Vacuum permeability

The vacuum permeability measures how strongly magnetic fields form in empty space. It links electric current to the magnetic field it produces and, together with the vacuum permittivity, fixes the speed of light.

Used in

• Speed of light: c = 1/√(ε₀μ₀)
• Magnetic field of a current-carrying wire B = μ₀I/(2πr)

Source: SI defining constants / CODATA 2018