SOME ASPECTS OF NOISE INDUCTION 479 



currents, etc., enter into the consideration of coupling. It is, of 

 course, impracticable to do more in this discussion than consider some 

 of the more important aspects of this phase of the subject. 



In general, it can be said that except for very small separations where 

 rapid changes in coupling may occur with changes in the relative 

 positions of the circuits, all of the types of coupling will become smaller 

 as the separation between the power and telephone circuits increases. 

 The rate at which the coupling falls off with increasing separation 

 depends on many factors. For example, the coupling involved in 

 direct metallic induction generally falls off faster with increasing 

 separation than does the coupling affecting the longitudinal telephone 

 circuit. Likewise the coupling affecting the induction from balanced 

 currents and voltages generally falls off faster than that from residual 

 currents and voltages. 



In order to demonstrate that, in general, the coupling is reduced by 

 increasing the separation, the telephone line in the exposure is moved in 

 such a way as to change the separation and it is noted that, as the 

 separation increases, the noise decreases and vice versa. 



For a uniform exposure, the amount of noise in an untransposed 

 telephone circuit exposed to an untransposed power circuit will gen- 

 erally be approximately proportional to the length of the exposure, 

 provided the total exposure is electrically short. (For long exposures, 

 this proportionality may not hold because of phase-shift, attenuation 

 effects, etc.) In order to illustrate the effect of changes in length of 

 exposure, one-third, two-thirds, and all of the telephone line in the 

 miniature exposure are employed successively and it is noted that the 

 volume of sound from the loud speaker is approximately proportional 

 to the length of the exposure. The direct proportionality between 

 noise and exposure length does not hold for exposures to which 

 coordinated transposition layouts have been applied as the resultant 

 noise in such cases depends largely on the effectiveness of the coordi- 

 nated layout. The effects of transposition are discussed in the following. 



Transpositions in Poiver Circuits 

 Transpositions in power circuits are used primarily to accomplish 

 two results. The first of these is the reduction, within exposures, of 

 the induction from balanced currents and voltages. The second is the 

 equalization of the admittances to earth and the series impedances of 

 the power wires in order to limit the residual voltages and currents. 

 In this discussion only the first of these two results (that is, reduction of 

 induction due to balanced currents and voltages within the limits of 

 inductive exposures) will be analyzed. 



