200 



MAGNETISM 



before they reach these points they become unstable and swing 

 round, thus dissipating energy. A remarkable conclusion was 

 predicted by Swinburne as a consequence of Ewing's theory, and 

 was subsequently verified by experiment, viz. that when the field 



in such a case as that repre- 

 sented in Fig. L5."> is exceed- 

 ingly strong, hysteresis should 

 disappear. If the field i- -o 

 strong that the molecular 

 magnets are held all the time 

 in the line of the magnetising 



FIG. 155. force and rotate round their 



centres, as it were, as the iron 



travels round, there is no instability, no conversion of magnetic 

 energy into energy of vibration, and no hysteresis. 



Temperature and magnetisation. We ha\e seen that 

 in a permanent magnet sgiall rise of temperature is accompanied 

 by diminution of magnetisation, the magnetisation returning to 

 its initial value when the temperature falls again. We mav 

 perhaps explain this by supposing that rise of temperature is 

 accompanied by increased vibration of the molecular magnet - 

 about their centres, so that they do not point so entirclv in the 

 direction of the magnetising force-. For hisl-mcc. lei \ >. Fig, \~>('^ 

 be the equilibrium position of a molecule 

 under a force H and let it vibrate In ! \\een 

 ! S 1 and 7f 2 .v.,. When at t^ .s-j, we may 

 represent the effect of the displacement of 

 N by superposing a small magnet ,y n l ; 

 when at 1I 2 *. by superposing a small 

 magnet s n v * n has more effect in 

 destroying the magnetisation in the direc- 

 tion of H than ft n l has in increasing i(. 

 since it is more nearly parallel to it. There 

 is another possibility which mav be men- 

 tioned. The permeability of iron increuM - 

 with rise of temperature. The magnetic 

 circuit of a molecule or of a group of 

 molecules is partly completed through the 

 iron, and it is only part which issues into 



the air. As the permeability rises with rise of temperature the 

 groups may find it easier to complete more of the circuit within 

 the iron, and some of the lines of force mav be withdrawn from 

 the air. 



The increase, of . permeability with rise of temperature \\hich 

 occurs in weak fields may probably be accounted for by supposing 

 that initially the iron is in the first stage of magnetisation, where 

 the molecular magnets are only slightly deflected, as in Fig. I IV 

 But as the temperature rises molecular vibration increases and 



