384 



ALTERNATING CURRENTS 



existing in the field between A and B is shown. The capacit;uico 

 between conductors of such a line can be shown to be approximately 



r 0.0194 , .. 



C . mf. per mile 



(87) 



where D is the distance between conductor centers and r is the 

 radius of each conductor, both expressed in the same units. 



The simplest method of treating transmission-line problems is 

 to work with voltages to neutral and with capacitances to neutral. 



(a) Neutral plane between two line conductors. 



yj 



o II- 



O 



(6) Line capacitance replaced by (c) Line capacitance replaced by 

 a single condenser. two series-connected condensers. 



FIG. 348. Substitution of equivalent condensers for transmission line 



capacitance. 



In Fig. 348 (a), an imaginary plane surface xy is shown midway 

 between conductors A and B and perpendicular to the plane 

 of the conductors. The electrostatic field between this sur- 

 face and each conductor is the same. As the plane bisects 

 every electrostatic flux line, the potential difference between 

 conductor A and any point in the plane is equal to the potential 

 difference between conductor B and this same point. That is, 

 the potential of every point on the plane xy is midway between 

 the potential of conductor A and that of conductor B. Hence, 

 every point in this surface is at the same potential and xy is 

 an equipotential surface. The plane xy may be replaced by a 



