350 JAMES PRESCOTT JOULE. 



not change its position, and this we call heat ; but when the sum of 

 the individual motions is equal to the motions of the first body, the 

 two bodies have the same amount of energy. One condition we call 

 mechanical, the other heat. In a proper sense they may all be called 

 mechanical conditions, but it is convenient to designate the varieties 

 by different names. Joule evidently saw this at the outset, for his 

 experiments were all devised to fit such hypothetical conditions, and 

 they therefore show how clear his mechanical conceptions were, and 

 how far in advance he was of any of his contemporaries. 



His experiments upon the mechanical equivalent of heat were con- 

 tinued from year to year for a long time. In 1847 he had reduced 

 the equivalent to 781.8, and in 1849 to 772 foot pounds, the same 

 number we use to-day. He followed out the fundamental idea in all 

 sorts of ingenious ways ; by friction in water, in oil, in mercury, in the 

 velocity of sound in air, in determining the velocity of a molecule of 

 hydrogen under standard conditions of temperature and pressure, in 

 calculating specific heats, in the efficiency of steam and air engines, 

 the effects of pressure, and the temperature of meteors while passing 

 through the atmosphere. By 1852, however. Joule's work became 

 pretty generally known in Europe, and he had been elected a member 

 of numerous societies there before he had been thus honored at home. 

 It was the old story. The wise man lives in the next town, he is not 

 our neighbor. While the learned of Europe were honoring him, the 

 people of Manchester were asking, " Who is this Joule of Manchester 

 we are hearing about ? " 



In 1864 he was made one of the Committee of the British Associa- 

 tion to devise a system of electrical units, which gave us the volt, 

 the ohm, and the rest of the unit terminology so indispensable now. 

 In his earliest work, it was necessary for him to know with greater 

 definiteness than was possible with such a system as was then in use, 

 or rather the lack of it, and he devised one which is easily trans- 

 formed into the absolute one we now employ, and his services in that 

 committee were invaluable. The determination of the ohm resulted 

 in a larger figure for the mechanical equivalent of heat than Joule 

 had found in other ways, and he offered to change its value to cor- 

 respond ; but it afterwards appeared that his value was nearer right 

 than the new one thus obtamed. In 1878 his determination of this 

 quantity was 772.55 pounds. This mechanical equivalent is some- 

 times called a Joule, and has J for its symbol in honor of the 

 discoverer. 



The statement of the relationship between heat and work is the 



