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The Mechanical Equivalent of Heat

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-287

2025-11-25

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The number of units of work required to produce one heat unit is called the mechanical equivalent of heat. Joule's experiments determined that the number of foot pounds of work necessary to heat 1 lb. of water 1° F. is 772, or to heat 1 lb. of water 1° C. is 1390. This is called Joule's equivalent. More recent determinations by Rowland give 778 and 1400 instead. To heat 1 kg. of water 1° C. requires 427 kilogram-meters of work. Joule's method was as follows: A cord attached to a weight was run over a fixed pulley and wound around the axle of a wheel, with paddles at the other end of the axle. This was arranged so that, on letting the weight fall, the paddles were caused to rotate in a known quantity of water in a vessel. The weight multiplied by the distance through which it falls gives the mechanical work, and the mass of water multiplied by the change of temperature gives the heat units. Joule's experiment showed that a 10-lb. weight falling through 77.2 ft. would raise the temperature of 1 lb. of water through 1° F.