96 Galvanism, from Galvani to Ohm. 



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 cus- 

 tomary 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 electro- 

 scopic 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 electro- 

 scopic forces." He accordingly supposed that each voltaic cell 

 possesses a definite tension, or discontinuity of electroscopic 



