326 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



direction while it is up. At the receiving station a neutral relay 

 and a polarized relay are connected in circuit with the line. The 

 neutral relay responds to the key which varies the strength of 

 the line current, and the polarized relay responds to the key 

 which reverses the line current. 



31. Duplex telegraphy. The sending of two messages in oppo- 

 site directions over one line wire simultaneously is known as 

 duplex telegraphy. This is accomplished as follows : Fig. 29 rep- 

 resents the arrangement of apparatus at one station. An exactly 



LINE similar arrangement is 



installed at the. other 





L|J BELAY station. Let c be the 



total resistance of the 

 line through the distant 

 station to tne ground. 

 -. Then the resistances a, 



GROUND r. i >r fin 



\ \ o, c and a form a Wheat- 



stone's bridge. When 



these resistances are so adjusted that a\b c]d, then the key 

 at the home station may be pressed without sending a current 

 through the home relay. When the key at the home station is 

 pressed, however, current flows over the line to the other station, 

 and it is easily seen from the figure that a line current coming to 

 a station divides, and flows in part through the relay at that sta- 

 tion. Therefore the relay at each station responds to the move- 

 ments of the key at the other station. 



32. Quadruplez telegraphy. The sending of two messages 

 each way over one line wire simultaneously is known as quadru- 

 plex telegraphy. This is accomplished by combining the arrange- 

 ments for diplex and duplex telegraphy. The single key repre- 

 sented in Fig. 29 is replaced by two keys, one for reversing the 

 current and the other for altering its strength ; and the single 

 relay is replaced by two relays, one a neutral relay and the other 

 a polarized relay. With this arrangement the polarized relay at 



