544 PARTICULAR CASES OF INDUCTION. 



magnetic induction will be null on the whole surface, and in the 

 interior of the layer which surrounds the molecule. 



If we assume that the molecule has the form of a sphere of 

 radius r, the currents on the surface produce in the interior a force 

 - X, equal and of opposite sign to the external magnetic force ; 

 the sphere is magnetised uniformly with an intensity (355) equal to 



, and the value of its magnetic moment is - - ^X. 



If we assume that in the unit volume there are n equal 

 spheres, so small and so far apart that they do not act on one 

 another, the mean strength of the magnetisation of the medium 



will be - - ^X = - - , h being the ratio of the sum of the volumes 



2 07T 



of the small spheres to the total volume of the space which contains 

 them. 



563. ABSOLUTE CONDUCTING SCREENS. It is clear that these 

 conclusions from formula (63) would apply also to a surface of 

 finite extent which possessed absolute conductivity ; the induced 

 currents which any variation of the field would produce in this 

 surface, would always be such that the flow relative to each portion 

 of the surface would be constant in other words, that the per- 

 pendicular component of the magnetic induction at each point would 

 retain a fixed value. If then this component part was null at a 

 given moment, it would remain null whatever were the variations 

 of the field. It follows from this that a closed or unlimited surface, 

 of resistance zero, is a complete screen for all points in the interior 

 against the effects of the variation of the field on the other side 

 of the surface ; these effects reduce to the production of surface 

 currents, which keep the field in the interior constant at zero. 



The distribution of magnetic forces about a perfect conductor 

 is quite comparable to the distribution of velocities in an incom- 

 pressible fluid which surrounded the same bodies. 



564. In order to explain magnetic phenomena on these con- 

 siderations, it must be assumed that the primitive current in a 

 molecule, or about a conducting channel, is not null, and we take 

 the general equation (64). Considering always the case of a circuit, 

 the perpendicular of which makes an angle with the direction 

 of the magnetising force, we shall have 



LI + XAcos(9 



L o> 



or _ _ XA 



I = I cos 



J_J 



