TELEGRAPHIC APPARATUS. 
251 
Duplex Telegraphy .—Our readers must be cautioned against 
the common mistake of supposing that duplex telegraphy 
consists in employing novel apparatus. It is, on the contrary, 
applicable to all the instruments we have described, and is the 
result merely of a particular mode of operating, which places 
the receiving apparatus in a state of unstable equilibrium, 
when it becomes sensitive to variations of currents from 
opposed stations. As this idea appears somewhat abstract 
and difficult to comprehend, we shall compare the simplest 
telegraphic circuit with the duplex communications. If the 
reader will turn back to figure 168, he will see that the Morse 
key puts the battery out of circuit when receiving, leaving only 
Fig. 178. 
the receiving apparatus in connection with the line as the 
signals arrive from the distant stations ; whilst, when the 
key is in use transmitting, the communication is cut between 
the receiving apparatus and the line, and the latter is put into 
circuit with the battery. 
In the duplex system, matters are differently arranged : 
during the transmission, as in the receiving of messages, the 
battery current of the stations always passes through the coils 
of the receiving apparatus ; it should, however, be understood 
that the coils of this apparatus must be so constructed as not 
to be affected by the passing-out current, but only by that 
which arises from signals made at the station in correspond¬ 
ence. Let us see how this result is obtained. 
A b, in fig. 178, represents the line; A is the station from 
which a message is sent, b the station at which it is to be 
received. At A cz is the battery of which the copper pole is 
