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IX. — On Some Relations between Magnetism and Twist in Iron and Nickel. Part I. 

 By Cargill G-. Knott, D.Sc. (Edin.), F.R.S.E., Professor of Physics, Imperial 

 University, Tokyo, Japan. 



(Read 16th July 1888.) 



In a former paper # I described certain experiments on the relations of magnetism 

 and twist in iron and nickel, the chief results of which it may be well to give briefly 

 here. When an iron or nickel wire is under the influence of longitudinal and circular 

 magnetisations, it twists in a direction which is definitely related to the direction of the 

 magnetising forces. This effect in iron was discovered by Wiedemann^ and for conven- 

 ience I shall call it the Wiedemann Effect. It was pointed out by Clerk Maxwell that 

 the Wiedemann effect might be explained as a consequence of the earlier discovery made by 

 Joule, that iron lengthens in the direction of magnetisation, and contracts at right angles 

 thereto.^ Led by a consideration of Barrett's discovery § of the shortening of nickel wire 

 in the direction of magnetisation, I determined to test nickel in the same way in which 

 Wiedemann had tested iron. It was quite obvious that, if Maxwell's explanation of the 

 Wiedemann effect were the true one, nickel wire should, ceteris paribus, twist in a sense 

 opposite to that in which iron twists. The experiment when made completely fulfilled 

 the expectation. Thus, when an iron wire, with one end fixed, is traversed by an electric 

 current in the direction in which it is at the same time longitudinally magnetised, the 

 wire is twisted so that the free end rotates right-handedly with reference to the traversing 

 current, or the longitudinal magnetisation. In nickel, on the contrary, the corresponding- 

 rotation is left-handed. This was the chief conclusion arrived at in my earlier paper ; 

 and a little consideration will show how very readily the Wiedemann effect, whether in 

 iron or in nickel, is explained in terms of the simpler strains studied by Joule and 

 Barrett. 



There were, however, other results of interest touched upon, especially with regard to 

 the influence of tension, which seemed to call for further investigation ; but it was not 

 till the spring of 1887 that I was able to return again to the subject. In that year I 

 was fortunate in obtaining the assistance of Mr Nagaoka, a graduating student of physics 

 in the Imperial University of Japan, who undertook a very thorough examination of the 

 influence of tension on the Wiedemann effect. His results, taken in conjunction with Mr 

 Bid well's recent elaborate measurements of the changes of length of iron and nickel in 

 varying magnetic fields, go far to establish the sufficiency of Maxwell's explanation, as 

 will be seen further on. 



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



t Wiedemann's Galvanismus, Bd. ii. § 491 (1st edit.). 



t Sturgeon's Annals of Electricity, vol. viii. ; also Phil. Mag., 1847. 



§ See Nature, vol. xxvi., 1882. 



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