Du′plex-tel′egraph.
A telegraph so arranged that messages can be simultaneously transmitted in opposite directions on the same line-wire.
The first telegraph of this kind was devised by
Dr. Gentl of
Austria, in 1853, and modified by Frieschen and
Siemens-Holske in 1854; but it is only within the past few years that any duplex systems have been put into successful operation, and, up to this time, only on American lines.
The system invented by
Joseph B. Stearns, of
[
764]
Boston, based upon
Gentl's plan, is represented in
Fig. 1806, in which the relay or receiving instrument is composed of two pairs of electro-magnets
m m acting in opposite directions upon a common armature lever
A. The key is the armature of an electro-magnet, which is in a local circuit controlled by a Morse key
K. L B is the local battery.
The main battery (
M B) current is equally divided between the relay-magnets
m m, one half passing through one set of magnets to the line
l, and the other half passing through the other magnets, and a rheostat
R — equal to the resistance of the main line — to earth
E. The relay-magnets are thus equally excited and their influence upon the armature neutralized, so that the outgoing current gives no signal at the sending station.
A current received, however, traverses only one set of the electro-magnets, destroying the equilibrium, and causing a signal.
The key is so constructed that it closes one circuit to the earth before breaking another, thus always preserving the continuity of the circuit, a condition essential in systems of this kind.
A condenser
C is placed in a shunt circuit to the magnets in the short or home circuit, in order to neutralize the effect of the extra current on the line-magnets of the relay.
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Duplex-telegraph. |
Another system of
Mr. Stearns is shown in the lower part of the same figure.
It is based on the arrangement of circuits known as the “Wheatstone bridge,” the relay or receiving instrument being placed on the bridge.
The current of the main battery is divided by the rheostats
r r and
R R′ to points
x z. If the resistances of circuits
C to
z and
C to
x are proportionately to each other as resistances of circuits
x y or line, and
z to earth, there will be no current in transmitting across the bridge
x z, in which the relay or receiving instrument is placed.
The larger portion of the incoming current, however, passes through and actuates the relay, as it offers the path of least resistance.
Moses G. Farmer, of
Boston, invented a duplex system in 1858, in which he used a key which preserved the continuity of the circuit, and also reversed the battery at the sending station, this reversal making the signals at the distant station, the relay being prevented from responding by the current of a local equalizing battery closed simultaneously by this key.
