Telegraph, Duplex, Differential.
A system of duplex telegraphy employing the differential action of two
exciting or magnetizing coils. The general principles are the following.
Suppose that at each of two stations, there is a magnet working as a
sounder or relay. Each magnet is differentially wound, with two coils of
opposite direction, of identical number of turns.
When the sending key at a station A is depressed two exactly equal
currents go through the magnet in opposite directions. One called the
compensation current goes to the earth at the stations. The other called
the line current goes through the line, through the line coil of the
distant station E, thereby actuating the relay or sounder armature.
The instrument of the sender A is unaffected because he is sending
opposite and equal currents through its two coils. A special resistance
is provided on the compensation circuit for keeping the currents exactly
equal in effect. Nothing the sender at A does affects his own
instrument.
Now suppose E desires to telegraph back at the same time that A is
telegraphing to his station. He works his key. This does not affect his
own instrument except by sending the equal and opposite currents through
its coils. When his key is depressed and A's key is untouched, he works
A's receiving instrument.