LAWS or StEvov Elk:clam! CUIMENTS. .1,1V:111 tage is token of the action of an electric current on a magnet to define :t 'unit current.' A unit pole in magnetism is defined as such a pole that, if two of them are at the distance of one centi meter apa•t in a vacuum, the fierce leetueen them will he one dyne. A unit current is defined as being one of such intensity that. if it is passing through a wire bent into the form of a circle of radius one centimeter. the force due to it on a unit pole at the centre of the circle is 27r, where 7r — 3.14159. This unit is palled the C. (:. S. electromagnetic limit and it has been shown by experiment that its ratio to the 0:. S. electro static unit' is 3 X lum. A current of intensity 2 around the same eirenlor circuit •onld produce the force 17' on a unit pole at the centre; and experiments show that if a current of intensity i is passed around a circular circuit of radius r centimeters, the force on a unit pole at the centre is 27ri f there are n turns of wire making up a tlat coil of radius r. the force on a unit magnetic pole placed at the centre is The 'dimensions' of a magnetic pole are AO see MAGNETISM) : and since the force en a pole of strength ice due to an mort trie current as just described is - ; the dimensions of this fraction must be those of a force, i.e. Since the dimensions of nit are given and r is a length, i, the current-strength has the dimensions p, Therefore. on the C. G. S. electromagnetic sy,tent, an electric quantity has the dimensions All I) µ ; IL in these expressions represent, the dimension, of magnetic- permeability. or inductivity. as it is called.
li a small magnetic needle is pivoted at the centre of a coil with its axis in the plane of the coil, there will be a couple (q.v.) acting on the needle' tending to make it turn at right to OP coil. This couple may lee balanced against a couple due to the earth's field of magnetic force; and in this manner the eurrent-=.tremith i can be measured in terms of the earth's field of force. Such an instrument is called a `galvanmneter.' because it measures a steady electric eurrent. Ily means of it Faraday's laws of electrolysis may ice verified. The number of grains of any substance
liberated each second when a unit current passes through au electrolyte is called the 'elect ro-chemi cal equivalent' of that substance. The values of this quantity for sonic (dements is given in the following table: livitrog•n eaantat oxygen. C4q.pe•r n ou:52:10 Zinc 11.IM3S5 Silver 0.0111sO The number of tubes of magnetic induction (see AlvbxETism I which pass through any coil of wire when it carries a unit electric current is called its 'coefficient of and the number of tithes which pass through a second coil of Wire owing to this unit current in the first coil is called the 't-oeflicient elf mutual in duction' of the two eoils. It may that this sallIC 11111111)er would pass through the first coil if there were a unit eurrent in the sec ond. These coefficients of self and mutual in ductimi—sometinies called simply 'the induction' —are mathematical functions of the size and shape of the coils. of their relative positions. and of the surrounding medium, lint are independent of the currents. If 1,, is the eoeflicient of self-induction of one coil, and Al that of mu tual induction between it and a second coil. and if emrrents of intensities i, and i,, respectively, are flowing in the two coils, the number of tubes mf induction through the tirst coil is Li, 11 the eleetro-inoti•e force applied to the ter minals of a metal wire are varied, and the cor responding current measured, it is observed that one bears a constant relation to the Other. If i is the eurrent strength and E the eleetro-mo tive force. E=-11i, where is a constant for the win.. is found to vary directly as the length of the wire and inversely as it S cross ,-eetion, and to lee different for different metals or for the same metal at different temperatures.
is called the 'electrical resistance' of the given wire at whose terminals the E. Al. F. E. is ap• plied. This law connecting E. 11. and i is called law: The heating effect in the conductor has been shown to he Lit; and so this may lee written showing that the effect is the smile for a plus as for a minus current, i.e. it is indepen dent of the direction of the current.