PERMEABILITY, MAGNETIC. The force between two magnetic poles varies as the product of the strengths of the poles, and inversely as the square of the distance between them. It de pends also on the nature of the intervening medium. In the C.G.S. electro-magnetic system of units, unit pole is defined as one which repels an equal pole at a distance of one centimetre in empty space with a force of one dyne. Let F be the force between two poles of strengths m2 at a distance r apart. Then, in a vacuum, F = mon More generally, F — mon2 ,u The constant depends on the medium, and is known as the permeability. The permeability may also be defined as the ratio of the magnetic in duction, B, to the magnetizing force, H; the induction being a measure of the magnetic force which would be exerted on unit pole placed in a narrow crevasse in the medium perpendicular to the direction of the magnetic force. Thus µ = The conception of permeability is due to Faraday, who spoke of it as "conducting power for magnetism," and the term was introduced by Kelvin in 1872.
Substances are divided into two main magnetic classes—dia magnetics, with the permeability, /.4, less than 1, and paramag
netics withµ greater than 1. In diamagnetics the directions of the magnetization and the magnetizing force are opposite, in paramagnetics the same. For most substances the permeability differs very little from 1, and does not vary with the magnetizing field. A few paramagnetic substances, below certain critical tem peratures, are ferromagnetic, having magnetic properties resem bling those of iron. The permeability of ferromagnetics (which include iron, nickel and cobalt and some of their alloys) is a function of the magnetic force, and generally depends on the previous history of the specimen. The permeability may be very large ; the maximum for some dynamo steels, and iron-nickel al loys being greater than 1 o,000. A knowledge of the permeability characteristics of iron and iron alloys, in particular, is of great technological importance. Material having a high permeability for definite ranges of magnetic force is generally required in the con struction of electrical machinery. (See MAGNETISM.)