On the Production of Transient Electric Currents. 125 



•angles of twist from 10° to 60°, the currents being observed after at 

 least one previous reversal of the magnetising force. 





Transient current given on 



Circular 



Angle of 



reversal of battery. 



magnetisation. 



twist 









(scale reading). 







mean 



A to B 



B to A 



z 



+ 10 



-25 



+ 25 



lni 



— 10 



+ ZL 



— ZL 



ini 

 IU2 



+ 20 



-41 



+ 43 



ZL 



-20 



+ 42 



-38 



20 



+ 30 



-55 



+ 56 



28 



-30 



+ 54 



-52 



26^ 



+ 40 



-67 



+ 67 



33^- 



-40 



+ 66 



-64 



32| 



+ 50 



-73 



+ 75 



37 



-50 



+ 71 



-72 



36 



+ 60 



-78 



+ 80 



39i 



-60 



+ 76 



-78 



38| 



The same results are shown graphically in the dotted line of fig. 2 ; 

 comparison of it with the fall lines of the same figure will serve to 

 show the part played by hysteresis in the changes of circular 

 magnetisation which are caused by changes of stress. 



§ 15. The torsions which have been hitherto spoken of were well 

 within the limits of elasticity. When the angle of twist was 

 increased beyond 60° the transient currents given on reversal of the 

 battery were not greatly augmented. They reached a maximum at 

 about 90°, at which angle the torsion began to produce distinct 

 permanent set, and for . greater angles they diminished slightly. 

 Moreover, after the wire was twisted beyond its elastic limit, and 

 allowed to come back to its new zero of torsional stress, reversal of the 

 longitudinal magnetism then gave transient currents of opposite sign 

 to those which were given while the stress which caused the set was in 

 operation ; the reason probably being, that the wire, though then free 

 from torsional stress, had acquired a helical quality with respect to 

 magnetic inductive susceptibility, the susceptibility being less along 

 the lines of permanent extension than in other directions. The lines 

 of induction were therefore screws of opposite sign to the lines of 

 permanent extension. Again, effects of the same sign as those 

 observed would be given by permanent twist if permanent extension 

 produced a state in which the electrical conductivity of the iron was 

 less along than across the lines of strain. After being twisted to 

 + 360° the wire was allowed to spring back: its new zero of stress 

 was at +125°. There, after several reversals, B to A gave — 7, and 



