336 ELECTRICITY. 



In order to communicate electricity to every part 

 of the side of a non-conducting substance, it is ne- 

 cessary to bring each part successively in contact 

 with an electrified body. 



To avoid this difficulty, it is customary to coat 

 the sides of the non-conductor with some conduct- 

 ing substance, such as tin-foil, by which means the 

 operation of charging and discharging is rendered 

 very easy j for when the electricity is communi- 

 cated to one part of the coating, it immediately 

 spreads itself through all parts of the electric that 

 are in contact with that coating ; and when the 

 discharge is to be made, it will be sufficient to 

 make a communication by means of a conducting 

 substance, between the coatings of both sides. 



In coating an electric plate, it is necessary to 

 keep the coatings from coming to the edges of the 

 plate ; because, otherwise, the contrary electricity 

 of one side would force its way through the air, 

 round the edge, to the coating of the other side, 

 and thus prevent any charge taking place. 



In order to communicate a considerable charge 

 to a plate, it is necessary that the side opposite to 

 that which receives the charge have a communi- 

 cation through conducting substances with the 

 earth ; otherwise, only a very small charge can be 

 given. 



Those effects take place in the same manner, 

 whether the glass be in the form of a plate, or 

 in any other shape, provided it be sufficiently 

 thin : it being not the form, but the thickness 

 of the glass, that renders it capable of receiving 

 an higher or lower charge. The thinnest glass 

 receives the highest charge, but is more liable to 

 be broken by it j because the attraction of the 



