Permeablity of free space, μO.
Defined in terms of the permittivity of free space εO and the speed of light c.
μO=1/(εOc²)
μO=4πx10-7 Weber/ampere-meter
The relative permeabilty μr of any material of permeability μ is given by μ/μO.
The magnetic flux density B and the magnetic field strength H are related by the permeability.
B=μ H=μr μO H