494 BELL SYSTEM TECHNICAL JOURNAL 



capacity and adjustments of the telephone repeaters usually used in 

 such circuits. 



Balance 



In order that a telephone circuit may be perfectly balanced, the 

 series impedances of the two sides must be identical in each element of 

 length and the admittances of the two sides to earth and to other 

 conductors likewise must be identical. 



Since it is impracticable to construct telephone circuits of perfect 

 symmetry, unbalances exist and these are classified as "series im- 

 pedance" and "shunt admittance" unbalances. By a "series impe- 

 dance" unbalance is meant a difference between the series impe- 

 dances of the two wires composing the circuit. Such an unbalance 

 may be caused, for example, by a joint which does not have a negligible 

 resistance. If a "bad" joint exists, the longitudinal currents due to 

 the induced voltages encounter unequal impedances in the two wires. 

 Consequently, the currents in the two wires tend to be unequal, the 

 difference causing current through the terminal impedances and 

 hence causing metallic circuit noise. The effect of a high resistance 

 joint depends upon the magnitude of longitudinal current along the 

 wires as well as the unbalance in resistance caused by the joint. To 

 illustrate the effects of a high resistance joint, the demonstration set-up 

 is arranged to minimize the noise-metallic due to direct induction (by 

 transposing it) and the high resistance joint is then inserted. (See 

 Fig. 18.) The large increase in the noise from the loud speaker indi- 



-lAAA/i 



Nm 



TELEPHONE LINE 

 IN PARALLEL 



UQ TO AMPLIFIER 

 NgT AND 



LOUDSPEAKER 



Fig. 18 — Arrangement for showing effect of high resistance joint in telephone line. 



cates the effect of the joint on the noise-metallic. On the other hand, 

 listening to the noise-to-ground when the joint is inserted, one can 

 detect no effect. 



Admittance unbalances are generally due to either unbalanced 

 capacitances or leakages to earth of the two wires. Such unbalances 

 when acted on by the noise to ground cause more current to flow to 

 ground from one side than from the other. Part of this current flows 

 around the metallic circuit and causes noise-metallic. To illustrate the 

 effect of an admittance unbalance, a small condenser or a high- 

 resistance leak can be bridged between one wire of the telephone circuit 



