ii8 



Mr. Henry Holden on the 



m 



27r/ 



dr 1 Ra . R2 



47rC 

 K 



or C = 



2-1 



Ri 

 K./ 



lO£^ 



^;ir. 4. — Cylinders of circular section, one inside the 

 other but not coaxal. 



Fig. 4. 



Fig-. 5- 

 Let A and A' be conjugate points with respect to the 

 circular sections of each of the cylinders. Then, it is known 

 that the lines of force between the cylinders are arcs of 

 circles passing through A and A', each line of force being 

 therefore defined by the condition 0= constant (see Fig. 4). 

 Let P be a point on a line of force, and suppose that the 

 direction of the force at that point makes angles a and o! 

 with AP( = p) and A'P( = pO respectively. Then since dr is 



