TWO EQUAL LINES OF OPPOSITE SIGNS. 



119 



The limiting line of force is therefore a circumference whose 

 centre is A', and which passes through the point A. 

 The equation of the equipotential surfaces is 



V = const- 2 1. ( ) = const + 2/. ( - 



from which 



= const e' 2 . 



136. Two EQUAL LINES OF OPPOSITE SIGNS. If we suppose 

 the two masses equal in absolute values, the equation of the lines of 

 force reduces to 



and that of the equipotential surfaces to 



The former represents segments of the circumference such as 

 ATA' (Fig. 30) passing through the two points A and A' and which 



Fig. 30- 



may have the angle ; the second represents circumferences 

 S, S' . . . having their centres on the right line AA', and such that the 

 two points are conjugate in reference to each of them. 



Considering the two equipotential surfaces S and S', a layer + m 



