μ

_{0}- Vacuum permeability (permeability of free space, permeability of vacuum or magnetic constant)
4πE-7
N/A^{2}

The physical constant μ0, commonly called the vacuum permeability, permeability of free space, permeability of vacuum or magnetic constant is an ideal, (baseline) physical constant, which is the value of magnetic permeability in a classical vacuum. Vacuum permeability is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum. In the reference medium of classical vacuum, µ0 has an exact defined value.