VIII. Hysteresis attending the Change of Length by Magnetization in Nickel and Iron. By H. NAGAOKA, Rigakushi*. SIN [Plates I. & II.] INCE Joule's discovery + that the length of iron is changed by magnetization, the subject has been studied by Mayer, Barrett §, Bidwell |, and Berget T. Bidwell carried the investigation into very strong magnetizing fields, and discovered several new facts concerning the changes of length in ferromagnetic substances. So far, however, nothing has been definitely established regarding the manner in which these substances change length during cyclic changes of magnetization. The object of the present investigation is to ascertain if there is any hysteresis in the changes of length during magnetic cycles, and at the same time to determine its amount. Several fruitless attempts were made before I obtained any definite result. The first method I had recourse to was that of interference-fringes. A small brass plate was brazed to the end of an iron wire, and a plane glass plate placed upon it. Separated by a thin air-film was a plano-convex lens of about 40 centim. focal length, with 23 fine dots on its plane face. The lens rested on a tripod. These different pieces of apparatus were detached from Fizeau's dilatometer. The change of length was determined by observing the displacement of the fringes produced by sodium-light. From the position of the dots it was possible to determine a change amounting almost to a hundredth part of a sodium wavelength D. But as each observation of the fringes required a few minutes, it was difficult to keep the temperature constant; and, moreover, owing to the uniformity of distribution of the fringes, it was not always easy to count the number displaced. Consequently it was necessary to devise a more delicate method, and, if possible, some means of compensating for temperature-effects. * Communicated by Prof. C. G. Knott, D.Sc., F.R.S.E. + Reprint of Papers, vol. i. p. 235. Phil. Mag. [4] xlvi. p. 177. § 'Nature, 1882. Proc. Roy. Soc. 1886; Phil. Trans. 1888; Proc. Roy. Soc. 1890. ¶ Compt. Rend. tom. cxv. p. 722. I also tried an experiment with a system of levers; but it did not work smoothly, so that the readings were capricious and could not be trusted. These faults were, to a great extent, removed by the apparatus which I describe below. The horizontal and vertical projections of the apparatus are represented in fig. 1 and fig. 2 (Plate I.) respectively. The essential part consists of a stout brass bar 53 centim. long, 1 centim. broad, and 1.1 centim. high. It is provided with three levelling-screws (1, 2, 3). A carefully polished V-groove is cut along the bar. A small rectangular brass pillar (p) is erected at one corner of the bar. A small vertical V-groove is cut on it, and on this two points of the lever rest. The lever with a mirror attached is shown in fig. 3, both from the front and from behind. It is a small rectangular piece of brass with three steel points (P1, P2, P3), of which two (P1, P2) rest on the V-groove in p. The other point (p3) comes in contact with a small plane glass plate, which is fixed to the end of the movable brass rod. The point of contact is in the prolonged axis of the wire whose change of length is to be determined. The distance of the line p1 p2 from p. is 1.125 millim. Preliminary testing showed that the relative positions of these three fine steel points were not directly affected by the magnetizing forces. The plate has three holes (h1, h2, h3). To the holes h2 he is attached a thin brass wire, which is pulled at its middle by means of a small spiral spring (82) of hard brass wire. Another spring (81), similarly made, is attached to the other hole. These springs can be adjusted by means of slide arrangements, n1 and no, attached to the sides of the bar. The circular mirror, m, attached to the lever was obtained from Hartmann and Braun. The greatest difficulty in the measurement of change of length by magnetization arises from the temperature-changes produced by a current passing into the magnetizing coil. On this account most experimenters have passed the current only for a very short time, and observed the change before the temperature produced any effect. The consequence is that the changes are traced only by jumps. I found that the temperature-effect could be greatly compensated for by applying the principle of the gridiron-pendulum. This end was achieved by using zinc rods of different lengths such that, in any combination, the total expansion due to small changes of temperature in particular lengths of zinc and iron (or nickel) was equal to that in a particular length of brass. Zinc rods 5 millim. thick were carefully turned on a lathe, and cut into proper lengths. The extremities of the zinc rod (z in fig. 1) are concave, so that the convex ends of the brass rods (a, w') come into contact at the axial points of the rod. To the ends of the wire (w) are brazed two short brass rods (w', w'), about 1 centim. long and 5 millim. thick, with their ends made convex. [In brazing the wire care was taken that the axis of the wire coincided with the axes of the rods.] The A brass rod (b), 5 millim. thick, is placed in contact with w'. At the end of the rod a plane glass plate is attached, so that the steel point ps of the lever comes in contact with it. At the other extremity of the row of rods is a stop. It consists of a triangular prism of brass to which a brass rod (d), 5 millim. thick, is attached. prism fits in the V-groove, and is fixed tightly by means of a clamping-screw, c. To adjust the length (bc) it is provided with a slit g. The screw (c) can be placed at any part of the slit, and the position of the movable system b, w', w, w', z, can be so adjusted that the plane of the lever is perpendicular to the axis of the system. The slight push exerted by the springs (81, 82) on the movable system prevents the play of different parts among each other, and was sufficient to overcome friction during contraction. 