3-40 Messrs. Nagabka and Honda on Change of Volume and 



It will be seen from the above table that in iron and steel 

 there is increase of magnetization in low fields till it reaches 

 a maximum, after which it gradually decreases. The decrease 

 does not take place continuously, but reaches a maximum, 

 whence the magnetization begins to recover. Although 

 the former result here arrived at is the well-known Villari 

 effect, we do not know whether the maximum decrease 

 due to longitudinal stress has as yet been experimentally 

 ascertained. 



There is nothing remarkable in nickel. Longitudinal pull 

 produces decrease of magnetization, which becomes gradually 

 less as the field-strength is increased. This fact is so well- 

 established by experiment that we need not enter into further 

 discussion of the subject. 



Effect of Hydrostatic Pressure. — We can easily see that the 

 change of magnetization SI due to decrease of volume — cr by 

 hydrostatic pressure is given by 



(*' + t) 



k" 

 As k'+ -5- is very small in nickel, the effect of hydrostatic 

 o 



pressure is very small compared to that of longitudinal 

 pull. (See fig. 3, dotted-line.) There is increase of mag- 

 netization with the volume-contraction of the magnet. Such 

 an increase reaches a maximum in a low field, whence the 

 effect gradually diminishes. Similar changes are also to be 

 noticed in iron and steel, quite contrary to experiment. The 

 agreement between theory and experiment is very close 

 in nickel, while there is wide discrepancy in iron and 

 steel. 



Effect of Torsion on Longitudinally or Circularly Magnetized 

 Wire. — There are other important consequences to be drawn 

 from the constant k" with regard to the effect of torsion on 

 longitudinally magnetized wire and on ferromagnetic wire 

 traversed by electric currents. The strain caused by twisting 

 a circular wire can be resolved in elongation and contraction 

 in directions perpendicular to each other and inclined to the 

 axis of the wire at 45°. Taking these two principal axes of 

 strain for those of x and y, we obtain the following expressions 

 for the strains : 



where g> denotes the amount of torsion and r the distance from 



