472 BELL SYSTEM TECHNICAL JOURNAL 



course, regardless of the characteristics of the power and telephone 

 circuits, if the separation between them could be very great, there would 

 be no "inductive coupling" and consequently, no induction from one 

 into the other. Practically, of course, neither power nor telephone 

 systems can be constructed so as to have zero influence or susceptive- 

 ness, and it is frequently impracticable to separate them sufficiently to 

 make the coupling negligible. The practical coordination problem, 

 therefore, is to work out the most convenient and economical method of 

 controlling the factors so that inductive interference is avoided. 



In the practical problem of inductive coordination between power 

 and telephone systems there are often two more or less distinct aspects 

 to be considered. One of these aspects is concerned with the possi- 

 bility of extraneous currents in the telephone circuits which have 

 frequencies within the range used in transmitting speech and which 

 may, therefore, cause "noise" in the telephone receivers at the ends of 

 the circuit. This phenomenon may arise during the normal operation 

 of power and telephone systems although abnormal conditions on 

 either system may result in increasing the noise during the existence of 

 such abnormal conditions. The other aspect commonly referred to as 

 "low frequency induction," is associated almost entirely with faults to 

 ground on power systems and is primarily concerned with the possi- 

 bility at such times of high induced voltages at fundamental power 

 system frequency. This article, however, is confined to the noise 

 aspect of the problem. 



Demonstration Apparatus 



In order to qualitatively illustrate some of the factors involved in 

 noise induction, a miniature inductive exposure as shown in the photo- 

 graph referred to previously, may be used. The demonstration cir- 

 cuits consist essentially of a miniature three-phase, three-wire power 

 line and a two-wire telephone line which are set parallel to each other 

 on a grounded copper screen and are connected as shown schematically 

 in Fig. 1. The power line can be energized in various ways from an 

 ordinary three-phase power distribution circuit through suitable 

 transformers. The telephone line is connected to an amplifier and 

 loud speaker so that the noise on the telephone circuit under various 

 conditions can be heard. Both lines can be transposed independently 

 or in a coordinated manner and unbalances can be inserted in the 

 telephone circuit. The particular connections and arrangements of 

 the lines and apparatus used in each of the demonstrations are de- 

 scribed as that demonstration is discussed. 



