Middle of the Nineteenth Century. 245 



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 magnetiza- 

 tion 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 * by the relation 



3 if 

 t\jp 



= SFTv 



where K 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 



B = H + 47rl, 



where I denotes the total intensity of magnetization : so if I 

 denote the permanent magnetization, we have 



B = H + 47rl + 47rl,,, 

 = )uH + 47rI , 



where //, denotes (1 + 4) : //, was called by Thomson the 

 permeability. 



In 1851 Thomson extended his magnetic theory so as to 

 include magnecrystallic phenomena. The mathematical founda- 

 tions 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 fluids," 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 symmetri- 

 cally 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 



* Cf. p. 65. 



