The Permeability of Free Space, `mu_0`, is also known as Permeability of a Vacuum and sometimes as the Magnetic Constant. This constant is used to measure the amount resistance to forming a magnetic field in a classical vacuum. This constant is used in a number of equations related to magnetic field density and its relationships to magnetic flux density.
The Permeability of Free Space is defined as `4pi x 10^-7 (V*s) / (A*m)`, which is approximately:
1.2566370614359173E-6 `(V*s) / (A*m)` or
1.2566370614359173E-6 `H /m` or
1.2566370614359173E-6 `N/A**2` or
1.2566370614359173E-6 `T*M/A` or
1.2566370614359173E-6 `(Wb)/(A*m)`
This is an exactly known constant since it is the product of a precise value with a precisely known number, Pi.