170 APPLIED SCIENCE 



sealed. Dry cells have the disadvantage, however, of hav- 

 ing a very high internal resistance, because the electrolyte 

 cannot so readily carry the current when in this form as it 

 can when fluid. Furthermore, the small amount of liquid 

 present and the method of construction do not allow the 

 free escape of the gases which form when the cell is in opera- 

 tion. For this reason, the cell becomes polarized very soon 

 and is satisfactory only where intermittent service is needed. 

 It should not be used where the current must flow continu- 

 ously for any length of time. 



In order that the internal resistance of the cells may be 

 reduced to its lowest point, the zinc and carbon are arranged 

 to present as great a surface as possible and to be as near to- 

 gether as circumstances will allow. This arrangement affords 

 a large conductor of short length for the current to flow through 

 inside the cell. The carbon should be as porous as possible, as 

 it can then absorb a great amount of oxygen and thus neutralize 

 the hydrogen gas produced by the cell when in operation and 

 prevent the cell from polarizing as soon as it would if there 

 were no oxygen present to combine with the hydrogen. 



210. Storage Batteries. The storage batteries of com- 

 merce (Fig. 76) are built up with electrodes composed prin- 

 cipally of lead peroxide (PbO 2 ) as the positive electrode, and 

 sponge lead as the negative electrode. The positive plate 

 is hard, like soapstone, while the spongy lead is so soft that 

 it may be cut by the finger nail. Both plates are immersed 

 in a dilute solution of sulphuric acid. On discharging the 

 battery, the metallic lead, peroxide, and sulphuric acid react 

 forming lead sulphate and water. On charging, the reverse 

 takes place; the lead sulphate forms metallic lead, lead 

 peroxide, and sulphuric acid. 



