484 Mr. J. P. Joule on the Heat disengaged 



In the year 1840* I commenced experiments on the calorific 

 effects of voltaic electricity, having at that time no knowledge of 

 what Riess had previously done in frictional electricity. By 

 these experiments it was proved, that when a current of voltaic 

 electricity is propagated along a metallic conductor, the heat 

 evolved thereby in a given time is proportional to the resistance 

 of the wire and the* square of the quantity of electricity trans- 

 mitted. 



Pursuing the inquiry, I found that the law applied very well 

 to liquid conductors ; and hence I inferred that the heat evolved 

 by any voltaic pile is proportional to its intensity or electromotive 

 force, and the number of chemical equivalents electrolysed in each 

 cell of the circuit ; or in other words, proportional to the intensity 

 of the pile and the quantity of transmitted electricity^ . 



The above law must be understood to hold good only when 

 the pile is free from local or secondary action ; for it is obvious 

 that the heat evolved by any action not directly engaged in pro- 

 pelling the current ought to be eliminated. Those parts of the 

 pile where these secondary and local actions are carried on, may 

 be regarded as minute voltaic circles respectively evolving heat 

 in quantities determined by the law ; but this heat is not to be 

 confounded with that due to the direct and useful action of 

 the pile. 



In applying the law, the intensity or electromotive force of 

 the pile must be taken at its maximum, and not when under the 

 influence of the polarization of Bitter, a phenomenon, which, as 

 is well known, is occasioned by the deposit of electro-positive 

 substances on the negative plates of the pile. When a pile is 

 under the influence of this polarization, its intensity is dimi- 

 nished; but I have shown that the diminution of heat due to 

 this diminution of the intensity of the pile is exactly counter- 

 balanced by the evolution of an additional quantity of heat at the 

 polarized plates ; and hence it appears that the heat evolved is at 

 all times proportional to the intensity of the pile when its plates 

 are in the proper condition and free from the polarization of 

 Bitter, multiplied by the quantity of transmitted electricity. 



In a memoir % " On the Heat evolved during the Electrolysis 

 of Water," I proved the three following propositions : — 



1st. That the resistance to conduction, whether it exists in 

 solid or in liquid conductors, occasions the evolution of a quan- 

 tity of heat, which, for a given time, is proportional to the mag- 



* Proceedings of the Royal Society. 

 t Philosophical Magazine, S. 3. vol. xix. p. 275. 



+ Memoirs of the Literary and Philosophical Society of Manchester, 

 2nd series, vol. vii. part 2. 



