XII. Magnetic qualities of nickel (supplementary paper)

Abstract

The present paper is a supplement to one with the same title, by the author and Mr. G. C. Cowan, which was read before the Royal Society on May 17 (p. 325, Suprà ). In that paper experiments were described in which the effects of stress, consisting of longitudinal pull, on the magnetic permeability and retentiveness of nickel had been examined, and it was shown that longitudinal pull had an immense influence in reducing both induced and residual magnetism in nickel. It was, therefore, to be expected (as Sir William Thomson pointed out in his first discussion of the effects of stress on magnetic quality) that longitudinal compression would make nickel more susceptible of magnetisation, and more ready to retain magnetic polarity. Experiments on the magnetisation of nickel under compression have now been carried out under the author’s directions by two of his students, Mr. W. Low and Mr. D. Low, and the results are described below. Further experiments have also been made to investigate the magnetisation of nickel, in very strong magnetic fields, by the method already used for iron by the author and Mr. W. Low, and the results of these are given at the end of this paper. In dealing with the effects of tensile stress on magnetic quality, it is convenient to test the metal in the form of a long wire, long enough to prevent the ends from materially affecting the magnetic field throughout the main part of the length. But in dealing with stress of compression this method of approximating to the condition of endlessness is impracticable. Dr. Hopkinson has shown that a short bar may be brought to a condition of endlessness, suitable for the measurement of its magnetic susceptibility, by sinking its ends in a massive yoke of iron, which affords an easy path for the return of the lines of induction from end to end, outside the bar, and he has made use of this plan in determining the form of magnetisation curves for various samples of iron and steel. This method lends itself well to experiments on the influence of compressive stress, for it is easy to fix the lower end of the bar in the yoke and apply weights directly, or by a lever, to the upper end. The arrangement adopted in the present experiments is shown in fig. 11. The sample under test was a bar of nickel supplied by Messrs. Johnson and Matthey (which was found on analysis to contain 0·75 per cent, of iron). It was 10 cms. long, and was turned to a diameter of 0·656 cms. The yoke was of soft wrought iron, with a cross-section on either side of 67 square cms. The lower end of the bar was supported in the yoke by resting on the end of a screw-bolt; on the upper end a short plunger of wrought iron pressed, and through this the desired stress of compression was applied by means of a lever (fig. 11). The clear length of the sample, within the yoke, was 5 cms. Over this there was wound a magnetising solenoid of 250 turns, inside of which there was a small induction coil wound close to the metal. The magnetisation was determined by reversing the magnetising current while the induction coil was connected to a ballistic galvanometer. To find the residual magnetism the magnetising current was broken after reversal, and the effect of this break was subtracted from half the effect of the reversal. In every case several reversals were made before a measurement was taken; and the process of demagnetising by reversals was resorted to whenever it was necessary to get rid of residual effects of previous magnetisation.