THE ORIGIN OF MOTIVE POWER. 423 



a thermometer to have risen by 15 Fahr. After 

 the rotation was continued for a few minutes more, 

 and again stopped, the ball was found to be so hot 

 that a piece of phosphorus applied to any point of 

 its surface immediately took fire. It is clear that 

 in this experiment the electric currents, discovered 

 by Faraday to be induced in the copper in virtue 

 of its motion in the neighbourhood of the magnet, 

 generated the heat which became sensible. Joule 

 first raised the question, Is any heat generated by 

 an induced electric current in the locality of the 

 inductive action ? He not only made experiments 

 which established an affirmative answer to that 

 question, but he used the mode of generating heat 

 by mechanical work established by those experi- 

 ments, as a way of finding the numerical relation 

 between units of heat and units of work, and so 

 first arrived at a determination of the mechanical 

 value of heat. At the same time (1843) he gave 

 another determination founded on the friction of 

 fluids in motion ; and six years later he gave 

 the best determination yet obtained, according to 

 which it appears that 772 foot pounds of work 



