CHAP, i.j ELECTRICITY AND MAGNETISM. 23 



number of charges of electricity from one single charge. 

 This instrument is the Electrophorus (Fig. 12). It 

 consists of two parts, a round cake of resinous material 

 cast in a metal dish or " sole," about 12 inches in 

 diameter, and a round disc of slightly smaller diameter 

 made of metal, or of wood covered with tinfoil, and 



Fig. 12 



provided with a glass handle. Shellac, or sealing-wax, 

 or a mixture of resin, shellac, and Venice turpentine, may 

 be used to make the cake. A slab of sulphur will 

 also answer, but it is liable to crack. Sheets of hard 

 ebonised indiarubber are excellent ; but the surface of 

 this substance requires occasional washing with ammonia 

 and rubbing with paraffin oil, as the sulphur contained 



