SOME ASPECTS OF NOISE INDUCTION 



491 



portion isolated from the three-phase portion by a transformer 

 and with it metalHcally connected can be compared. With the 

 transformer connected (thereby creating a condition similar to 

 Fig. 15-C) the noise in the loud speaker is much lower than when 

 a metallic connection is used and thus indicates a substantial 

 reduction in the residuals. 

 3. The demonstration setup is so arranged that the single-phase portion 

 can be transposed. With the single-phase portion metallically 

 . connected to the three-phase portion, transposing the single- 

 phase portion causes relatively little change in the noise from 

 the loud speaker. However, when the single-phase portion is 

 isolated from the three-phase portion by the transformer, 

 transposing it further reduces the noise materially. When the 

 single-phase portion is connected metallically to the three-phase 

 portion, the induction is largely due to residual voltage and as 

 such is not affected by the power circuit transpositions. When 

 it is connected through the isolating transformer, however, there 

 is no residual voltage present and the induction, being due to 

 balanced voltages, is materially reduced by the power transposi- 

 tions. 



TELEPHONE LINE 



TO 

 AMPLIFIER 



Fig. 16 — Influence of single-phase extension to three-phase power line. 



Three-phase portion 



. As far as the three-phase portion of the line is concerned, the single- 

 phase extension acts as additional admittance to ground on two 

 of the wires. Consequently if the single-phase extension is long, 



