In electrical engineering, the magnetic susceptibility is the degree of magnetization of a material in response to a magnetic field. The dimensionless mass magnetic susceptibility is represented by the symbol χ (lowercase Greek letter chi).

where

J is the magnetization of the material (the magnetic dipole moment per unit mass), measured in A/m (i.e. Amperes per metre)

H is the applied field , measured in A/m (where the magnetic induction B is related to H by the relationship B = μ₀(H + J) where μ₀ is the permeability of vacuum (see table of physical constants).)

If χ is positive the material is called paramagnetic, and the magnetic field is strengthened by the presence of the material. If χ is negative then the material is diamagnetic and the magnetic field is weakened in the presence of the material.

The magnetic susceptibility of a ferromagnetic substance is not linear. Response is dependent upon the state of sample and can occur in directions other than that of the applied field. To accommodate this, a more general definition using a tensor derived from derivatives of components of M with respect to components of H

called the differential susceptibility describes ferromagnetic materials. When the coercivity of the material parallel to an applied field is the smaller of the two, the differential susceptibility is merely a function of the applied field.

The magnetic susceptibility and the magnetic permeability (μ) are related by the following formula: