496 



BELL SYSTEM TECHNICAL JOURNAL 



It will be recognized that the general principles involved in this last 

 demonstration are essentially the same as those which were involved in 

 the demonstration of the effect of a single-phase extension to a 3-phase 



'f^ 



Fig. 19 — Effects of interconnecting metallic and grounded circuit. 



power line. In the case of the single-phase extension, it was possible 

 to reduce the inductive influence by isolating the single-phase part 

 from the 3-phase part by means of an isolating transformer. Following 

 the same line of reasoning, it should be possible to reduce the effect of 

 the connection between the metallic and grounded parts of the tele- 

 phone circuit by means of an isolating transformer. Inserting a 

 repeating coil between the metallic and grounded portions provides 

 such isolation and it is noted from the reduction in noise when this 

 repeating coil is inserted, that the conditions are essentially the same as 

 when the grounded portion is disconnected from the metallic portion. 

 (This whole analysis and demonstration, of course, applies only when 

 the grounded portion is unexposed since the grounded circuit is totally 

 unbalanced and hence would quite likely be noisy if it were subjected 

 to direct induction.) 



Carrying the similarity of these two demonstrations a step farther, 

 it will be recalled that it was shown that when the single-phase and 

 3-phase portions of the power circuit were metallically connected, 

 transposing the single-phase portion resulted in relatively small reduc- 



