In electromagnetism, **permeability** is the degree of magnetisation of a material that responds linearly to a magnetic field. Absolute permeability is represented by the symbol μ. In SI units, permeability is measured in henrys per metre. is known as the Permeability of free space (absolute permeability below) - Permeability in linear materials owes its existance to the approximation:
- Where is a dimensionless scalar called the Magnetic Susceptibility
- According to the definition of the Auxillary Field,
- Thus
where
*μ* is the permeability, measured in henrys per metre
*B* is the magnetic flux density (also called the magnetic induction) in the material, measured in teslas
*H* is the magnetic field strength, measured in amperes per metre
## Absolute permeability
Absolute permeability is represented by the symbol μ_{0} and is the permeability of the vacuum, where μ_{0} = 4π × 10^{−7} N A^{−2} (exactly). Together with permittivity, permeability defines the speed of light.
## Relative permeability Relative permeability, sometimes denoted by the symbol μ_{r}, is the ratio of the permeability of a specific medium to the permeability of free space μ_{0}: ## SI magnetism units |