142 ELECTRICITY 



potential. This discharge along the wire is the electric- 

 current. The discharge would, of course, bring the two 

 charges to the same potential ; but the action of the liquid 

 upon the two strips is such that it renews the difference 

 in potential just as rapidly as the discharges are made. 

 Thus the difference in potential is kept up and the cur- 

 rent continues to pass along the wire. 



The two metal strips are called plates or poles ; the 

 copper plate, having the higher potential, is said to be 

 positive, while the zinc plate is called negative. Note that 

 the plates must not be connected within the cell by any 

 conductor except the liquid, nor outside the cell by 

 any conductor but that through which it is intended the 



current shall flow. Other- 

 wise all or a part of the 

 current would be lost to 

 useful work. 



163. Kinds of Cells. 

 Different substances may 

 be used in cells, for poles or 

 for liquids. For the poles, 

 copper and zinc, or carbon 

 and zinc are most com- 

 monly used. The liquid is 



generally water, in which some acid or salt has been 

 dissolved. Sometimes the liquid is a weak solution of 

 sulphuric acid; but as this destroys the zinc plate rapidly 

 it is not often used in practice. More commonly the 

 liquid is a solution of sal ammoniac ; cells of this sort 

 are used for ringing call bells and for light work; the 



