Magnetic field Intensity H
(A) Magnetization M
When a substance is placed in an external magnetic field, the substance experiences a torque due to the field and aligns in the same direction as the field. The magnetization so produced in the substance is called Induced magnetization. It is denoted by symbol M. Magnetization in magnetic field is analogues to polarization of dielectric material in electrostatic field.
(B) Magnetic field Intensity H
The magnetic field in "empty" space is denoted by the symbol B. It is calculated from Ampere's Law or Biot-Savart’s Law and measured in tesla. However, when the magnetic field passes through a magnetically responsive material, such as iron, the material itself contributes its internal magnetic field. Then a second quantity, H called as magnetic field intensity is used to characterize the strength of external field i.e. the magnetic field due to the external sources (electric current) only, excluding the contribution due to material’s internal magnetic field. H is related to B through permeability μ as B = μH.
Magnetic field Intensity H is also called as Magnetizing force or Auxiliary Magnetic field. H is also expressed in terms of M as follows
Or in the equivalent form as
B = μo(H + M)
H and M have the same units, amperes/meter.
More discussion about M and H is given in the chapter on Magnetism in Matter.