THE ELECTRIC CURRENT. 



the zinc electrode where it combines with the zinc forming zinc sul- 

 phate, and the hydrogen is liberated at the carbon electrode where 

 it is oxidized at the expense of the oxygen in the chromic acid. 



In the chromic acid cell, the zinc wastes away rapidly even 

 when the cell is not delivering current, and it is therefore desirable 

 to lift the zinc out of the solution when the cell is not in use. 

 Figure 7 shows a chromic acid cell arranged so that the zinc 

 electrode may be conveniently lifted out of the solution. In this 

 figure the cell is shown with a zinc electrode placed between 

 two carbon plates. The two carbon plates are connected together 

 and constitute one electrode. 



The Edison- LaLande cell consists of a zinc plate and a compact 

 block of copper oxide (CuO) immersed in a strong solution of 

 caustic potash (KOH). The cell shown in Fig. 8 has two zinc 



Hg.so* 



Paste 



Fig. 8. 



Fig. 9. 



plates on opposite sides of the copper oxide plate. These 

 two zinc plates are connected together and constitute a single 

 electrode. When this cell delivers current, the KOH is decom- 

 posed, potassium ions are liberated at the copper oxide plate, the 

 copper oxide is reduced to metallic copper, and the potassium is 

 oxidized and goes into solution as KOH. At the same time 

 hydroxyl ions (OH) are liberated at the zinc electrode where 

 they break up into free oxygen and water (2OH = O -f- H 2 O), 

 the free oxygen combines with the zinc forming zinc oxide, and 



