1911 Encyclopædia Britannica/Permeability, Magnetic

PERMEABILITY, MAGNETIC, the ratio of the magnetic induction or flux-density in any medium to the inducing magnetic force. In the C.G.S. electromagnetic system of units the permeability is regarded as a pure number, and its value in empty space is taken as unity. The permeability of a metal belonging to the ferromagnetic class—iron, nickel, cobalt and some of their alloys—is a function of the magnetic force, and also depends upon the previous magnetic history of the specimen. As the force increases from zero the permeability of a given specimen rises to a maximum, which may amount to several thousands, and then gradually falls off, tending to become unity when the force is increased without limit. Every other substance has a constant permeability, which differs from unity only by a very small fraction; if the substance is paramagnetic, its permeability is a little greater than 1, if diamagnetic, a little less. The conception of permeability (Lat. per, through, and meare, to wander), is due to Faraday, who spoke of it as “conducting power for magnetism” (Experimental Researches, xxvi.), and the term now in use was introduced by W. Thomson (Lord Kelvin), in 1872, having been suggested by a hydrokinetic analogy (Reprint of Papers on Electrostatics and Magnetism, xxxi., xlii.) It is generally of importance that the iron employed in the construction of electrical machinery should possess high permeability under the magnetic force to which it is to be subjected. (See Electromagnetism and Magnetism.)