CHAP, v.] TELEGRAPHIC LINES. 629 



VI. DUPLEX TELEGKAPHY. 



Very early in the development of the telegraph it became 

 known that a wire would transmit more than one current at 

 the same instant of time, and that when the currents were passed 

 into the wire in the same direction, the effect of the duplication of 

 the currents upon the directive force of the needle at the distant 

 station was greatly increased, and when the currents were in 

 opposite directions the movement of the needle was almost imper- 

 ceptible, and that if the currents were accurately balanced the 

 needle would remain stationary. The application, of these known 

 facts to the indicating of distinct signals constitutes Duplex Tele- 

 graphy. In ca.rrying out this system of transmission it is necessary 

 that the coils of the instruments at the sending and transmitting 

 station shall be so arranged that whenever the transmitting station 

 sends a current into the line, although it may be indicated at the 

 distant station, it is neutralized upon the coil of the sending 

 instrument, and no signals are shown; but the instrument remains 

 free to receive signals from the distant station. This neutralization 

 or balance is obtained by winding the coils of the instruments 

 with two parallel wires, . after the manner of a differential 

 galvanometer. 



Therefore when the distant station sends a current, it either 

 increases or reduces the effect of the local current, but in passing- 

 through the coil of the transmitting instrument it is equally divided, 

 neutralizing its effect upon the needle of that instrument, but at 

 the distant receiving instrument the current passes through both 

 coils in the same direction, and therefore exercises a directive force 

 upon the needle, and indicates a signal. The Duplex system is 

 capable of increasing the transmitting capacity of a wire, two, 

 three, or four-fold as may be required. The system is now exten- 

 sively employed both upon the Postal Telegraph lines in Great 

 Britain and on many telegraph lines in the United States and 

 Europe. 



