Telegraph, Double Needle.
A telegraph system in which the message is read by the motions of two
vertical needles on the face of the instrument in front of the receiving
operator. An identical instrument faces the transmitting operator. By
two handles, one for each hand, the needles are caused by electric
impulses to swing to right and to left so as to give a telegraphic code.
It has been generally superseded by the single needle telegraph.
Telegraph, Duplex.
A telegraph capable of transmitting simultaneously two messages over one
wire. The methods of effecting it are distinct from those of multiplex
telegraphy. This term is used as a distinction from diode multiplex
telegraphy, in which the work is done on other principles. There are two
systems of duplex telegraphy, the differential and the bridge systems.
Telegraph, Duplex Bridge.
A system of duplex telegraphy employing the principle of the Wheatstone
bridge. The other or differential system depends on equality or
difference of currents; the bridge method on equality or difference of
potentials. The cut shows the system known as Steam's Plan.
At the ends of the line wire are two cross connections like duplicate
galvanometer connections in a Wheatstone bridge, each including a
receiving relay. The rest of the connections are self-explanatory.
When A depresses his key the current splits at the point indicating the
beginning of the bridge. One portion goes through the line to B and to
earth, the other goes to earth at A through the rheostats indicated by
the corrugated lines.
On reaching B's end the current divides at the cross-connection and part
goes through the receiving relay shown in the center of that
cross-connection.
Thus if A sends to B or B to A it is without effect on the home
receiving instrument. Now suppose that both simultaneously are sending
in opposite directions. If the connections be studied it will be seen
that every movement of the transmitting key will affect the balance of
the distant or receiving end of the bridge and so its instrument will
record the signals as they are sent.
As shown in the cut the sending keys are on local circuits, and work
what are known as duplex transmitters. These are instruments which send
line signals without breaking the connection.
Fig. 313. STEARN'S PLAN OF DUPLEX BRIDGE TELEGRAPHY.
In Stearn's plan condensers are introduced as shown. By this plan
different receiving instruments can be used. The inventor once worked a
Morse instrument at one end of the line, and a Hughes' instrument at the
other end.