196 BELL SYSTEM TECHNICAL JOURNAL 



open-wire subscribers' telephone circuits, the shielding effect of the 

 various telephone wires on each other. 



For telephone cable circuits when the sheath is grounded at either 

 one or both ends, the inductive effect of the power circuit voltages on 

 the wires enclosed is negligible as compared to that of the power circuit 

 currents. Furthermore, because of the close association of the wires 

 of a pair in the cable and the frequent twist, the metallic-circuit in- 

 duced voltages are negligibly small as compared to the longitudinal 

 \oltages so that, in general, only the magnetic longitudinal coupling 

 factors are of importance in these situations. 



The work further indicates that, for most practical problems involv- 

 ing overhead distribution lines of the multi-grounded type and sub- 

 scribers' cable circuits, a knowledge of the coupling for the residual or 

 unbalanced currents is sufficient, the effect of the balanced currents be- 

 ing relatively unimportant. However, in cases where the line currents 

 are particularly heavy or contain exceptionally large harmonic com- 

 ponents, the balanced currents become important. 



In the range of frequencies used for telephone transmission the ratio 

 of open-circuit voltage induced on a telephone line through electric 

 induction to inducing voltage on the power circuit is substantially 

 independent of frequency. When the exposed section of line is con- 

 nected to the remaining section of the telephone line or to terminal 

 apparatus, a current is set up which is approximately proportional to 

 the frequency of the induced voltage. The circuit will perform as if 

 there were a small condenser connected between the power circuit and 

 telephone circuit and the induced current experienced will be propor- 

 tional to the frequency and the magnitude of the inducing voltage on 

 the power circuit. 



The coupling between power and telephone circuits for currents is 

 in the nature of a mutual inductance, so that the voltage induced in the 

 telephone circuit is proportional to the magnitude of the inducing cur- 

 rent in the power circuit and its frequency. 



This statement applies strictly only to induction from the balanced 

 current components. Induction from residual current in the power 

 circuit is complicated by the effect of the finite conductivity of the 

 earth. With increasing frequency the earth currents tend to be closer 

 to the surface and the coupling with the telephone circuit tends to 

 increase less rapidly than would follow from proportionality with 

 frequency. The departure from linearity is not large in the frequency 

 range from 250-2750 for highway separations and for joint use. 



Transpositions afford one of the most powerful means available for 

 controlling coupling of power and open-wire telephone circuits in given 



