MAGNETIC

PERMEABILITY  , 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 sub-stance 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 .