It has been explained how the pole of a magnet either of the bar or solenoidal form will exert a repellent or attractive force upon the pole of another magnet according as the poles concerned are like or unlike. Advantage is taken of this fact for the measurement of magnetic forces. Thus a pole is said to be a unit pole, or to possess a unit of magnetic force, when it exerts a force of one dyne upon a similar pole placed at a distance of one centimetre from it.

## Unit Of Electric Current

This may be measured by its magnetic effect upon a unit pole under specified conditions; thus if a current passing along a conductor, one centimetre in length and bent into an arc of a circle of one centimetre radius, exerts a force of one dyne upon a unit pole placed at the centre, the current is said to be one absolute unit of current.

## Ampere

The absolute unit current is considered to be too large for practical purposes, and in consequence the practical unit has been taken as 1/10 of the absolute unit. This is called the Ampere. The ampere is represented practically by the current which, when passed through a solution of nitrate of silver, in accordance with a definite specification, deposits silver at the rate of 0.00118 gramme per second. The ampere does not give an idea of the quantity of electricity passing through a conductor, but serves more as a gauge to indicate the size or volume, so to speak, of a current. The quantity of electricity is given by multiplying the current by the time during which it flows, and an unit quantity of electricity is said to pass any point in a conductor when one ampere flows for one second. This unit of quantity, which is called a Coulomb, is rarely made use of in practice, but serves by inference to indicate the value of an ampere. An analogy may be drawn between the flowing of water through pipes and the passage of current through a conductor. In the former case the water has a certain definite volume per unit length of pipe; but no idea of the quantity of water delivered by the pipe is obtained until the number of such volumes which have flowed through the pipe in a definite time is known. So with a current flowing in a conductor, no idea of the quantity of electricity can be formed until the time during which it flows is taken into consideration.