SOME ASPECTS OF XOISE INDUCTION 483 



Coordination of Transpositions 

 In order to summarize the effects of power and telephone circuit 

 transpositions, Fig. 7 has been prepared. While this table applies only 

 to transpositions within an exposure, it will be recalled that telephone 

 and power system transpositions outside of exposures may have an 

 important bearing on the balance of the circuits. 



Induction Effect on Telephone Noise 



Transpositions From Met To Ground 



Power Balance V. Yes * Yes 



Power Residual V. No No 



Power Balance I. Yes * Yes 



Power Residual I. No No 



Telephone All Types Yes No 



* Power transpositions will reduce metallic noise on untransposed telephone lines. 

 With telephone lines transposed the effects of power transpositions on metallic noise 

 due to direct induction may be small. 



Fig. 7 — Summary of effects of transpositions within inductive exposures. 



In some cases, it may be desirable to reduce not only the noise- 

 metallic due to direct metallic-circuit induction but also the longitu- 

 dinal-circuit noise due to balanced currents or voltages. An inspection 

 of the table indicates that this may be done by transposing both the 

 power and telephone circuits. In order to secure the greatest value 

 from the transpositions in such cases they should be installed in such a 

 way as to effectively "coordinate" with each other. In such co- 

 ordinated layouts, the power circuit transpositions (where used) are 

 largely relied on for reducing the longitudinal-circuit noise on the 

 telephone circuits due to induction from balanced components and the 

 telephone transpositions are largely relied on for minimizing the noise- 

 metallic due to direct induction between the wires. Fig. 8 is a 

 schematic diagram illustrating the principle of coordinated transposi- 

 tions. It will be noted that the following considerations have been 

 adhered to: 



1. The telephone circuits are balanced, that is, both wires occupy both 



pin positions for equal lengths, between successive power circuit 

 transpositions. This is necessary in order to ensure as close an 

 approach as practicable to equality of induction on both sides 

 of each telephone transposition. 



2. The power circuit is transposed in a complete barrel. If the expo- 



sure is long or irregular, more than one barrel might be required. 



In multi-wire telephone lines, the telephone transpositions are, of 

 course, much more complex than those illustrated in Fig. 8, but in the 

 systems designed for use in inductive exposures, so-called "neutral" 



