John Prescott Joule 25 



In a paper " On the Production of Heat by Voltaic 

 Electricity," he announced the most important law, that 

 heat generated in a circuit is proportional to the time, the 

 resistance and the square of the current. 



In the early stages of his investigations, Joule tacitly 

 adopted the then accepted view that heat is a substance, 

 which could not be generated or destroyed, but he soon 

 altered his opinion. In 1843 he expressed himself as 

 follows : 



" The magnetic electrical machine enables us to 

 convert mechanical power into heat by means of the 

 electric currents which are induced by it. And I have 

 little doubt that, by interposing an electro-magnetic 

 engine in the circuit of a battery, a diminution of the 

 heat evolved per equivalent of chemical change would 

 be the consequence, and this in proportion to the 

 mechanical power obtained." 



It seems that Joule was not then aware of the previous 

 experiments by Count Rumford, in which heat had been 

 generated by means of mechanical work (see page 108). 



He assumed a more decisive attitude in a subsequent 

 paper, which is introduced with the words : 



"It is pretty generally, I believe, taken for granted 

 that the electric forces which are put into play by the 

 magneto-electrical machine possess, throughout the whole 

 circuit, the same calorific properties as currents arising 

 from other sources. And indeed when we consider heat 

 not as a substance, but as a state of vibration, there appears 

 to be no reason why it should not be induced by an action 

 of a simply mechanical character, such, for instance, as 

 is presented in the revolution of a coil of wire before 

 the poles of a permanent magnet. At the same time, it 

 must be admitted that hitherto no experiments have 

 been made decisive of this very interesting question ; for 

 all of them refer to a particular part of the circuit only, 

 leaving it a matter of doubt whether the heat observed 

 was generated or merely transferred from the coils in which 

 the magneto-electricity was induced, the coils themselves 

 becoming cold. The latter view did not appear untenable 

 without further experiments. . . ." 



