158 



SCIENCE OF HOME AND COMMUNITY 



handling. It is not really a dry cell but a moist cell, as the 

 chemicals are brought into contact in the form of a moist 

 paste. Most of these dry cells are modifications of the 



Leclanche cell. The elements used 

 are carbon and zinc, and the chief 

 chemical is sal ammoniac. The 

 outer shell of the cell is made of 

 zinc. Inside this is a mixture of 

 sal ammoniac and plaster of Paris 

 to make a stiff paste. In the center 

 is a rod of carbon surrounded with 

 manganese dioxid. The two chem- 

 icals are separated by blotting 

 paper. After the cell is moistened, 

 it is sealed with pitch to prevent 

 the water from evaporating. If 

 the cell really did become dry, it 

 would be worthless. 



FIG. 56. Dry cell. 



LABORATORY EXERCISE 19 



Purpose. To learn how the cell 

 used to operate the electric door bell is made. 



Apparatus. Strip of sheet zinc, strip of sheet copper, each 

 with a wire attached. Sulfuric acid, compass, tumbler, sal 

 ammoniac cell, dry cell sawed lengthwise into two equal 

 parts. 



Directions, i. A simple cell may be made as follows. Fill 

 a tumbler about two thirds full of water, add to this about 

 one twelfth as much sulfuric acid. Put the strip of copper and 

 zinc in this solution in such a way that they do not touch. 

 Notice what takes place on the surface of each metal. Connect 

 the two wires and see if there is any difference in what happens 

 on the surface of the two metals. Hold the wire north and 

 south and place a compass directly over it. What happens to 

 the needle? 



2. The sal ammoniac cell is very commonly used to operate 



