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

by F.C.S. Richard Anderson

Fig. 37.

He says: ‘What we really wish to prevent is the possibility of an electric discharge taking place within a certain region—say, in the inside of a gunpowder manufactory. If this is clearly laid down as our object, the method of securing it is equally clear.

‘An electric discharge cannot occur between two bodies unless the difference of their potentials (i.e. their electrical conditions) is sufficiently great, compared with the distance between them. If, therefore, we can keep the potentials of all bodies within a certain region equal, or nearly equal, no discharge will take place between them. We may secure this by connecting all these bodies by means of good conductors, such as copper wire ropes, but it is not necessary to do so, for it may be shown by experiment that if every part of the surface surrounding a certain region is at the same potential, every point within that region must be at the same potential, provided no charged body is placed within the region.

‘It would therefore be sufficient to surround our powder-mill with a conducting material, to sheath its roof, walls, and ground-floor with thick sheet-copper, and then no electrical effect could occur within it on account of any thunderstorm outside. There would be no need of any earth connection. We might even place a layer of asphalt between the copper floor and the ground, so as to insulate the building. If the mill were then struck with lightning, it would remain charged for some time, and a person standing on the ground outside and touching the wall might receive a shock, but no electrical effect would be perceived inside, even on the most delicate electrometer. The potential of everything inside with respect to the earth would be suddenly raised or lowered as the case might be; but electric potential is not a physical condition, but only a mathematical conception, so that no physical effect would be perceived.

‘It is therefore not necessary to connect large masses of metal, such as engines, tanks, &c., to the walls, if they are entirely within the building. If, however, any conductor, such as a telegraph-wire, or a metallic supply-pipe for water or gas, comes into the building from without, the potential of this conductor may be different from that of the building, unless it is connected with the conducting shell of the building. Hence the water or gas supply-pipes, if any enter the building, must be connected to the system of lightning conductors; and since to connect a telegraph-wire with the conductor would render the telegraph useless, no telegraph from without should be allowed to enter a powder-mill, though there may be electric bells and other telegraphic apparatus within the building. I have supposed the powder-mill to be entirely sheathed in thick sheet copper. This, however, is by no means necessary in order to prevent any sensible electrical effect taking place within it, supposing it struck by lightning. It is quite sufficient to enclose the building with a network of a good conducting substance. For instance, if a copper wire, say No. 4, B. W. G. (0·238 inch diameter) were carried round the foundation of the house, up each of the corners and gables, and along the ridges, this would probably be a sufficient protection for an ordinary building against any thunderstorm in this climate. The copper wire may be built into the wall to prevent theft, but should be connected to any outside metal, such as lead or zinc on the roof, and to metal rain-water pipes. In the case of a powder-mill, it might be advisable to make the network closer by carrying one or two additional wires over the roof and down the walls to the wire of the foundation. If there are water or gas-pipes which enter the building from without, these must be connected with the system of conducting wires; but if there are no such metallic connections with distant points, it is not necessary to take any pains to facilitate the escape of the electricity into the earth; still less is it advisable to erect a tall conductor with a sharp point in order to relieve the thunder-clouds of their charge.

Fig. 38. ARRANGEMENT OF PROFESSOR CLERK MAXWELL’S LIGHTNING CONDUCTORS.

‘It is hardly necessary to add, that it is not advisable, during a thunderstorm, to stand on the roof of a house so protected, or to stand on the ground outside, and lean against the wall.’

Prof. Clerk Maxwell, in a letter to Mr. Charles Tomlinson, F.R.S., the author of ‘The Thunderstorm,’ says: ‘My plan is to convert a building into a closed conducting vessel by a sufficient number of wires enclosing it. For an ordinary house, a skeleton of its edge is quite enough. A a may be a zinc ridge, B b and C c water-gutters of zinc or iron; but the pieces A B D, A C E, a b d, a c e, and the circuit D E e d should be of stout copper wire or rope, built into the wall as a security against theft, but connected to every other piece of metal on the outer surface of the house, and to every gas or water-pipe which enters the house from without, but not to any masses of metal wholly within the whole, unless this is desirable for other purposes.’