342 On Change of Volume and Length by Magnetization. 



the longitudinal magnetization is the same in iron, steel, and 

 nickel. 



It has been established by G. Wiedemann * that the longi- 

 tudinal magnetization produced by twisting iron wire traversed 

 by an electric current is opposite to that in nickel. The 

 opposite character of the transient current in these two 

 metals has also been observed by Zehnder f, and independently 

 by one of us f. The existence of a maximum transient current 

 in these two metals has also been clearly established by the 

 latter, although there is some difference in the field-strength 

 between iron and nickel. It appears from the experiments 

 of Dr. Knott § that the area of the hysteresis-curve in the 

 longitudinal magnetization produced by twisting circularly 

 magnetized wire reaches a maximum as the field-strength is 

 increased ; but on account of the feebleness of the current, the 

 existence of the maximum in the longitudinal magnetization 

 is not well established. Judging from the course of the curve 

 given by the same experimenter, it seems highly probable 

 that the maximum would be reached if we could push the cir- 

 cularly magnetizing force a little further. The conclusion 

 (3) is still an open question, although some experiments of 

 Matteucci || seem to corroborate the view above stated. 



Looking at the curves of £"H (fig. 3], we are struck with 

 the close resemblance of the curves representing the amount 

 of torsion produced by the combined action of the circular and 

 longitudinal magnetizing forces on a ferromagnetic wire. We 

 can no doubt coordinate the effect of torsion on a magnetized 

 wire with the Wiedemann effect. The discussion of the last- 

 mentioned effect we hope to publish in the near future. 



In spite of the qualitative explanations which KirchhofPs 

 theory affords with regard to the effect of longitudinal pull, of 

 hydrostatic pressure in nickel, and of torsion, there are 

 instances in which the theory apparently fails in several 

 quantitative details, so that it necessarily calls for modification. 

 We may remark that k f and k" are physically functions of the 

 strain, as borne out by the numerous experiments on the 

 effect of stress on magnetization. The present state of the 

 theory of magnetostriction may perhaps be compared with 

 that stage in the history of the theory of magnetism when the 



* G. Wiedemann, Elektricitat, Bd. iii. 

 f Zehnder, Wied. Ami. Bd. xxxviii. p. 68 (1889). 

 % Nagaoka, Phil. Mag. vol. xxix. p. 123 (1890) ; Journal of the College 

 of Science, Tokyo, vol. iii. p. 335 (1890). 



§ Knott, Trans. Roy. Soc. Edinb. vol. xxxvi. p. 485 (1891). 

 II Matteucci, Annates de Chimie et de Physique, 1858. 



