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IX. Magnetic-Elongation and Magnetic- Twist Cycles. 

 By Prof. C. G. Knott, D.Sc, F.R.S.E* 



IN measuring the successive changes of length of iron 

 and nickel wires subjected to cyclic magnetizations, Mr. 

 Nagaoka has worked out a problem of extreme difficulty. Not 

 only is the quantity to be measured excessively minute, but, 

 unless very refined precautions are taken, it is certain to be dis- 

 turbed beyond recognition by inevitable temperature-changes. 

 With admirable experimental skill, Mr. Nagaoka has applied 

 the principle of the gridiron-pendulum ; and the remarkable 

 smoothness of the curves which embody his instructive results 

 is an evidence of the perfection of the compensation. Joule's 

 original experiments, and all similar experiments of later date, 

 demonstrated the existence of what might be called a residual 

 elongation when the magnetizing force was removed. Mr. 

 Nagaoka has now given us the whole history of the magnetic- 

 elongation cycle, and has done for this very much more 

 difficult enquiry what Warburg and Ewing did for the mag- 

 netization cycle. 



It may not be without interest to compare Mr. Nagaoka's 

 results with what I believe to be closely related results obtained 

 by myself several years ago. As long ago as J 85 8 Wiedemann 

 discovered f that, when a current passes along a longitudinally 

 magnetized iron wire, the wire twists. In 1883 I observed 

 the same phenomenon with nickel wire similarly treated, and 

 more recently also with cobalt J. This phenomenon is, for 

 present purposes, conveniently called the Wiedemann Effect. 

 Maxwell suggested that it could be explained in terms of 

 the Joule Effect, as we may also conveniently term the 

 phenomenon which has been engaging Mr. Nagaoka's atten- 

 tion. In my papers on the subject I have brought this probable 

 explanation prominently forward. The general features of 

 the Wiedemann Effect in the three magnetic metals were 

 just what was to.. be expected if Maxwell's explanation were 

 admitted. .Not. only so, but certain of these features sug- 

 gested corresponding characteristics in the Joule Effect — 

 ^characteristics which Mr. Bidwell has subsequently observed § . 



In my paper of 1891 (Trans. R. S. E. vol. xxxvi.) I have 

 given the magnetic-twist cycles both for iron and nickel. 

 While a steady current was passing along the wire the longi- 



* Communicated by the Author. 

 " t See Wiedemann's JElectricitqt, Bd. iii. 

 -'----' f- See Trans. Roy. Soc. Edin. vols; xxxii., xxxv., and xxxvi'. 

 § See Proceedings Roy. Soc. vol. i. (1892), p. 495. 



