and Gunpowder Magazines. 375 



ductors. But it is not necessary to do so ; for it may be 

 shown experimentally that if every part of the surface sur- 

 rounding a certain space is at the same potential, every point 

 within that space must be at the same potential, provided no 

 charged body be placed within that space. It would there- 

 fore be sufficient to surround a powder-magazine with a good 

 conductor, to sheath its roof, walls, and ground-floor with 

 thick sheet copper, and under such conditions no electrical 

 effect could occur within it on account of any thunderstorm 

 outside. In such case there would be no need of any earth- 

 connexion ; a layer of asphalt might be placed between the 

 floor and the ground so as to insulate the building. Should 

 the magazine be struck by 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*. 



For ordinary buildings it is sufficient to enclose the build- 

 ing with a network, say, of copper wire, such as No. 4 British 

 wire-gauge (0*238 inch diameter), carried round the founda- 

 tion of the house, up each of the corners and gables, and along 

 the ridges. The copper wire may be built into the wall to 

 prevent theft ; and it should be connected to any outside 

 metal, such as the lead or zinc on the roof, and to metal rain- 

 water pipes, &c. 



Highgate, N., 

 August 1889. 



* On this subject see Faraday, 'Experimental Researches/ §§ 1173-4. 

 An insulated cubical chamber was constructed, coated on the outside with 

 metal, and charged with electricity from a powerful machine. "I put a 

 delicate gold-leaf electrometer within the cube, and then charged the 

 whole by an outside communication very strongly for some time together ; 

 but neither during the charge nor after the discharge did the electrometer 

 or the air within show the least signs of electricity." 



Going further back, we may refer to Priestley's ' History of Electricity ' 

 (4to, 1775) for some experiments by Franklin and the author. A pair of 

 pith-balls suspended within an insulated tin quart vessel were not in the 

 least affected when a charge was given to the vessel (pp. 688 et seq.). It 

 was also found that a wire net could be substituted for the tin quart. 

 Hence a wire netting would protect a gunpowder magazine as effectually 

 as plates of metal, but would not be so durable. 



