22 NEWBERY, The Theory of Overvoltage. 



gaseous hydrogen in the experimental electrode. (The 

 pressure of hydrogen round the standard electrode is of 

 course unity.) 



Then the overvoltage 



^ ^ RT 

 ^~E Q = —lnp Bi 



= 0-029 log /V, at i8°C 



Hence for an overvoltage of 0"5 volt 

 0-029 log/ //o _ = 0-5 

 . *. p B% = 4-8 x io 17 atmospheres. 



It is evident that a change of pressure of 100 atmos- 

 pheres will produce a negligible effect upon such an 

 electrode, a conclusion which is fully borne out by ex- 

 periment. 



Temperature has a relatively small but appreciable 

 effect upon overvoltage, the rule being that rise of tem- 

 perature decreases overvoltage. This effect has not been 

 very fully studied, partly owing to the great difficulty in 

 eliminating the effects of other confusing factors (time 

 specially) while the temperature is changing. 



Townsend {Phil. Mag., 1898 [v] , 45, 126) has shown 

 that electrolytic gases carry with them a considerable 

 electrical charge, and it is probable that this charged gas 

 exercises an inductive effect upon the electrode. Gases 

 may also be ionised by bubbling them through certain 

 liquids, and the smaller the bubbles the greater the degree 

 of ionisation. Thus hydrogen acquires a positive charge 

 when bubbled through dilute acid by means of a very fine 

 glass jet. 



If hydrogen is liberated easily from an electrode of 

 low overvoltage, one would expect the bubbles to be 

 smaller than when it is liberated under higher pressure 

 from one of high overvoltage, and further, that any 



