Force, being measured by the change of momentum in unit time, is expressed in terms of the same units in which unit mo mentum is defined. The natural British unit is the "poundal," the force which in one second accelerates the velocity of a mass of one pound by one foot per second; but it has not come into general use. The metric (and scientific) unit, named the "dyne," is derived in the same way from the centimetre, gramme, and second. The poundal and dyne are related as follows poundal =13,825.5 dynes.
A common unit of force among engineers is the "weight of one pound," by which is meant the force equivalent to the gravi tational attraction of the earth on a mass of one pound. This unit obviously depends on gravity; and since this varies with the lati tude and height of the place of observation (see EARTH, FIGURE OF), the "force of one pound" of the engineer is not constant. Roughly, it equals 32.17 poundals. A pressure is a force per unit area and can be measured in terms of the above units; for exam ple, as lb. wt. per square foot. The standard atmosphere (corre sponding to a barometric height of 760 cm.) is often taken as a secondary unit; it is equal to 1,013,600 dynes per centimetre.
Energy or work is measured by force acting over a distance. The scientific unit is the "erg," which is the energy expended when a force of one dyne acts over 1 centimetre. (See ENERGY.) This unit is too small for measuring the quantity of energy asso ciated, for instance, with engines; for such purposes a unit ten million times as great, termed the "joule," is used. As we no ticed in the case of units of force, common-life experience has led to the introduction of units dependent on gravitation, and there fore not invariable; the common British practical unit of this class is the "foot-pound"; in the metric system its congener is the "kilogramme-metre." Power is the rate at which force does work; it is therefore ex pressed by "units of energy per second." The metric unit in use
is the "watt," being the rate equal to 1 joule per second. Larger units in practical use are: "kilowatt," equal to i,000 watts; the corresponding energy unit being the kilowatt-second, and 3,600 kilowatt-seconds or 1 kilowatt-hour called a "Board of Trade unit" (B.T.U.). This last is a unit of energy, not power. In British engineering practice the common unit of power is the "horse-power" (HP), which equals 55o foot-pounds performed per second, or 33,00o foot-pounds per minute; its equivalent in the metric system is taken as 746 watts.
Empirical units of "quantity of heat" readily suggest them selves as the amount of heat necessary to heat a unit mass of any substance through unit temperature. In the metric system the unit, termed a "calorie," is the quantity of heat required to raise a gramme of water through one degree Centigrade. This quantity, however, is not constant, since the specific heat of water varies with temperature. (See CALORIMETRY.) In defining the calorie, therefore, the particular temperatures must be speci fied; consequently there are several calories particularized by spe cial designations :—(i) gramme-calorie, the heat required to raise i gramme of water between 15° C and 17° C through I° C; (2) "mean or average gramme calorie," one-hundredth of the total heat required to raise the temperature of I gramme of water from o° C to 100° C; These units are thus related common calorie= o.987 mean calories=o.992 zero calories. A unit in com mon use in thermo-chemistry is the greater calorie, which refers to one kilogramme of water and I° C. In the British system the common unit, termed the "British Thermal Unit" (B.Th.U.), is the amount of heat required to raise one pound of water through one degree Fahrenheit; a centigrade unit (C.H.U.) has also been introduced and is rapidly being adopted.