Model of Ferromagnetic Induction, 495 



Let W, fig. J, be a pivoted magnet controlled by two 

 fixed magnets A and B, which point in opposite directions. 



Assume, in the first instance, that the attracting pole of 

 A acts on W somewhat more strongly than the repelling 

 pole o£ B, so that there is a feeble control due to this differ- 

 ence. Assume further that the clearance between W and 

 the adjacent poles of A and B is very small. Let an 

 external field H be applied. Then as H is increased VV will 

 deflect stably through a small angle, however nearly balanced 

 are the forces of control exerted by A and B. When the 

 deflexion exceeds a certain small angle, W becomes unstable. 



Fiff. 1. 



If there is a second pair of fixed magnets placed as at C and 

 D, W will tend to assume a stable relation to them, but a 

 further increase of H will make it break away from their 

 control also into a position nearly parallel to the field. To 

 make such a model completely imitate the process of ferro- 

 magnetic induction we have only to think of it as extended 

 into three dimensions, letting W be pivoted with two degrees 

 of rotational freedom, and surrounding it by say lour pairs 

 of fixed controlling magnets, situated on the four diagonal 

 axes of a cube. The sketch, drawn for a diagonal plane, 

 shows two such pairs. It is obvious that if a substance were 

 made up of atoms on this model all the familiar character- 

 istics of ferromagnetic induction would be reproduced in it 

 when a field was applied, removed, reversed and so on, or 

 when the piece was made to rotate in a constant field. 

 Since the strength of the control may be reduced to any 

 desired extent, a quantitative agreement with the results of 

 experiment becomes possible. 



