TILE C. (:. ELECTROSTATIC system starts with the following definition: A unit electric charge is such a one that if two particles of matter, placed in a vacuum at a distance of one centimeter apart, are each charged with a unit charge. the force of action of one on the other will be (al• dyne. Based on this definition are the following units: Two points are at a unit difference of potential, if it requires one erg to carry a unit charge from one point to the other. The capacity of a condenser is unity. if, when charged to a unit difference of potential. there is a unit charge on each plate of the condenser. A unit current is such that if it flows for one second. an electric eharge equal to unity is carried. on this unit current all the magnetic quantities could lie based.
Tut: C. O. S. Eta:crnic-mAGNEnc system starts the following definition: A unit magnetic pole is such that if two such poles are at a dis tance apart of one centimeter in a vacuum, the force of action of one on the other is one dyne. On this definition is then based that of a unit electric current, viz., it is such a current that, if it is flowing through a linear conductor bent. in the form of a circle of radius r cm.. the force on a unit magnetic pole placed at the centre of the circle is 2 7r /r, where as usual 7r = 3.14159. A unit quantity is the charge carried by a unit current in ( ne second, and to this can be referred units of potential and capacity: a unit eleetro InCliye force between two points is such that it requires one erg to carry a unit quantity between them, or if a unit current is flowing between them an amount of energy equal to one erg is set free in one ceeondt a linear conduetor of unit re sistance is such that if a unit electro-motive force is applied at its ends a unit current will be pro duced a unit of induetion is the induction in a cir•uit when the electro-motive force induced in this circuit is unity while the inducing cur rent varies at the rate of a unit current per second.
Experiments have established the conneetiou between the units for the saute quantity in the two systems. Knowing this ratio for any quan tity. e.g. electric current, it may be at once de duced for others. It is thus estaldished that Electrostatic unit Electro-magnetic unit of quantity or current = of quantity or current-I v of potential or E.INI.F.= of potential or v of capacity = of capacity • : where r = 3 X or 30,000,000.000 very ap proximately. This is the same number as that ex pressing the velocity of light as it should be Re cording, to the electro-inagnetie theory of light, which identities the luminiferous ether with the medium serving for the propagation of disturb ances due to electric oscillations. see LIGHT: ELECTRICITY.
These S. eleetro-magnetie and electrostatic units are, in nearly every ease, of inconvenient mag nitude for practical purposes; and. further, their definitions are not such as lead easily and direct ly to methods of measurement. Consequently, certain so-called 'practical units' have been adopt ed which are of convenient magnitude and bear simple relations to the absolute units. so far as
possible. The following are these practical units, as legally defined: The unit of resistance shall be what is known as the international ohm. which is substantially equal to 1,000.000.000 units of resistance of the centimeter-gram-second system of clectro-ma,Lmetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at the tempera ture of melting ice, 14.4521 grams in mass. of a constant •ross-se•tional area, and of the length of 106.3 centimeters. (This would require a cross-section of 1.00003 square millimeter-. ac cording to the accepted value for the density of mercury.) One million 10G (duns, is called a `nicgolun! The unit of current shall he what is known as the international ampere, which is 0.1 of the unit of current of the centimeter - gram - second system of eleetro-magnetie units, and is the prac tical equivalent of the unvarying current. which. When passed through a solution of nitrate of silver in water. in accordance with standard specifications (see .\alt'ERr.I, deposits silver at the rate of 0.001118 of a gram per second. (inc thousandth of an ampere is called a milliampere.
The unit of electro-motive force shall be what is known as the international volt, whieli is the vslietro-motive force that. steadily applied to a conductor whose resistance is one international ohm, will produce a eurrent of an international ampere. and is practically equivalent to +71 of the electro-motive force between the poles or •leetrodos of the voltaic cell known as Clark's cell, at a temperature of 15° C., and prepared in the Ili:11111(•r described in the standard specifica tions. Fee VOLT.
The unit of quantity shall he what is known as the international eoulomb, which is the quantity of electricity t ransferred by a current of one inter tuitional ampere in one See Cot The unit of capacity shall be what is known as the international farad I se 1. F AD) which is the eapaeity of a condenser charged to 11 po tential of one international volt Iry one inter national coulomb of electricity. One-millionth of a farad is called a mierofarad.
The unit of work shall be the joule. which is equal to 10.000.000 units of work in the (2116 ineter-gram-sccond system. and which is practi cally equivalent to the energy expended in one second by an international ampere in an inter national ohm.
The unit of power shall be the watt, which is equal to 10,000.000 units of power in the centi meter-gram-second system. and which is prac tically equivalent to the work done at the rate of one joule per second. One thousand watts is called a kilowatt.
The unit of induction shall be the henry, which is the induction in a circuit when the electro motive force induced in this circuit is one inter national volt while the inducing current varies at the rate of one ampere per second. See HENRY.