CALORIE (also spelt CALORY). A unit of heat defined as the quantity of heat required to raise a unit mass of water one degree in temperature on the Centigrade scale, in which the freezing point of water is taken as o°C. and the boiling point as 00°C. The magnitude of the unit is proportional to the unit of mass selected, so that it is also necessary to specify the unit of mass in defining the quantity of heat measured. The following units of this type are in general use for various purposes.
The gramme-calorie (or small calorie) defined as the quantity of heat required to raise the temperature of 1 gramme of water by 1°C., is the unit of heat most commonly employed for scien tific purposes for which the gramme is taken as unit of mass. The kilogramme-calorie (kilocalorie or large calorie) is equal to ',coo gramme-calories, and is commonly employed by engi neers in countries where the metric system is adopted. The pound calorie is similarly employed in connection with the British sys tem of weights, and is equal to 453.6 gramme-calories, if 1 lb. is taken as 453.6 gm.
The term calorie is also employed in stating the total heat or total energy of a substance, such as water, or steam, when measured per unit mass on the Centigrade scale. It is unnecessary in this case to specify the unit of mass because the total heat measured in pound-calories per pound is obviously the same for any given substance and range of temperature as that measured in gramme-calories per gramme, provided that the same unit of mass is employed in either case in measuring the substance and in defining the heat unit. In this connection the mean calorie is most commonly employed, defined as one hundredth part of the increase of total heat of water per unit mass between o° and ioo°C. under atmospheric pressure.
Similar units of heat may be defined in terms of the Fahrenheit and Reaumur scales of temperature, but are rarely, if ever, called by the name calorie, or employed for purely scientific purposes. By far the most important of these is the British Thermal Unit (B.Th.U.), defined as the quantity of heat required to raise one pound of water one degree on the Fahrenheit scale. One B.Th.U. is equal to 453.6/1.8 or 252 gramme-calories pre cisely.
The absolute values of all the above units of heat depend on the properties of water in a manner which is explained in the article CALORIMETRY. Their relations to other units are further illustrated in the article HEAT.