470 BELL SYSTEM TECHNICAL JOURNAL 



actually construct, operate and maintain the plants and into the 

 considerations of management. Naturally, the best results can be 

 secured if all concerned have a thorough understanding of the subject 

 and appreciate each other's requirements and points of view. 



In promoting the mutual understanding of this subject which is so 

 desirable, it has been found helpful in some cases to use demonstrations 

 of the principles underlying the work accompanied by explanations in 

 everyday language. One of the demonstrations which has been shown 

 before a number of audiences of power and telephone people with this 

 in mind has to do with noise frequency induction and employs the 

 miniature lines and apparatus shown in photograph No. 1. A con- 

 siderable amount of interest has been aroused by these demonstrations 

 and many of the people in the audiences have found complete or 

 partial explanations of some specific problems which have been 

 troubling them. 



In order to illustrate the manner in which the miniature lines and 

 apparatus may be used to demonstrate principles of noise frequency 

 induction, there follows a description of this apparatus and a discussion 

 of the processes of induction along the lines usually followed in the 

 demonstrations. 



Fundamentals of Problem 



The problems concerned with inductive coordination arise due to the 

 fact that wires transmitting electricity necessarily have electric and 

 magnetic fields about them which may under certain conditions cause 

 voltages to appear in other wires which are in these fields. This 

 phenomenon is called induction. The voltages and currents used in 

 power transmission are much greater than those used in speech trans- 

 mission so that there are practically no situations in which the currents 

 and voltages on telephone systems affect power system operation due 

 to induction, but situations do arise in which power system voltages 

 and currents affect telephone system operation. 



The effects of induction in a given situation of proximity between 

 power and telephone circuits are dependent upon the characteristics of 

 both the power and telephone systems and upon the coupling (due to 

 the electric and magnetic fields) between them. It is theoretically 

 possible for a power line to be so constructed and maintained that it 

 would cause no induction into a nearby telephone circuit. Such a 

 power line would be said to have zero "inductive influence." Like- 

 wise, it is theoretically possible to have a telephone circuit so con- 

 structed and maintained that it would be unaffected by any electric 

 or magnetic fields set up by power systems. Such a telephone circuit 

 would be said to have zero "inductive susceptiveness." Also, of 



