III. Electrodynamic qualities of metals.—Part VII. Effects of stress on the magnetization of iron, nickel, and cobalt

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198. In a preliminary notice of investigations regarding the 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 that the diminution of magnetism in a soft iron wire, which I had found 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 that when the magnetizing force was below that critical value the effect of pull was to increase the magnetism—a result which I afterwards found had been previously obtained by Villari. The critical value of the magnetizing force I found to be about twenty-four times the vertical component of the terrestrial magnetic force at 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 that 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, but 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, that 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 magnetism 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 the earth’s magnetic force. The reversed effects of the “ons” and “offs,” observed after this change, were really augmentations and diminutions of magnetism induced by the earth’s vertical force, and were therefore the proper effects for soft iron when subject to a magnetizing force of less than the Villari critical value. Further experimental investigation is necessary to explain the greater amount of effect, the same in kind as those observed before the stretching by 28 lbs., which the wire showed after it had been stretched by this weight. 199. The experiments indicated in my preliminary notice of June 10,1875, were the commencement of an elaborate series of investigations by Mr. Andrew Gray and Mr. Thomas Gray, which have been continued with little intermission from that time until now, and which are still in progress, with the general object of investigating the effects of longitudinal and transverse stress upon the magnetization of different qualities of iron and steel, and of nickel and cobalt. A separate series of investigations was made nearly two years ago by Mr. Donald Macfarlane on the effects of torsion on the magnetization of soft iron, bringing out some very remarkable results, also included in this paper (§§ 223—229, below).
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