Electric Current

A current of electricity is the quantity of electricity which passes in one second across any section of the conductor in which it is flowing.

Thus, if in one second the quantity which crosses any section is the unit quantity, the measure of the current is unity.

A unit current is said to flow in a conductor when unit quantity of electricity is transferred across any section in one second.

But as yet we have no definition of the unit quantity of electricity. To obtain this, we shall consider certain other properties of an electric current.

A current flowing in a conductor is found to produce a magnetic field in its neighbourhood. Magnetic force is exerted by the current, and the pole of a magnet placed near the conductor will be urged by a force definite in direction and amount. If the conductor be in the form of a long straight wire, a north magnetic pole would tend to move in a circle round the wire, and the direction of its motion would be related to the direction of the current in the same way as the direction of rotation is related to that of translation in a right-handed screw.

If instead of a magnetic pole we consider a compass needle placed near the wire, the needle will tend to set itself at right angles to the wire, and if we imagine a man to be swimming with the current and looking at the needle, then the north end will be turned towards his left hand.

As to the intensity of the force, let us suppose that the length of the wire is l centimetres, and that it is wound into the form of an arc of a circle, r centimetres in radius; then when a current of intensity i circulates in the wire, it is found that the magnetic force at the centre is proportional to li/r² and acts in a direction at right angles to the plane of the circle, and if i be measured in proper units, we may say that the magnetic force is equal⁽¹⁾ to li/r². If we call this F, we have

Let the length of the wire be one centimetre, and the radius one centimetre, and let us inquire what must be the strength of the current in order that the force on a unit magnetic pole may be one dyne.⁽²⁾

We have then in the equation

and it becomes therefore

that is, the strength of the current is unity, or the current required is the unit current. Thus, in order that the equation

may be true, it is necessary that the unit current should be that current which circulating in a wire of unit length, bent into the form of an arc of a circle of unit radius, exerts unit force on a unit magnetic pole placed at the centre.

But we have seen already that the unit current is obtained when unit quantity of electricity crosses any section of the conductor. We have thus arrived at the definition of unit quantity of electricity of which we were in search.

This definition is known as the definition of the electromagnetic unit of quantity.

(1)	See p. 394.
(2)	See chap. ii. p. 18.