198 BELL SYSTEM TECHNICAL JOURNAL 



in the terminal apparatus to the induced voltages, the sensitivity of the 

 receiving apparatus and the operating power level of the telephone 

 circuits. 



Propagation Effects and Balance. — Important differences exist with 

 respect to propagation effects and balance between open-wire and cable 

 circuits and between toll and exchange systems. 



As pointed out in the discussion of coupling, only the magnetically 

 induced longitudinal voltages and currents affect telephone cable cir- 

 cuits. Because of the absence of electric induction and direct metallic- 

 circuit induction and because of the important shielding effects exerted 

 by the cable sheath and the various telephone circuits on each other, 

 telephone cable circuits are much less susceptive than open-wire cir- 

 cuits. 



In open-wire telephone systems consideration must be given both to 

 electric and magnetic induction and to voltages directly induced in the 

 metallic circuit as well as to those induced in the longitudinal circuit. 

 In a line composed of a number of circuits, the currents set up in any 

 one circuit depend, not only upon the voltage induced in that circuit 

 and its impedance, but also upon the currents and voltages which are 

 set up in the rest of the telephone circuits on the line. It is not pos- 

 sible, therefore, to calculate the induced currents merely from a knowl- 

 edge of the magnitudes of the currents and voltages on the power 

 circuits and the coupling between the power circuits and isolated pairs 

 of wires on the telephone line, considered independently. 



These mutual effects among the various telephone circuits exist both 

 within and without the exposed sections. Thus, the propagation of 

 the induced voltages and currents along any one circuit is influenced 

 both by the electrical conditions of this circuit and also by the condi- 

 tions of all other wires on the line. Additional complexities arise in 

 the propagation of the induced voltages and currents, because of non- 

 uniformity in impedances to ground at terminals, points where circuits 

 join or leave the line, and where lengths of cable may be used at termin- 

 als or at intermediate points. The impedances to longitudinal induced 

 voltages and currents vary over a wide range depending on the number 

 of wires on the line, the relative position of the exposure and the circuit 

 terminals and the occurrence of sections of cable. Due to reflection 

 effects from these irregularities, peaks of current and voltage may 

 exist along the circuits which are large as compared to the correspond- 

 ing magnitudes at the exposure terminals. If circuit unbalances 

 happen to exits at these maximum points, metallic-circuit voltages and 

 currents thereby introduced are increased. 



While the distribution of longitudinal voltages and currents among 

 the various wires upon the telephone line depends uj^on the nature of 



