${\displaystyle \frac{F}{l}=\frac{{\mu }_0}{2\pi }\cdot \frac{\left|I_1{I}_2\right|}{d}}$

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.