Cbe Eelautograpb* 169 



in ordinary telegraph systems, the telephone 

 circuit is completed by using a second wire in- 

 stead of the earth. 



As a complete defense against the effects of 

 induced currents tin- wires should be exactly 

 alike as to cross-section (or size) and resist- 

 ance. They should be insulated and laid to- 

 gether with a slight twist. This latter is to 

 cause the two wires so twisted to average al- 

 ways the same distance from any contiguous 

 wire. 



One factor in determining the intensity of 

 an induced current is the distance the wire in 

 which it flows is from the source of induction. 

 A telephone put in circuit at the end of the 

 two wires that are thus laid together will be 

 practically free from the effects of induced 

 currents that are set up by the working of 

 contiguous wires for this reason: Whenever 

 a current is induced in one of the slack- 

 twisted wires it is induced in both alike; the 

 two impnl-es lu-ing of the same polarity meet 

 in the telephone, where they kill each other. 

 del to have a perfect result we must have 

 perfect conditions, which nro never attained 

 absolutely, but nearly enough for all practical 

 j.i ir poses. 



In thr rarly days of telephony great diffi- 

 culty wa- .-,.) iii n-inir a single wire 

 :uled at ea<-h end in thr ordinary way, if 

 it ran near other wires that were in ju.-li\- 



