26 Britain's Heritage of Science 



The crucial experiment was performed by Joule with the 

 result again in his own words " that we have therefore in 

 magneto-electricity an agent capable by simple mechanical 

 means of destroying or generating heat." The second part 

 of the same paper, entitled " On the Mechanical Value of 

 Heat," begins as follows : 



" Having proved that heat is generated by the magneto- 

 electrical machine, and that by means of the inductive 

 power of magnetism we can diminish or increase at 

 pleasure the heat due to chemical changes, it became an 

 object of great interest to enquire whether a constant 

 ratio existed between it and the mechanical power gained 

 or lost. For this purpose it was only necessary to repeat 

 some of the previous experiments and to ascertain, at the 

 same time, the mechanical force necessary in order to 

 turn the apparatus." 

 He thus finds that 



" The quantity of heat capable of increasing the 

 temperature of a pound of water by one degree of Fahren- 

 heit's scale is equal to, and may be converted into, a 

 mechanical force capable of raising 838 Ibs. to the 

 perpendicular height of one foot." 



The particular method adopted to determine what we 

 now call the mechanical equivalent of heat was beset with 

 many experimental difficulties, and it is not therefore sur- 

 prising that his first result was nearly 9 per cent, in error. 

 Osborne Reynolds observed that the paragraph quoted really 

 overstates the conclusions Joule was entitled to draw, because 

 he has only shown that work could be converted into heat, 

 but not the inverse process, and that, at that time, he had 

 no clear ideas as to the conditions under which heat may be 

 converted into work. In fact he had dealt only with the 

 first law of thermodynamics, and it took some years before 

 the second law could be formulated with precision. It must 

 be remembered, however, that Joule was only twenty-five 

 years old at the time of his great discovery, and that he 

 was worldng alone, unsupported, and opposed by all the 

 prejudices of the recognized authorities, 



It is not necessary to refer here in detail to the skill with 

 which Joule extended his investigations in many directions, 



