SOME ASPECTS OF NOISE INDUCTION 



485 



1. The wave shape of the currents and voltages. 



2. The magnitude (and wave shape) of residual voltages and currents. 



(Residuals were discussed briefly in connection with inductive 



coupling.) 



Wave Shape 

 It is recognized as commercially impossible to build rotating ma- 

 chinery entirely free from harmonics. It is further recognized that 

 some distortion of wave form is inherent with power transformers 

 which must employ iron in their magnetic circuits. Harmonics are of 

 interest from the standpoint of noise induction, since they may induce 

 voltages of frequencies within the range ordinarily used in telephone 

 message circuits. Induced voltages at such frequencies have much 

 greater interfering effects (from the standpoint of noise) than does the 

 voltage normally induced at the fundamental frequency. The ap- 

 proximate relative interfering effects of voltages of different frequencies 

 in typical telephone circuits are shown in Fig. 9 which is a so-called 

 "noise weighting" curve. 



200 400 600 



800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 

 FREQUENCY IN CYCLES PER SECOND 



Fig- 9 — Curve showing approximate relative interfering effects of voltages of different 

 frequencies across a telephone circuit. 



The demonstration set-up for impressing voltages of two different 

 wave shapes on the untransposed power line is shown in Fig. 10. With 

 the switch in the "normal" position, the wave shape is that taken 

 directly from the commercial power supply. A wave shape of voltage 

 having greater harmonic content than that of the commercial voltage, 



