, is to lay the conductor deep enough into the ground to reach permanent moisture. When this exists in a considerable mass, the single conducting rope, touching it, will be quite sufficient; but when the quantity is deficient, or doubtful, it will certainly be advisable to spread out the rope, so as to run in various directions, similar to the root of a tree, likewise in search of moisture. There are various modes of accomplishing this, shown in [figs. 46 and 47].