Permeability, also called magnetic permeability, is a constant of proportionality that exists between magnetic induction and magnetic field intensity. This constant is equal to approximately 1.257 x 10-6 henry per meter (H/m) in free space (a vacuum). In other materials it can be much different, often substantially greater than the free-space value, which is symbolized µo.
Materials that cause the lines of flux to move farther apart, resulting in a decrease in magnetic flux density compared with a vacuum, are called diamagnetic. Materials that concentrate magnetic flux by a factor of more than 1 but less than or equal to 10 are called paramagnetic; materials that concentrate the flux by a factor of more than 10 are called ferromagnetic. The permeability factors of some substances change with rising or falling temperature, or with the intensity of the applied magnetic field.
In engineering applications, permeability is often expressed in relative, rather than in absolute, terms. If µo represents the permeability of free space (that is, 1.257 x 10-6 H/m) and µ represents the permeability of the substance in question (also specified in henrys per meter), then the relative permeability, µr, is given by:
µr = µ / µo
= µ (7.958 x 105)
Diamagnetic materials have µr less than 1, but no known substance has relative permeability much less than 1. Certain ferromagnetics, especially powdered or laminated iron, steel, or nickel alloys, have µr that can range up to about 1,000,000. When a paramagnetic or ferromagnetic core is inserted into a coil, the inductance is multiplied by µr compared with the inductance of the same coil with an air core. This effect is useful in the design of transformers and chokes for alternating current (AC), audio frequencies (AF), and radio frequencies (RF).