358 H. NAGAOKA AND K. HONDA; 



of several previous investigators on the change of volume produced by 

 magnetization. 



It was generally admitted that there is no change of volume by 

 magnetization, but it will be easily seen that most of these experimenters 

 tried to increase the volume of the magnet by unusually increasing 

 the thickness instead of length, thereby incurring the risk of in- 

 creasing the demagnetizing factor. They did not therefore arrive 

 at a field strength sufficient to produce appreciable change of 

 volume. 



Joule was I he first to call attention to the change of volume 

 which may accompany the magnetization of iron. The result was in 

 the negative, but as he did not give the strength of the magnetizing 

 current or the intensity of magnetization, it is difficult to com- 

 pare his result with that of his successors. It is beyond doubt 

 that the change of volume was very minute, and there was suf- 

 ficient evidence that the elongation in the direction of magnetiza- 

 tion is accompanied by contraction in the direction perpendicular 

 to it. 



The elaborate researches of Cantone 2) on the strain of a ferromagne- 

 tic ovoid are not free from the fault above mentioned. The length of the 

 major axis of the ovoid was 1G.7 times that of the minor, so that the 

 demagnetizing factor = 0.1134. As his results are given in terms of 

 the magnetizing current and the moment of the magnet, we have 

 thought it advisable to recalculate the result in H ( = H — N I where 

 H stands for the magnetizing field in the coil) and I (in C.G.S. 

 units). 



1). Joule, Phil. Mar/. 3Q, 7G, 1847, Collected Works I. 



2). Cantone, Memoria d. Accad. d. Lincei 6, 1S90 ; Rendiconti d. Accad. d. Lincei ß, (1), 

 257, 1890. 



