The permeability of free space (a vacuum ) is a physical constant equal to approximately 1.257 x 10 -6 henry per meter . It is symbolized µ o . Permeability in general is symbolized µ, and is a constant of proportionality that exists between magnetic flux density and magnetic field strength in a given medium. In certain metals, notably iron and nickel and alloys containing them, µ is substantially greater than µ o .
If µ o represents the permeability of free space and µ represents the permeability of a particular substance (also specified in henrys per meter), then the relative permeability µ r of that particular substance is given by:
µ r = µ / µ o
= µ o x 7.96 x 10 5
Also see Table of Physical Constants .