Middle of the Nineteenth Century. 237 



mutual action of the molecular magnets themselves. In the 

 unmagnetized condition the molecules " arrange themselves so 

 as to satisfy their mutual attraction by the shortest path, and 

 thus form a complete closed circuit of attraction," as D. E. 

 Hughes wrote* in 1883 ; when an external magnetizing force 

 is applied, these small circuits are broken up ; and at any stage 

 of the process a molecular magnet is in equilibrium under the 

 joint influence of the external force and the forces due to the 

 other molecules. 



This hypothesis was suggested by Maxwell,t and has been 

 since developed by J. A. Ewing;J its consequences may be 

 illustrated by the following simple example : 



Consider two magnetic molecules, each of magnetic moment 

 m, whose centres are fixed at a distance c apart. When 

 undisturbed, they dispose themselves in the position of stable 

 equilibrium, in which they point in the same direction along 

 the line c. Now let an increasing magnetic force H be made 

 to act on them in a direction at right angles to the line c. 

 The magnets turn towards the direction of H ; and when 

 H attains the value Sm/c 3 , they become perpendicular to the 

 line c, after which they remain in this position, when H is 

 increased further. Thus they display the phenomena of induc- 

 tion initially proportional to the magnetizing force, and of 

 saturation. If the magnetizing force H be supposed to act 

 parallel to the line c, in the direction in which the axes 

 originally pointed, the magnets will remain at rest. But if H 

 acts in the opposite direction, the equilibrium will be stable 

 only so long as H is less than ra/c 3 ; when H increases 

 beyond this limit, the equilibrium becomes unstable, and the 

 magnets turn over so as to point in the direction of H\ when 

 H is gradually decreased to zero, they remain in their new posi- 

 tions, thus illustrating the phenomenon of residual magnetism. 



* Proc. Roy. Soc. xxxv (1883), p. 178. 

 f Treatise on Elect. $ May., 443. 



I Phil. Mag. xxx (1890), p. 205 ; Magnetic Induction in Iron atid other Metals,. 

 1891. 



E. G. Gallop, Messenger of Math, xxvii (1897), p. 6. 



