.24 



cos a 1 sin a 

 . and as — = 



c c r 



the axial fraction is proportional to 



sin a cos a 



This expression has a maximum for a = 45°. 



As the same reasoning may be used for every other strip situated 

 on the planes of the prism, we ought to cut the prism in such a 

 way that the bi-phme-angle at the rib amounts to exactly 90°. 



This is only true if the planes of the prisms terminate in the line 



of intersection. This case however never occurs: the planes are 



always cut off by a plane parallel to the ribs so that the inter- 



ferricnm is enclosed between two planes i)arallel to each other and 



perpendicular to the lines of force. In such a case we can, however, 



still calculate the maximal field-intensity at the line of intersection 



of the planes. The field is then formed by two different components, 



i.e. by the magnetic lines passing through the side-planes and by 



those issued by the two limiting planes. If these be IJ^ and H^ 



we have for the total field: H = B,-\- H,. 



We found for each strip H^ the value of 



sin a cos a 



. In order to obtain an expression lor 



r 



the magnetization due to an entire side-plane 



we suppose that the breadth of the strip is 



(/r, and that consequently its action is propor- 



dr ^ 

 tional to sin a cos a ~. It we integrate this ex- 

 /' 



pression between the limits q and R, in which 



Q represents half the depth of the interferricum, 



R half the thickness of the magnet, we find 



(he value looked for. It amounts to : 



Fig. 3. 



R 



r . dr 



//j — I sin a cos a — = sin a cos a Ign 

 J »■ 



R 



In order to calculate the magnetic field caused by the free mag- 

 netism in the parallel boundary plane, we divide it again into 

 length-wise strips. 



The attraction exercised on the rib by each strip is inversely 



1 



proportional to their mutual distance b, consequently —. 



