Patent GB-151871-A (GB)
Title: Improvements in method and apparatus for duplex cable signalling
IPC: H04L5/14
Filed: 1919-12-29
Abstract: 151,871. Cohen, L. Dec. 29, 1919. Duplex telegraphs.-In a method of duplex signalling, the grounded cable or line is coupled to two arms of a closed wheatstone bridge, and the source of electromotive force connected to two junction points of the bridge and also to an auxiliary circuit. The indicating-instrument is included in a circuit coupled to the bridging-arm and the auxiliary circuit. When the key 29, Fig. 1, is closed to send a signal, the current divides at the junction points 30, 31, part flowing through the parallel auxiliary circuit 19, 20 and 21 and part flowing to the junction points 34, 35 of the bridge and completing circuits through the bridge arms. The connexions of the transformers 18, 17 are arranged so that the electromotive forces induced in the cable are in the same direction, whilst the cable couplings and the elements of the four arms of the bridge are suitably adjusted to balance out completely in the bridging-arm, current reactions from the cable. Any current flow in the bridging-arm therefore depends upon the sending electromotive force and the impedances of the bridge, and can be readily duplicated in the auxiliary circuit 19, 20, 21. By coupling the indicating-instrument circuit to the bridging-arm and the auxiliary circuit through transformers 36, 23 so that the currents induced in the windings 14 and 22 oppose one another, the transmitting current is completely neutralized in the indicating-circuit. An incoming signal induces currents in opposite directions in the two bridge arms b, d, but since the mutual inductance of one transformer is made much greater than the other, the difference in electromotive forces sets up a current flow in the circuit 35, 32, 13, 33, which reacts upon the indicating-circuit through the transformer 36. In a modification, two independent sources of electromotive force are used, one for the bridge circuit and the other for the auxiliary circuit, the two keys being mechanically linked so that current variations in the two circuits are effected simultaneously. According to another modification, direct coupling replaces the transformers 36, 23. Fig. 4 shows another modification of the invention in which two closed wheatstone bridges are used connected in parallel, the cable being coupled to two arms of each bridge. When the key 46 is depressed, a current flow is set up which divides at the points 52 and 53, part flowing through each bridge. The electrical constants of the arms of the two bridges are adjusted so that the currents flowing in the two bridging- arms 23, 24, and 25, 26, from the transmitting source due to lack of balance are oppositely directed. The current, however, due to the received signal flows in the same direction in the bridging-arms and by means of suitable transformers 31, 32 can be made to add in the indicator circuit, while residual currents due to imperfection of balance will oppose and neutralize one another. A further modification is described in which the two closed wheatstone bridges are connected in series, the principle of operation being analogous.