1. 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.
Definitions of telegraph, duplex bridge
