314 



A Theory of Voltaic Action. 



[Nov. 18, 



These considerations apply very much in fact to Sir W. Thomson's 

 gravity battery with copper filings and zinc funnel. 



38. But now suppose that any atmosphere capable of acting che- 

 mically on the plates has been removed. On bringing them close 

 together and making metallic connexion between them, they will 

 not be able to return to their original difference of potential, for the 

 energy required to effect the electrical charge is no longer forth- 

 coming. The facts thus far encourage us therefore to hold the view 

 that the difference of potentials is not an intrinsic property of zinc 

 and copper in themselves, but is the result and manifestation of the 

 fact that their contact in the atmosphere has started a chemical 

 action which has proceeded till choked off by its own results. 



According to the ideas of the theory of chemical equilibrium of 

 Clausius, Gribbs, and Helmholtz, it has proceeded until the accumula- 

 tion of its products has secured that further progress shall not any 

 longer lead to a dissipation of energy. 



39. If this argument which refers the difference of potential to an 

 initial chemical action on the surface of the plates where they are in 

 contact with the atmosphere or to a condensed liquid surface film 

 derived from it, be allowed, the next step would be the assignment of 

 some mode of action which would account for the result. And here 

 the idea of a double electrical layer, which has been elaborated by 

 Helmholtz from the phenomena of electrolysis, and so successfully 

 applied by him in explanation of the polarisation of voltaic cells, comes 

 up hopefully. 



The oxygen of the moisture in the atmosphere will under proper 

 circumstances combine chemically with the zinc, giving rise to 

 electrical effects, as in Sir W. Thomson's gravity battery already 

 mentioned. When the circumstances are such that actual combina- 

 tion is not possible, the two substances will feel each other's presence, 

 and take up along their surface of contact a conformation of mole- 

 cular equilibrium; we have a right to assume that this configuration 

 will present a sheet of positively charged atoms, facing an equal 

 sheet of negatively charged atoms, and thus forming a double layer. 

 A precise explanation of how this comes about could hardly be 

 required of us, for so long as we continue to apply to such questions 

 our ordinary notions of electrical attractions at all, there is no way 

 open for explaining the dynamical fact of a difference of potential 

 between two substances in contact, except the assumption of such a 

 layer along the surface of contact. How it is formed may be at any 

 rate illustrated from the phenomena of voltaic polarisation. 



40. We may then provisionally contemplate a zinc plate as 

 surrounded by such a double layer, negative inside and positive out- 

 side ; and a copper plate as surrounded by a similar layer of smaller 

 moment, the moment being measured in the same way as for magnetic 



