370 ELECTROSTATICS AND MAGNETISM. [PT. II. CH. IX. 



field is uniformly polarized, and the field inside the ellipsoid is 

 uniform. The field outside will not be uniform. 



We obtain from equations (9) for the components of the polari- 

 zation 



1 + fcL' 1 + icM- 1 + tcN' 



and for the field in terms of the undisturbed field 



V ~V P 



The field in the polarized body is not in the direction of the 

 undisturbed field, unless either L = M = N (sphere), or the un- 

 disturbed field has the direction of one of the axes of the ellipsoid, 

 when two of the components X 0) F , Z vanish. The force due to 

 the polarization, Fi, has the components 



/cN 



If K is positive, this force is in the opposite direction to the 

 undisturbed force. If the body be magnetic, and hard, so that 

 when the field F is removed, the polarization is retained, the 

 force Fi alone acts, and tends to produce a magnetization in the 

 reverse direction, or to demagnetize the body. The force Fi is 

 accordingly called the self-demagnetizing force, and since we may 

 write 



(13) Xi = -LA, Yi = -MB, Zi = -NC, 



L, M, N are called the self-demagnetizing factors for the three 

 axes. 



193. Couple experienced by Ellipsoid. The couple ex- 

 perienced per unit of volume has, by 120 (5), the components 



BZ, -CT , CX,-AZ, % AY -BX . 



Suppose that the force of the undisturbed field is in one of the 

 principal diametral planes of the ellipsoid, then the couple tends 



