574 BELL SYSTEM TECHNICAL JOURNAL 



Since the inductive coordination of power and telephone plants 

 inherently involves the characteristics of both systems as well as the 

 physical relations between them, problems can be effectively handled 

 only by joint consideration in each specific case. As pointed out in a 

 previous article dealing with noise induction/ effective cooperative 

 action depends upon an adequate mutual understanding of the 

 principles involved in coordination. Many of these principles can be 

 demonstrated using comparatively simple apparatus. 



This article describes a demonstration that has been developed to 

 illustrate some of the more important factors concerned with low- 

 frequency induction between power and telephone circuits, together 

 with a discussion of this subject along the lines which would be followed 

 in presenting the demonstration. 



Demonstration Apparatus 

 The demonstration apparatus consists of two separate arrangements 

 as follows: 



(a) For many of the demonstrations, a miniature inductive exposure, 

 consisting of a three-wire power line and a two-wire telephone 

 line can be used. The power line is energized at a compara- 

 tively low voltage from a three-phase bank of transformers. 

 The telephone line can be grounded at either or both ends and 

 by means of a voltage measuring device, consisting of an 

 amplifier and a projecting meter, a qualitative indication of 

 the voltage along it or between it and ground can be obtained. 



{h) For other demonstrations, a fairly high voltage in the telephone 

 circuit is required, but since it is impracticable to secure this 

 voltage using the miniature inductive exposure, it is necessary 

 to use an iron core transformer. In order to improve the safety 

 conditions when using this higher voltage, the miniature lines 

 are not used and the circuit is entirely separate from that used 

 in the low-voltage demonstrations. 



A power supply frequency of 60 cycles per second is used. The 

 phenomena illustrated are, however, applicable for all other frequencies 

 commonly encountered in power transmission and distribution circuits. 

 In a demonstration of this kind, where the exposure is compressed 

 into a small space and where the amount of power available is limited, 

 it is obvious that the results can have no quantitative significance. 

 This demonstration, therefore, is designed only to provide qualitative 

 illustrations of some of the principles involved. 



* "Some Theoretical and Practical Aspects of Noise Induct ion," by R. F. Davis 

 and H. R. Huntley, published in Bell System Technical Journal, October, 193.?. 



