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

by F.C.S. Richard Anderson

‘Lightning,’ Professor Arago tells his story, ‘having struck a rather thick rod erected on a Mr. Raven’s house, in Carolina, United States, afterwards ran along a wire carried down the outside of the house to connect the rod on the roof with an iron bar stuck in the ground. The lightning in its descent melted all the part of the wire extending from the roof to the ground-storey, without injuring in the least the wall down which the wire was carried. But at a point intermediate between the ceiling and the floor of the lower storey things were changed: from thence to the ground the wire was not melted, and at the spot where the fusion ceased the lightning altered its course altogether, and, striking off at right angles, made a rather large hole in the wall and entered the kitchen. The cause of this singular divergence was readily perceived, when it was remarked that the hole in the wall was precisely on a level with the upper part of the barrel of a gun which had been left standing on the floor leaning against the wall. The gun barrel was uninjured, but the trigger was broken, and a little further on some damage was done in the fire-place.’ Commenting upon this case, Professor Arago goes on: ‘Here the lightning went off horizontally through the wall, in order to strike a fowling-piece standing upright in the kitchen. How much injury might not have resulted from this lateral movement, if the lightning had not had to traverse a thick wall?’ Consequently, he argues, Voltaire is right in his jocular-oracular declaration about the perils of indoor lightning conductors, in their being ‘great lords’ dangerous to approach.

It is really difficult to understand how a man like Professor Arago could be misled into such false reasoning as this about an accident which, in itself, was of the simplest, and of the very easiest explanation. That the stroke of lightning falling upon Mr. Raven’s house, in Carolina, should have melted the wire of the conductor points to one cause, and to one only, namely, that there was no proper earth connection. Had it existed, the wire, although thin, could not possibly have been ‘melted all the part extending from the roof to the ground-storey,’ nor could the electric force have left its appointed path to seek a better one through a wall, and, still more astounding, ‘striking off at right angles.’ It is abundantly clear that such cases, and others to the same effect, brought against the fixing of lightning-conductors inside the walls of buildings, prove absolutely nothing. What is beyond controversy is, that a good conductor, in proper condition, is absolutely harmless to surrounding objects, including human beings. A man, even with a ‘fowling-piece’ in his hands, might lean full length against half-an-inch copper rod carrying off a heavy stroke of lightning into ‘good earth’ without so much as becoming aware of the passing of the electric discharge. If certainly a ‘grand seigneur,’ as Voltaire remarks, the electric force has this in common with some of the greatest of men, of not wasting its time, but following a clear aim.

Fig. 31.

Fig. 32.

A very common, and, it may be added, a very mischievous opinion is prevalent, that lightning conductors should be carefully insulated from the buildings to which they are attached, and consequently many conductors are made to pass through insulators of glass and other materials of low conductivity. This practice of separating the building from the lightning conductor is not only utterly useless but positively dangerous. It is not unusually thought that by insulating the conductor the electric discharge will be prevented from entering the building. Such an idea is ipso facto absurd, for it is preposterous to suppose that a flash of lightning which can travel through thousands of feet of air—itself a very bad conductor of electricity—and then shatter to pieces the most compact bodies, would be stopped in its course by means of a few inches of glass, or a few feet of air. It may therefore be confidently asserted that no insulator can possibly be made that would be capable of preventing the electric discharge leaving the lightning conductor provided it could find an easier path leading to the earth. Mr. Phin, in his work on ‘Lightning-Rods’ says very pertinently:—But not only are insulators worthless—they are positively dangerous if the principle upon which they are adopted is fully carried out, which, however, is but rarely done. A little consideration will show this. Thus, if a house be furnished with a carefully-insulated lightning-rod, and should also have any large surface of metal, such as a tin roof, an extensive system of gutters, or such like, connected with it, it is easy to see that the house must resemble a large Leyden jar, of which the tin roof, or other mass of metal, constitutes one coating, and the lightning-rod and the earth constitute the other, while the insulators and the dry material of the house represent the glass of the jar. If both the outside and the inside of this jar (the tin roof and the earth) had been connected together, it would have been impossible to have brought one coating into a condition opposite to that of the other. But the rod being carefully insulated from the roof, it is obvious that the inductive action of the cloud will bring the roof and the earth into opposite conditions; and if a man were to form the path of least resistance between them, the discharge would take place through his body, and he would probably be destroyed. It is obvious, then, in the first place, that lightning-rods should be connected with all large masses of metal which may exist in or upon the house, such as metallic roofs, tin or iron gutters, or pipes, iron railings, &c. In the second place, the rod should be attached to the house in the neatest and least obtrusive manner possible.’

Fig. 33.

It is indeed desirable for various reasons that the copper rope or band forming the lightning conductor should be affixed to the building in the neatest and least obtrusive manner possible. The conductor may be fastened by means of ordinary metal staples made of stout copper wire. A better method however is indicated in [figs. 31] and [32], one showing the rope conductor formed of forty-nine wires, usually employed by Messrs. R. S. Newall and Co. for the protection of ordinary houses and buildings, and the other the copper band used by them for the same purpose. This fastening is simply a strap of copper bent to the required shape and pierced with two holes, by means of which it is fixed to any building by copper nails or screws. This method possesses several advantages; it is very sightly and neat, it can be easily applied without injury to any building, and as it allows the conductor a certain freedom of movement, it readily permits the contraction and expansion caused by the variations of temperature. The band conductor shown here is one inch wide by one-eighth of an inch thick, and weighs ·44 pound per foot. The rope conductor, although it appears less, has more metal in it; it measures five-eighths of an inch in diameter, and weighs ·67 pound. [Fig. 33] shows a different mode of attaching the lightning conductor. It is generally used for the heavier ropes.