Theory of Induced Magnetism. 



211 



part of the group becomes unstable, and hysteresis immediately 

 comes into play. At the same time , there begins to be a 

 marked augmentation of susceptibility — that is to say, a 

 marked increase in the rate at wbich resultant moment is 

 acquired. It is not difficult to arrange groups in -which the 

 state of instability is reached with one and the same value of 

 .0 throughout the group. But. in general, we shall have 

 different elementary magnets, or different lines of them, 

 reaching instability with different values of «£). The range of 

 <§, however, which suffices to bring about instability through- 

 out the whole, or nearly the whole, of the members in most 

 groups is not large ; we, therefore, find in the action of the 

 model a close analogy to the second stage (b, fig. 3) of the 

 process of magnetization, in which the magnetism rises more 

 or less suddenly, as well as to the first stage (a). 



Fiir. 3. 



During the second stage (b), the magnetic elements fall for 

 the most part into lines which agree more or less exactly 

 with the direction of ,£)• If, at the end of this stage, we 

 remove <£), we find that a very large proportion of the mo- 

 ment which the group has acquired remains ; in other words, 

 there is a great deal of residual magnetism. To take an 

 instance, suppose we have a group lying initially as in fig. 4, 

 and apply a magnetic force, <£), in the direction sketched, the 

 first stage (a) deflects all the molecular magnets slightly, 

 without making any of them become unstable ; the second 



