Patent GB-191511655-A (GB)
Title: Improvements in Telephone Systems.
IPC: H04M5/08
Filed: 1915-08-12
Abstract: 11,655. Relay Automatic Telephone Co., and Aitken, W. Aug. 12. Exchange systems. - Relates to private branch exchange systems with automatic disconnexion in the cord circuits. The line indicators of the extension lines are short-circuited during conversation over the third wire of the cord circuit, but as soon as the subscriber replaces his receiver, the short circuit is broken and further calls can be made although the plug remains in the jack. Means are provided to prevent the closing of this short circuit when connexion is made with an exchange circuit. The line indicator of the exchange line is disconnected by a cut-off relay during conversation, but as soon as a subscriber replaces his receiver, a resistance is thrown into the circuit of the cut-off relay, which de-energizes and brings the line indicator under the control of the exchange line although a plug remains in the jack. In the system described, the plugs may be used interchangeably in exchange and extension line jacks, and the holding-coil is only temporarily associated with a connexion. The Figure shows an exchange line jack J<3> with the usual line relay 8<E>, call lamp 18L, and cutoff relay 22<E>, and an extension line jack J<4>, the indicator 28<E> having one end of its winding connected to the bush of the jack. The cord circuit is provided with a quick-acting supervisory relay 21<E> controlling a slow-release disconnect relay 19<E>. A change-over relay 22<B> disconnects the local microphone battery LB when connexion is made with an exchange line. Relays 22<A>, 31<E> associated with the third wire of the answering and calling plugs respectively open the short circuit previously referred to, when the corresponding plug is inserted in the exchange jack. Call from main exchange to extension line. In response to signal 18<L>, the branch operator inserts the answering-plug A in jack J<3> thereby completing a circuit 22 through relays 22E, 22<A>, 22<B> and a resistance 22B which prevents the relays from attracting their armatures at this instant. The operator depresses her key K to cut in her speaking set over circuit 34, and in so doing closes initial energizing circuits 19, 20 for the relays 18<E>, 20<E>. The disconnect relay 19<E> closes circuit 21 through the local battery LB, the two windings of supervisory relay 21E, and the retaining-winding of the holding-relay 20<E>. The relay 21<E> short-circuits the resistance 22<R>, so that the relays 22<E>, 22<A>, 22<B> all attract their armatures. The relay 22<E> disconnects the call signal, relay 22<A> opens a connexion between the third wire c and the strand a of the answering-plug, and the change-over relay 22<B> cuts off the local battery LB, connecting the two windings of the relay 21<E> in series over contact 23, and in parallel with a condenser 35. The relay 22<B> also closes the circuit of the supervisory lamp 24<L>, which is, however, shortcircuited over contact 27 of the relay 21<E>. The operator then inserts the calling-plug in the extension jack J<4>, the line indicator 28<E> of which is short-circuited over circuit 26. The supervisory lamp 26<L> is lit over circuit 25 until the subscriber answers, whereupon the holding- relay 20<E> is shunted by the local line and deenergizes, cutting off the bridge connexion and opening the circuit of the 1 amp 25<L>. The microphone circuit of the called subscriber is now through to the main exchange, with only the condenser 35 shunted by the windings of relay 21<E>. The extension subscriber may cal in the branch operator by moving the switch hook, the relay 21<E> momentarily opening the short circuit of lamp 21<L>. When the plug is withdrawn at the main exchange, or the extension receiver is replaced, relays 21<E>, 19E deenergize. Resistance 22<R> is cut into circuit, causing the cut-off relay 22<E> to de-energize to reconnect the line relay 8<E>, so that a new call can be received at once. The lamp 24L gives a clearing signal to the operator. Relay 19<E> opens the short-circuit 26 across the call indicator 28E so that a new call can at once be made by the extension subscriber. Calls from branch exchange to main exchange. In response to a call on an extension line, the operator inserts the answering-plug A and depresses her key K. The relays 19<E>, 20<E> are energized as before, and the relay 21<E> energized over the extension line and circuit 21. The call indicator is short-circuited over circuit 30. The insertion of the calling-plug C in the exchange jack energizes the relays 22<E>, 31<E>, 22B over circuit 31. The holding-coil 20<E> forms a bridge connexion across the exchange line, thus giving a call signal at the main exchange. On the insertion of the plug at the main exchange, the line circuit is momentarily opened to deenergize the holding-coil 20<E>. Fresh calls can be made in the manner previously described. Local connexions. When both the answering and calling plugs are inserted in extension jacks, there is no circuit for either of the relays 22<B>, 22<A>, or 31<E>. When the subscriber answers and the holding-relay 20<E> de-energizes, a circuit 32 is completed for a relay 32<E>, which locks itself in a lamp circuit 33. The relay 32<E> cuts off relays 22<A> 31<E>, which thus maintain both line-indicators short-circuited. As the relay 22<B> is not energized, both subscribers have a common microphone circuit from the battery LB through the upper winding of relay 21E, calling- line, b-strand, called line, and lower winding of relay 21<E>. When either subscriber replaces his receiver, relays 21<E>, 19<E> de-energize, the latter opening the short circuits across the indicators, so that a new call can be sent in by either subscriber before the plugs are withdrawn. The shunt across lamp 25<L> is opened so that this lamp gives the clearing-signal. The Provisional Specification describes main exchange apparatus in which the junctions to the branch exchange are divided into first and second choice groups as described in Specification 20,453/14, and the multiple of the firstchoice lines is divided. For example, in the case of ten junctions to the branch exchange arranged as eight first-choice and two secondchoice junctions, the multiple of the first-choice junctions is divided into four parts, two junctions being multiped over each quarter of the multiple, and the second-choice junctions over the whole multiple. Each operator thus has access to four jacks instead of ten, but each of the two first-choice jacks has access over line-finders of the relay-switching type as described in Specification 1470/15, to all the eight first-choice junctions. Each jack of the firstchoice junctions is arranged to test busy only when all the eight first-choice junctions are in use.