Vacuum Permeability

The physical constant μ₀, commonly called the vacuum permeability, permeability of free space, or magnetic constant is an ideal, (baseline) physical constant, which is the value of magnetic permeability in a classical vacuum. Vacuum permeability is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field production in a vacuum. In the reference medium of classical vacuum, µ₀ has an exact defined value:

µ₀ = 4π×10−7 V·s/(A·m) ≈ 1.2566370614...×10−6 H·m−1 or N·A−2 or T·m/A or Wb/(A·m)

in the SI system of units.

As a constant, it can also be defined as a fundamental invariant quantity, and is also one of three components that defines free space through Maxwell's equations. In classical physics, free space is a concept of electromagnetic theory, corresponding to a theoretically perfect vacuum and sometimes referred to as the vacuum of free space, or as classical vacuum, and is appropriately viewed as a reference medium.