THE MAGNETIC CIRCUIT
In an electric circuit, a current is established due to the existence of an electromotive force. In the same way, in a magnetic circuit, a flux is established due to the existence of a magnetomotive force. The mmf is produced by the current flowing in the coil and its value is the product of the current and the number of turns on the coil.
Magnetomotive Force = Current x Number of Turns on the Coil
Note that, although mmf is quoted in ampere turns, the actual unit dimension is in amperes.
MAGNETISING FORCE
The magnetomotive force can be expressed in terms of the length of the magnet. It is then referred to as the magnetising force or magnetic field strength and given the symbol H. The magnetising force is a measure of the intensity of the magnetic effects at any given point in the magnetic field.
Magnetising Force (H) = Magnetomotive Force / Length of magnet
Note that:
• The unit of field strength is ampere per metre, although it may be quoted as ampere turns per metre.
• The length of a solenoid ‘l', is the length along its axis and not the length of wire from which the solenoid is made.
It will therefore be seen that a solenoid having 10 turns per metre carrying a current of 6A (10* 6 = 60 ampere/metre) will produce the same strength of magnetic field as one of 12 turns per metre carrying 5A (12 *5 = 60 ampere/metre).