ELECTROSTATICS 



23 



and therefore the capacity of A per centimeter length between the 

 wire and the neutral or point of zero potential is 



rt _ JL _ y 



u ^ ~" 7T 7 n E> 



^ 2glog^ 



2 log 



-r- rr electrostatic units. (44) 



L) it 



R 



and the capacity of each wire in farads per mile is 



38.8 X 10- 9 



log 



10 



D-R 

 R 



(45) 



(d) Determine the capacity of a single wire of radius R cm., 

 suspended at a height H cm. above the earth. If the wire is 

 raised to a potential E above the earth potential, lines of dielec- 

 tric flux will pass from the wire to earth as shown in Fig. 18. 



FIG. 18. Capacity of a wire to earth 

 If ^ is the flux from each centimeter length of A, then q = j is 



^t 7T 



the charge on each centimeter of A, and a corresponding negative 

 charge q appears on the earth, but it is not evenly distributed 

 being most concentrated directly beneath the wire. 



The flux passing from the wire A to the earth would be un- 

 changed if the charge on the earth were collected on a second 



