Patent GB-411443-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/58
Filed: 1932-12-05
Abstract: 411,443. Automatic exchange systems. AUTOMATIC ELECTRIC CO., Ltd., Strowger Works, and BEALE, C. E., 59, Towers Road, Wavertree, both in Liverpool. Dec. 5, 1932, No. 34461. [Class 40 (iv).] In a system in which several subscribers who cannot intercommunicate are connected to a sub-office connected to an exchange by a single two-wire line, the sub-office selective switch, which may be of the type described in Specification 396,738, is stepped synchronously with apparatus at the exchange by impulses generated at the exchange independently of any action at the sub-office and transmitted over one of the line wires, the driving magnet being connected to the line through a rectifier which acts as a very high impedance under speech conditions. Two systems are described. In that shown in Figs. 1-4 a further digit is transmitted after the final selector has seized the line to the sub-office. In register controller systems where it is undesirable for suboffice subscribers to have 5 digit numbers, the final selectors giving access to sub-offices are reached from the first selectors. In the system shown in Figs. 5 and 6, standard final selectors are used and the sub-office subscribers are given 4-digit numbers which may have any value, selection of the wanted subscriber being controlled by a switch in the sub-office line circuit which hunts for a position marked by the final selector. Outgoing call-both systems. A call by any subscriber shunts the normally energized relays A-D to enable ST, Fig. 2, at the exchange to pull up over the upper junction wire and connect the driving magnet WM to relay S in a common interrupter set. WM is not operated in this circuit, but S interacts with and is intermittently short-circuited by R to enable WM to pull up driving its wipers and sending impulses over the lower junction wire and a metal rectifier MRA to magnet WRM of the selector at the sub-office. WRM releases A-D and in due course a circuit is made over the calling loop for SZ which stops the switches and locks itself in series with the ordinary line relay L which starts a finder. When the line is found, K is operated over a holding winding of the line relay and cuts off its line winding, and CT also operated over the test wire switches through cutting off SZ. The call is now established in the usual way and a booster impulse is given to the meter selected by the switch WM. At the end of the connection the relapse of K, CT makes a driving circuit for WM over its off-normal contacts and both switches go home. It may be arranged that the off-normal springs of WM are not restored until it has made a full revolution, whereas those of WRM are restored after half a revolution. In this way it is ensured that the switches are restored correctly even if they were out of step previously. The line circuit shown in Figs. 5 and 6 is operated in the same way. Incoming call-final selector takes three digits. On levels over which sub-offices are reached, post springs IN, P1 in the final selector shown in Figs. 3 and 4, are operated and an additional relay SE operated in parallel with the holding circuit of the switching relay 1H prevents the application of ringing current and switches the impulse circuit to the magnet WF of a switch mechanism having off-normal springs WFN but no banks and wipers. WF repeats the digit over the positive junction wire to the selector at the suboffice, CT having been operated over the private wire. Owing to the change-over of springs WFN2, SE holds only until the end of the digit and the line is then rung in the usual way. The relapse of 1B at the end of the call re-operates SE which connects WF to an interrupter set on 11. WF is thus restored sending an impulse at each step to restore WRM. The restoration of springs WFN stops the switches and releases SE, 1H to restore the final selector. Incoming call-system using standard final selectors. On the arrival of a call, 1K cuts off the line relay and NC, also energized over the private wire, starts the switch 1WM which sends impulses as before to WRM at the suboffice. The positive wires from the final selectors are led to the bank 1WM 2c and the ringingreturn battery connected up in the calling final selector in due course energizes 1CT to stop the switches and complete the ringing circuit. CZ, operated when the switch moves off normal, cuts off NC and 1SZ and locks up. At the end of the connection CZ, 1K, 1CT fall and the .switches are driven home. Subscriber with out-service only. The suboffice and line-circuit selecting switches may have an additional outlet and the subscriber corresponding to this may make outgoing calls in the ordinary way, but cannot be called.