Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/116

 the electric forces existing in the two particles; just as, in the theory of heat, the flow of caloric between two particles is regarded as proportional to the difference of their temperatures."

The comparison between the flow of electricity and the flow of heat suggested the propriety of introducing a quantity whose behaviour in electrical problems should resemble that of temperature in the theory of heat. The differences in the values of such a quantity at two points of a circuit would provide what was so much needed, namely, a measure of the "driving-power" acting on the electricity between these points. To carry out this idea, Ohm recurred to Volta's theory of the electrostatic condition of the open pile. It was customary to measure the "tension" of a pile by connecting one terminal to earth and testing the other terminal by an electroscope. Accordingly Ohm says: "In order to investigate the changes which occur in the electric condition of a body A in a perfectly definite manner, the body is each time brought, under similar circumstances, into relation with a second moveable body of invariable electrical condition, called the electroscope; and the force with which the electroscope is repelled or attracted by the body is determined. This force is termed the electroscopic force of the body A."

"The same body A may also serve to determine the electroscopic force in various parts of the same body. For this purpose take the body A of very small dimensions, so that when we bring it into contact with the part to be tested of any third body, it may from its smallness be regarded as a substitute for this part: then its electroscopic force, measured in the way described, will, when it happens to be different at the various places, make known the relative differences with regard to electricity between these places."

Ohm assumed, as was customary at that period, that when two metals are placed in contact, "they constantly maintain at the point of contact the same difference between their electroscopic forces." He accordingly supposed that each voltaic cell possesses a definite tension, or discontinuity of electroscopic