388,752. Automatic exchange systems. ASSOCIATED TELEPHONE & TELEGRAPH CO., 1033, West Van Buren Street, Chicago, U.S.A. Aug. 28, 1931, No. 24311. Convention date, May 11. [Class 40 (iv).] In a private automatic exchange, all mainexchange calls are dealt with by certain of the subscribers themselves, incoming calls being transferred if necessary by the answering subscriber to the wanted subscriber. The system shown comprises three types of subscriber, type S, able to make and receive local calls only ; type S\ able to make and receive local and main exchange calls, to transfer mainexchange calls, and to hold a main-exchange junction whilst making or receiving a local call ; and type S<2>, who in addition to the facilities of type S<1> can hold one junction whilst making, receiving, or transferring a second mainexchange call. Certain of the subscribers of types S<1> and S<2> may be prevented from making outgoing junction calls. The two-way junctions 840, 841, terminate in finder-switches, Fig. 7, over which on the operation of a substation key O by subscriber S<1> or S<2> the subscriber is connected to an idle outgoing junction, and on the operation of an answering key A the subscriber is automatically connected to the particular junction over which a call is being received. Local calls are made over finderconnector links, Figs. 3 and 4, the connectors being of single-motion type set directly by two series of impulses. In order to call a subscriber who is absent from his substation, the dialling of a special digit connects the calling line over a special finder, Fig. 5, to a code-ringing circuit in which code-ringing signals are produced by dialling long and short digits, the wanted party being able to connect himself to the other end of the circuit from any substation by dialling another special digit. Local call (to S<1>, No. 42) ; call-finder, Fig. 3. When subscriber S removes his handset, common line relay 7 operates in series with the upper winding of two-step cut-off relay 3 which operates only its armature 5 to mark the line calling. Relay 7 effects the grounding of start lead 377 through a relay group SR which so operates that, when the next call is initiated, lead 378 will be grounded and an alternate finder used. Relay 310 operates connecting up stepping relay 308 and magnet 307 which interact to step the wipers until the marked contact is reached. Cut-off relay 3 then operates completely in series with the upper winding of relay 309, which is of the type described in Specification 272,872, [Class 38 (v), Electric switches &c.], comprising a central magnetic spacing member with a backwardlyextending projection forming an armature backstop and is operative with only a small current in the upper winding. Relay 312 operates and switches through to the connector line relay 407. Connector, Fig. 4. Relays 407, 408, 413 operate and dialling-tone generated by relay 453 is fed over wiper 406 to the calling line. The impulses of the first digit are repeated to magnet 415 which advances the connector wipers four steps, relay 410 operating at the first impulse over wiper 404. At the end of the digit the relapse of relay 409 completes a self-interrupting circuit for the magnet over wiper 404 until position 14 is reached. (If the first digit is 3, wiper 404 does not find ground so that relay 410 releases and the wipers remain in their third position until the second digit is received.) The two impulses of the second digit advance the wipers to position 16 to which line S<1> is connected. If the line is busy, relay 412 operates over wiper 403 connecting up busy-tone lead 459. If the line is idle, switching relay 414 energizes connecting up ringing lead 457 and when the called subscriber replies relays 411, 410, 314 operate to complete the connection. When the calling subscriber hangs up, magnet 415 returns the connector wipers to normal over a circuit including wiper 404. Incoming call from main exchange answered by subscriber S<1>. Intermittent ringing-current over junction 840, 841 operates relays 806, 807, 804, 803, 710, relay 807 locking up and connecting up relay 808 which operates a series of scattered calling signals connected to circuit 835. When subscriber S<1> removes his handset and operates his answering key A, a circuit is completed over springs 155 for relay 141 and over springs 151, 156 for common line relay 161 in series with the upper winding of two-step cut-off relay 140. Relay 711 operates over start lead 164 completing a circuit for stepping magnet 707 and relay 708 which interact to step the wipers until wiper 703 engages conductor 166. Relay 712, similar to relay 309, Fig. 3, then operates in series with the lower winding of relay 140 which pulls up completely. Relay 713 extends the substation loop to the junction operating relay 801 and cutting off the ringing current. Relay 805 also operates, locking 803 and energizing relay 716. Outgoing call to main exchange set