Experiments show that one unit of heat is equivalent to 772 ft.-lbs. of work, and when this quantity of work disappears in friction, one unit of heat is generated. Other experiments show that the unit should be 778 ft.-lbs. It is not of much importance which number is used; some use one, and some use the other, but all agree in naming this quantity of work after the discoverer of the relationship, James P. Joule of Manchester, England. The unit is therefore called Joule's equivalent, or the mechanical equivalent of heat.

Example. - One pound of good coal gives out on complete combustion, 14,500 B. T. U. of heat. Find the amount of work stored in one pound of coal.

Units of heat X Mechanical equivalent = Work in foot-pounds.

14,500 X 772 = 11,194,000 ft.-lbs.