482 BELL SYSTEM TECHNICAL JOURNAL 



purpose, which is closely allied with the first and second, is the limita- 

 tion of crosstalk coupling between the various telephone circuits on the 

 same line.) 



Within an inductive exposure, slightly different voltages may be 

 induced on or along the two wires of a telephone circuit as pointed out 

 previously. By transposing the wires frequently, they can both be 

 exposed to the power system more or less equally and the voltages 

 induced in them will tend to be equalized. The difference and hence 

 the noise-metallic due to direct metallic-circuit induction thus is 

 reduced. This is illustrated in Fig. 6. If the induction on the two 

 sides of a transposition is identical in magnitude and phase, complete 

 neutralization can be secured. In actual cases, however, these vol- 

 tages are not identical in magnitude and phase because of irregularities 

 in the exposure, irregularities in pole spacing, etc., and because of the 

 phase shift and attenuation which were discussed in connection with 

 power system transpositions. 



POWER LINE 



TO AMPLIFIER 

 TELEPHONE LINE V LOUDSPeVr 



INDUCTION METALLIC INDUCTION METALLIC 



SECTION I SECTION 2 



Fig. 6 — Effect of telephone transposition on metallic noise. 



Since the voltage to ground and the longitudinal circuit current due 

 to either electric or magnetic induction, act on the telephone wires in 

 parallel, telephone transpositions do not reduce them. 



To demonstrate the effects of telephone circuit transpositions, the 

 miniature telephone circuit is transposed. It is noted from the de- 

 crease in the noise from the loud speaker that, when the telephone 

 circuit does not contain high resistance joints or other important 

 unbalances, a substantial reduction in the noise metallic occurs when 

 the telephone transpositions are cut in. However, no effect can be 

 noted on the noise to ground. 



