LO W-FREQ UENC Y IND UCTION 



577 



out above. This coupling is a function of the strength and frequency 

 of alternation of the flux set up at the location of the telephone line 

 by a given amount of ground return current in the power line. The 

 second factor is concerned with the amount of ground return current 

 in the power circuit at the time of a ground fault since, for a given 

 coupling, this will determine the strength of the magnetic field. The 

 third factor is concerned with the conditions in the telephone plant 

 which determine its reactions to a given induced voltage. Each of 

 these three factors is taken up individually in the following dis- 

 cussion. 



Since low-frequency induction between power and telephone circuits 

 involves a series of separate and distinct occurrences, it is evident that 



POWER LINE 



TELEPHONE LINE 



1 



T 



Fig. 2 — Demonstration of nature of low-frequency induction. 



the frequency with which ground faults occur on the power lines, the 

 locations and circumstances of the various faults, the particular 

 conditions in the telephone system at the time of such faults, etc., 

 are also important. These matters are all subject to fortuitous 

 variations so that there are many probability factors that must be 

 considered in the study of any low-frequency induction problem. 

 These probability factors cannot be demonstrated with the apparatus 

 available. 



In order to illustrate the general nature of the electrical phenomena, 

 Fig. 2, which shows parallel power and telephone systems with the 

 power line supplying a load, has been simulated using the miniature 



