Popular Science Monthly/Volume 10/December 1876/The Protection of Buildings from Lightning
By Professor J. CLERK MAXWELL.
Most of those who have given directions for the construction of lightning-conductors have paid great attention to the upper and lower extremities of the conductor. They recommend that the upper extremity of the conductor should extend somewhat above the highest part of the building to be protected, and that it should terminate in a sharp point, and that the lower extremity should be carried as far as possible into the conducting strata of the ground, so as to "make" what telegraph engineers call "a good earth."
The electrical effect of such an arrangement is to tap, as it were, the gathering charge by facilitating a quiet discharge between the atmospheric accumulation and the earth. The erection of the conductor will cause a somewhat greater number of discharges to occur at the place than would have occurred if it had not been erected; but each of these discharges will be smaller than those which would have occurred without the conductor. It is probable, also, that fewer discharges will occur in the region surrounding the conductor.
It appears to me that these arrangements are calculated rather for the benefit of the surrounding country, and for the relief of clouds laboring under an accumulation of electricity, than for the protection of the building on which the conductor is erected.
What we really wish is, to prevent 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 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-wife 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 sheathe its roof, walls, and ground-floor, with thick sheet-copper, and then no electrical effect could occur within it on account of any thunder-storm 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, etc., 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 he allowed to enter a powder-mill, though there may he electric bells and other telegraphic apparatus entirely 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 inclose 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 thunder-storm 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 at 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.
It is hardly necessary to add that it is not advisable, during a thunder-storm, to stand on the roof of a house so protected, or to stand on the ground outside and lean against the wall.—Nature.