Manchester Memoirs, Vol. Ix. (1916), No. 11 15 



its overvoltage falls greatly. If the surface be roughened 

 by making it the anode in dilute sulphuric acid for a 

 short time and then reversing the current, its overvoltage 

 rises to a very high value. Hence the kind of roughness 

 of the surface as well as the nature of the surface itself 

 may produce very great changes of overvoltage. Since 

 the nature of the surface depends largely on the previous 

 history of the metal, a knowledge of this is frequently of 

 value in estimating the best conditions for attaining a 

 certain overvoltage in electrolytic work. 



It has long been known that the nature of the surface 

 of the electrode affects the liberation of a gas from it, but 

 only recently has it been shown that the converse of this 

 is also true. When a polished platinum electrode is used 

 as anode or cathode in dilute acid or alkali at high current 

 density, its surface after a time becomes roughened, and 

 if electrolysis is continued for a long period, a black coat- 

 ing is observed, which chemical tests show to be pure 

 platinum. Other electrodes show similar behaviour and 

 microscopic examination shows the presence of innumer- 

 able small craters, suggesting most forcibly that the 

 surface has been blown open by a series of tiny explosions. 

 See Plate. 



b. hi the Electrolyte. 



The nature of the ion liberated at the electrode affects 

 the overvoltage in much the same way as the nature of 

 the electrode. If hydrogen is liberated at an electrode, 

 its overvoltage will as a rule be quite different from that 

 when oxygen or chlorine is being evolved. Platinum has 

 a very high oxygen overvoltage but a very low hydrogen 

 overvoltage. Nickel is low in both cases and lead high. 



The overvoltage when a metal is depositing or being 

 dissolved is, with rare exceptions, very small compared 

 with that when a gas is being liberated. 



