Patent GB-502539-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M3/60
Filed: 1937-09-17
Abstract: 502,539. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., McCLEW, J. W., and PEARCE, O. A. Sept. 17, 1937, No. 26311.. [Class 40 (iv)] A high-voltage signal operates a relay over discharge tube to cut out the feeding bridges of a connection or to release it. The invention is described with reference to a system, such as is described in Specification 502,545, comprising a number of exchanges connected in tandem to a main exchange and in which subscribers on the main and adjacent satellite set up internal calls themselves, all other calls including internal calls in the more distant exchanges being set up by an operator at the main exchange. The feeding bridges are cut out when it is desired to make a routine test of lines in the tandem exchange without providing special test junctions ; and the connection released is a checking connection set up by the operator to a calling party to verify the number he has given as his own, signals of opposite polarity being used for the two purposes. The test connection is also released by a high-voltage signal of the same polarity as the bridge cut-out signal. The repeater shown is situated at the adjacent satellite and  is seized from its left-hand side by a selector, when a call is initiated by an operator at the main exchange, and repeats further digits over the both - way junction to more distant exchanges. A call from a more distant exchange incoming to the right-hand side of the repeater is directly routed over back contacts of relays B and C to an incoming switch giving. access to a junction to the main exchange. Call initiated by operator. On seizure of the repeater, A and B pull up, differential relay OC remaining inert. The outgoing junction is looped over D and I and further digits are repeated thereover by A, C pulling up during the trains to shunt D and I and also to disconnect the shunt circuit over discharge tube NL to prevent its response to inductive surges. When the called party replies, D operates followed slowly by DD. If the wanted party is busy, the operator may cause breaking in by grounding both speaking wires to operate differential relay OC which repeats the signal over the junction. Cutting out the feed bridges for a test connection. Relay TC in series with rectifier MRA and tube NL operates on the high-voltage signal followed by Rand TS. Relay R prevents the operation of C and makes a clean metallic circuit over the repeater so that the high-voltage signal passes to the next exchange. When the signal, ceases, R restores, but the metallic circuit is maintained by SW which now operates in series with TS. Relay B is held by SW. Release of test connection. The high-voltage is re-applied to operate TC which holds SW and releases TS. When the signal ceases, SW falls back and the bridge is re-inserted, release now taking place in normal manner when the calling loop is opened. Release of a checking connection set up by the operator to a distant calling party. The connection will have been set up. as described in Specification 502,545 and'involves operations at the incoming switch over which the calling party reached the main. exchange as well as setting the local link. The high-voltage release signal of opposite polarity to that previously considered, operates a relay (not shown), at 'the incoming switch, in series with a discharge tube and rectifier which restores the switch and also makes. a metallic connection through to the junction so that the high-voltage pulse may similarly restore the connecting side of the local link. The Provisional Specification describes a circuit in which the two relays and oppositely poled rectifiers are connected to a single discharge tube.