298,474. Associated Telephone & Telegraph Co. Oct. 8, 1927, [Convention date]. Automatic exchange systems.-Relates to systems for small exchanges in which the calling and wanted lines are extended by line-switches to opposite ends of the same feed-relay-group, the called line being selected by a temporarily-used impulse-recorder. In one system, plunger-type line-switches controlled by a master-switch are employed, and either the same line switches and a single master-switch are used for both incoming and outgoing calls or two line-switches controlled by separate master-switches may be associated with each line. The impulse-recorder consists of relays only and comprises a set of four counting- relays energized in various combinations to record successively the tens and units digits, and a set of four storage-relays on which the tens digit is stored whilst the units digit is being recorded. The second system, shown diagrammatically in Figs. 6 and 12, employs line-switches of the usual rotary type, each switch having two test wipers (represented by separate banks A, B, Fig. 6), for establishing connections respectively with the calling end of an idle feed-relay-group TRG and with the called end of the particular group over which a connection is being extended. The recorder R comprises tens and units marking- switches and is associated with the feed-group TRG by a finder RF. Calls to the main exchange are set up by dialling a first digit O which causes the recorder to select a junction-relay-group MTGR associated with a two-way junction TL. A finder associated with the relay-group MTGR then hunts for the called end of the feed-group TRG in use whereupon the feed-group and recorder ane released and the connection is completed over a line-switch TLS and selectors D,E, F, Fig. 12, at the main exchange. Calls from the main to the local exchange are set up over a line-switch SLS, selectors D, E, relay-group BFG, and junction TL to the relay-group MTRG which then hunts by means of a further test wiper for an idle feed-group TRG over which the connection is completed. In both systems, the marking circuits for starting the called subscriber's line-switch include circuits common to a plurality of lines. If no idle recorder is available, a busy signal is given when dialling commences. Certain features of the systems shown form the subjectmatter of a divided Specification 308,366. First system, Figs. 1-5, employing plungertype switches. The system is designed for 49 subscribers numbered 11 - - 17, 21 - - 27, &c. and has five feed-relay groups TRG1 - - TRG5 permitting five simultaneous conversations. Each switch has ten sets of springs to which the incoming and outgoing ends of the feed-relay group are alternately connected, the first set of springs to the incoming end of TRG1, the second set to the outgoing end of TRG5, the third to the incoming end of TRG2, the fourth to the outgoing end of TRG1, &c. With this arrangement, the master-switch MS which controls both incoming and outgoing connections, makes a minimum movement since assuming that it has taken two steps to preselect TRG2 after a call has been initiated on TRG1 it will only need to take one further step to complete the call over TRG1. A modified arrangement is described operating in a generally similar manner in which each subscriber has two plunger-switches controlled by separate master-switches for outgoing and incoming calls, respectively. In the following description it is assumed that subscriber No. 22, Fig. 1, is calling subscriber No. 56, Fig. 3. Connection to idle impulse-recorder. Line relay LR22 energizes the pull-down winding of magnet PM22 which operates springs 17 - - 20 thus extending the calling loop to the feed-relaygroup TRG1, Fig. 2, at which relays LR, RR, operate. Relay RTR of the recorder selector also operates, connecting earth to the upper windings of relays SR1, SR2. If the first recorder RRG1, Figs. 4, 5, is busy, relay SRI is short-circuited over contacts similar to 158, 27, and relay SR2 operates. If both are busy, a busy tone is given to the subscriber when he dials by the operation of relay BR over contacts 45, 44, 42, 40. Assuming however that RRG1 is idle, relays SRI, HR1, SCR, operate. Reception of digits. The first impulse is repeated over leads 43, 52, to relay FR1. Relays CH1, ACH1 also operate and at the end of the impulse relay FR1 locks in series with FRTA. The next three impulses similarly operate and lock FR2, FR3, FR4, releasing FR1, FR2, FR3, respectively. For the fifth impulse, relays FR1, FR1A are again operated and FR4, FR4A remain locked so that when relay CH1 releases at the end of the digit, storage relays Y, Z, operate in series over their lower windings and contacts 80, 77, 188, 75, 74 and lock over their upper windings in parallel. When relay ACH1 relapses, the counting chain is released and relay DR is included in the locking circuit of Y, Z, releasing SCR. The second digit of six