Vacuum permeability

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This article is about the magnetic constant. For the analogous electric constant, see vacuum permittivity.

The physical constant μ₀, (pronounced "mu nought" or "mu zero"), commonly called the vacuum permeability, permeability of free space, permeability of vacuum, or magnetic constant, is the 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.

As of May 20, 2019, the vacuum permeability μ₀ will no longer be a defined constant (per the former definition of the SI ampere), but rather will need to be determined experimentally; The 2018 CODATA value is given below. It is proportional to the dimensionless fine-structure constant with no other dependencies.^[1][2][3]

μ₀ = 1.25663706212(19)×10⁻⁶ H/m

Before this, in the reference medium of classical vacuum, μ₀ had an exact defined value:^[4][5]

μ₀ = 4π×10⁻⁷ H/m = 1.2566370614…×10⁻⁶ N/A² (1 henry per metre ≡ newton per square ampere)