James Prescott Joule.
[James Prescott Joule] commenced the experimental investigations which have made him famous at some time previous to 1843, at which date he published, in the Philosophical Magazine, his earliest method. His first determination gave 770 foot-pounds. During the succeeding five or six years Joule repeated his work, adopting a considerable variety of methods, and obtaining very variable results. One method was to determine the heat produced by forcing air through tubes; another, and his usual plan, was to turn a paddle-wheel by a definite power in a known weight of water. He finally, in 1849, concluded these researches.
The method of calculating the mechanical equivalent of heat which was adopted by Dr. Mayer, of Heilbronn, is as beautiful as it is ingenious: Conceive two equal portions of atmospheric air to be inclosed, at the same temperature—as at the freezing-point—in vessels each capable of containing one cubic foot; communicate heat to both, retaining the one portion at the original volume, and permitting the other to expand under a constant pressure equal to that of the atmosphere. In each vessel there will be inclosed 0.08073 pound, or 1.29 ounce, of air. When, at the same temperature, the one has doubled its pressure and the other has doubled its volume, each will be at a temperature of 525.2° Fahr., or 274° C, and each will have double the original temperature, as measured on the absolute scale from the zero of heat-motion. But the one will have absorbed but 63∕4 British thermal units, while the other will have absorbed 91∕2. In the first case, all of this heat will have been employed in simply increasing the temperature of the air; in the second case, the temperature of the air will have been equally increased, and, besides, a certain amount of work—2,116.3 foot-pounds—must have been done in overcoming the resistance of the air; it is to this latter action that we must debit the additional heat which has disappeared. Now, (2,116.3/23∕4) = 770 foot-pounds per heat-unit—almost precisely the value derived from Joule’s experiments. Had Mayer’s measurement been absolutely accurate, the result of his calculation would have been an exact determination of the heat-equivalent, provided no heat is, in this case, lost by internal work.
Joule’s most probably accurate measure was obtained by the use of a paddle-wheel revolving in water or other fluid. A copper vessel contained a carefully weighed portion of the fluid, and at the bottom was a step, on which stood a vertical spindle carrying the paddle-wheel. This wheel was turned by cords passing over nicely-balanced grooved wheels, the axles of which were carried on friction-rollers. Weights hung at the ends of these cords were the moving forces. Falling to the ground, they exerted an easily and accurately determinable amount of work, W × H, which turned the paddle-wheel a definite number of revolutions, warming the water by the production of an amount of heat exactly equivalent to the amount of work done. After the weight had been raised and this operation repeated a sufficient number of times, the quantity of heat communicated to the water was carefully determined and compared with the amount of work expended in its development. Joule also used a pair of disks of iron rubbing against each other in a vessel of mercury, and measured the heat thus developed by friction, comparing it with the work done. The average of forty experiments with water gave the equivalent 772.692 foot-pounds; fifty with mercury gave 774.083; twenty with cast-iron gave 774.987—the temperature of the apparatus being from 55° to 60° Fahr.
Joule also determined, by experiment, the fact that the expansion of air or other gas without doing work produces no change of temperature, which fact is predicable from the now known principles of thermo-dynamics. He stated the results of his researches relating to the mechanical equivalent of heat as follows:
1. The heat produced by the friction of bodies, whether solid or liquid, is always proportional to the quantity of work expended.
2. The quantity required to increase the temperature of a pound of water (weighed in vacuo at 55° to 60° Fahr.) by one degree requires for its production the expenditure of a force measured by the fall of 772 pounds from a height of one foot. This quantity is now generally called “Joule’s equivalent.”
During this series of experiments, Joule also deduced the position of the “absolute zero,” the point at which heat-motion ceases, and stated it to be about 480° Fahr. below the freezing-point of water, which is not very far from the probably true value,-493.2° Fahr. (-273° C.), as deduced afterward from more precise data.
The result of these, and of the later experiments of Hirn and others, has been the admission of the following principle:
Heat-energy and mechanical energy are mutually convertible and have a definite equivalence, the British thermal unit being equivalent to 772 foot-pounds of work, and the metric calorie to 423.55, or, as usually taken, 424 kilogrammetres. The exact measure is not fully determined, however.