372 MAGNETIC INDUCTION. 



and, if k is very great, 



The external potential at P, of the layers M x and M 2 , at a 

 distance r from the centre, and on a radius which makes the angle to 

 with the force of the field, is 



cos q> (ji- i) (i +p) (i - /3) , cos (o 



<?> 



putting 



from which is deduced, for the action of the induced magnetism in 

 two points at a distance r on the line OA and on the line OB, that 

 is at the pole and at the equator, 



v a/I- 



y\.p ^ I 



390. For a solid sphere we have /? = 0, and therefore 



The ratio of the actions excited at the same point by a hollow 

 sphere, and by a solid one of the same external diameter, is then 



A 

 A~o 



