EQUILIBRIUM OF A DIAMAGNETIC BODY. 381 



the couples tends to bring the common axis into the direction of the 

 field. In this position the magnetisation is a maximum. 



If, on the contrary, the axis is perpendicular to the direction of 

 the field, the reciprocal actions tend to diminish the magnetisation of 

 each of the insulated balls ; the equilibrium is unstable, and the 

 magnetisation of the system of the spheres is minimum. 



Thus the existence of a position of stable equilibrium for a 

 magnetic needle in a uniform field, implies the existence of inter- 

 actions between the different magnetic elements which constitute it, 

 and in contradiction with Poisson's hypothesis on the constitution of 

 magnetic bodies, which presupposes that such actions do not take 

 place. 



400. The conclusions would be almost the same for the equi- 

 librium of a diamagnetic body, although the actions are in the 

 opposite direction. 



For the induced magnetisation is then in the opposite direction 

 to the magnetising force. A series of balls B, B', B", . . ., arranged 

 on a straight line perpendicular to the field, becomes magnetised in 

 a direction opposite to that of the field ; the reactions increase then 

 the magnetising force on each of the balls. This direction corre- 

 sponds thence to a maximum of magnetisation and to a state 

 of equilibrium. 



Let us now suppose that the line of the balls forms an angle a with 

 the direction of the field ; since the direction of magnetisation is 

 inverse, and each pole tends to develop a pole of the same kind in 

 the nearest part of another ball, the reactions diminish the magnetis- 

 ing force, and modify its direction; the effect is further the more 

 marked the smaller the angle a. The couples which act on the 

 spheres tend to set the axis in a direction parallel to the field. The 

 magnetisation is then a minimum. 



Hence a diamagnetic needle should also take up a direction 

 parallel to that of the field to be in stable equilibrium. 



Nevertheless the coefficient of magnetisation for diamagnetic 

 bodies is so feeble, that the reactions of the particles may be 

 neglected and their effect escape all means of observation. 



For a diamagnetic needle, provided it is not crystalised, is in 

 mobile equilibrium in a uniform magnetic field ; in all experiments 

 in which there seem to be phenomena of direction, the effect is due 

 to the magneto-crystalline properties (397) of the body in question. 



401. EQUILIBRIUM OF BODIES IN A VARIABLE FIELD. In a 

 variable field the phenomena are more complicated. 



Diamagnetic bodies simply follow the law of Faraday that is to 



