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XLV. Experiments on the CI tana e in Dimensions caused by 

 Magnetization in Iron. By Edwakd Rhoads, Ph.D., 

 Lecturer in Physics, Haverford College, Perm., U.S.A.* 



THE object with which these experiments were undertaken 

 was to investigate the relation between the changes of 

 length and of width in a long thin specimen due to its 

 magnetization. The work of Joule, Barrett, Bidwell, Knott, 

 Nagaoka and Houda f 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 J 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, Laving 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 uni- 

 formly 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. 



t 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. Rov. Soc. Edin. vol. xxxviii. p. 527, vol. xxxix. p. 457 (1898) ; 

 Nagaoka and Houda, Phil. Mag. vol. xlvi. p. 262 (1898). 



X See previous paper, Phys. Rev. vii. p. 05, for the effect that non- 

 isotropic condition of the metal may have. 



