ELECTRICITY IN THE HOME 



'57 



a current is formed, as may be shown by holding the wire 

 over, and parallel to, the needle of a compass. The needle 

 will be turned from its north and south position. The cause 

 for this current is the difference between the action of the 

 acid on the two metals, the zinc being acted on more than 

 the copper. If the cell is watched, bubbles of gas will be 

 seen to form around the zinc and, passing across, gather on 

 the copper. These are bubbles of hydrogen, which are poor 

 conductors of electricity. After a while so many bubbles 

 collect on the copper that the current cannot pass through, 

 and so the cell ceases to give any current. 



In order to make a cell that will be of practical use, it is 

 necessary to find some way of preventing the accumulation 

 of this gas on the metal. This is generally done by putting 

 in the cell a second chemical, which unites with the hydrogen 

 before it collects on the 

 metal. A number of dif- 

 ferent chemicals may be 

 used. In one cell copper 

 sulfate is. used, which causes cop- 

 per to be deposited on the copper 

 strip. In other cells a compound 

 is used which gives off oxygen, and 

 this unites with the hydrogen. 



Classes of cells. Two types of cells 

 are commonly distinguished, the 

 wet and the dry. The Leclanche* 

 cell is a type of wet cell sometimes 

 used for door bells . (See figure 55.) 

 Carbon and zinc are used and 

 placed in a solution of sal ammoniac. The carbon is placed 

 in a porous cup and surrounded with a compound which 

 oxidizes the hydrogen. 



Dry cells. The dry cell (see figure 56) is now largely re- 

 placing the wet cell on account of its greater convenience for 



FIG. 55. Leclanche" cell. 