2, Perhaps the following short explanation of figs. 1 and 2 will make the various parts of the apparatus clear :-11, 12, 13, levelling-screws; a, lever with three steel points (dots in the figure); p, pillar with vertical groove; 81, 82, brass springs attached to the lever for adjustment; ni, ng, slide arrangement for adjusting the strength of the springs; kɩ k2, kз k4, clamping-screws for ni, n, respectively; b, brass rod with plane glass; w', w', brass rods attached to the extremities of the wire w; w, wire whose change of length is to be measured; z, compensating zinc rod; d, brass rod attached to the stop; g, slit in the stop; c, clamping-screw. The rod was placed inside a solenoid 30 centim. long, which lay in a horizontal position magnetic east and west. It had a resistance of 0.63 ohm, and gave a field of 37.97 C.G.S. units for a current of one ampere. The internal diameter of the solenoid was 3.0 centim., and no part of the measuring apparatus came in contact with it. Care was taken to place the apparatus along the axis of the solenoid, and so to place the wire that its middle point coincided with that of the solenoid. These precautions were always necessary, especially when the wire was thick. 134 H. Nagaoka on Hysteresis attending Change of The optical method for observing the change of angle requires little explanation. When the Gauss-Poggendorff method is used at a great distance, it requires very strong illumination of the scale and a good observing-telescope; or when the reflected spot of light is read on the scale, electricor lime-light must be used. This inconvenience can be removed in the following manner (see fig. 4). A fine glass thread t is placed vertically in the focus of a small achromatic lens c, and illuminated by a lamp. The ray, after passing the lens, is reflected by a right-angled prism p, and thrown on the mirror m. After reflexion in the plane mirror, the ray traverses an achromatic lens L (whose focal length was about 70 centim. in my experiment). The image of the glass thread is then observed by means of a microscope, M, provided with a micrometer. I used a microscope detached from a geodetic comparator; the magnifyingpower was about 40, but if great exactness be desired it can be increased about five times, provided sufficient illumination be given. In place of the glass thread at t I tried a fine slit, spider or silk thread, and diamond traces on glass; but it was found best to use a glass thread of such thickness that its image was a little greater than the movable double threads of the micrometer. In the present experiment, 1776 divisions of the micrometer were equal to 524"-1, so that a single division was equal to 0" 295. The displacement of one micrometer division, therefore, gave a change of length where represents the wave-length of the sodium-line D. ( changes of angle can be advantageously used in various other researches The magnetizing current was supplied by Bunsen cells, and its strength was measured by a Thomson Graded Galvanometer. The galvanometer was gauged by means of a deciampere balance. The current was always changed continuously by means of a liquid slide included in the circuit. A rough plan of the arrangement of the different parts is given in fig. 4. The hysteresis accompanying the change of length in nickel is not so complicated as in iron. I will first describe a few experiments made with the former metal. A nickel wire, 194 centim. long and 2.04 millim. thick, was carefully annealed by placing it due magnetic east and west in a porcelain tube, and heating it red-hot in a charcoal fire. The wire was then placed in the V-groove of the apparatus, and inserted in the solenoid. The strength of the current was gradually increased, and, at convenient intervals, the corresponding readings of the micrometer and galvanometer were noted, till the magnetic field was 10.2 C.G.S. units. The contraction of the wire was at first very slow, but when the field was about 8 the rate of change was greatly increased. As the field was diminished the wire tended to return to its former state, but not by its former course. There was lagging, so that the wire, for the same strength of field, was more contracted than when the field was on the increase. In fact, when the field was diminished to zero, the wire still remained contracted 38.2 × 10-8 of its original length. When the current was reversed, the wire continued in its tendency to recover its former length until the reversed field became equal to 5. There the recovery stopped, and the wire began once more to contract, and the contracted length in field -10.1 was nearly the same as that in field +10·2. On decrease of current, the same succession of changes took place. The changes are graphically represented in fig. 5 (Pl. I.), where the course is in the order of the letters of the alphabet. The curve cdefgh of a cycle from the highest field and back to it is nearly symmetrical with respect to the line of zero field, forming complete loops on both sides. The measurements are given in Table I. at the end of the paper. On reannealing and experimenting between fields + 30, * I found lately that it is more advantageous to replace the prism p and the mirror m by a small rectangular prism attached at m. The positions of telescopes and collimators must be suitably changed. |