JOINT DEVELOPMENT AND NOISE FREQUENCY INDUCTION 199 



the inducing field in which it is placed, the experiments of the committee 

 have shown that a satisfactory degree of approximation for studying 

 propagation effects can be obtained by energizing all wires on the line 

 simultaneously at the same potential from a common source. An 

 extensive experimental study has been made in this way by the 

 committee in which the magnitudes of the longitudinal voltages and 

 currents at various points along the line have been measured as well 

 as metallic-circuit currents set up through the unbalances at the send- 

 ing and receiving ends of the line. 



By making measurements of this sort on a considerable number of 

 lines of different types of construction and different transposition 

 arrangements, it is hoped to obtain statistical data whereby the metal- 

 lic-circuit voltages and currents at the circuit terminals may be deter- 

 mined from the magnitudes of longitudinal voltages and currents as 

 measured at exposure terminals. 



Unbalances in toll circuits are the result of commercial variation 

 from the balanced condition, since the circuits are designed to be 

 symmetrical. These unbalances may consist of resistances in joints, 

 capacitance or inductance unbalances due to irregularities in trans- 

 position spacing or to omitted or unspecified transpositions, or differ- 

 ences in the impedances of apparatus connected in series with the wires 

 or between them and ground. These unbalances are fortuitous both as 

 regards their magnitudes and location along the toll circuits. Some in- 

 crease in importance with frequency and others decrease. These, com- 

 bined with the irregularities in the propagation of the longitudinal 

 voltages and currents, cause the resulting metallic-circuit currents in 

 individual circuits to vary in an erratic fashion with frequency. The 

 general trend is one of proportionality, independent of frequency within 

 the important range, between the longitudinal currents and voltages at 

 the exposure and current in the metallic circuit at the terminals. Tak- 

 ing into consideration the effects on coupling, the currents at the ter- 

 minals increase approximately in direct proportion to the frequency of 

 the inducing voltage or current on the power circuits. 



Because of the lower susceptiveness of cable circuits together with 

 the high degree of balance of the terminal apparatus and because of the 

 more general use of private rights-of-way, cases of noise-frequency 

 induction into toll cable circuits have been comparatively infrequent. 

 For this reason the attention of the subcommittee as far as toll systems 

 are concerned, has been directed toward open-wire circuits. 



In exchange circuits certain inherent unbalances exist due to the 

 arrangements employed for supervisory signaling, for selective ringing, 

 and for coin box service. The supervisory system utilizes a low im- 



