JOINT DEVELOPMENT AND NOISE FREQUENCY INDUCTION 195 



to the power circuit, such transpositions do not change the magnitude 

 of the longitudinal voltages, but do reduce the metallic-circuit voltages. 



The relative magnitudes of inducing voltages and currents differ 

 widely among various power circuits, and may vary greatly with time 

 on any given circuit. They will also differ considerably at a given 

 time and on a given circuit among the various frequencies involved. 

 For this reason it is necessary to consider separately the coupling 

 arising through the electric and magnetic fields. 



X'oltages induced in metallic circuits for the separations between 

 lines usually encountered are practically proportional to the spacing 

 of the wires of the telephone circuit. Voltages induced in eight-inch 

 spaced pairs are thus approximately two-thirds of those induced in 

 12-inch pairs, while those induced in phantoms on 12-inch spaced side 

 circuits are twice those induced in the sides. The longitudinal voltages 

 are, however, practically independent of the wire spacing so that the 

 contributions which these voltages make to noise in the metallic circuit 

 are unchanged except as the change in spacing may affect the balance 

 to ground. 



Spacing of the wires on the power circuit and their configuration also 

 have an important effect on the coupling for the balanced voltages and 

 currents, the coupling, in general, increasing as the spacing increases. 

 Coupling for the residual components is, however, affected only to a 

 minor degree by the spacing and configuration. Much information 

 bearing on these matters is included in the material on coefficients of 

 induction published by the California Commission referred to above. 



Measurements of coupling have been made by the subcommittee in 

 a number of situations. These have included cases of (1) exposure of 

 overhead transmission lines and open-wire toll telephone circuits at 

 highway separations, (2) overhead distribution lines and subscribers' 

 telephone cables in joint use and at street separations and (3) overhead 

 distribution lines and subscribers' open-wire circuits in joint use. 

 Information was obtained on coupling both for voltages and currents 

 and for the balanced and residual components. The results of the 

 work on overhead distribution lines and subscribers' telephone cables 

 have already been published.^ The other data are to be published as 

 soon as they are prepared in suitable form. 



The work on overhead distribution lines and subscribers' circuits is 

 relatively complete, covering a wide range of conditions typical of 

 those encountered in the field. Various arrangements of primary and 

 secondary conductors covering single-phase and three-phase, three- 

 wire and three-phase, four-wire systems were investigated. The 

 shielding effect of the telephone cable was determined and, with the 



