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BELL SYSTEM TECHNICAL JOURNAL 



increased when the resistance of the conductor and its ground con- 

 nections is reduced. 



In order to illustrate these effects, the demonstration shown in 

 Fig. 5 has been set up. With the shield wire on the power line, the 

 shielding efifect can be shown under two conditions as follows: 



(a) When the switch directly grounding the shield wire is closed the 

 induced voltage in the telephone circuit goes down materially 

 due to the shielding effect of the current in the shield wire. 



{h) When, instead of grounding the shield wire directly, it is grounded 

 through a small resistance, the reduction in induced voltage is 

 much smaller because the resistance limits the current in the 

 shield wire. 



In order that a conductor may exert a shielding effect, it must have 

 a substantial coupling to either the power or telephone line; i.e., it must 

 be fairly close to one or the other. By moving the wire shown in 

 Fig. 5 it can be demonstrated that: 



MOVABLE SHIELD WIRE 



POWER LINE 



TO AMPLIFIER 

 AND METER 



TELEPHONE LINE 



1 



Fig. 5 — Demonstration of shielding effect of grounded wire. 



(a) With the shield wire on the power line or on the telephone line, 



substantial shielding is secured. 

 {h) If the wire is moved outside the exposure, the shielding is reduced 



to a small value. 



