Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/265

Thomson introduced a number of new terms into magnetic science—as indeed he did into every science in which he was interested. The ratio of the measure of the induced magnetization I_i, in a temporary magnet, to the magnetizing force H, he named the susceptibility; it is positive for paramagnetic and negative for diamagnetic bodies, and is connected with Poisson's constant k_p^[1] by the relation

$\kappa ={\frac {3}{r\pi }}{\frac {k_{p}}{1-k_{p}}}$,

where κ denotes the susceptibility. By an easy extension of Poisson's analysis it is seen that the magnetic induction and magnetic force are connected by the equation

$\mathbf {B} =\mathbf {H} +4\pi \mathbf {I}$,

where I denotes the total intensity of magnetization: so if I₀, denote the permanent magnetization, we have

${\begin{matrix}\mathbf {B} &=&\mathbf {H} +4\pi \mathbf {I_{i}} +4\pi \mathbf {I} _{0}\\&=&\mu \mathbf {H} +4\pi \mathbf {I} _{0}\end{matrix}}$

where μ denotes (1 + 4πκ): μ was called by Thomson the permeability.

In 1851 Thomson extended his magnetic theory so as to include magnecrystallic phenomena. The mathematical foundations of the theory of magnecrystallic action had been laid by anticipation, long before the experimental discovery of the phenomenon, in a memoir read by Poisson to the Academy in February, 1824. Poisson, as will be remembered, had supposed temporary magnetism to be due to "magnetic fields," movable within the infinitely small "magnetic elements" of which he assumed magnetizable matter to be constituted. He had not overlooked the possibility that in crystals these magnetic elements might be non-spherical (e.g. ellipsoidal), and symmetrically arranged; and had remarked that a portion of such a crystal, when placed in a magnetic field, would act in a manner depending on its orientation. The relations connecting

1. Cf. p. 65.