380 ELEMENTARY LESSONS ON [CHAP. xi. 



and oxygen tend to reunite and set up an opposing 

 electromotive -force of no less than 1-45 volts. This 

 opposing electromotive-force, which is in fact the measure 

 of their " chemical affinity " is termed the electromotive- 

 force of polarisation. It can be observed in any water- 

 voltameter (Art. 208), by simply disconnecting the 

 wires from the battery, and joining them to a galvan- 

 ometer, when a current will be observed flowing back 

 through the voltameter from the hydrogen electrode, 

 toward the oxygen electrode. The polarisation .in a 

 voltaic cell (Art. 163) produces an opposing electro- 

 motive-force in a perfectly similar way. 



Now, since the affinity of hydrogen for oxygen is 

 represented by an electromotive-force of 1-45 volts, it is 

 clear that no cell or battery can decompose water unless 

 it has an electromotive -force at least of 1-45 volts. 

 With every electrolyte there is a similar minimum 

 electromotive-force necessary to produce complete con- 

 tinuous decomposition. 



414. Theory of Electrolysis. Suppose a current 

 to convey a quantity of electricity Q through a circuit 

 in which there is an opposing electromotive -force E : 

 the work done in moving Q units of electricity against 

 this electromotive-force will be equal to E x Q. (If E 

 and Q are expressed in "absolute" C.G.S. units, E-Q 

 will be in ergs.) The total energy of the current, as 

 available for producing heat or mechanical motion, will 

 be diminished by this quantity, which represents the 

 work done against the electromotive-force in question. 



But we can arrive in another way at an expression 

 or this same quantity of work. For the quantity of 

 electricity in passing through the cell will deposit a 

 certain amount of metal : this amount of metal could be 

 burned, or dissolved again in acid, giving up its potential 

 energy as heat, and, the mechanical equivalent of heat 

 being known, the equivalent quantity of work can be 

 calculated. Q units of electricity will cause the depo- 



