PROFESSOR IN BERLIN 319 



opposite one another like the plates of a condenser, and if they 

 are then moved farther apart, the copper will be negatively, 

 and the zinc positively charged. 



The necessary experimental laws had already been expressed 

 by Helmholtz in the ' Conservation of Energy ', to the effect that, 

 so long as only conductors of the first class are involved, i. e. 

 conductors which undergo no electrolytic dissociation during 

 conduction, and so long as these conductors are at the same 

 temperature and at rest, the passage of the electric current 

 always tends to equilibrium of the electricity ; it is only when 

 the conductors are moved by an external force that electrical 

 currents, or concentrated accumulations of electricity, arise. 

 This class of experiments further comprises those with dry 

 metal plates, which are connected by dry and insulated metal 

 wires. Since in this case, with each new arrangement of such 

 conductors at the same temperature, the electric motion soon 

 produces equilibrium, we must assume its cause to be forces 

 of a simple character, which obey the law of the conservation 

 of energy. In this essay Helmholtz brought forward the view 

 that these phenomena arise from forces of attraction, which 

 the different substances possess in different degrees for the 

 two electricities, and which only act at perceptibly small 

 distances. When two pieces of copper and of zinc are in 

 contact, and the zinc attracts the positive electricity more 

 strongly than the copper, it flows to the zinc, and charges it 

 positively, while the copper remains negative, until the electrical 

 attraction produced by this charge, and acting at a distance, 

 which draws the positive electricity back to the copper, 

 restores equilibrium with the attraction of the zinc. 



Moreover the vis viva which a particle of positive electricity 

 acquires from the influence of the zinc and copper alone, in 

 its transfer from copper to zinc, will be equivalent to the 

 vis viva lost by the same electric particle through the attraction 

 of the negative charge of the copper, and repulsion of the 

 positive from the zinc, in the same way. This last magnitude 

 calculated for the unit of positive electricity is, however, termed 

 difference of electrical potential-function between the copper 

 and the zinc. 



This theory thus demands that the electricity contained in 

 the copper and zinc when they come into contact shall be so 



