JOULE 289 



circuit. He constructed for this purpose an Induction 

 machine with steel magnets, the rotating, current-producing 

 armature of which was bulk Into a calorimeter, while the 

 current was led away to the outside to be measured. He 

 found by comparison of the results on open and closed cir- 

 cuits, that here also only heat appears in the source of current, 

 as in every other conductor, and he immediately regarded 

 the heat appearing in the whole circuit as the equivalent, 

 not of chemical transformation, as In the cells, but of 

 the mechanical work used by the machine, just as In the 

 case of Rumford's friction experiment. He now allowed 

 the machine to be driven by falling weights, in order to 

 measure the mechanical work, and he thus determined 

 the number of foot-pounds which are equivalent to a 

 calorie. 



While Joule's law for the heat of the current had already 

 been published before Robert Mayer's first paper (1840 and 

 1 841), the experiments last described were published after 

 Mayer, In 1843. The agreement, within the limits of ac- 

 curacy then obtainable, of the equivalent found by Joule In 

 so curious a manner, that is to say by introducing electrical 

 current generation (calculated In metre-kilograms it was 460 

 for one degree centigrade and i kilogram calorie), with the 

 result found by Robert Mayer in a totally different con- 

 nection, namely by the warmth produced in gases (365 In 

 the same units), must have immediately led to full recogni- 

 tion of the importance of Robert Mayer's discovery, and also 

 of Joule's similar line of thought, that Is of the energy prin- 

 ciple, by all those who could understand it; there could be no 

 further doubt that new insight of the widest possible range 

 had been actually won Into the interconnection of natural 

 processes, as Robert Mayer believed from the start, and all 

 that remained was to see how far the new ideas agreed with 

 what was already known. This agreement was demonstrated 

 In principle and on general lines by Robert Mayer in his 



Us 



