32 NEWBERY, The Theory of Overvoltage. 



As the composition of the alloy varies, its back E.M.F. 

 will also vary, and hence the most powerful oxidising or 

 reducing action will be exerted when this back E.M.F. 

 is greatest, i.e. when the overvoltage is greatest. Part of 

 the fall of overvoltage at very high current densities is also 

 due to this variation of the composition of the alloy, since 

 one with very high gas content may have a lower back 

 E.M.F. than one with less gas. 



(ii.) is probably the main source of high-efficiency 

 oxidations and reductions. When an easily oxidisable 

 or reducible body is present, the discharged ions react at 

 once before time is given to form a higher alloy or pro- 

 duce any appreciable pressures in the electrodes. Hence 

 the presence of these bodies will reduce the overvoltage 

 to an extent dependent upon the speed of the secondary 

 reaction, a conclusion fully proved by the extensive 

 researches of Le Blanc. 



(iii.) only occurs when the oxidisable or reducible 

 body is itself ionised or capable of being ionised. It is, 

 however, of very common occurrence : thus copper sul- 

 phate is reduced to metallic copper by adding a negative 

 or subtracting a positive charge from the copper ion ; 

 ferrous sulphate is oxidised to ferric sulphate by adding 

 a positive charge to the ferrous ion, etc. 



Summary. 



The following suggested theory of overvoltage is com- 

 posed of parts of the theories of Nernst, Le Blanc and 

 Foerster, with additions. 



The overvoltage of a metal is determined by four 

 factors. 



