THE ELECTRIC CURRENT 



143 



current is not very strong, but the cells serve for 

 months sometimes. 



On telegraphs and signal work, gravity cells are used 

 (Fig. 104). The plates are zinc and copper, and the 

 liquid is a solution of copper sulphate in water. Many 

 cells have to be used in order to give a current of any 

 strength, but they need little care except to be filled 

 with water now and then. Many so-called " dry cells '' 

 are in use; these contain a liquid, but the outside 

 covering (which usually forms the zinc plate) is sealed 

 so that the liquid cannot get out. 



164. The Circuit. In order that a current may flow 

 from the positive to the negative plate these must not 



Bell 



} Push 

 Button 



only be connected by a conductor but there must be 

 another conducting path back again to the starting 

 point. In other words, there must be a complete path 

 of conductors from the positive plate through wires or 

 instruments to the negative, then through the liquid to 



