Manchester Memoirs, Vol. Ix. (1916), No. 11. 7 



E.M.F. was applied to produce a steady current through 

 the electrolyte. Caspari's results are therefore true over- 

 voltages under the condition of indefinitely small current 

 density. 



b. The \Knickpunkt 3 method. 



This method was adopted by Coehn and Dannenberg 

 and others, and consists in applying a gradually increasing 

 potential to an electrolytic cell and observing the current 

 which passes. At a certain potential, when decomposition 

 sets in, the current increases very rapidly with slight 

 increase of the applied potential. Hence the curve 

 obtained by plotting current against applied potential 

 bends sharply upwards and the position of the bend 

 (Knickpunkt) gives the decomposition voltage of the 

 electrolyte. The overvoltage of the cell as a whole is 

 then found by subtracting the normal formation voltage of 

 the decomposition products, and if one of the electrodes 

 is unpolarisable, the overvoltage of the other is given. 

 The method is unsatisfactory firstly, because it only can 

 give information as to the overvoltage at indefinitely low 

 current densities, and secondly because the exact position 

 of the Knickpunkt is ill defined and very doubtful, as- 

 sumptions have to be made to decide on the voltage at 

 which the curve really 'bends.' Consequently the method 

 has given considerably varying results in the hands of 

 different experimenters. 



c. The bubble angle method. 



This ingenious method was devised by Muller, who, 

 starting from the conception that overvoltage was due to 

 surface tension forces at the electrodes, attempted to 

 measure it by determining the angle which the surface of 

 a gas bubble makes with the electrode surface just before 



