xl Annual Report of the Council. » 



of an electro-magnet with the strength of the current and the 

 length of the wire; in 1840 he employed a Voltameter and 

 measured the zinc used in supplying the current by means of 

 the hydrogen liberated. His experiments led him to the 

 remarkable generalisation, published in his first paper to this 

 Society, that the heat of combustion of zinc was identical with 

 the heat developed in the circuit of a zinc battery, and that the 

 heat of combustion is the consequence of resistance to electrical 

 conduction between Oxygen and the combustible. In 1843 

 came his classical experiment of measuring the heat developed 

 in the coil of an electro-magnet made to rotate between the poles 

 of a magnet by mechanical power or by a battery, in which he 

 proved not only the equivalency between the heat developed 

 and the chemical and electrical energies spent in driving an 

 electro-magnetic engine, but also the equivalency between the 

 heat developed and the mechanical effort in driving a magneto- 

 electric machine. 



Joule's first determination of the mechanical equivalent of 

 ^heat was given to the British Association at Cork, in 1843. 

 Immediately after the meeting he began his experiments on the 

 friction of water, and then proceeded to work at the changes of 

 temperature produced by the rarefaction and condensation of 

 the air, which led him to the conception of the "absolute zero 

 of temperature." But the leading physicists of this date were not 

 prepared for the revolution of ideas inaugurated by Joule. His 

 papers were rejected by the Royal Society ; and for nearly six 

 years no one in authority accepted, or even alluded to, Joule's 

 views. The reasons for this silence were probably two-fold : the 

 apparent contradiction between Joule's results and Carnot's 

 theorem, and the small differences of temperature on which 

 Joule relied. 



In June, 1847, at the Oxford meeting of the British 

 Association came the memorable meeting of Kelvin and Joule, 

 when the latter read his paper before the Chemical Section, and 

 -was requested by the President "to cut it short." "This I 



