222 



ELEMENTS OF GENERAL SCIENCE 



ring the bell originally, a change occurred while the current 

 was passing through the acid solution. Examination will 

 show that one of the plates is now coated with a brown 

 deposit called lead peroxide, a compound of lead and the 

 oxygen which was set free at the plate connected with the 

 positive terminal of the battery. 



t As the current rings the electric bell the brown deposit 

 may partially disappear, and the two 

 lead plates soon reach such a condi- 

 tion that the bell fails to ring. The 

 cell can be charged again, however, 

 and the entire process repeated. 



Electricity is not stored in this cell, 

 however, in the same way that milk 

 is kept in a bottle. As the current 

 flows through the solution the surface 

 of one of the lead plates is changed 

 chemically into lead peroxide. This 

 process is known as charging the bat- 

 tery. It produces the necessary con- 

 ditions for any electric cell; that is, 



FIG. ill. Hydrometer for two unlike plates in a solution that 

 will conduct the electric current. As 

 the current is taken from the cell the 



The hydrometer syringe is , , . , ,. , ,. , 



so constructed that by its use lead peroxide disappears, thus discharg- 

 the specific gravity (density) i n g the cell. This simple device illus- 



of the acid may be determined , . , ,. , , , 



trates the principle of the lead storage 



cell that has such an extensive use. In the commercial battery 

 two or more cells are used, the plates are closer together, are 

 strongly supported, and differ in other details of construction. 

 229. The automobile storage battery. The automobile 

 makes constant use of a storage battery. The battery is 

 usually found in a heavily constructed box (fig. 110) in 

 which two or more cells are connected in series. Electric 

 current at voltages depending upon the number of simple 



testing the acid in storage 

 cells 



