In electromagnetism, permeability is the degree of magnetisation of a material that responds linearly to a magnetic field. Magnetic permeability is represented by the symbol μ. In SI units, permeability is measured in henrys per metre. The constant value is known as the magnetic constant, or by the older term permeability of a vacuum (absolute permeability below).

Permeability in linear materials owes its existence to the approximation:

Where is a dimensionless scalar called the magnetic susceptibility

According to the definition of the auxiliary 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 (the magnetic constant) is represented by the symbol μ₀ and is the permeability of the vacuum, where μ₀ = 4π×10⁻⁷ N A⁻² (exactly).

The permittivity of free space (the electric constant) and the magnetic constant are related to the speed of light (c) by the formula

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 μ₀:

Magnetic susceptibility is defined as:

Magnetic susceptibility for some materials
Medium
Hydrogen	0.008 × 10-6	1.2566371 µN/A2
Copper	−6.4 × 10-6	1.2566290 µN/A2
Water	−8.0 × 10-6	1.2566270 µN/A2
Aluminium	22.2 × 10-6	1.2566650 µN/A2
Platinum	265 × 10-6	1.2569701 µN/A2

SI magnetism units