Theory of Galvanic Polarization. 423 



The notion of a condenser, therefore, gives a complete account 

 of the principal feature of the galvanic polarization. 



Direct measures of the charge by Kohlrausch showed that 

 on dividing this polarization-fall of potential equally between 

 the anode and kathode plates, the distance between the faces of 

 the condenser comes out to be about the fifteen-millionth part 

 of a millimetre; while more careful observations by Helmholtz 

 on cells in which absorbed gases have been removed from the 

 fluid, give the greater value of a ten-millionth of a millimetre. 

 And Helmholtz makes out the very important fact that for 

 all electromotive forces which do not exceed a certain moderate 

 value, the capacity, and therefore the distance of the surface- 

 layers, is very sensibly constant*. 



2. The most accurate and convenient method of observing 

 the polarization at the common surface of two liquids is pro- 

 bably the electro-capillary method invented and applied by 

 Lippmann. 



When a surface of separation can persist between the fluids, 

 the energy, reckoned as potential, of pairs of particles close 

 to the surface must exceed that of the same particles when 

 in the interior of their respective fluids. The difference may, 

 as Gauss pointed out, be reckoned as surface-energy, and 

 specified by its amount per unit area of surface. If T re- 

 present this amount, it follows, as is well known, that the 

 forces which arise from it may be represented by a surface- 

 tension equal to T across each unit of length, tending to con- 

 tract the surface in all directions. 



Now, if the common surface is polarized with constant 

 charges + Q and — Q on its two faces, there will exist an ad- 

 ditional electrical energy, which is also reckoned by its amount 

 per unit surface, and whose total value is 



E=iQV, 

 or, what is the same, ™ 



Tji 1 W> 



2 0S J 

 where S is the area of the surface and C is its electrical capa- 

 city per unit area. 



The effect of this surface-energy will therefore, the system 

 being conservative, be represented by a surface-tension T', 

 where m/_^E 



dS' 



_E 



~ S ; 



• Faraday Lecture, p. 296; Wissen. Abh. i. p. 858. 

 2H2 



