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XVIII. — On Some Relations between Magnetism and Twist in Iron and Nickel (and 

 Cobalt). Parts II, and III. By Cargill G. Knott, D.Sc. (Edin.), F.R.S.E., 

 Professor of Physics, Imperial University, Tokyo, Japan. (With Five Plates.) 



(Read 1st June 1891.) 



Part II. 



1. This paper forms the continuation of a paper already communicated to the Royal 

 Society of Edinburgh, and published in the Transactions (vol. xxxv. pp. 377-390). 

 Strictly speaking, it is third of a series bearing on the same subject. In the first* I 

 showed that when a current is passed along a longitudinally magnetised nickel wire a 

 twist is produced similar in character but opposite in direction to the twist which 

 Wiedemann discovered to be produced in iron similarly treated. This was the chief 

 result arrived at ; but other results were also obtained, especially with regard to the 

 influence of tension, which called for further investigation. In the later paper, the 

 influence of tension was more thoroughly studied ; and also the effect of change of 

 temperature. I also discussed more fully the suggestion first made by Maxwell t that 

 the effect discovered by Wiedemann can be explained in terms of the changes of length 

 which Joule observed in iron when magnetised. Obviously the same explanation must 

 be extended to the case of nickel, which Barrett had found also to be subject to changes 

 of length when magnetised. Mr Bidwell's recent elaborate measurements of these 

 changes of length enabled me to make an approximate calculation of the twist in an iron 

 tube longitudinally and circularly magnetised. The comparison of the result as calcu- 

 lated for a tube with the result observed for a wire of the same diameter, established, in 

 my opinion, the sufficiency of Maxwell's explanation. This conclusion, however, Wiede- 

 mann does not seem to admit (see Beiblatter, xiii. p. 718). 



2. The present paper consists of two main sections. The first is a further discussion 

 of the properties of the twist due to superposed magnetisms in iron and nickel ; and the 

 second is mainly an investigation into the longitudinal polarity produced by twisting 

 wires along which a current is flowing. Paragraph (12) is concerned with cobalt, and 

 paragraph (28) relates to some rather curious experiments on magnetically neutral wires. 



In these latest experiments on the Wiedemann effect (as, for brevity, I have termed 

 the twist in the magnetic metals due to superposed magnetisms) a modified method of 

 experiment was adopted. In all previous work on the subject, whether by Wiedemann, 



* "On Superposed Magnetisms in Iron and Nickel," Trans. Roy. Soc. Edin., vol. xxxii. p. 193, 1883. 



t In an appendix to Gore's paper on " Electrotorsion " {Phil. Trans., 1874), Sir W. Thomson suggests the 

 same explanation, apparently unaware that Maxwell had anticipated him, or that Wiedemann had anticipated 

 Gore. 



VOL. XXXVI. PART II. (NO. 18). 4 D 



