154 



PARTICULAR CASES OF EQUILIBRIUM. 



168. Two UNEQUAL MASSES OF OPPOSITE SIGNS. Let +m 

 and m' be two masses of opposite signs situate at A and A' (Fig. 45) 

 at a distance of 20, m being greater than m' in absolute value ; let 

 us put 



m 



The equation of an equipotential surface is 



r r 



r r 



rr 



One of these surfaces corresponds to potential zero, and its 

 equation is 



m m' 



7-7=. 



whence 



It is a sphere to which the point A' is internal ; the two points 

 A and A' are conjugate in reference to this sphere. 



To determine the radius R, and the centre O of the sphere, we 

 shall use the ratios 



BA B'A R OA 



