In Electromagnetic,the magnetic constant μ0 (also known as vacuum permeability or permeability of free space) is a universal physical constant, an electromagnetic property of classical vacuum, relating mechanical and electromagnetic units of measurement. In the International System of Units (SI), its value is exactly expressed by:
μ0 = 4π × 10−7 N/A2 = 4π×10−7 henry/metre (H/m) , or approximately 1.2566×10−6 H/m
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 o . Permeability in general is symbolized , 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, is substantially greater than o .
If o represents the permeability of free space and represents the permeability of a particular substance (also specified in henrys per meter), then the relative permeability r of that particular substance is given by:
r = / o
= o x 7.96 x 10 5