PROTECTION OF BUILDINGS FROM LIGHTNING. 155 



atmospheric accumulation and the earth. The erection of the conduct- 

 or 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 dis- 

 charges 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 experi- 

 ment 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 be- 

 tween 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 touch- 

 ing the wall might receive a shock, but no electrical effect would be 

 perceived inside, even on the most delicate electrometer. The poten- 

 tial of everything inside with respect to the earth w T ould 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- 



