250 PRINCIPLES OF THE MECHANICAL THEORY OF HEAT. 



and the same operation was repeated. After this had been done 20 times the 

 elevation of the temperature produced by the above means in the water of the 

 reservoir A was measured, and was found to amount to nearly 0°.6 F. The 

 mechanical power expended in the production of this effect is obtained by 

 multiplying the weights by the total distance through which they have fallen, 

 with allowance, however, for the acceleration with which,, each time, they have 

 descended to the floor. 



In the mode here described Joule has conducted a long series of researches, 

 and calculates, as a mean after the application of the necessary corrections, that 

 an expenditure of power equivalent to 773.64 foot-pounds develops under the 

 above circumstances as much heat as is required to raise the temperature of one 

 pound of water 1° F., or, in other terms, that a unit of heat (French) is the 

 thermal equivalent of a mechanical expenditure of j)oicer of 435 ¥dogram-metres. 



The friction of an iron paddle-wheel in quicksilver gave 776.3 foot-pounds, 

 and the friction of cast-iron plates with one another 774.88 foot-pounds as the 

 expendture of power which is necessary to raise the temperature of one pound of 

 water 1° F. 



A not very different result was obtained hy Joule when he compared the 

 quantity of heat set free in the coils of an electro-magnet rotating between 

 strong magnetic poles, with the mechanical power necessary to produce this rota- 

 tion, (Phil. Mag. vol. xxiii.) For determining the heat developed in the coils of 

 the rotating electro-magnet, the latter was introduced into a glass tube in such 

 a manner that the interval between the magnet and the glass wall formed a ves- 

 sel closed on all sides, which was filled with water. Through the heat devel- 

 oped by the rotation of the electro-magnet, the temperature of this water was 

 raised, and the increase of temperature carefully ascertained. In order to deter- 

 mine the mechanical power required to produce the rotation, a string was wound 

 around the prolongation of the axis of rotation, and the revolution of the magnet 

 effected by a weight suspended to the string. From this experimenti Joule com- 

 puted that, for the production of an amount of heat capable of raising one pound 

 of water 1° F. a mechanical power of 838 foot-pounds is requisite, and thus the 

 unit of heat corresponds to an expenditure of poiver of 460 kilogram-metres. 



To the same physicist we owe an experiment for determining the mechanical 

 equivalent of heat through that which is liberated by compression of the air, 

 (Kronig's Journal, iii). Into a copper reservoir A, 12 inches in length, 136^ 

 cubic contents, ^-inch thickness of wall, by means of a compression pump screwed 

 to it, air was pressed, as into the bulb of an air-gun, until an elastic force of 

 nearly 22 atmospheres was attained. During this operation the copper reservoir, 

 together with the pump, was immersed in a vessel which held 45 pounds 3 ounces 

 of water. By 300 strokes of the piston the air in the vessel was condensed from 

 1 to 21.654 atmospheres, and so much heat was thereby developed that the tem- 

 perature of cool water rose 0°.645 F. This increase of temperature, how- 

 ever, did not arise alone from the compression of the air, but also from the 

 friction of the piston. To elinunate this last, the tube through which the air 

 had been introduced was closed, and it was found that, through 300 strokes of 

 the jjiston, which now were not attended by a compression of the air in the 

 reservoir, the temperature of the cool water was raised 0°.297 F. By this 

 first experiment, therefore, on computing the results of compression of the air, 

 an elevation of temperature of 0°.348 F. is given. 



After making the necessary reductions and corrections, it now resulted that 

 through the compression of 2956 cubic inches of dry air, of atmospheric density, 

 into a space of 136.5 cubic inches, such a quantity of heat was developed as was 

 necessary to raise the temperature of one pound of ^tater 13°.628 F. This is 

 equivalent to the quantity of heat required to raise the temperature of 3437 

 grams of water 1° vJ. 



