Manchester Memoirs, Vol. Ix. (1916), No. 11. 23 



changes which tend to raise overvoltage will lower the 

 ionisation of the escaping gas. All this is fully justified 

 by experiment and it has been found that in all cases the 

 overvoltage of a metal and the degree of ionisation of the 

 gas liberated show an inverse relationship. 1 Since the 

 pores in the metal are very much smaller than any which 

 can be produced artificially, the ionisation of electrolytic 

 gas is always much greater than -that produced by 

 bubbling through glass jets, etc. At high current densities 

 the inductive effect is so great that in some cases the 

 measured overvoltage is reduced to a negative quantity. 



3. Theories of Overvoltage. 



Sufficient has perhaps been said to show how complex 

 a phenomenon that of overvoltage really is. We are 

 probably dealing with a mixture of chemical, electrical, 

 and mechanical forces, and a complete theory of over- 

 voltage must take account of all. 



The following outline of the chief theories so far 

 suggested is given but it is impossible to do full justice 

 to each theory in a paper of this type. For fuller infor- 

 mation the references given at the end of this paper should 

 be consulted. 



Nernst considers that before a gas can be liberated 

 from an electrode it must first penetrate into the metal 

 which therefore occludes a certain amount of gas. If the 

 solubility of the gas in the metal is small, the process of 

 electrolysis must compress more gas into the electrode 

 before it can be liberated. Hence the overvoltage should 

 vary inversely with some function of the solubility. This 

 theory fails to account for metal overvoltage, fall of over- 

 voltage with high current density etc., but it certainly 

 contains part of the truth. 



l J. Client. Soc, 105, 1914, 242S — 2431. 



