ABSOLUTE CONDUCTING SCREENS. 545 



If the primitive current I was null, the moment of the action 

 of the field on the molecule will be 



X 2 A 2 X 2 A 2 



IAX sin 6 = sin cos = sin 29. 



L 2 L 



7T 



The equilibrium will be stable, for = - , that is when the plane of 



the current is perpendicular to the direction of the field. This is 

 the case, for instance, of a ring which is suddenly brought into 

 a very powerful field. 



In the general case, the moment of the couple which acts on 

 the molecule may be written 



X 2 A 2 



- cos 6 sin - I AX sin 6 = mX sin [BX cos - i J, 



putting w = I A and B= -. 

 LI 



If we assume with Weber (427), that the reaction D of the 

 medium is constant in magnitude and in direction, we shall 

 have the equation of equilibrium, 



X sin 0(i - BX cos 6) = D sin(a - 0). 



The component, parallel to the magnetising force X, of the 

 magnetic moment of the molecule is 



(XA 

 I o cos ) A cos 6 = m cos 6>(i - BX cos (9). 



If the coefficient B is very small that is, if the primitive 

 molecular current I is very powerful we arrive again at Weber's 

 formula for the magnetisation of magnetic substances. If this co- 

 efiicient B is very great we obtain the phenomena of diamagnetism. 



In intermediate cases, magnetisation will first of all be pro- 

 portional to the magnetising force for small forces, it will then 

 pass through a maximum, and afterwards decrease. Experiment 

 does not seem to show any phenomenon of this kind, so that the 

 occurrence of currents of molecular induction cannot be con- 

 sidered proved. 



NN 



