IX. The influence of stress and strain on the physical properties of matter.— Part III. Magnetic induction (continued).—The internal friction of nickel, and cobalt, studied by means of magnetic cycles of very minute range

Abstract

Three years ago the author had the honour of presenting to the Royal Society two memoirs relating to the internal friction of metals, as studied by the logarithmic decrement of arc of torsionally oscillating wires these were followed by a third, dealing with the effect of magnetisation on the internal friction of iron. The results recorded in these memoirs seemed to favour the view adopted by G. Wiedemann, that the main part of the dissipation of energy met with in a torsional cycle arises from the permanent rotation of the molecules about their axes , and served as an encouragement to attempt to ascertain how far the dissipation of energy which accompanies cyclic changes of magnetisation would be amenable to the laws which regulate the dissipation of energy in torsional cycles. The cyclic changes of magnetisation which attend on cyclic changes of magnetising force have been studied by Ewing and others,| but mainly for comparatively great magnetising forces—for forces, in fact, far exceeding those for which there seemed any likelihood of any simple laws holding good. Lord Rayleigh has shown, however, that when the magnetising force is sufficiently minute the ratio of induction to force (permeability) in iron, tends rapidly to a finite constant value as the force diminishes, and that the dissipation of energy in any cycle can be very simply expressed. The author proposed, therefore, to make use of very feeble magnetising forces, and to render their effects sufficiently evident by very sensitive arrangements. Since the same experiments would suffice to determine both the constant permeability mentioned above, and the dissipation of energy in any cycle, the two have been studied side by side. The metals which have been examined are iron, nickel, and cobalt; they have been tested not only at the ordinary temperature of the room but also at 100° C. Finally, the effects of mechanical stress on the constant permeability and on the dissipation of energy have received somewhat careful attention. The “ballistic” method of observation has been adopted in these researches.