63, 64] Charges +e, -e 51 



doublet of strength yu, having for its direction the positive axis of x. Its 

 potential is 



or, if we turn to polar coordinates and write x = r cos 0, is 



^ .................................... (24). 



The lines of force are shewn in fig. 18. Obviously the lines at the 



FIG. 18. 



centre of this figure become identical with those shewn in fig. 17, if the 

 latter are shrunk indefinitely in size. 



42 



