Precision measurement and calibration :

Abstract

Consequently, a variety of "calories" originated, such as the 4°-calorie, the 15°-calorie, the 20°-calorie, the mean (0-100°) calorie, etc. Although the comparative heat measurements could be made with relative ease, it was recognized very early that it was necessary to determine the quantity of energy corresponding to a given calorie. The energy corresponding to a specific calorie was called J, the mechanical equivalent of heat 10 . 8-7Investigations of this quantity were carried out over most of a century with either mechanical or electrical sources of energy.With the development of accurate electrical standards and instrumentation, it became possible to measure electrical energy more precisely than heat could be measured in terms of the heat capacity of water. After this time, the calorie and the mechanical equivalent of heat had little significance except as measurements of the heat capacity of water. Out of habit, however, the calorie continued to be used as a heat unit, so that it became desirable in accurate calorimetry to separate the definition of calorie from the heat capacity of water. As a result, there are two "dry" or defined calories used today 11 .In engineering steam tables, the I.T. calorie (International Table calorie