MOLECULAR HYPOTHESIS 



197 



On still further increasing H the magnets tend to set more and 

 more nearly parallel to H, as in Fig. 149, and then the limit of 

 magnetisation is practically reached. 



The curve of magnetisation is represented in Fig. 150, where O A 

 represents the initial stage with 

 no hysteresis, A B the sudden ^^^x^, 



turning round, and BC the 

 subsequent approach to par- 

 allelism with H. 



If H be removed the return 

 will be from Fig. 149 to Fig. 

 148 that is, a very large 

 amount of residual magnetism 



H 



remains in the direction of H. 

 Reversal of H will ultimately FIG. 148. 



send the magnets round in the 



opposite direction and we shall have a sudden reversal of mag- 

 netisation. The behaviour of such a group may be studied by 

 placing four compass needles in a square. The earth's field must 



be as nearly as possible neutral- 



) ised by means of a large magnet, 



placed so that its field is equal 

 ) and opposite to that of the 



H ^ earth. On gradually approach- 



. ing a second magnet towards 



the group the phenomena will 

 v ) be observed. 



If there are many such square 

 groups near together, and with 

 the sides of the squares at 



different inclinations to H, instability will set in at different values 

 of H for the different squares, and "the transition from one stage 



Fu;. 100. 



