NETISM.
The practical unit of current is the ampere, which is defined to be one-tenth of the electromagnetic unit of current. Thus, for example, the field inside a long coil having n turns of wire per centimetre is equal to 4lrCn/io, when the current C is ex pressed in amperes.
Since a current produces a magnetic field and so exerts a force on a magnetic pole, we should expect there to be a force on a current in a magnetic field. As a simple example, consider a circular wire, of radius r, carrying a current C, with a magnetic pole of strength m at the centre of the circle. The force on the pole will be 2lrCm/r dynes, in a direction perpendicular to the plane of the circuit. There must therefore be an equal and opposite force on the wire. The force on one centimetre of the wire is therefore equal to
But
is the field strength at the wire due to the pole, so we see that there is a force on a current C, in a perpendicular magnetic field F, equal to CF per centimetre. The direction of the force is perpendicular to the plane containing C and F. If the circuit and the pole are in a medium of magnetic permeability µ differing from unity, then the field F due to the pole m is equal to
so that the force per centimetre on the circuit is equal to p.CF, instead of CF. (See MAGNETISM.) A magnetic field, parallel to a wire carrying a current, exerts no force on the wire, for the field is symmetrical about the wire so that there can be no force. The force on a wire carrying a current C in a mag netic field F is therefore equal to µCF sin 0 per centimetre, where 0 is the angle between C and F, for F sin 0 is the component of F perpendicular to C. Fig. 19 shows the lines of force due to a current in a perpendicular magnetic field. On one side of the wire there is a stronger field than on the other, because the two fields are in the same direction on one side and in opposite directions on the other side. The force on the wire is in the direction from the stronger field to the weaker field. To a person looking along the direction of a current the field of the current is from left to right above the wire and from right to left below it. A magnetic field from left to right therefore gives a downward force on the wire.
Two straight parallel wires carrying currents in the same di rection attract each other, but repel each other when the cur rents are in opposite directions.
If d is the distance between the wires and C and C' the currents, then the field at one wire due to the other one is 2C/d, so the force per centimetre is 2CC'/d dynes. Electrodynamometers are instruments for determining a current by measuring the force exerted by one portion of a cir cuit carrying the current on an other portion. The current is passed through a fixed coil, and also through another coil which is suspended from a balance by means of which the vertical force between the two coils is meas ured. The current can be calculated when the distance between the coils and the size of the coils are known (see INSTRUMENTS,
