320 HERMANN VON HELMHOLTZ 



distributed between the two that the difference of electrical 

 potential shall be of a definite magnitude, determined by the 

 nature of the metals. From this it is at once obvious that 

 conductors of this sort must be subject to the laws of the 

 galvanic tensional series, and that chains consisting of three 

 or more conductors of the first class at the same temperature 

 can never produce an electric current, since the attractive 

 forces of the metals for the electricities can only cause the 

 electricity to assume the state of equilibrium, which is imposed 

 by these forces of attraction. Volta assumed a separating force 

 residing in the surface of contact, and believed that the positive 

 electricity, having once entered the zinc by the surface of 

 contact with the copper, could flow out again without further 

 obstacle into any conductor in which it was not opposed by 

 any new force of separation at the contact surfaces; in 

 Helmholtz's opinion, on the contrary, the positive electricity 

 was held fast in the zinc by attraction, and this attraction must 

 be overcome by a corresponding expenditure of work, before 

 the positive electricity can be removed from the zinc again 

 by any other natural force. Electrolytic conductors, however, 

 do not follow the tensional series, because they are dissociated 

 by each electric motion, and cannot, of course, while this dis- 

 sociation is proceeding, arrive at a state of resting equilibrium. 

 Helmholtz confirmed his theory by a long series of elabo- 

 rate experiments, which he performed with the help of 

 W. Thomson's quadrant electrometer. 



In his earlier critical work on the Theory of Electro- 

 dynamics, Helmholtz had evinced more and more disposition 

 to adopt the Faraday- Maxwell hypothesis, but he had always 

 insisted that a complete understanding of the theory of electro- 

 magnetic phenomena, and an ultimate decision as to the claims 

 of the several hypotheses, would only be possible after an exact 

 investigation of the processes that obtain for unclosed currents 

 of very brief duration. Weber had, indeed, endeavoured to 

 minimize certain difficulties and contradictions in his electro- 

 dynamic hypotheses, by attributing to electricity a certain 

 amount of inertia, such as pertains to heavy bodies ; Helmholtz, 

 however, very soon recognized that true inertia would be 

 proportional to the mass of the electricity in motion, inde- 

 pendent of the position of the conductor, and that this would 



