An epitome of electricity & galvanism

by Jacob Green; Erskine Hazard

6th. The electricity of the kite is generally stronger or weaker, according as the string is longer or shorter; but it does not keep any exact proportion to it; the electricity, for instance, brought down from a string of an hundred yards, may raise the index of an electrometer to 20°, when with double the length of string, the index of an electrometer will not go higher than 25°.

7th. When the weather is damp, and the electricity pretty strong, the index of an electrometer, after taking a spark from the string, or being presented to the knob of a coated phial, rises surprisingly quick to its former place; but in dry and warm weather it rises exceedingly slowly.

CHAP. XV.
The structure and use of lightning rods.

Since the discovery of the identity of lightning and the electric matter, long rods of iron, or other metals, have been made use of, with a view to protect buildings from the effects of lightning. This is the most practical and important part of our whole subject, and deserves to be treated with the utmost attention. Iron and copper are the metals which, on account of their conducting power, their cheapness, and the quantity required for a lightning rod, are principally used. Copper is preferable to iron. Care should be taken that the rod be not less than half an inch in diameter. It is best to have it, if possible, of one continued piece. If this be not practicable, the pieces should be screwed into each other; or at least so constructed that the rust will not separate the perfect metal of one piece from that of another; because metallic rust is almost a non-conductor of electricity. The rod should be fastened to the house by wooden cramps or staples, rather than by those of metals of any kind; because wood is neither so good a conductor of electricity, nor so likely to promote the rust of the metal which it touches. The rod should be raised above the top of the building or chimney to which it is attached, at least five or six feet. The point or points should be made very sharp, and for a few inches should taper off in the form of a pyramid, having all the corners or edges sharp. It is not of much importance whether there be, or be not, more points than one. If the means afterwards to be mentioned be not used to preserve the points from rust, it may be of use to gild them; and the gilding should extend downwards a foot or more. It is better to paint the point of a rod, than to leave it wholly unprotected against rust. The lower end of the rod should be driven or sunk at least five or six feet into the ground, and in a direction from the building. If it can be connected with the water of a spring, a well, or a cistern, it will be so much the better. At powder-mills, arsenals, and all depots of inflammable materials, it is better to attach the rod to a post, raised for the purpose, a foot or two from the building, than to the building itself. If the building be large, there should be a rod at each end; and it is an additional security, if these rods be connected by a piece of metal, running from the one to the other, on the roof of the house. If there be but one rod, it should, in this country, be put on the western end of the house; because thunder storms oftenest arise from that quarter. If the position of the house affords but little choice in this respect, the rod should be placed either on the kitchen chimney, or as near to it as possible; because smoke and heat are conductors, and in the summer, smoke and heat seldom ascend from any other chimney than that of the kitchen. When there is a copper spout to a house, the rod, if convenient, may be connected with it as a part of the conductor. In this case however, care should be taken to make the connexion complete, both at top and bottom. Large barns and barracks, have even more need of a rod to preserve them from lightning than a dwelling house, because the vapour which ascends from them when filled with vegetable substances, imperfectly dried, is a powerful conductor.

Ships, and all vessels which have high masts, have as much need of conductors as houses on the land. Copper conductors are in every view the best for ships, as they will not contract rust from sea water. A conductor, of this metal, should be attached to the highest mast of the vessel, and extend three or four feet above its top. It should be inserted into the side of the mast, so as to leave the surface smooth, be carried across the deck and over the side of the ship to the keel; so that it may terminate where the lower extremity may be always under the water. Chains are often used as conductors to ships, but they are far inferior to a piece of metal, whose parts are not separated.

In the above directions it has been our aim to show in what manner structures may be best and most effectually protected against danger from lightning, and whenever it is practicable the best means ought certainly to be used. But it is to be remembered that where means the most effectual cannot be applied, those of an inferior kind are not to be neglected. A small rod, however pointed or fastened to a house, is unspeakably better than none, and a chain should always be used in a ship, if a rod cannot be obtained. In ninety nine cases out of a hundred, any metallic conductor, reaching from the top to the bottom of a structure, will preserve it from destruction by lightning, and save the lives or property of the inhabitants, when the whole might otherwise have been destroyed.

The points of rods have often been found melted by lightning, and both they and the lower extremities are often injured by rust. For an effectual method of preventing both these inconveniencies, the public are indebted to Robert Patterson Esq. professor of mathematics in the University of Pennsylvania, and director of the Mint of the United States.—His memoir on the subject is as follows:—

“From the instances which now and then occur of houses being struck with lightning, that are furnished with metallic conductors, and the frequent instances of these conductors having their tops melted off by a stroke of lightning, it appears that this admirable contrivance for guarding houses against the dangerous effects of lightning is, in some degree, still imperfect. Some improvement seems yet to be wanting at both extremities of the rod—at the upper extremity, to secure it against the accident of being melted, which renders it afterwards unfit to answer its original intention, viz. drawing off the electricity, or lightning, from the passing cloud, in a silent imperceptible manner, for it is only pointed conductors that possess this property—and at the lower extremity, to afford a more ready passage for the fluid into the surrounding earth.

The first of these intentions, would I am persuaded, be effectually answered by inserting in the top of the rod a piece of black lead, of about two inches long, taken out of a good pencil, and terminating in a fine point, projecting but a very little above its metallic socket; so that if the black lead point should happen to be broken off by any accident, of which however I think there can be but little danger, still the point of the rod would be left sharp enough to answer the purpose of a metallic conductor.

This substance is well known to be infusible, by the greatest heat, and hence its use in making crucibles; nor is it evaporable as remarked by Cronstedt, in his mineralogy, Sect. 231, except in a slow calcining heat, to which it could never be exposed at the top of a lightning rod.