Ans. In British thermal units (B.t.u.).
Ques. What is the British thermal unit?
Ans. The quantity of heat required to raise the temperature of 1 lb. of pure water 1° Fahr., at or near 39.1° F., the temperature of maximum density.
Ques. What is the mechanical equivalent of heat?
Ans. The number of foot pounds of mechanical energy equivalent to one British thermal unit.
Joule’s experiments 1843-50 gave the figure 772 ft. lbs. which is known as Joule’s equivalent. Later experiments gave higher figures, and the present accepted value is 778 ft. lbs., that is: 1 B.t.u. = 778 ft. lbs.
Electrical Horse Power.—It is desirable to establish the relation between watts and foot pounds in order to determine the capacity of an electric generator or motor in terms of horse power.
One watt is equivalent to one joule per second or 60 joules per minute. One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal:
60 × .7374 = 44.244 ft. lbs.
Since one horse power = 33000 ft. lbs. per minute, the electrical equivalent of one horse power is