from Faraday to J. J . Thomson. 381 



rapidly increasing e.g., a dropping electrode; that is to say, 

 the difference of potential between an ordinary mercury 

 electrode and the electrolyte, when the surface-tension has its 

 maximum value, is equal to the difference of potential between 

 a dropping-electrode and the same electrolyte. This result has 

 been experimentally verified by various investigators, who have 

 shown that the applied electromotive force when the surface- 

 tension has its maximum value in the capillary electrometer, is 

 equal to the electromotive force of a cell having as electrodes a 

 large mercury electrode and a dropping electrode. 



Another memoir which belongs to the same period of 

 Helmholtz' career, and which has led to important develop- 

 ments, was concerned with a special class of voltaic cells. The 

 most usual type of cell is that in which the positive electrode 

 is composed of a different metal from the negative electrode, 

 and the evolution of energy depends on the difference in the 

 chemical affinities of these metals for the liquids in the cell. 

 But in the class of cells now considered* by Helmholtz, the 

 two electrodes are composed of the same metal (say, copper) ; 

 and the liquid (say, solution of copper sulphate) is more con- 

 centrated in the neighbourhood of one electrode than in the 

 neighbourhood of the other. When the cell is in operation, the 

 salt passes from the places of high concentration to the places 

 of low concentration, so as to equalize its distribution ; and this 

 process is accompanied by the flow of a current in the outer 

 circuit between the electrodes. Such cells had been studied 

 experimentally by James Moser a short time previously! to 

 Helmholtz' investigation. 



The activity of the cell is due to the fact that the available 

 energy of a solution depends on its concentration ; the molecules 



of mercury, until the upper layer of the solution is so much impoverished that the 

 double layer can no longer be formed. The impoverishment of the upper layer of 

 the solution has actually been observed by Palniaer, Zeitsch. Phys. Chem. xxv 

 (1898), p. 265 ; xxviii (1899), p. 257 ; xxxvi (1901), p. 664. 



* Berlin Monatsber., 1877, p. 713 ; Phil. Mag. (5) v (1878), p. 348; reprinted 

 with additions in Ann. d. Phys. iii (1878), p. 201. 



t Ann. d. Phys. iii (1878), p. 216. 



