348 JAMES PRESCOTT JOULE. 



he concludes that the heat produced is proportional to the resistance 

 of the conductor, independent of the shape or kind of metal which 

 closes the circuit. He proved also that the " heat evolved in a given 

 time IS proportional to the resistance of the conductor multiplied by 

 the square of the electric intensity." By electric mtensity he meant 

 what we now call current, and the above we now recognize as RC^. 



In the early part of the century, Davy and Count Rumford had 

 proved from their experiments that heat was but a Idnd of molecular 

 motion, not an entity as had long been supposed, but this conception 

 did not become general among those who had most to do with thermal 

 phenomena. Text-books, as well as special treatises, continued to rep- 

 resent the old conceptions as yet valid till within about twenty-five 

 years. Nowadays we are so familiar with the expression " heat as a 

 mode of motion " that few give to it more than a passing thought, 

 and a large number of persons who deal with the subject of heat in 

 its mechanical relations speak of it as a "■ form of energy," an ex- 

 pression which fails to convey with any definiteness the character of 

 the motions that differentiate heat from other kinds of energy motion. 

 As energy is a product with motion as one of the factors, it follows 

 that a mode of motion and a form of energy cannot be identical. 

 Joule was not muddled on that. In 1843 he read a paper before the 

 British Association on the " Mechanical Value of Heat." In that 

 paper he says, " 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." The Italics 

 are his. He then describes at length numerous experiments in which 

 the heat developed in an electric circuit was measured, when the 

 electric current was produced by a magneto-electric machine. The 

 amount of work required to turn the magneto-electric machine when 

 producing the current was carefully measured also, and from these he 

 reached the first determination of the mechanical equivalent of heat, 

 namely. " The quantity of heat capable of increasing the tetnperature 

 of a pound of water by one degree of Fahrenheit's scale is equal to, 

 and may be converted into, a mechanical force capable of raising 838 

 pounds to the perpendicular height of one foot.''' This result, as we now 

 know, is too large, and in 1881 he accounted for the excess by noting 

 that no allowance was made for the heat absorbed by the magnet itself, 

 which at first he thought neglectable ,- but at the latter date he con- 

 cludes it would account for all the difference between 838 and 772. 



The story of the reception of this remarkable paper by British 

 physicists, giving as it did the first law of thermodynamics, is strik- 



