Electrodynamic Qualities of Metals. Part VII. Effects of Stress on the Magnetization of Iron, Nickel, and Cobalt

W. Thomson
1879 Philosophical Transactions of the Royal Society of London  
P l a t e s 2-13.] § § 198 and 199. 1 9 8 . I n a preliminary notice of investigations regarding tlie effects of stress on inductive magnetization in soft iron, communicated to the Royal Society on the 10th of June, 1875, I described experiments which afforded a complete explanation of the seeming anomalies referred to in § §194 and 195,'* which had at first been so perplexing. These experiments showed th a t the diminution of magnetism in a soft iron wire, which I had found to be produced by
more » ... to be produced by pull, while the wire was under the influence of a constant magnetizing force, was to be observed only when the magnetizing force exceeded a certain critical value, and th a t when the magnetizing force wras below th a t critical value the effect of pull was to increase the magnetism-a result which I after wards found had been previously obtained by ViLLARi.t The critical value of the magnetizing force I found to be about tw enty-four times the vertical component of the terrestrial magnetic force a t Glasgow. Hence the magnetizing force which I had used in my first experiment, which ( § 183) was nearly 300 times the vertical component of the terrestrial force, must have been about twelve times as great as the critical value. Further (which was most puzzling), I found the absolute amount of the effects of pull to be actually greater with the small magnetizing force of the earth than th a t of the opposite effects of the 300-fold greater magnetizing force of my early experiments. Thus the effect of the terrestrial force was not only in the right direction, b u t was of amply sufficient amount to account for the seeming anomalies which had at first been so perplexing ; and in going over the details of the old observations I find all the anomalies quite explained. One of them, th a t particularly referred to in § 195, is still interesting. The alternate augmentation of the residual magnetism by " on" and diminution of it by " off," with the weight of 14 lbs., corresponded to the normal effect on residual magnet ism in soft iron. The elongation of 8 per cent, produced when the 28 lbs. was hung on, was no doubt accompanied by a shaking out of nearly all the residual magnetism, and an inductive magnetization in the opposite direction by the vertical component of * Phil. Trans, for 1876, p. 710 (R ead May 27, 1875). t P oggendorff's ' A nnalen,' 1868 ; also W iedemann's ' G alvanism us,' vol. ii., § 499. * Compare § §178, 179. Phil. Trans, for 1876, p. 693. El e c t r o d y n a m i c q u a l i t i e s o e m e t a l s . 57
doi:10.1098/rstl.1879.0063 fatcat:kr7jdxdd7ndgdogvngdgo2hb2i