Physical Units

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. From the scientific standpoint the current and voltage are most readily defined in terms of forces. The ampere is defined as one-tenth of that current which, flowing in a circle of 1 cm. radius, exerts a force of 27r dynes upon a unit magnetic pole placed at its centre. This unit pole is itself defined in terms of the mechanical force between two such poles ; so the ampere is made to depend on me chanical units. Again, voltage X current X time is mechanical work and is expressed in terms of ergs or joules. Resistance is voltage divided by current, and thus the corresponding unit of resistance is defined. But for electric engineering (power and light) the choice is decided by the nature of the quantities them selves. Since resistance is a permanent quality of a substance, it is possible to select a certain piece of wire or tube full of mer cury, 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 practical electrical units.

It is known that 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 electrolyses 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, there is the method already described. Thirdly, the unit of elec tromotive 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 estab lish a simple relation between the latter and the absolute me chanical 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 i unit of energy in I unit of time.

British Association Units.

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 re sults of the committee's work, together with a large amount of most valuable matter on the subject of electric units, were col lected in a volume edited by Prof. Fleeming Jenkin in 1873, en titled Reports of the Committee on Electrical Standards. This committee continued to sit and report annually to the British Association since that date until 1912; the whole of the reports have since been reprinted in book form. It is to the labours of this committee, acting in friendly correspondence with interna tional committees, that the present system of electrical measure ments is due.

Electrostatic and Electromagnetic Units.—It has through out been recognized that a second consistent system of electrical units is possible. The unit electrical charge (or quantity of elec tricity) can be based upon the force between two electric charges. Accordingly, on the electrostatic system the unit of electric quan tity is such that f =-- / where q is the quantity of each of two equal charges, d is the distance between them and K is the dielectric constant (or specific inductive capacity) of the inter vening medium, that of air being taken as unity. (See ELECTRO STATICS.) The current is then defined as the amount that flows in unit time, and the energy is specified in terms of ergs or joules; thence the unit of resistance is defined by means of the equation The relation between the electromagnetic and electrostatic system of units is that the absolute EMU unit of current (i.e., amperes) is equal to 3 X mu' electrostatic units. (See the various articles on ELECTRICITY.) Practical Electric Units.—The committee of the British Association charged with the duty of arranging a system of absolute and magnetic units settled also on a system of practical units of convenient magnitude, and gave names to them as follows :— Tog absolute electromagnetic units of resistance = I ohm units of electro motive force = I volt of an „ „ unit of current =I ampere of an „ unit of quantity = I coulomb units of capacity =I farad