66] Charges +e, +e, +e 55 



surrounding the former equipotential, and finally reducing to circles at in- 



2 2*25 2'5 2'75 



finity. The curves drawn correspond to potentials - , , and . 



a a a a 



3*04 



There remains the region between the point D and the equipotential - . 



a 



At Z) the potential is - , so that the potential falls as we recede from the 

 a 



equipotential * - and reaches its minimum value at D. The potential at 



D is of course not a minimum for all directions in space : for the potential 

 increases as we move away from D in directions which are in the plane 

 ABC, but obviously decreases as we move away from D in a direction per- 

 pendicular to this plane. Taking D as origin, and the plane ABC as plane 

 of xy, it will be found that near D the potential is 



FIG. 22. 



Thus the equipotential through D is shaped like a right circular cone in 

 the immediate neighbourhood of the point D. From the equation just 

 found, it is obvious that near D the sections of the equipotentials by the 

 plane ABC will be circles surrounding D. 



