TWO UNEQUAL MASSES OF OPPOSITE SIGNS. 



155 



Remarking that OA - OA' = 2 a, we easily deduce from this 



(30) 



I 



4 -I 



The potentials are negative inside the sphere, and positive 

 outside. 



Fig. 46. 



All the equipotential surfaces are closed surfaces with one sheet 

 or with two distinct sheets, except a single one which has two 

 adjacent sheets S f and S' i5 and which passes through the point I 

 (Fig. 46) where the force is zero. The position of this point is given 

 by the equation 



m m' 



or 



