structural:expansion, information:highlighting
The vacuum magnetic permeability is the magnetic permeability in a classical vacuum. It is a physical constant, conventionally written as μ0. It quantifies the strength of the magnetic field induced by an electric current. Expressed in terms of SI base units, it has the unit kg⋅m⋅s−2·A−2. It can be also expressed in terms of SI derived units, N·A−2. Since the redefinition of SI units in 2019, μ0 is an experimentally determined constant, its value being proportional to the dimensionless fine-structure constant, which is known to a relative uncertainty of 1.6×10−10,‍ with no other dependencies with experimental uncertainty. Its value in SI units as recommended by CODATA is: From 1948 to 2019, μ0 had a defined value, equal to: The deviation of the recommended measured value from the former defined value is within it uncertainty. The terminology of permeability and susceptibility was introduced by William Thomson, 1st Baron Kelvin in 1872. The modern notation of permeability as μ and permittivity as ε has been in use since the 1950s.