190 



ADVANCED ELECTRICITY AND MAGNETISM. 



no. The condenser type of insulating bushing again con- 

 sidered. The radial electric field surrounding a line charge 

 decreases in intensity at increasing distances from the line charge, 

 and to produce a radial field of constant intensity in a homogene- 

 ous dielectric free charge would have to be distributed in a certain 

 definite manner throughout the dielectric. This condition is 

 approximately realized in the condenser type of insulating 

 bushing as described in Art. 89, the free charge being on the tin- 



Fig. 130. 



foil sheets. A sectional view of such a bushing is shown in Fig. 

 129. The theoretically correct shape of the curve cc in Fig. 

 129 is an equilateral hyperbola, because the successive tinfoil 

 cylinders must have the same area, as explained in Art. 89. 



No attempt is made to show the lines of force in the region 

 surrounding the bushing in Fig. 129. As a matter of fact this 

 electric field is greatly concentrated in the region immediately 

 surrounding the rod at the ends of the bushing. This concentra- 

 tion of electrical stress is obviated by Mr. Fortescue's method as 

 follows : 



