ON THE PROTECTION OF BUILDINGS FROM LIGHTNING. 539 



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 shewn 

 by experiment that if eveiy 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 con- 

 ducting 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 

 thunderstorm outside. There would be no need of any earth connection. We 

 might even place a layer of asphalte 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 poten- 

 tial 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 con- 

 dition, 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 con- 

 ductor 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 con- 

 ductors, 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 appa- 

 ratus 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 conduct- 

 ing substance. For instance, if a copper wire, say No. 4, B.W.G. (0'238 inches 

 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 



682 



