| scalar constant linking the electromagnetic quantities and the mechanical quantities, obtained from the relation
where F/l is the magnitude of the lineic force between two straight parallel conductors of infinite length and negligible circular cross-section, placed at a distance d apart in vacuum and carrying electric currents I1 and I2
Note 1 to entry: In a vacuum, the product of the magnetic constant and the magnetic field strength H is equal to the magnetic flux density B:
B = μ0H
Note 2 to entry: The magnetic constant is related to the electric constant ε0 and to the speed of light in vacuum c0 by the relation
μ0ε0c02 = 1.
Note 3 to entry: The value of the magnetic constant is equal to 1,256 637 062 12(19) × 10−6 N A−2.