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BELL SYSTEM TECHNICAL JOURNAL 



points are established between which the circuits may be subjected to a 

 uniform exposure. Consequently in a coordinated system of trans- 

 positions, it is ordinarily desirable that the neutral points in the tele- 

 phone transposition system fall opposite or nearly opposite transposi- 

 tions in the power system or other important electrical changes in the 

 power system or in the exposure. 



X 



X 



TELEPHONE 



X 



X 



:zx 



X 



X 



X 



X 



x^: 



Fig. 8 — Schematic layout of coordinated transpositions. 



To illustrate the above, the demonstration apparatus is arranged to 

 secure a coordinated layout. When the coordinated layout is cut in, 

 only a relatively small amount of noise from the loud speaker is heard, 

 and it is observed that the insertion of small series or shunt unbalances 

 in the telephone circuit does not materially increase this noise (i.e., the 

 telephone circuit is not particularly critical as regards unbalances) as 

 long as the supply system is energized by balanced voltages only. 

 When residual voltage is used on the miniature supply line, the 

 longitudinal-circuit noise on the telephone system is higher and the 

 telephone circuit is more critical as regards unbalances. 



Inductive Influence of Power Lines 

 In considering some of the factors affecting the inductive influence 

 of power lines, it should be recalled that, theoreticall}', a power system 

 could be so constructed that it could set up no external electric or 

 magnetic fields and consequently would have negligible influence. It 

 is, as previously mentioned, impracticable to construct power lines in 

 this way and consequently, the factors controlling the deviations from 

 this condition require consideration. 



Among the factors affecting the inductive influence of a power line 

 are the amount of line current, the operating potential, the configura- 

 tion of the wires, etc. It does not seem necessary to demonstrate these, 

 but there are two additional factors of importance, as follows, which 

 will be discussed: 



