RESEARCHES 03 MAGNETOSTRICTION. 371 



caused by magnetization and the effect of compressional stress on 

 magnetization is established. 



Results in nickel : — The curves of a pressure cycle are shown in 

 F\g&. 15 and 16. The change of magnetization due to hydrostatic 

 pressure is also very small, hut is comparatively greater than 

 that in iron and the hysteresis during the cycle is more decided. 

 Whereas hydrostatic pressure causes the diminution of magnetization 

 in iron, it causes increase of magnetization in nickel. Like other 

 effects of stress such as stretching or twisting, we find that the change 

 in iron is opposite to that in nickel. 



The curves of the change of magnetization by constant pressure 

 (Figs. 17, IS, 19, 20) (PI. XIX) in différent fields show that there is 

 increase of magnetization in weak fields until it reaches a maximum 

 in moderate fields ; it then goes on slowly decreasing. This feature is 

 characteristic of nil pressures up to 250 atmospheres. Plotted against 

 magnetization, the general appearance of the curves is the same as 

 that for magnetizing fields. 



Comparing these curves with those obtained from change of 

 volume by magnetization, we notice that whereas increase of magne- 

 tization produces diminution of volume in nickel, diminution of 

 volume produces increase of magnetization. 



It will he shown later on that the minuteness of the effecl of 

 compression on the magnetization of iron and nickel leads to an 

 important conclusion in the theory of magnetostriction. 



Effect of transverse stress on the magnetization of an iron tube: — 

 Lord Kelvin, 1 ' in his series of experiments on the electrodynamic quality 

 of metals, investigated the effect of transverse stress on the magnetiza- 

 tion of an iron tube, by subjecting the inner surface of a gun barrel to 



1) Kelvin. lot: cit. 



