Magnetic Field Strength (H) – also called Magnetising Force – is defined as the magnetomotive force (MMF or Fm) divided by the mean length (l) of the path taken by the magnetic flux. Magnetic Field Strength is measured in amperes per metre (A/m) – and not as you might expect ampere-turns per metre – since ‘turns’ is not an SI unit.

A circular coil consisting of a number of turns of wire wrapped around a ferrite core, is an example of a simple electromagnetic system.

Passing current through the coil causes a circular magnetic flux to be generated within the toroidal core, the direction of which may be predicted using the right hand corkscrew rule or the right hand grip rule. The mean length of the magnetic path (l) is taken to be path through the centre of the circular ring, as shown at the left in the above illustration.

#### Worked Example 1

Calculate the magnetic field strength H in a circular coil of mean radius 4 cm, wrapped with 200 turns of wire, if the current flowing through the coil is 0.1 A.
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The mean length of the magnetic path is given by the circumference of a circle of radius 0.04 m. Hence, l = 2 π r = 0.251327412 m

H = I N / l

= 0.1 × 200 / 0.251327412

= 79.58 A/m to 2 d.p.