44 REPORT — 1876. 



relief of clouds labouring under an accumulation of electricity, tlian for the pro- 

 tection of tlie 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, 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 suificiently great compared with the distance between them. If, 

 therefore, we can heep the potentials of all bodies within a certain region equal or 

 nearly equal, no discharge will take place between them. We may secure this hj 

 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 cei-tain 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 oiu- 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-connexion. We might even place a layer 

 of asphalt between the copper floor and the groimd, 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 every thing 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 could 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 svipply-pipe for 

 water or fias, comes into the building from without, the potential of this conductor 

 may be diflerent 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 elec- 

 tric bells and other telegi-aphic 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 necessaiy in order to prevent any sensible electric 

 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 in- 

 stance, if a copper wire, say No. 4, B.W.G. (0-238 inch in diameter), were carried 

 round the foundation of a 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 thimder-storm in this climate. The copper wire may be built into the 

 wall to prevent theft, but it 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 sj'stem of conducting-wires ; but if there 

 are no such metallic connexions with distant points, it is not necessary to take any 

 pains to facilitate the escape of the electricity into the earth. 



It is desirable, however, to provide for the safety not only of the building itself, 

 but of the system of conductors which protects it. The only parts of this system 

 which are in any danger are the points where the electricity enters and leaves it. 

 If, therefore, the system terminates above in a tall rod with a sharp point, and 

 do-svnwards in an " earth wire," the external discharge will be almost certain to 

 occur at the ends of these electrodes, and the only possible dan;nge ■^^•ill be the loss 



