Patent GB-620157-A (GB)
Title: Improvements in or relating to electrical control systems
IPC: H04Q1/18
Filed: 1946-06-14
Abstract: 620,157. Automatic exchange systems. STANDARD TELEPHONES & CABLES, Ltd. June 14, 1946, Nos. 18001, 18002 and 18004. Convention dates, July 7, 1945, July 7, 1945, and July 16, 1945. [Class 40 (iv)]  An electrical control system comprises means for applying first and second D.C. potentials to the cathode and control grid of a triode valve, which controls relay means to prevent further variation of the second potential when it bears a predetermined relationship to the first potential, and means independent of the anode circuit to prevent operation of the relay means when the potential applicable to the control grid is higher than that applicable to the cathode. The invention is applied to an automatic telephone system of the type described in Specification 592,846 in which a register is set according to the number dialled and a selector hunts until it finds the corresponding terminal, at which time the potentials on the selected register and selector terminals are equal. In Fig. 1, the register and selector brushes are connected through gridtransfer and cathode-transfer relays GTR, CTR and a rectifier S in such a manner that when the selector brush is more negative than the register brush it is connected to the cathode of a tube V and when more positive, to the grid, the register brush being connected to the other of the two electrodes. The characteristics of the tube are that only when the potentials match is the plate current sufficient to energize relay VR which releases relay GR, whereupon clutch magnet P relapses to stop the selector hunting. In Fig. 2, a resistor R4 connected to a battery prevents false operation when the selector brush is passing from one terminal to another. When the selector potential is more negative, the anode current of a pentode V is insufficient to operate a relay VR, and when the selector potential is more positive than the register potential, a relay PCR in the screen grid circuit energizes to break the circuit to the relay VR. When the potentials match, relay PCR remains inoperative and the anode current is sufficient to energize relay VR so that the circuit to the clutch magnet P is broken and hunting ceases. In Fig. 3, when the selector potential is higher relay PCR energizes to break the circuit to relay VR. If the selector is at lower potential, the valve V does not conduct. When the potentials match, the anode current is sufficient to energize relay VR to release the clutch magnet P. A relay PCR1 cuts in extra resistance when the potential difference between the selector and register is excessive. The Specification as open to inspection under Sect. 91 also contained the following matter. In Fig. 4 (Cancelled), the selector and register feed a pair of cross-connected triodes V1, V2, the anode circuits of which include relays VR3, VR2. A relay VR1 energizes only when both relays VR2, VR3 are energized, i.e. only when both tubes V1, V2 are conducting, which occurs when the potentials on the selector and register brushes match. In Fig. 5 (Cancelled), the relay VR1 energizes only when its tube V1 conducts at the same time as relay VR2 energizes, i.e. when tube V2 also conducts. This subject-matter does not appear in the Specification as accepted.