368 ELECTROSTATICS AND MAGNETISM. [PT. II. CH. IX. 



so that the polarization is proportional to the total force of the 

 field, that is the sum of the external field and the field of force due 

 to the polarization. The coefficient 



. 



21 



is called the magnetic susceptibility, in the magnetic case. In the 

 electric case the quantity K has never come into practical use. 

 The equation 



I= K F, 



was the basis of Poisson's theory of magnetic induction, K being 

 supposed a quantity inherent in the body, and equal to zero for 

 air. We see however that K depends on the medium by which the 

 body is surrounded, as well as on the body itself: /e 21 may therefore 

 be called the relative susceptibility of the body of inductivity /JL Z 

 in a medium of susceptibility fa. If rc 21 is positive, the polariza- 

 tion is in the direction of the polarizing force, and the body is said 

 to be paramagnetic, or simply magnetic. If /c 21 is negative, the 

 polarization is in the opposite direction to the force, and the body 

 is said to be diamagnetic. Accordingly any body immersed in a 

 medium of greater inductivity than its own will appear diamag- 

 netic. If we consider always the polarization of a body with 

 respect to a vacuum, so that /^ = 1, we may put 



fJi = 1 -f 4!7TK. 



Bodies are accordingly magnetic or diamagnetic as //, is greater 

 or less than unity. It is evident that the assumption that K is 

 zero for a vacuum is arbitrary, in the same degree as, but inde- 

 pendently of the assumption that the inductivity of vacuum is 

 unity, for we might assume all apparent polarizations to be the 

 differences of the polarizations of bodies from the polarizations of 

 vacuum. 



192. Uniform Polarization by Induction. When a body 

 of different inductivity from the rest of the medium is inserted 

 into a field of force, the configuration of the field is disturbed 

 owing to induction, the polarization due to which produces new 

 forces FI which must be added to the forces of the undisturbed 



