Lightning, Thunder and Lightning Conductors

by Gerald Molloy

THE RETURN SHOCK ILLUSTRATED.

Let us suppose ABC to represent the outline of a thundercloud which dips down toward the earth at A and at C. The electricity of the cloud develops by inductive action a charge of the opposite kind in the earth beneath it. But the inductive action is most powerful at E and F, where the cloud comes nearest to the earth. Hence, bodies situated near these points may be very highly electrified as compared with bodies at a point between them, such as D. Now, when a flash of lightning passes at E, the under part of the cloud is at once relieved of its electricity, its inductive action ceases, and, therefore, a person situated at F suddenly ceases to be electrified. This sudden change from a highly electrified to a neutral state involves a shock to his system which may be severe enough to stun or even to kill him. Meanwhile, people at D, having been also electrified to some extent by the influence of the thundercloud, must in like manner undergo a change in their electrical condition when the flash of lightning passes, but this change will be less violent because they were less highly electrified.

Many experiments have been devised to illustrate this theory of Lord Mahon. But the best illustration I know is furnished by this electric machine of Carré’s. If you stand near one end of the large conductor when the machine is in action and sparks are taken from the other end, you will feel a distinct electric shock every time a spark passes. The large conductor here takes the place of the cloud, the spark that passes at one end represents the flash of lightning, and the observer at the other end gets the return shock, though he is at a considerable distance from the point where the flash is seen.

An experiment of this kind, of course, cannot be made sensible to a large audience like the present. But I can give you a good idea of the effect by means of this tuft of colored papers. While the machine is in action I hold the tuft of papers near that end of the conductor which is farthest from the point where the discharge takes place. You see the paper ribbons are electrified by induction, and, in virtue of mutual repulsion, stand out from one another “like quills upon the fretful porcupine.” But, when a spark passes, the inductive action ceases, the paper ribbons cease to be electrified, and the whole tuft suddenly collapses into its normal state.

While fully accepting Lord Mahon’s theory of the return shock as perfectly good so far as it goes, I would venture to point out another influence which must often contribute largely to produce the effect in question, and which is not dependent on the form of the cloud. It may easily happen, from the nature of the surface in the district affected by a thundercloud, that the point of most intense electrification—say E in the figure—is in good electrical communication with a distant point, such as F, while it is very imperfectly connected with a much nearer point, D. In such a case it is evident that bodies at F will share largely in the highly-electrified condition of E, and also share largely in the sudden change of that condition the moment the flash of lightning passes; whereas bodies at D will be less highly electrified before the discharge, and less violently disturbed when the discharge takes place.

This principle may be illustrated by a very simple experiment. Here is a brass chain about twenty feet long. One end of it I hand to any one among the audience who will kindly take hold of it; the other end I hold in my hand. I now stand near the conductor of the machine; and will ask some one to stand about ten feet away from me, near the middle of the chain, but without touching it. Now observe what happens when the machine is worked and I take a spark from the conductor: My friend at the far end of the chain, twenty feet away, gets a shock nearly as severe as the one I get myself, because he is in good electrical communication with the point where the discharge takes place. But my more fortunate friend, who is ten feet nearer to the flash, is hardly sensible of any effect, because he is connected with me only through the floor of the hall, which is, comparatively speaking, a bad conductor of electricity.

Summary.—Let me now briefly sum up the chief destructive effects of lightning. First, with regard to good conductors: though it passes harmlessly through them if they be large enough to afford it an easy passage, it melts and converts them into vapor if they be of such small dimensions as to offer considerable resistance. Secondly, lightning acts with great mechanical force on bad conductors; it is capable of tearing asunder large masses of masonry, and of projecting the fragments to a considerable distance. Thirdly, it sets fire to combustible materials. And lastly, it causes the instantaneous death of men and animals.

Franklin’s Lightning Rods.—The object of lightning conductors is to protect life and property from these destructive effects. Their use was first suggested by Franklin, in 1749, even before his famous experiment with the kite; and immediately after that experiment, in 1752, he set up, on his own house, in Philadelphia, the first lightning conductor ever made. He even devised an ingenious contrivance, by means of which he received notice when a thundercloud was approaching. The contrivance consisted of a peal of bells, which he hung on his lightning conductor, and which were set ringing whenever the lightning conductor became charged with electricity.

Franklin’s lightning rods were soon adopted in America; and he himself contributed very much to their popularity by the simple and lucid instructions he issued every year, for the benefit of his countrymen, in the annual publication known as “Poor Richard’s Almanac.” It is very interesting at this distance of time to read the homely practical rules laid down by this great philosopher and statesman; and, though some modifications have been suggested by the experience of a hundred and thirty years, especially as regards the dimensions of the lightning conductor, it is surprising to find how accurately the general principles of its construction, and of its action, are here set forth.

“It has pleased God,” he says, “in His goodness to mankind, at length to discover to them the means of securing their habitations and other buildings from mischief by thunder and lightning. The method is this: Provide a small iron rod, which may be made of the rod-iron used by nailors, but of such a length that one end being three or four feet in the moist ground, the other may be six or eight feet above the highest part of the building. To the upper end of the rod fasten about a foot of brass wire, the size of a common knitting needle, sharpened to a fine point; the rod may be secured on the house by a few small staples. If the house or barn be long, there may be a rod and point at each end, and a middling wire along the ridge from one to the other. A house thus furnished will not be damaged by lightning, it being attracted by the points and passing through the metal into the ground, without hurting anything. Vessels also having a sharp-pointed rod fixed on the top of their masts, with a wire from the foot of the rod reaching down round one of the shrouds to the water, will not be hurt by lightning.”