io NEWBERY, The Theory of Overvoltage. 



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Text-fig. 3. 



With platinum electrodes in a solution of iodine in 

 potassium iodide, a similar graph was obtained, showing 

 that the velocity of depolarisation in this case was prac- 

 tically infinite. 



With platinum electrodes in normal sulphuric acid, 

 the graph obtained was of the type shown in Text-fig. 4. 



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Text-fig. 4. 



The actual potential of the platinum electrodes at any 

 instant is shown by these curves. The slope of the curves 

 is due, of course, to the gradual absorbtion of hydrogen 

 and oxygen by the electrodes, and its disappearance by 

 diffusion or re-combination. Hence these curves show 

 not only the maximum E.M.F. from which the overvol- 

 tage can be calculated, but also the velocity of polarisation 

 and depolarisation. The method, therefore, is of wide 

 applicability but unfortunately fails in at least one im- 

 portant respect, — it cannot be used for determining the 

 effect of passing the current for long intervals of time, 

 and since this is essential for practical purposes, other 

 methods must be used. 



