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-conduct-