OPEN-WIRE TELEPHONE LINES 



739 



Capacitance 



The capacitance of two parallel wires in space with a distance be- 

 tween centers which is negHgible compared with their length is 



C = 



0.019415 

 logio^ 



X 10~^ farads per loop mile. 



(4) 



It will be noted that the capacitance varies in inverse relation to the 

 separation between wires. 



As in Equation (3), it is assumed in this formula that the two wires 

 are suspended in space or at a considerable distance from the ground 

 and from other wires. On an actual line the capacitance of a pair is 

 changed to an appreciable extent by the presence of other wires, and 

 to a slight extent by the capacitance to ground. The true capacitance 

 between the two wires under actual conditions may be derived from 

 the direct capacitances between all wires and the direct capacitances of 

 all wires to ground.^ The capacitance is not changed to any great ex- 

 tent by skin effect. 



The means of insulation and support provided at each pole have an 

 appreciable effect on the capacitance of a pair of wires, especially in 

 wet weather. This is due to the fact that the insulators and, under 

 certain conditions, the pins and parts of the crossarms, act as the dielec- 

 tric of small condensers which are, in effect, shunted between the line 

 wires. These effects are being discussed in a companion paper.'^ The 

 percentage increase in capacitance due to the insulators varies with 

 different weather conditions and different types of insulators, ranging 



* See Technical Report No. 54 of the Railroad Commission of the State of Cali- 

 fornia, Joint Committee on Inductive Interference, entitled "Inductive Interference 

 Between Electric Power and Communication Circuits," 1919. 



''See "A Study of Telephone Line Insulators," by L. T. Wilson, printed in this 

 issue of the Bell System Technical Journal. 



