Galvanism , from Galvani to Ohm. 95 



that the conductivity varied with the temperature, being 

 " lower in some inverse ratio as the temperature was higher." 



He also observed that the same magnetic power is exhibited 

 by every part of the same circuit, even though it be formed 

 of wires of different conducting powers pieced into a chain, 

 so that " the magnetism seems directly as the quantity of 

 electricity which they transmit." 



The current which flows in a given voltaic circuit evidently 

 depends not only on the conductors which form the circuit, 

 but also on the driving-power of the battery. In order to form 

 a complete theory of voltaic circuits, it was therefore necessary 

 to extend Davy's laws by taking the driving-power into 

 account. This advance was effected in 1826 by Georg Simom 

 Ohm* (b. 1787, d. 1854). 



Ohm had already carried out a considerable amount of 

 experimental work on the subject, and had, e.g., discovered that 

 if a number of voltaic cells are placed in series in a circuit, the 

 current is proportional to their number if the external 

 resistance is very large, but is independent of their number if 

 the external resistance is small. He now essayed the task 

 of combining all the known results into a consistent theory. 



For this purpose he adopted the idea of comparing the flow 

 of electricity in a current to the flow of heat along a wire, the 

 theory of which had been familiar to all physicists since the 

 publication of Fourier's Theorie analytique de la chcdeur in 

 1822. " I have proceeded," he says, " from the supposition that 

 the communication of the electricity from one particle takes 

 place directly only to the one next to it, so that no immediate 

 transition from that particle to any other situate at a greater 

 distance occurs. The magnitude of the flow between two 

 adjacent particles, under otherwise exactly similar circum- 

 stances, I have assumed to be proportional to the difference of 



*Ann. d. Phys. vi (1826), p. 459 ; vii, pp. 45,117; Die Galvanische Eette 

 mathematisch bearbeitet : Berlin, 1827 ; translated in Taylor's Scientific Memoirs, 

 ii (1841), p. 401. Cf. also subsequent papers by Ohm in Kastner's Archiv fur 

 d. ges. Naturkhre, and Schweigger's Jahrbuch. 



