The Heat Unit *

Abstract

This paper gives the history of various calories and British thermal units which have been in use.

International agreement has now been reached that the joule (10⁷ ergs) should be the fundamental unit of heat. The advantages of this unit are that it is (1) independent of the intrinsic properties of any particular substance, (2) independent of the temperature scale, and (3) easily realizable in terms of the electrical units.

Calorimetric apparatus nowadays is calibrated by electrical heating and the electrical units are known with high precision. Since the international system of electrical units has now been superseded by the absolute system, it is necessary to express in absolute units the various heat units formerly defined in terms of international electrical units.

The international steam table calorie defined as 1/860 of an international watt hour, when expressed in absolute joules, becomes 4.1868. The other calorie which is defined in terms of the international electrical units is the thermochemical calorie. This, in absolute joules, becomes 4.1840.

Table 1 shows conversion factors to joules.