WEBER'S THEORY. 401 



The condition of equilibrium is 



mX sin = mD sin ft = mD sin (a - 6) 



from which we deduce 



D sin a 



(n) tan 



X + Dcosa' 



428. The structure of the medium being symmetrical in 

 reference to the axis of x, the strength of magnetisation is given 

 by the sum of the projections of the magnetic moments of all the 

 molecules on the axis of x. 



The projection of the moment of a molecule is expressed by 

 mcosd; the number of those which originally made the angle a 



with the axis of x, is - sin a da. : the resultant is then 



2 



cos0sina<a, 



C ir n f inn 



= m cos0-sinadfa= - 



Jo 2 J, 2 



or 



M 

 1= -- 



The triangle SOP gives the equation 



from which is deduced 



We have further 



D 2 = R 2 + X 2 -2RXcos0. 



Expressing in this way the angles a and 9 by their values as a 

 function of R, we get 



D D 



