458 H. Nagaoka and E. T. Jones on the Effects 



The change of length of a soft-iron ellipsoid has, however, 

 heen investigated over a much greater range of field-intensities 

 by one of us*, and the results represented by curves as 

 functions of magnetizing force, of the square of mag- 

 netization, and of the former, respectively. If the change 

 of length due to Kirchhoff's system of stresses be calculated 

 from Cantone's formula, and the result subtracted from the 

 observed change of length, these curves (K' and K" being pro- 

 visionally disregarded) assume the forms shown in figs. 1, 2, 3. 



Since the coefficients K' and K" are not known with any 

 accuracy, for the iron used, the correction due to them cannot 

 at present be calculated. The ordinates of the full-line curves 

 represent therefore the changes of length produced by uniform 

 magnetization, corrected on Kirchhofr 's theory for the partial 

 stresses represented by terms containing H and K, which 

 were measured at the same time. The residual change of 

 length, represented by the ordinates in figs. 1, 2, 3, must be 

 due to the other unknown terms in the expressions for the 

 stresses, or to some change of molecular arrangement accom- 

 panying the magnetization. 



A similar process was followed by Moref, who measured 

 the change of length of part of an iron wire magnetized by a 

 coil. It was assumed, however, that there existed in the wire 



B 2 



a contracting stress tt- , the reason given for this assumption 



being that if the wire were cut in two, the two ends would 



B 2 



be held together by a force — per unit area. Now, accord- 

 ing both to Maxwell's theory and to that of KirchhofF, the 



. . . B 2 



tension m a narrow air-gap in the wire is -7-, and this 



has been experimentally verified by one of us over a large 

 range of magnetizing field % ; but both theories give different 

 values for the stress inside the iron. Even if the wire were 

 cut in two, the force holding the parts together would not be 



B* 



— per unit area unless the coil were also cut into two parts, 



into which the pieces of wire were rigidly fixed, so that part 

 of the total tractive force is due to the mutual attraction of the 

 two coils. 



* H. Nagaoka, Wied. Ann. liii. p. 487 (1894). 

 t Phil. Mag. Oct. 1895. 



X See du Bois, Mognetische Kreise, § 104 ; E. T. Jones, Wied. Ann. lvii 

 p. 271 (1896) ; Phil. Mag. March 1896; Wiedemann's Electr. iii. p. 640.' 



