Page:EB1911 - Volume 27.djvu/765

 kilowatt-second, and 3600 kilowatt-seconds or 1 kilowatt-hour called a “Board of Trade unit” or a “kelvin.” This last is a unit of energy, not power. In British engineering practice the common unit of power is the “horse-power” ( HP &#8198;), which equals 550 foot-pounds performed per second, or 33,000 foot-pounds per minute; its equivalent in the metric system is about 746 watts, the ratio varying, however, with gravity.

Units of Heat.—In studying the phenomena of heat, two measurable quantities immediately present themselves:—(1) temperature or thermal potential, and (2) quantity of heat. Three arbitrary scales are in use for measuring temperature (see ), and each of these scales affords units suitable for the expression of temperature. On the Centigrade scale the unit, termed a “Centigrade degree,” is one-hundredth of the interval between the temperature of water boiling under normal barometric pressure (760 mm. of mercury) and that of melting ice; the “Fahrenheit degree” is one-hundred and-eightieth, and the “Réaumur degree” is one-eightieth of the same difference. In addition to these scales there is the “thermo-dynamic scale,” which, being based on dynamical reasoning, admits of correlation with the fundamental units. This subject is discussed in the articles and .

Empirical units of “quantity of heat” readily suggest themselves 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 ). In defining the calorie, therefore, the particular temperatures must be specified; consequently there are several calories particularized by special designations:—(1) conventional or common gramme calorie, the heat required to raise 1 gramme of water between 150° C. and 17° C. through 1° C.; (2) “mean or average gramme calorie,” one-hundredth of the total heat required to raise the temperature of 1 gramme of water from 0° C. to 100° C.; (3) “zero gramme calorie,” the heat required to raise 1 gramme of water from 0° C. to 1° C. These units are thus related:—1 common calorie= 1·987 mean calories=0·992 zero calories. A unit in common use in thermo-chemistry is the major calorie, which refers to one kilogramme of water and 1° 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 correlation of these units of quantity of heat with the fundamental units of mass, length and time attended the recognition of the fact that heat was a form of energy; and their quantitative relationships followed from the experimental determinations of the so-called “mechanical equivalent of heat,” i.e. the amount of mechanical energy, expressed in ergs, joules, or foot-pounds, equivalent to a certain quantity of heat (cf. ). These results show that a gram-calorie is equivalent to about 4·2 joules, and a British thermal unit to 780 foot-pounds.

Electrical Units.—The next most important units are the electrical units. We are principally concerned in electrical work with three quantities called respectively, electric current, electromotive force, and resistance. These are related to one another by Ohm’s law, which states that the electric current in a circuit is directly as the electromotive force and inversely as the resistance, when the current is unvarying and the temperature of the circuit constant. Hence if we choose units for two of these quantities, the above law defines the unit for the third. Much discussion has taken place over this question. The choice is decided by the nature of the quantities themselves. Since resistance is a permanent quality of a substance, it is possible to select a certain piece of wire or tube full of mercury, and declare that its resistance shall be the unit of resistance, and if the substance is permanent we shall possess an unalterable standard or unit of resistance. For these reasons the practical unit of resistance, now called the international ohm, has been selected as one of the above three electrical units.

It has now been decided that the second unit shall be the unit of electric current. As an electric current is not a thing, but a process, the unit current can only be reproduced when desired. There are two available methods for creating a standard or unit electric current. If an unvarying current is passed through a neutral solution of silver nitrate it decomposes or electrolysis it and deposits silver upon the negative pole or cathode of the electrolytic cell. According to Faraday’s law and all subsequent experience, the same current deposits in the same time the same mass of silver. Hence we may define the unit current by the mass of silver it can liberate per second. Again, an electric current in one circuit exerts mechanical force upon a magnetic pole or a current in another circuit suitably placed, and we may measure the force and define by it a unit electric current. Both these methods have been used. Thirdly, the unit of electromotive force may be defined as equal to the difference of potential between the ends of the unit of resistance when the unit of current flows in it.

Apart, however, from the relation of these electrical units to each other, it has been found to be of great importance to establish a simple relation between the latter and the absolute mechanical units. Thus an electric current which is passed through a conductor dissipates its energy as

heat, and hence creates a certain quantity of heat per unit of time. Having chosen our units of energy and related unit of quantity of heat, we must so choose the unit of current that when passed through the unit of resistance it shall dissipate 1 unit of energy in 1 unit of time.

A further consideration has weight in selecting the size of the units, namely, that they must be of convenient magnitude for the ordinary measurements. The founders of the modern system of practical electrical units were a committee appointed by the British Association in

1861, at the suggestion of Lord Kelvin, which made its first report in 1862 at Cambridge (see B. A. Report). The five subsequent reports containing the results of the committee’s work, together with a large amount of most valuable matter on the subject of electric units, were collected in a volume edited by Prof. Fleeming Jenkin in 1873, entitled Reports of the Committee on Electrical Standards. This committee has continued to sit and report annually to the British Association since that date. In their second report in 1863 (see B.A. Report, Newcastle-on-Tyne) the committee recommended the adoption of the absolute system of electric and magnetic units on the basis originally proposed by Gauss and Weber, namely, that these units should be derived from the fundamental dynamical units, but assuming the units of length, mass and time to be the metre, gramme and second instead of the millimetre milligramme and second as proposed by Weber. Considerable differences of opinion existed as to the choice of the fundamental units, but ultimately a suggestion of Lord Kelvin’s was adopted to select the centimetre, gramme, and second, and to construct a system of electrical units (called the C.G.S. system) derived from the above fundamental units. On this system the unit of force is the dyne and the unit of work the erg. The dyne is the uniform force which when acting on a mass of 1 gramme for 1 second gives it a velocity of 1 centimetre per second. The erg is the work done by 1 dyne when acting through a distance of 1 centimetre in its own direction. The electric and magnetic units were then derived, as previously suggested by Weber, in the following manner: If we consider two very small spheres placed with centres 1 centimetre apart in air and charged with equal quantities of electricity, then if the force between these bodies is 1 dyne each sphere is said to be charged with 1 unit of electric quantity on the electrostatic system. Again, if we consider two isolated magnetic poles of equal strength and consider them placed 1 centimetre apart in air, then if the force between them is 1 dyne these poles are said to have a strength of 1 unit on the electromagnetic system. Unfortunately the Committee did not take into