22 Dr. H. J. S. Sand on the Measurement of the 



adopt as an empirical fact that the velocity of reduction at the 

 electrode v depends on the concentration of hydrogen c in it 

 according to the relation 



in which k and y are constants depending on the conditions 

 of the experiments. In contradiction to this, as will he under- 

 stood from what has already been said, Nernst's view would 

 require that once the conditions of equilibrium have been 

 overstepped, the above velocity should be infinitely great. 

 As a matter of fact, Haber's results have been doubted by 

 Brunner, who believes that changes of concentration in the 

 liquid adjoining the electrode may be partly responsible for 

 them. To this Haber and Russ have replied, stating reasons 

 for the correctness of the results obtained by the former *. 

 In view of the fact that the investigation of the dependence 

 of electrode-potentials on current-density seems one of the 

 most important problems of contemporary electrochemical 

 research, it appears very desirable to have independent 

 methods for the determination of the rate at which changes 

 of concentration occur in the layer of electrolyte adjoining 

 the electrode. 



When the diffusion coefficients required for calculating 

 these changes of concentration are not known, it would seem 

 that the most reliable method for arriving at an estimation 

 of their rapidity is afforded by the investigation of the 

 behaviour of stationary liquids free from convection-currents. 

 This was the method adopted by me for the examination 

 of copper solutions. When hydrogen began to be evolved 

 owing to complete depletion of the copper-salt at the electrode, 

 a sudden, very large fall of cathode-potential took place, 

 which caused a corresponding increase of voltage between 

 the two electrodes that could be easily observed. This method 

 is only applicable where very sharp breaks of potential occur, 

 and the anode is not polarizable in the liquid employed. 

 Another method has since been described by Cottrell f. In 

 it a constant E.M.F. is maintained between the electrodes ; 

 the current, however, must remain so minute, or the re- 

 sistance of the electrolyte be so small, that practically no 

 E.M.F. is required to overcome it according to Ohm's law. 



It is the object of the present paper to describe a method 

 which will allow the electrode-potential in stationary liquids 

 of the most varying kind to be measured, when very much 

 larger currents are employed than those which can be made 



* Ztschft. Phys. Chem. xlvii. p. 260 (1904). 

 t Ibid. xlii. p. 385 (1903). 



