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XLV. Experiments on the Change in Dimensions caused by Magnetization in Iron. By EDWARD RHOADS, Ph.D., Lecturer in Physics, Haverford College, Penn., U.S.A.*

THE with me the relation between the changes of

THE object with which these experiments were undertaken

length and of width in a long thin specimen due to its magnetization. The work of Joule, Barrett, Bidwell, Knott, Nagaoka and Houda + bears on the subject; but only the two last and Bidwell deal with the case of uniform magnetization. Their results are opposite in character, Bidwell obtaining a diminution of volume, at least in low fields, while Nagaoka and Houda obtain an increase in all fields. In the results of neither does any simple relation between the longitudinal and transverse changes appear. It seems to me that the lack of isotropy in the metal used may be responsible for this, and also that the lack of uniformity in the winding of Bidwell's ring which is necessary to leave space for the connecting rods must have differently affected the changes of width and length.

My experiments have not been successful in settling this point as yet, the change of width apparatus not giving reliable curves, but that for change of length, shown in the figure, similar to one I used before, worked fairly well; and some of the curves taken with it are interesting as throwing light on disputed points, and as illustrating the form of the change of length cycle. The specimens consisted of narrow strips of thin tinned sheet-iron, 21 centims. long, having a ratio of length to mean diameter of about 220 to 1. The change of only 7 cm. in the central portion was measured so as to avoid the parts near the ends which will not be uniformly magnetized. Two small concentric tubes fitting over the specimen were attached one to each end of this length, so that when it expanded or contracted they moved relatively to each other. The outer tube, which was of course 7 cm. longer than the other, had 5 cm. of this length of platinum, the rest of this tube and all the other one being of brass. This gave temperature-compensation. These two tubes projected some distance beyond the end of the specimen, and the relative motion of their ends actuated the short arm of *Communicated by Prof. J. J. Thomson.

† Joule, Phil. Mag. vol. xxx. pp. 76 & 225 (1847); Barrett, Nature, vol. xxvi. p. 485; Bidwell, Proc. Roy. Soc. vol. lvi. p. 94 (1894); Knott, Trans. Roy. Soc. Edin. vol. xxxviii. p. 527, vol. xxxix. p. 457 (1898); Nagaoka and Houda, Phil. Mag. vol. xlvi. P. 262 (1898).

See previous paper, Phys. Rev. vii. p. 65, for the effect that nonisotropic condition of the metal may have.

a lever, about 1.75 cm. in length, whose long arm, about 9 cm., carried two legs of the tilting mirror. The scale was placed at a distance such that a movement of 1 mm. represented a change of one ten-millionth of the 7 cm.

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length between the clamps. The specimen with its surrounding tubes was placed in a coil, water-jacketed on the inside, 24 cm. long, giving a field of 98 c.G.s. to the ampere, constant almost to the ends of the specimen. This coil was suspended by indiarubber so that all vibration-effects were done away with. The whole was very skilfully made by Mr. Pye, instrument-maker of the laboratory. The curves obtained are very smooth and free from the effects of temperature; but they are not always quite the same after taking down the tubes, &c., and putting them up again apparently the same as before. The cause of this is not clear at present.

The first curve given (fig. 2) serves to illustrate the form taken by several change of length cycles going up to different

Fig. 2.


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field-strengths. Their character was first discovered and discussed by Nagaoka. It will be seen that the specimen which is lengthening as the field is being increased, continues to lengthen when the field is gradually withdrawn (except in the very smallest cycle), then a maximum length is reached and shortening begins and continues till the field is entirely removed and put on in the opposite direction with sufficient strength to drive out the residual magnetism. As the field is further strengthened the specimen is magnetized in the reverse direction and begins to elongate again. We may roughly distinguish four classes of cycles dependent upon the limits between which the field is varied. First, in very low fields, two or three c.G.S. units, there is no change of length; so the cycle is simply a horizontal line. Second, from these fields up to about ten units the length begins to decrease on diminishing the field, after having remained constant just at first. Thirdly, we have the cycle already alluded to in which increase of length takes place on reducing the current, even though the length had been previously increasing with increasing current. And, fourthly, in fields so great that the maximum length has been reached and the specimen has then considerably shortened, on decreasing the field again there is a loop on the lower side of the original curve (not shown in curve given).

The two curves given in fig. 3 bear on a point that has been several times brought up, namely, whether the actual thickness of the specimen has any effect on the change of length. It is natural to assume that it has not, as these curves go to show, provided the condition of "endlessness" in the specimen is fulfilled to a sufficient degree; but reference to a paper by Lochner (Phil. Mag. vol. xxxvi. p. 498)

will show to what extent this has been questioned. It is, however, important to provide in experiments on this subject not only that the magnetization of the part whose expansions

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and contractions are measured should be uniform, but also that that of the metal adjacent on all sides should be so; for if not, its distortions will be communicated elastically to the part under consideration.

The next set of curves (fig. 4, p. 467) bears on the effect of annealing. Those which are marked "hardened by shearing," are for the specimens just as they were cut from the sheet of soft iron by a die made for the purpose. Then they were annealed in a stream of hydrogen at a full red-heat, though this was not hot enough to anneal them completely, as is shown by the greater increase of retentiveness obtained in

fig. 5 (p. 468), for a specimen of the same kind of iron heated directly in a flame after being wrapped in asbestos. Mr. Bidwell has obtained a diminution of the maximum by annealing

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Upper curves for specimens cut out in one direction.
Lower curves for specimens cut out at right angles to above.

which would not be expected from the intimate relation which undoubtedly exists between the change of length and the magnetization-curves. The possibility that his specimen may have been burned in annealing suggests itself.

These curves (fig. 4) are also arranged to show the effect of the direction in which the specimens were cut from the original sheet.

The apparatus to show the change of width consisted of a strip of thin sheet-iron 5 x 36 cm., in the centre of which was an arrangement for making the change of width operate a lever, parallel to the specimen, on the end of which rested

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