He continued: ‘When, however, the electrical agency is confined to bodies, such as the metals, and 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.’
After these theoretical explanations, Mr. Snow Harris went into the practical part of the business of protecting buildings, and, more especially, powder magazines and others containing explosive materials, against the effects of lightning. ‘It follows,’ he remarked,’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. 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 neutralised upon the terminating planes of action--viz., the surface of the earth and opposed clouds. 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. To this end, one or more conducting channels of copper, depending upon the magnitude and extent of the building, should be systematically applied to the walls. These conducting channels should consist either of double copper plates, united in series one over the other, as in the method of fixing such conductors to the masts of her Majesty’s ships, the plates being not less than 3½ inches wide, and of 1/16th and ⅛th of an inch in thickness; or the conductors may with advantage be constructed of stout copper pipe, not less than 1/16th of an inch thick, and 1½ to 2 inches in diameter; in either case the conductors should be securely fixed to the walls of the building, either by braces, or copper nails, or clamps. They should terminate in solid metal rods above, projecting freely into the air, at a moderate and convenient height above the point to which they are fixed, and below they should terminate in one or two branches leading outward about a foot under the surface of the earth; if possible, they should be connected with a spring of water or other moist ground. It would be proper, in certain dry situations, to lead out, in several directions under the ground, old iron or other metallic chains, so as to expose a large extent of metallic contact in the surface of the earth.’