196 MAGNETISM 



poles. The magnetic circuit is completed within the group and 

 there is practically no external field exhibited at a distance. 



If a small magnetising force is put on, represented by H, each 

 magnet will be turned round a little, its N pole tending to move in 

 the direction of H, as in Fig. 145. But the displacement of a 



FIG. 1-H. Fi<;. 1 !'-. 



magnet from a position N S to a position N'S', Fig. 146, may be 

 represented by the superposition of two other magnets with equal 

 poles, for superpose on NS, Fig. 146, a magnet SjN'and a magnet 

 NjS', then N and S x neutralise each other, and Sand N, neutralise 

 each other, and we have N'S' left. 



Thus the effect of the rotation of the four magnets in Fig. 145 

 may be represented by superposing eight little magnets as in Fig. 

 147, and these will have a field generally in tin- direction of H. 



.1- 



N 



*N 



- 



% 



FIG. 146. FIG. 1 17. 



While the magnetising force H is small, these deflections, and 

 therefore the lengths of the small magnets in Fig. 147, will be 

 proportional to the magnetising force, and the net magnetisation 

 will also be proportional to it. But when a certain point is reached 

 in the value of the magnetising force the configuration becomes un- 

 stable and the needles swing round into some such position as that 

 in Fig. 148, and their external field suddenly and greatly increases. 



