Chap, xxxiv.] MECHANICAL EQUIVALENT OF HE A T. 453 



indicates heating of the junction against which the 

 current was directed. The energy required to com- 

 press the air and drive it out of the bellows has 

 partly heated the air. The converse experiment, 

 the transformation of heat into mechanical energy, 

 has teen, beautifully shown by an experiment of 

 Tyndall's. Air was compressed in a small metallic 

 box by means of a pump, and the stopcock then closed. 

 Heat was developed in the process. By letting the 

 box stand for a time the heat disappeared. The 

 stopcock was then opened, and the current, expelled by 

 the force of the compressed air, was directed against 

 a face of the thermopile, which indicated cooling by 

 a deflection of the needle ; the expansion of the air 

 had used up heat. 



But there is a definite relationship between the 

 mechanical energy and the amount of heat. This 

 relationship Joule worked out, and called " the 

 mechanical equivalent of heat." He used a weight 

 which was permitted to fall a certain distance. 

 During its fall it turned a brass paddle-wheel, rotating 

 about a vertical axis in a copper vessel tilled with 

 water. The agitation of the water by the wheel 

 raised the temperature, the increase of temperature 

 being measured at the conclusion of the experiment. 

 He found that by permitting a weight of 1 pound 

 to fall through a distance of 772 feet sufficient heat 

 was generated to raise the temperature of 1 pound 

 of water 1 Fahr. The work done is that of a weight 

 of 1 pound falling through a distance of 772 feet, or, 

 what is the same thing, 772 pounds falling through a 

 distance of 1 foot. This is expressed shortly by the 

 phrase 772 foot-pounds. The same amount of work 

 would be required to raise 1 pound 772 feet high, or 

 772 pounds 1 foot high. This amount of work, then, 

 is equivalent to an amount of heat sufficient to raise 

 the temperature of 1 pound of water 1 Fahr. But 



