194 BELL SYSTEM TECHNICAL JOURNAL 



various physical relationships of power and telephone lines. These 

 coefficients indicate the voltages induced in short, isolated, untrans- 

 posed telephone circuits by unit voltage and current on similarly un- 

 transposed power circuits. They do not include the small separations 

 involved with jointly used poles. 



These curves and others based on them have been used for many 

 years in determining relative coupling, when comparing different 

 exposures, different routes involving various degrees of exposures, 

 different configurations of power and telephone circuits and for other 

 comparisons where all factors were substantially equal in the situations 

 being compared, except those involved in determining the coefficient of 

 induction. For these purposes they have been very useful. Methods 

 have not, however, been available whereby these coefficients could be 

 used for computing noise where transposed circuits were involved and 

 where many telephone wires were on the line, which exert an important 

 shielding effect on each other. 



The Joint Subcommittee on Development and Research has been 

 conducting experimental studies both for highway and wider separa- 

 tions, and those occurring with jointly used poles, so that the effects of 

 transpositions and of mutual shielding of the many wires involved might 

 be properly taken into account in determining the noise currents in the 

 metallic circuits. 



In determining the coupling between power and telephone circuits, 

 it is desirable to differentiate between the effects of the balanced and 

 residual components of the voltages or currents of the power circuit, 

 between the effects of voltages and those of currents, and on the tele- 

 phone line between induced voltage which acts directly in the metallic 

 circuit, termed "metallic-circuit induction," and that which acts in the 

 circuit composed of the wires with ground return, termed "longitudinal- 

 circuit induction." 



Since the residual components act in a circuit having ground as one 

 side with the wires in parallel for the other, while the balanced com- 

 ponents are confined to the wires of the system, the coupling for the 

 residual components is much greater than for the balanced components. 

 The coupling for the balanced components may be reduced by the use 

 of power-circuit transpositions, while such transpositions have no effect 

 on coupling for the residual components. 



The distance between the power and telephone wires is usually large 

 as compared to the spacing of the wires of the telephone circuit, so that 

 the longitudinal induced voltages are large as compared to the metallic- 

 circuit voltages. The effect of the telephone transpositions being 

 merely to equalize the relations of the two sides of the telephone circuit 



