356 Recent Advances in Telegraphy, July, 
earth. This circuit is only interrupted when the lever or 
handle of the key is up. At such a time, of course, no 
current flows through the circuit. But when the sending 
clerk depresses the key the circuit is complete, a current 
traverses the line, and a signal is received at Station B. 
Combined according to the Morse code, such signals make 
up the message. 
At first sight it seemed impossible that two messages 
could be sent, each in an opposite direction to the other, 
at the same time through the line. How was it possible 
that the messages could pass each other without interfering ? 
If currents of equal strength were sent in opposite direc- 
tions, would they not neutralise each other ? In that case 
no current would traverse the wire at all, and what would 
become of the messages ? Yet it is precisely on the ground 
that two currents of equal intensity do eliminate each other 
that duplex telegraphy is accomplished. The feat has been 
done, not by discovering or inventing a means whereby two 
currents could pass each other in a wire without disturbing 
one another, but by ingeniously joining up the sending and 
receiving apparatus at each end of the wire, so that the 
very interference of the currents, the one with the other, 
should cause the instruments to signal. 
Such an arrangement was necessarily different from that 
used in ordinary transmission, as described. There was 
necessarily both a battery and a receiving instrument 
at each end ; but the whole secret of the duplex system 
consists in the placing of the receiving instrument. All the 
methods hitherto invented agree in this — that the receiving 
instrument at either end is so placed that currents leaving 
the station where it is cannot cause it to signalise, whereas 
currents coming in from the distant station can. Again, 
should the currents leaving the station where it is be by any 
means stopped, this will have the effedl of making it sig- 
nalise. In describing the leading methods we shall show in 
each case how these conditions were obtained. 
Dr. Gintl, a director of Austrian telegraphs, may be re- 
garded as the founder of duplex telegraphy. Gintl, in 1853, 
described his system to the Academy of Sciences, Vienna, 
and practically tried it on the land-line between Vienna and 
Prague. Gintl’s original system is shown in Fig. 1, which 
represents the arrangement of apparatus at Vienna and at 
Prague, with the line wire between. Here the receiving 
instrument is placed between the battery and the line, so 
that, when the circuit is completed, by depressing the key 
the current flows through the receiving instrument into the 
