122 



Prof. J. A. Ewing. 



(0° to —60°) brings us to the value —33, almost identical with that 

 which was reached in § 9 by twisting at once from +60° to —60°. 



This tendency on the part of the circular magnetisation to follow at 

 a distance instead of accompanying the changes of torsion is still more 

 clearly shown if we divide the whole angle from +60° to —60° into 

 several steps. The following figures give the transient current so 

 obtained after the torsion had been reversed sufficiently often to bring 

 the changes into a steady and sensibly cyclic state. The current A 

 was kept on throughout. 





Transient 



Circular 



Torsion. 



current. 



magnetisation. 



-60° to +60° 



, ... +64 .., 



, .,, +32 



+ 60 „ +30 ... 



... - 3 ... 



+29 



+ 30 „ ... 



... -17 ... 



, , . , -12 



„ -30 . . 



. .. -28 .., 



, ... -16 



-30 „ -60 . . . 



... -16 .., 



.... -32 



with similar values for the return stages. 



The full lines in fig. 2 give the relation of circular magnetisation 

 to angle of twist during this cyclic operation. They show well 

 how the changes of the former lag behind those of the latter. 



Fig. 2. 



§ 11. The same tendency towards persistence of previous state 

 is exhibited whenever we change the magnetisation of a piece of iron 

 or steel by the alternate application and removal of any kind of stress. 

 It is exhibited also when magnetisation is changed by changing the 

 magnetising force, when it appears as the characteristic ordinarily 

 called retentiveness, to which the existence of residual magnetism is 



