The Ampere Defines Vacuum Permeability
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2×10−7 newton per meter of length.
Adopted in 1948, the effect of this definition is to fix the magnetic constant (permeability of vacuum) at exactly 4π×10−7 H·m−1. To further illustrate:
Two thin, straight, stationary, parallel wires, a distance r apart in free space, each carrying a current I, will exert a force on each other. Ampère's force law states that the force per unit length is given by
The ampere is defined so that if the wires are 1 m apart and the current in each wire is 1 A, the force between the two wires is 2×10−7 N m-1. Hence the value of μ0 is defined to be exactly
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