128 



Prof. J. A. Ewing. 



circular magnetisation of the steel wire, starting from the norma 

 value for +90°, was altered by successive twistings between +90° 

 and — 90° under the action of a constant longitudinal magnetising 

 force of 20 c.g.s. units. It exhibits very strikingly the lessening of 

 range brought about by successive twistings, which has been already 

 alluded to (§ 8) as occurring (though to a less extent) in iron. Finally, 

 after several twistings, a sensibly cyclic set of changes was instituted 

 which is shown by the full lines in fig. 4. [The "normal" curve 



Fig. 4. 



(see § 9) is shown by the dotted line in the same figure.] A remark- 

 able feature here is that when the direction of twisting is reversed 

 the first effect is to continue the same kind of change of circular 

 magnetism as was going on before. The curve rises before it begins 

 to fall, and vice versa. 



Many other observations have been made, which it is needless to 

 describe, since they are all in obvious agreement with the explanation 

 already offered. It only remains to point out this agreement in the 

 case of those features of the experiments where it is not at once 

 evident. 



§ 20. It was discovered by Villari,* and rediscovered by Sir W. 

 Thomson,! that if an iron rod in a magnetic field be subjected to pull 



* Wiedemann's " GralTanismus," II, § 499. 

 f " Phil. Trans.," Part I, 1879, p. 55. 



