132 On Transient Electric Current from Iron and Nickel. 



is, by stretching, rendered less in the elongational line than 

 in lines perpendicular to it. The result is that the lines 

 of induction of twisted wire are no longer straight, but 

 are changed into helical lines. They are inclined toward 

 the direction of relatively increased magnetic susceptibility. 

 The direction of the transient current will be thus deter- 

 mined by that of compression. In iron, on the contrary, 

 the direction of the current is that corresponding to stretching* . 

 Consequently, the direction of the current in nickel must be 

 opposite to that in iron. 



In nickel there is nothing corresponding to Villari reversal 

 in iron. The magnetization of nickel is always diminished by 

 stretching, while it is increased by compression. The conse- 

 quence is that the circular component of the lines of induction 

 is always on the increase as the twist becomes greater. This 

 will account for the reason why the transient current does 

 not reach a maximum even when the angle of torsion 

 amounts to 100 . 



Further investigations on the subject will be published in 

 the Journal of the College of Science, Imperial University, 

 Japan. 



Tokyo, Oct. 17, 1889. 



an 



.es 



[Note on the Direction of the Induced Longitudinal Current in 

 Iron and Nickel Wires by Twist when under Longitudinal 

 Magnetizing Force. By Sir W. Thomson. 



To avoid circumlocutions suppose the iron or nickel wire 

 to be vertical, and the magnetizing current to be in the 

 opposite direction to that of the motions of the hands of a 

 watch held with its face up. The undisturbed magnetization 

 is downwards f. Now suppose a right-handed twist to be 



* For explanation of this see Note by Sir W. Thomson added below. 



t Much of circumlocution is avoided, and of clearness gained, through- 

 out dynamics and physics, by introducing the substantive noun ward (as 

 has been done by my brother Prof. James Thomson in his lectures on 

 Engineering, and in lithographed sheets put into the hands of his 

 students, in the University of Glasgow) to signify line and direction in a 

 line ; — that which is represented ordinarily by a barbed arrow. Taking 

 advantage of this usage I now define the ward of magnetization as the 

 ward in which the magnetizing force urges a portion of the ideal northern 

 magnetic matter or northern polarity. By northern polarity, I mean 

 polarity of the same kind as that of the earth's northern hemisphere. It 

 is that which is marked blue by Sir George Airy to distinguish it from 

 southern magnetic matter or southern polarity, which he marked red. 

 According to a usage condemned 300 years ago by Gilbert, but not yet 

 quite dead, English instrument-makers still sometimes mark with au N 

 the true south pole, and with an S the true north pole, of their steel bar- 

 magnets. All confusion due to this unhappy mode of marking magnets 

 is done away with by Sir George Airy's red and blue. 



