14 NEWBERY, The Theory of Over volt age. 



b. In the electrolyte — 



Nature of ions present in electrolyte. 

 Concentration of electrolyte. 

 Impurities or additions to electrolyte. 



c. General — 



Current density. 



Time. 



External pressure. 



Temperature. 



Ionisation of gas liberated. 



a. hi the Electrodes. 



The table of overvoltages already given is sufficient 

 to show how greatly the metal chosen affects the 

 magnitude of the overvoltage. Of the pure metals, 

 platinised platinum has the lowest known cathodic over- 

 voltage, and zinc or mercury under certain conditions the 

 highest. 



Slight impurities in a metal frequently produce great 

 changes in the overvoltage. Scratching the surface of a 

 lead plate with a platinum wire renders the plate useless 

 as a cathode for powerful electrolytic reductions, while a 

 trace of mercury covering an impure zinc plate raises 

 the overvoltage sufficiently to prevent the zinc dissolving 

 in dilute acid. 



Hard forms of a metal have different overvoltages 

 from soft forms, sometimes higher sometimes lower, so 

 that hammering, tempering, annealing, etc., all exert a 

 considerable effect on the overvoltage of some metals. 

 This effect however usually disappears after the electrode 

 has been in use for some time. 



Generally speaking, rough surfaces show lower over- 

 voltages than smooth ones but this is not always the case. 

 A plate of polished lead when subjected to a high cathodic 

 current density, becomes roughened, and at the same time 



