350 H. NAGAOKA. 



Unfortunately wc have not many experimental data on this subject, 

 so full and detailed as those we have on the Villari critical point. 

 According to the experiments of Ewing and Low*, the effect of 

 compression in altering the susceptibility of iron has a certain 

 similarity to the effect of stretching ; and there is a similar reversal 

 corresponding to that of Aillari. Under constant compressional 

 stress, the curve of magnetization for compressed iron lies below that 

 for the unstrained, in weak magnetizing; fields. In strong inagnetiz- 

 ing fields, the former acquires greater susceptibility, and the curve lies 

 above the latter. I do not know of any experiment establishing' the 

 relation between susceptibility and the compressional stress, when 

 either the magnetizing force or the amount of compression is made 

 to vary. Without these data, it is difficult to examine the effect on 

 the transient current either by varying the magnetizing force or by 

 varying the amount of twisting. 



The fact that compression reduces the susceptibility of iron in 

 weak fields shows why the transient current increases with increase 

 of twist. So long as the magnetizing force does not exceed a certain 

 limit, the susceptibility diminishes in the direction of compression, 

 while it increases in the direction of stretching provided the Villari 

 critical point is not exceeded. Thus the circular component of the 

 lines of magnetic induction is constantly increasing as the twisting is 

 increased and will therefore give rise to an increasing transient current. 

 The increase of current will take place so long as the difference of 

 susceptibilities in these two directions do not diminish. Observation 

 shows that the maximum transient current occurs for larger twist- 

 ings as the magnetizing force is taken greater. Since the stretching 

 stress giving the maximum susceptibility becomes smaller as the 

 magnetization of the wire is increased, it seems that compression 



* Phil. Mag. 18S8. 



