250 



MAGNETISM 



which the pores were smaller and more resisting than in the 

 surrounding medium then a sphere placed in a previously uniform 

 field would produce a distribution of lines of force like that given 

 in Fig. 194. 



Let us assume as before that the lines of force are parallel within 

 the sphere. In order that the level surfaces may cut the lines of 

 force at right angles, they must be nearer together within the 

 surface than they are at a distance outside. The intensity within 

 the body is therefore greater than it was before the body was 



FIG. 194. 



introduced, while the flux of induction, which we suppose to be 

 continuous, is less. If then we take B = yuH, yu must be less than 

 1. If we represent the alteration of field by induced polarity a- 

 scattered over the surface, as on p. 



and, as yu<Cl? <r becomes negative, or the polarity is reversed 

 and the forces are reversed. This is just what we observe with 

 diamagnetic bodies. A small diamagnotic body is repelled from 

 the pole of a magnet. 



Hence we interpret the distinction between paramagnetic 

 and diamagnetic bodies by saying that for the former p. is greater 

 than 1, while for the latter it is less than 1. If we use the 



magnetic susceptibility K = ^7 , K is positive for the former, 



negative for the latter. If we exclude the ferromagnetic bodies, 

 /UL 1 and K are always very small and for a given substance 

 practically constant, and the value of a- is very small. 



Magnetic induction in a body when the surrounding 

 medium has permeability differing from unity. In the 



