a physical quantity that characterizes the relationship between the magnetic induction B and the magnetic field H in a substance. Its designation is JUL; for isotropic substances μ = B/H (in the cgs system of units), or μ = B/μoH (in the International System of Units; μ₀ is the magnetic constant).

For anisotropic solids (crystals) the magnetic permeability is a tensor. It is related to the magnetic susceptibility K by the formula μ = 1 + 4πK (in the cgs system of units), or μ = 1 + K (in the International System of Units), where μ is measured in dimensionless units. For a physical vacuum K = 0 and μ = 1.

For diamagnetic materials K < 0 and μ < 1; for paramagnets and ferromagnets K > 0 and ju > 1. Magnetic permeability is called static or dynamic, respectively, depending on whether μ for ferromagnetic materials is measured in a constant or variable magnetic field. The values of the two types of permeability do not agree, since eddy currents, magnetic viscosity, and resonance phenomena affect the magnetization of ferromagnets in a variable magnetic field. Magnetic permeability is a complicated function of H, and the concepts of differential, initial, and maximum magnetic permeabilityhave been introduced to describe the relationship.