MK. S. \V. J. SMITH ON THK NATURE OF ELECTKOCAPILLAKV I'HKNn.MKNA. in 



mena. LIPPMANN found that the curve showing the relation between the surface 

 tension and the E.M.F. applied between the terminals of a capillary electrometer was 

 (for a jKirticular solution of sulphuric acid) approximately j>arallic through a con- 

 siderable portion of its course, and it appeared from this that c was a constant and 

 that y was independent of ir. 



It is not a necessary consequence of considerations such as the above, that c should 

 be constant ; but if the only effect of the potential difference is to produce an electro- 

 static surface energy represented by fair per unit surface, then the observed surface' 

 tension should have a maximum value when IT = 0, even although c may be variable 

 The assumption, that the maximum surface tension corresponds to zero potential 

 difference between the mercury and the solution, has been much employed in recent 

 years in the deduction of values for the contact potential difference between various 

 electrodes and electrolytes. It must be remembered, however, that the observed 

 variation in the surface tension need not be due solely to variation in the quantity 

 ^cir. The non-electrical surface energy, represented by y , may vary with the 

 potential difference. A variation in the potential difference at the surface of 

 separation between the mercury and the solution may be accompanied not only by a 

 variation in the electrostatic surface energy, but also by a variation in the distribution 

 of the matter in the neighbourhood of the surface. 



If a variation of the kind just mentioned can be traced, it is clear that we cannot 

 consider the phenomena as if they were due to a certain non-electrical distribution 

 ujxm which is superposed an electrostatic double-layer, producing no other effect than 

 that represented by its electrical energy. Hence it nerd not happen that the 

 maximum surface tension corresponds to zero potential difference, for the maximum 

 surface tension may arise from the fact that non-electrical effects, accompanying the 

 change in the potential difference and tending to reduce the surface tension, pass 

 through a minimum value as the potential difference changes. This minimum value 

 need not necessarily correspond to zero potential difference. The possible nature of 

 non-electrical effect*) which may accompany changes in the potential difference is 

 discussed later. The first part of the paper contains an experimental analysis of the 

 Lippmann-Hehnholtz theory. 



THE LIPPMANN-HELMHOLTZ THEOIIY OF ELECTROCAPILLARY PHKNOMKNA. 



In theLippmann Helmholtz theory of the capillary electrometer there are in reality 

 two distinct hypotheses, either of which may be separately justifiable. The first 

 concerns the manner in which the potential difference at the capillary varies with the 

 electromotive force applied between the terminals of the electrometer. The second 

 deals with the relation between the above potential difference and the tension of the 

 surface separating the mercury and the solution. 



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