from Faraday to J . J. Thomson. 379 



of which the other face is the electrode itself.* If a denote 

 the surface-density of electricity on either face of this quasi- 

 condenser, we have, therefore, 



de = - d(Sa) ; so a = dyfd V. 



This equation shows that when dyldV is zero i.e., when 

 the surface-tension is a maximum a must be zero ; that is to 

 say, there must be no difference of potential between the 

 mercury and the electrolyte. The external electromotive force 

 is then balanced entirely by the discontinuity of potential at 

 the other electrode J7" ; and thus a method is suggested of 

 measuring the latter discontinuity of potential. All previous 

 measurements of differences of potential had involved the 

 employment of more than one interface ; and it was not known 

 how the measured difference of potential should be distributed 

 among these interfaces ; so that the suggestion of a means of 

 measuring single differences of potential was a distinct advance, 

 even though the hypotheses on which the method was based 

 were somewhat insecure. 



A further consequence deduced by Helmholtz from this 

 theory leads to a second method of determining the difference 

 of potential between mercury and an electrolyte. If a mercury 

 surface is rapidly extending, and electricity is not rapidly 

 transferred through the electrolyte, the electric surface-density 

 in the double layer must rapidly decrease, since the same 

 quantity of electricity is being distributed over an increasing 

 area. Thus it may be inferred that a rapidly extending 

 mercury-surface in an electrolyte is at the same potential as 

 the electrolyte. 



This conception is realized in the dropping-electrode, in 



* The conception of double layers of electricity at the surface of separation of 

 two bodies had been already applied by Helmholtz to explain various other 

 phenomena e.g., the Volta contact-difference of potential of two metals, fiictional 

 electricity, and *' electric endosmose," or the transport of fluid which occurs when 

 an electric current is passed through two conducting liquids separated by a porous 

 barrier. Cf. Helmholtz, Berlin Monatsberichte, February 27, 1879 ; -Ann. d. Phys. 

 vii (1879), p. 337 ; Helmholtz, Wiss. Abh. i, p. 855. 



