22 ELECTRICAL ENGINEERING 



(c) Determine the capacity of two parallel wires A and B in 

 Fig. 17. 



When a difference of potential E is applied between them a 

 dielectric flux passes across from A to B and a positive charge 



Q = -p- appears on A and an equal negative charge Q = ~ 



4 7T 4 7T 



onB. 



A B 



FIG. 17. Capacity of parallel wires. 



If q is the charge per centimeter length on A, the potential 

 difference between A and B is 



,., 



where R is the radius of the wires and D is the distance between 

 centres. 

 The capacity of the two wires per centimeter length is 



C = ^ = - TT - n - H - n electrostatic units. (42) 

 hi , D K . . v K 



4 q log ^ 4 log ^ 



The capacity per mile of a line consisting of two wires is 

 2.54 X 12 X 5280 _ 



4 X 2.303 Iog 10 - X 9 X 10 11 

 K 



19.4 X 10- 9 



~ 

 D 



, 



farads ......... (43) 



It is sometimes useful to separate the capacity of the line into 

 the capacities of the two wires forming it. The potential of the 

 point midway between the wires is zero and the actual potential 

 of wire A is 



=^,Eq.24, 



