ANISOTROPIC BODIES. 665 



If the principal coefficients are constant without being very 

 small, the body should have the shape of a sphere. The mag- 

 netisation parallel to the component </>A of the field is then 



and the coefficient's for each of the principal axes will play 

 4?r 



the same part as the coefficients k. 



Supported by this observation, all the properties we are about 

 to establish, on the hypothesis of very small coefficients, might be 

 applied to anisotropic spheres, the coefficients of magnetisation of 

 which are of any order of magnitude. 



1227. When the structure of a body is symmetrical in respect 

 of a right line, the same symmetry is again met with in the mag- 

 netic properties, and two of the coefficients (for instance k and k') 

 are equal. 



In this case, if the body is movable about its axis of symmetry, 

 the couple D for this axis is null, and the body is in neutral 

 equilibrium in all directions. 



If the body is movable about a perpendicular to the axis of 

 symmetry, the component H of the field perpendicular to the axis 

 is alone operative. If 8 is the angle of this force with the axis of 

 symmetry, the expression for the couple D" becomes 



D" = H 2 (-'; sin 8 cos 8; 



equilibrium takes place when the axis of symmetry is parallel or 

 perpendicular to the effective component H, according as k>k' 

 or k<k'. 



This couple may be measured by a method of torsion (for in- 

 stance by a bifilar C) which has been turned through an angle w 

 from the direction of equilibrium ; we have then 



2C sin(a>-S) 



K k = . 



#H 2 sin 28 



If the body is suspended by a wire without torsion, and os- 

 cillates freely, -the directive couple for very small deflections is 



