What is Magnetic Field Intensity

The m.m.f. required to magnetize a unit length of a magnetic path is termed the magnetizing force or magnetic field intensity (H) for that portion of the magnetic circuit.

The magnetic field strength only applies to the part of the magnetic circuit that is made of the same material throughout and has a uniform cross-section. The ampere-turns per meter (H) unit is denoted by the symbol.

As with magnetomotive force, the ‘turns’ component of the expression is meaningless and should be removed to leave only the amperes per meter in pure SI units. For everyday use, however, “ampere-turns per meter” is more accurate. To clarify:

H = IN/I


l = length of magnetic circuit in meters

Remember to use the metric system when calculating distance! Never confuse magnetomotive force with magnetizing force.


In the following magnetic circuit, determine the magnetic field intensity (H):

figure 1 magnetic field intensity
figure 1 magnetic field intensity


In this case, we’ll use the following formula to determine the strength of the magnetic field (H).

figure 2
figure 2

The value of H is sensitive to the size of the magnetic path, so if we were to alter those parameters, the value of H would shift. If we double the length of the magnetic path, for instance, we can expect the magnetic field intensity, or H, to drop to half of what it was before.

If the above magnetic circuit has the dimensions, the magnetic field strength (H) will be;

figure 3
figure 3

Therefore, it is evident that the magnetizing force varies inversely per unit length of the total magnetic path for a fixed number of ampere-turns.