280 



MAGNETISM 



force can just overcome the friction F. Thereafter the pull l\ will 

 be constant and equal to F as long as the motion is from right to 

 left. 



To reverse the motion the force must be reversed from 



P a = -f F to P, = F, i;i \ing a di-^l.u rm< nt of ( M</j IK ton- 

 CF begins to move. Then P, remains constant at F till another 

 reversal. Again there is a change of 2F and a displacement 



meanwhile of 9d ; then motion with con>tant F. The 



Fio. 215. 



giving the relation between P, and d will be as in Fig. 215. After 

 the first motion from the centre it is a parallelogram. 



Now let us add the two forces P t -f P! = P for a given 

 displacement and we get Fig. 216, a hysteresis loop very much 



