6 NEWBERY, The Theory of Overvoltage. 



In this diagram {Text-fig. i), A represents the experi- 

 mental electrode, C the standard hydrogen electrode. A 

 gradually increasing voltage was applied to the electrodes 

 A and B from the battery E until the first gas bubbles 

 were observed on A, and then the potential difference 

 between the electrodes A and C was measured by means 

 of the potentiometer F. The electrolyte, normal sulphuric 

 acid, was contained in the reservoir D by means of which 

 the height could be adjusted. 



This method gives reliable values for the overvoltage 

 when an indefinitely small current is flowing, but unfor- 

 tunately is not applicable to practical cases where 

 appreciable current densities are employed. 



Some investigators have attempted to use it in such 

 cases and have drawn conclusions from their results which 

 are not justified by facts. 



Imagine the case when a fairly strong current is 

 passing between A and B. Measure the difference of 

 potential first between A and C, and then between B and 

 C. These added together will of course give us the value 

 of the applied E.M.F. Each of the above measurements 

 will therefore give us little or no information about the 

 overvoltage of either electrode, but merely shows the way 

 in which the E.M.F. of the battery is distributed between 

 the two electrodes and this depends on the relative size 

 of the electrodes, the resistance of the film of gas on each 

 electrode, the resistance of the solution and even of the 

 battery and leads. The futility of such measurements 

 may be realised when it is seen that changing the size or 

 material of one electrode alters the apparent overvoltage 

 of the other. 



In Caspari's work, however, the whole of the applied 

 E.M.F. was used up in overcoming the back E.M.F. of 

 the two electrodes, and only an indefinitely small extra 





