84 BELL SYSTEM TECHNICAL JOURNAL 



part of these triple-harmonic exciting currents as well as part of the 

 unbalanced non-triple and fundamental currents and under certain 

 conditions may substantially decrease the overall inductive influence 

 of a feeder by reducing the ground-return current flowing through an 

 exposure. The effect of such a connection in reducing the noise is 

 dependent upon the location of the bank with respect to the exposure 

 and its relative impedance to the various harmonics as compared to 

 that of the path back to the substation. From the power operating 

 standpoint such a bank tends to supply part of the unbalanced load 

 and also, in case of the interruption of one phase between it and the 

 substation, tends to supply the power to that portion of the phase still 

 connected to it. Under certain conditions, the action of such a bank 

 may prove detrimental to the operation of the power feeder due to its 

 action in attempting to balance the voltages at the point of its connec- 

 tion to the feeder. Under other conditions the neutral of an existing 

 bank can readily be connected to the feeder neutral with distinctly 

 beneficial effects on the inductive influence and with little or no adverse 

 effects on the power-system operation. The tendency of such banks 

 toward noise reduction and towards unbalanced load supply is shown in 

 two of the illustrative examples in the Appendix. 



Telephone Circuits 



The voltages induced into a telephone circuit may be divided into 

 (1) metallic-circuit induction, that is, a voltage induced between the 

 two sides of the circuit with a resultant current flowing around the 

 circuit, and (2) longitudinal-circuit induction, that is, a voltage in- 

 duced along the conductors such that the resultant current flows in a 

 circuit having the telephone conductors as one side and the earth as 

 the other. This latter voltage may also result in noise, due to its 

 action upon telephone circuit unbalances, setting up currents in the 

 voice channel (metallic-circuit). For either type of voltage, the 

 induction may be "electric," that is, from the voltage on the power 

 circuit, or "magnetic" from the current in the power circuit. 



The local telephone circuit may be divided into three parts: (a) the 

 central office equipment, {h) the line conductors and (c) the subscriber 

 equipment. Inter-office circuits include only the first two items. 



{a) Central Offices Equipment 



The central office equipment associated with a subscriber circuit 

 consists essentially of two elements: (1) line signaling equipment 

 connected to the circuit for indicating to the operator, or to the dial 

 equipment, the desire of a subscriber to start a call and (2) a linking 

 or switching circuit or circuits for interconnecting two subscriber cir- 