impulses similarly results in the operation of relays FR1, FR2, FR4, FR1A, FR2A, FR4A, the first pair not being released by the second pair owing to the closure of contact 72. (If the subscriber dials more than seven impulses in a train, the eighth impulse passes over contacts 68, 71 to operate the busy relay BR). Completion of connection. The relapse of relay CH1 at the end of the second digit energizes relay SDR which closes a holding circuit for ACH1 and a starting circuit for relay SRM of the master-switch. Relay SRM disconnects normal wiper 606, energizes relay OMR to prevent the initiation of a call whilst the master-switch is hunting, completes a circuit for the lower winding of magnet LM which releases the switch so that it commences hunting, and connects the upper differentially-wound winding of magnet LM to wiper 103. When wiper 103 reaches contact 105 corresponding to the outgoing end of the feedgroup TGR1, magnet LM de-energizes to stop the master-switch and relay SIR energizes over leads 107, 104, and wiper 103. May SIR opens the holding circuit of relay ACH1 and closes circuits, over lead 125 for units relay UR, and over contacts 112, 113, 114 for the fifth tens relay TD5. Relay TD5 completes circuits such as 123, 600 for relays such as UCR56 in each of the seven lines 51 - - 57, and the circuit for the pull-down winding of the plunger-magnet PM56 of the wanted subscriber is then made over contact 603, leads 133, 136, contacts 134, 77, 188, and the upper winding of relay FR. When relay ACH1 has fallen back, the switching relay SR, Fig. 2, operates over the circuit 150, 640, 145, 144, connecting up ringing current and operating the cut off relay COR which releases SR1 and the recorder. When the called subscriber replies, relay RTR operates releasing SR, and the back-bridge relay BBR switches through. Called line busy. If the wanted line is busy, relays UCR56, FR will not operate and the relapse of ACH1 completes a circuit for the busy relay BR which operates the cut-off relay COR and gives a busy signal. Preselection of idle feed-relay-group by masterswitch. When springs 17 - - 20 are operated on the initiation of a call as described above, earth over contacts 19, 23 operates magnet LM to release the master-switch which hunts until wiper 606 reaches an ungrounded terminal. When the end of the bank is reached arm MSA operates springs 623 to close the circuit of the trip magnet TM which is locked by catch 624. Magnet TM closes a circuit for the solenoid SM which restores the wipers to the beginning of the bank when cam MSB operates springs 623, 102 and unlocks magnet TM. Hunting continues as before. Faults. If a feed-relay-group is in use for more than a predetermined time without switching through, a thermostat T completes a circuit for the lower winding of busy relay BR in series with an alarm relay AR. Relay BR releases the impulse-recorder and gives a busy tone. Second system, employing rotary line-switches; connection to idle impulse-recorder. Line relay LR22, Fig. 7, causes line-switch LS22 to hunt in the usual way under the control of test wiper P22 and when an idle feed-relay-group TRG, Fig. 8, is reached, relays COR22, LRT, RRT, operate. Relay RRT completes a circuit for finder-startrelay FSR in the first idle impulse-recorder which connects finder-magnet MF to the test wiper 257. When the feed group TRG is reached, relay FCR operates over wiper 257 in series with relay FTR, and operates relay AFCR. If all the recorders are busy the non-operation of relay FTR causes on the transmission of the first impulse the operation of busy relay BRT. Reception of digits. The first impulse of the first digit (5) is repeated over contacts 226, 233, 264, 267, 268, wiper 271 of the tens switch TRS, normal contact 269, relay TLRR, to magnet TM. Relay TLRR operates so that subsequent impulses pass over contact 270 and the off-normal bank of wiper 271. At the end of the digit, relay TLRR releases so that the units digit (6) passes in a similar manner over wiper 271 and contact 274 to relay ULRR and magnet UM of the units switch. At the end of this digit, the relapse of relav ULRR closes a circuit from off-normal units wiper 277 over contacts 282, 280 for relays ICO, MSR1, the latter being so related to similar relays of other recorders that only one such relay can energize at one time. Completion of connection. Relays MSR1, ICO connect earth over conductor 265 and contacts 264, 234 to operate relay MPR which removes earth from the test contact 293 to render the outgoing end of the feed-group TRG selectable by the called subscriber's line-switch. Relay MSR1 also connects battery over wiper 273 and lead 297 to parallel circuits over lower windings of relays such as CR56 in each of the seven lines numbered 51 to 57, such circuits only being completed when the associated line is idle. Assuming that the wanted line 56 is idle, relay CR56 operates and closes a circuit for line relay LR56 ove