556 PHENOMENA OF INDUCTION IN NON-LINEAR CONDUCTORS. 



CHAPTER VII. 



PHENOMENA OF INDUCTION IN NON-LINEAR 

 CONDUCTORS. 



573. MAGNETISM OF ROTATION. Following up an observation 

 by Gambey on the deadening of the oscillations of a compass 

 needle, Arago showed, in 1824, that a magnetised needle placed 

 above a disc which is in rapid rotation, is carried along by the 

 disc, and tends to acquire a rotation in the same direction. 



The action exerted on a pole in these three conditions has 

 three components; the one tangential which impels the pole in 

 the direction of the rotation, another perpendicular which tends 

 to remove it from the disc, and finally a third directed along the 

 radius. This latter is null when the pole is at a distance from 

 the axis equal to about two-thirds of the radius of the disc ; nearer 

 the axis the pole is attracted to the centre ; when nearer the edges, 

 it seems repelled towards the edges. 



The motion of the needle is more marked with a good conductor 

 like copper, than with a metal which does not conduct so well, 

 such as brass, and particularly antimony. When there are breaks 

 in the continuity of the disc (as for instance with a radial saw cut) 

 the effect produced is enfeebled. 



These phenomena were at first ascribed to a special form of 

 magnetisation, and were known as magnetism of rotation. They 

 are really produced by induction currents developed in the metal; 

 but it was only after Faraday's great discovery that they were 

 ascribed to the true cause. 



574. CONDUCTING SHELLS. The problem raised by Arago's 

 experiment is that of induction in a conductor of two dimensions. 

 Maxwell solved this problem in a very elegant manner, by the use of 

 a method analogous to that of electrical images. 



