476 BELL SYSTEM TECHNICAL JOURNAL 



to those in all other short lengths, the total induced voltage is directly 

 proportional to the exposure length in a uniform (electrically short) 

 exposure. Also, since the rate of change of magnetic flux is propor- 

 tioned to frequency, the induced voltage will be proportional to the 

 frequency of the harmonics in the inducing current. 



The demonstration which shows the fundamental difference in the 

 action of electric and magnetic induction is shown in Fig. 4. 



1. In Fig. 4-^ the arrangements for demonstrating electric induction as 



well as the way the induced voltage acts through the impedance 

 to earth in the exposure are shown. In the setup the power line 

 is energized at about 200 volts, balanced 3-phase, but since the 

 far end is open the current in it is negligible. Consequently 

 only electric induction is present in appreciable amount. Since 

 the voltage to ground of the telephone circuit is the same over 

 its entire length, grounding it at any point reduces the voltage 

 at all points. This is shown in the demonstration by the great 

 reduction in the noise to ground as heard in the loud speaker 

 when the switch at the far end of the line is closed thus ground- 

 ing the line. 



2. In Fig. 4-J5 the arrangements for demonstrating magnetic induction 



as well as the manner in which the induced voltage acts are 

 shown. In this setup the power line is energized at about 17 

 volts, 3-phase and has a load such that the current is about 15 

 amperes in each wire. Due to the low voltage and the rela- 

 tively large current, magnetic induction is predominant. Since 

 the induced voltage acts along the circuit, it can be prevented 

 from acting on the amplifier input by opening the circuit at any 

 point. This is indicated in the demonstration by the fact that 

 the noise in the loud speaker is much greater when the switch at 

 the far end of the line is closed than when it is open. (This is, of 

 course, the exact reverse of the conditions when electric induc- 

 tion was being demonstrated.) 



In the demonstrations the lines used are very short electrically. For 

 circuits which are long enough so that propagation effects must be 

 considered, the results of grounding or opening the far end of the circuit 

 may be considerably different than for electrically short circuits. 



Inductive Coupling 

 General 

 In discussing inductive coupling, it is necessary to consider not only 

 the metallic power circuit and the metallic telephone circuit but also the 



