LIGHTNING CONDUCTORS.
Principles and Instructions relative to their application to Powder Magazines, by Sir W. Snow Harris, F.R.S. Extracted from Army List for July, 1859.
1.—Thunder and lightning result from the operation of a peculiar natural agency through an interval of the atmosphere contained between the surface of a certain area of clouds, and a corresponding area of the earth’s surface directly opposed to the clouds. It is always to be remembered that the earth’s surface and the clouds are the terminating planes of the action, and that buildings are only assailed by Lightning because they are points, as it were, in, or form part of, the earth’s surface, in which the whole action below finally vanishes. Hence buildings, under any circumstances, will be always open to strokes of Lightning, and no human power can prevent it, whether having Conductors or not, or whether having metals about them or not, as experience shows.
2.—Whenever the peculiar agency, (whatever it may be), active in this operation of nature, and characterized by the general term Electricity, or Electric Fluid, is confined to substances which are found to resist its progress, such, for example, as air, glass, resinous bodies, dry wood, stones, &c., then an explosive form of action is the result, attended by such an evolution of light and heat, and by such an enormous expansive force, that the most compact and massive bodies are rent in pieces, and inflammable matter ignited. Nothing appears to stand against it. Granite rocks are split open, oak and other trees, of enormous size, rent in shivers, and masonry of every kind frequently laid in ruins. The lower masts of ships of the line, 3 feet in diameter, and 110 feet long, bound with hoops of iron half an inch thick and 5 inches wide, the whole weighing about 18 tons, have been, in many instances, torn asunder, and the hoops of iron burst open and scattered on the decks. It is, in fact, this terrible expansive power which we have to dread in cases of buildings struck by Lightning, rather than the actual heat attendant on the discharge itself.
3.—When, however, the electrical agency is confined to bodies, such as the metals, which are found to oppose but small resistance to its progress, then this violent expansive or disruptive action is either greatly reduced, or avoided altogether. The explosive form of action we term Lightning, vanishes, and becomes, as it were, transformed into a sort of continuous current action, of a comparatively quiescent kind, which, if the metallic substance it traverses be of certain known dimensions, will not be productive of any damage to the metal. If, however, it be of small capacity, as in the case of a small wire, it may become heated and fused. In this case, the electrical agency, as before, is so resisted in its course as to admit of its taking on a greater or less degree of explosive and heating effect, as in the former case. It is to be here observed, that all kinds of matter oppose some resistance to the progress of what is termed the Electrical Discharge, but the resistance through capacious metallic bodies is comparatively so small, as to admit of being neglected under ordinary circumstances; hence it is that such bodies have been termed Conductors of Electricity, whilst bodies such as air, glass, &c., which are found to oppose very considerable resistance to electrical action, are placed at the opposite extremity of the scale, and termed Non-conductors or Insulators.
The resistance of a metallic copper wire to an ordinary electrical discharge from a battery, was found so small, that the shock traversed the wire at the rate of 576,000 miles in a second. The resistance however, through a metallic line of Conduction, small as it be, increases with the length, and diminishes with the area of the section of the Conductor, or as the quantity of metal increases.
4.—It follows from these established facts, that if a building were metallic in all its parts, an iron magazine for example, then no damage could possibly arise to it from any stroke of Lightning which has come within the experience of mankind; e.g., a man in armour is safe from damage by Lightning; in fact, from the instant the electrical discharge in breaking with disruptive and explosive violence through the resisting air, seizes upon the mass in any point of it, from that instant the explosive action vanishes, and the forces in operation are neutralized upon the terminating planes of action, viz., the surface of the earth, and opposed clouds.
5.—All this plainly teaches us, that in order to guard a building effectually against damage by Lightning, we must endeavour to bring the general structure as nearly as may be, into that passive or non-resisting state it would assume, supposing the whole were a mass of metal.