Galvanism, from Galvani to Ohm. 97 



force, which is to be regarded as its contribution to the driving- 

 force of any circuit in which it may be placed. This assumption 

 confers a definite meaning on his use of the term " electroscopic 

 force " ; the force in question is identical with the electrostatic 

 potential. But Ohm and his contemporaries did not correctly 

 understand the relation of galvanic conceptions to the j 

 electrostatic functions of Poisson. The electroscopic force 

 in the open pile was generally identified with the thickness 

 of the electrical stratum at the place tested ; while Ohm, 

 recognizing that electric currents are not confined to the 

 surface of the conductors, but penetrate their substance, 

 seems to have thought of the electroscopic force at a place in 

 a circuit as being proportional to the volume-density of 

 electricity there an idea in which he was confirmed by the 

 relation which, in an analogous case, exists between the 

 temperature of a body and the volume-density of heat 

 supposed to be contained in it. 



Denoting, then, by S the current which flows in a wire of 

 conductivity y, when the difference of the electroscopic forces at 

 the terminals is E, Ohm writes 



S = yE. 



From this formula it is easy to deduce the laws already given 

 by Davy. Thus, if the area of the cross-section of a wire 

 is A y we can by placing n such wires side by side construct 

 a wire of cross-section nA. If the quantity E is the same 

 for each, equal currents will flow in the wires ; and therefore 

 the current in the compound wire will be ?i times that in 

 the single wire ; so when the quantity E is unchanged, the 

 current is proportional to the cross-section; that is, the 

 conductivity of a wire is directly proportional to its cross-section, 

 which is one of Davy's laws. 



In spite of the confusion which was attached to the idea of 

 electroscopic force, and which was not dispelled for some years, 

 the publication of Ohm's memoir marked a great advance 

 in electrical philosophy. It was now clearly understood that 

 the current flowing in any conductor depends only on the 



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