60 



Dr. S. Bidwell. On the Magnetic Changes of [May 16, 



"On the Magnetic Changes of Length in Annealed Kods ot 

 Cobalt and Nickel." By SHELFORD BIDWELL, M.A., Sc.D., 

 F.K.S. Keceivud May 16, Read June 2, 1904 



The dimensions of a piece of magnetic metal are, in general, changed 

 by magnetisation. When subjected to a longitudinal field gradually 

 increasing from a small value, an ordinary iron wire is at first extended, 

 then, after passing a maximum, it recovers its original length, and 

 finally becomes shorter than when unmagnetised.* In a paper com- 

 municated to the Society in 1894f I showed that the changes usually 

 observed were modified if the iron had been annealed. In annealed 

 iron the maximum extension is diminished and contraction begins in a 

 weaker magnetic field; the elongation curve is, in fact, lowered to an 

 extent dependent upon the completeness of the annealing. In the 

 case of a certain soft-iron ring which had been raised to a bright red 

 heat and allowed to cool slowly for about 14 hours, there was no 

 preliminary elongation at all, retraction beginning (just as in nickel) 

 with a very small magnetising force. It is of interest to note that 

 after the lapse of 10 years the ring still retains its peculiar quality; 

 on May 9, 1904 a perceptible diminution of its diameter was observed 

 with a force of no more than 3 C.G.S. units, the diminution, of course, 

 Incoming greater with stronger forces. An unannealed ring of the 

 same iron attained its greatest elongation, 33 ten-millionths, in a field 

 of 80, and did not begin to contract until the field reached 420. 



In the course of some recent work it became desirable to ascertain 

 whether the changes of length exhibited by magnetised cobalt were 

 analogously affected. Cobalt in the ordinary condition behaves 

 oppositely to iron, contracting in weak fields and lengthening in strong 

 ones. It might be expected, therefore, that if the metal were annealed 

 it would begin to lengthen at an earlier stage of the magnetisation, 

 powibly without any initial contraction. In searching the literature 

 of the subject before undertaking the experiment, I found a very 

 interesting paper by the Japanese physicists Honda and Shimizu 

 entitled " Change in Length of Ferromagnetic Substances under High 

 and Low Temperatures by Magnetisation."! Among the numerous 

 curves given are two showing the changes of length produced by 

 magnetisation in "cast cobalt" and "annealed cobalt" at ordinary 

 temperatures. The curve for " cast cobalt " is of the same character as. 



Bidwell, ' Phil. Trans./ A, 1888, p. 205. 

 4 Proc. Roy. Soc.,' vol. 56, p. 228. 



'Tokyo Sugaku-Buturigukkwai IKkoku,' No. 19, p. 197. The authors were 

 lough to .end me a copy of the paper in 1903; unfortunately it was not. 

 1th the care it deserved until May, 1904. 



