TWO EQUAL MASSES OF OPPOSITE SIGNS. 



127 



Its value at P a on the axis AA', at a distance d from the centre, is 



2d 



n _ ^2x2 



and on the transverse axis at the same distance d from the centre, or 

 at the distance p from either of the masses, 



The product 2ma, of one of the masses by the distance separating 

 them, which is called the magnetic moment in the corresponding 

 problem in magnetism, may be called the electrical moment of the 

 system. 



A' 



Fig- 35- 

 147. When the force is perpendicular to the axis 



sn = cos a 



equation (12) becomes 



COS 0> COS 0) 



This is the equation of the curve APX (Fig. 35) which passes 

 through all the points of the plane where the force is vertical. It 



