130 



Prof. J. A. Ewing. 



§ 22. It lias been mentioned that the removal and re-application of 

 longitudinal magnetising force acting on an iron wire under torsion 

 produced almost no change of the circular magnetism, and that when 

 a small change was visible, it had sometimes the apparently anomalous 

 character of increase of circular magnetism with removal of longitudinal 

 magnetising force. My experiments show that the residual magne- 

 tism of soft annealed iron is generally very great, amounting to as 

 much as 80 or even 90 per cent, of the induced magnetism, provided 

 that care be taken, by the use of a very long rod or a ring magnet, to 

 prevent the existence of any self-demagnetising force when the 

 inducing force is withdrawn (a condition present in these torsion 

 experiments), and also provided that mechanical disturbance be 

 avoided, which, if it occurs, will remove by far the greater part of 

 this large residue. Now any change of circular magnetism which 

 takes place when the longitudinal magnetising force is withdrawn 

 from a twisted wire must be due to the fact that the effect of torsion in 

 producing circular magnetism is either less or greater on the residual 

 than on the total longitudinal magnetisation. The two are so nearly 

 equal that but little change occurs, and in general, when the longitu- 

 dinal magnetising force has not been very intense, the change is of 

 the nature of a diminution. But when the longitudinal magnetisation 

 has been very strong, the 45° components of it, on which the effects 

 of pull and push are felt, may approach so closely to the YiJlari 

 critical point that the reduction of them which takes place when the 

 magnetising force is removed makes them more susceptible to the 

 action of stress, and so causes an increase of circular magnetism. 



§ 23. To complete the explanation of the results, allusion need be 

 made to only one other point. It was mentioned in § 19 that when 

 by successive twistings from one side to the other, a cyclic condition 

 was established in steel, then at the beginning of each release from 

 torsional stress a slight increase of circular magnetism took place (see 

 fig. 4). My experiments on the effects of stress on magnetised wires 

 show that at the beginning of each loading or unloading the initial 

 change of magnetism is nil relatively to the initial change of stress, if 

 great care be taken to avoid mechanical disturbance. When there is 

 any disturbance at the beginning of the change of load, its effect is to 

 make the magnetisation approach the value which it would assume 

 under vibration. In the torsion experiments the changes of stress 

 were necessarily effected suddenly, and must have been accompanied 

 by some slight vibration. This affords a sufficient explanation of the 



of the transient currents be correct, it applies equally to "Wiedemann's result. 

 There, too, the intensity of magnetisation on which the differential effect of pull and 

 1 



push is felt, being of the whole circular magnetism, cannot be increased suffi- 

 ciently to cause the Villari reversal. 