up by subscriber S<2>. When subscriber S<2> removes his handset and presses key O, a circuit is completed over springs 251, 258 for common line relay 260 in series with the upper winding of two-step cut-off relay 240. Relay 711 energizes over start lead 164 energizing relay 709, and hunting proceeds as for an incoming call but in this case under the control of wiper 702. When the calling line is found, relays 712, 713 operate completing a circuit from ground at relay 802 (if the main exchange is manual) over the calling loop and relay 801 to battery on conductor 841 at the main exchange. Relays 801, 802 operate, energizing 805, 716, and connecting direct ground to conductor 841. When the distant operator completes the call, ground sent back over conductor 840 energizes relay 803 which switches through, releasing 802. If the main exchange is automatic, relay 802 is short-circuited and impulses are transmitted directly over a loop from direct ground on conductor 754 to battery at the selector line relay connected to conductor 841. Holding main-exchange call whilst making or answering local call. Subscriber S<1> may be called locally whilst engaged in a main-exchange call, by the ringing of his bell RB. Restoration of key O or A restores springs 151, 156 to bridge his set across the local apparatus, but springs 152, 155 remain locked as long as the handset is off the cradle, completing a holding circuit over the junction. Further operation of key O or A releases the local connection and reconnects the handset to the junction. In a similar wav the subscriber can make a local call whilst holding a main-exchange call. Holding main-exchange call whilst making or answering a further main-exchange call. Subscriber S<2>, whilst engaged in a mainexchange call over conductors 271, 272 can answer a further main-exchange call by pressing hold key H which operates relays 280, 283. Relay 283 completes a holding bridge over conductors 271, 272, connects the substation loop to conductors 294, 296, and completes a circuit for line relay 261 in series with the upper winding of two-step relay 282. The finder associated with the second junction then hunts for conductors 294, 295, 296. Release of key H holds the second junction whilst re-associating the handset with the first. Both junctions may be held whilst making or answering a local call by restoring key O or A as in .the previous paragraph. Operation of key PB will release whichever junction is connected to the handset. Transferring main-exchange call from one subscriber to another. If subscriber S<2> after answering an incoming call wishes to transfer it to subscriber S<1>, he restores key A as before to hold the junction and then makes a local call to subscriber S<1> over a local finder-connector link. When S<1> answers and is informed of the call he operates key A completing circuits for relays 141, 161, and the upper winding of 140. A circuit 732, 703, 266, 248, 268, 302, 313, 402, 166, 148 is now completed for operating relay 140 fully. Relay 313 grounds conductor 359 to operate line relay 607 of transfer switch TSS, Fig. 6, which hunts for the local finderconnector link. Relay 606 then operates connecting ground over the circuit 601, 356, 305, 354, 705 to operate relay 715 which completes a bridge over relay 801 to hold the junction and releases relay 713. Relay 711 then operates and the finder, Fig. 7, hunts for line S<1> which is now marked in the bank of wiper 704 over the circuit 469, 405, 370, 315, 361, 602. When this line is found, relay 315 operates to connect a busy tone to subscriber S<2> to inform him that the transfer has been effected and relay S<2> then hangs up and releases the local connection. S<1> can similarly transfer a call to S<2>, and S<2> whilst holding one incoming call can transfer a second to another subscriber. Restricted service. A subscriber S<1> is prevented from making outgoing junction calls by jumpering terminal 104 to terminal 105 instead of to 103 so that if such a subscriber operates key O, a circuit is completed for highresistance relay 223 which connects up a busy tone. Conference calling. The 13th terminals in banks 401, 406 of all connectors are connected together through a common impedance 468, so that by each dialling 30, a plurality of subscribers may be connected in parallel for conference purposes. Calling subscriber absent from their substations. A subscriber obtaining no reply initiates a new call and dials digit 2. Switching relay 414 energizes over wiper 403, lead 464, and relay 509, and ring cut-off relay 411 is operated over wiper 401, lead 461, and relay 516. Relay 509 initiates the operation of the finder switch, Fig. 5, which acts in a similar manner to the local finder, Fig. 3, and relay 516 closes a circuit over relay 311, and wiper 304 for marking the calling line in the bank of wiper 502. When the marked contact is found, relay 311 operates to release the local link, and relays 508, 51