MAGNETIC FIELD INTENSITY (H)

When two magnetic poles of strengths 𝑚₁ and 𝑚₂ webers are placed at a distance r meters in a medium of relative permeability 𝜇ᵣ , they experience a force 𝑭 of attraction or repulsion, given by

F=m1×m24π⋅μ0⋅μr⋅r2  N

Since the force exerted on a unit north pole (of strength 1 weber) placed at a point in a magnetic field would be the measure of the intensity of the magnetic field. The "field strength" at that point therefore is the formula for magnetic force. 

If we put 𝑚 = 1 weber and 𝑚₂ = 𝑚 weber (say), then we shall get the field strength 𝑭 as

F=m4π⋅μ0⋅μr⋅r2  N/Wb

This force F experienced by a unit pole, i.e., the field strength, is also denoted by H, and its measure not only newtons/weber but also ampere turns/m.

i.e.,                  H=m4π⋅μ0⋅μr⋅r2  N/Wb


or                      H=m4π⋅μ0⋅μr⋅r2  ampere−turns/meter (AT/m)

By definition the field intensity or magnetic field strength 𝐻 at any point in magnetic field is the force experienced by a unit north pole (1 weber) placed at that point.

Field strength 𝐻, involves the idea of the force, which has both magnitude and a direction, is a vector quantity.