644,112. Automatic exchange systems. WESTERN ELECTRIC CO., Inc. Feb. 20, 1948, No. 5097. Convention date, Jan. 5, 1940. [Class 40 (iv)] When signalled, an incoming trunk is connected to a cordless toll operator's position over a cross-bar link, Fig. 3, which is locked automatically if the connection established is of a type requiring supervision. The type is determined by the marker used by the operator in establishing the connection on a train of cross-bars of which contact groups 282, 285 of only the first primary cross-bar and the last secondary cross-bar are shown. The operator may lock the link of her own accord. A link comprises primary 300, 301 and secondary access to a different channel. A position circuit is shown in Figs. 7, 10, 11 and a control circuit therefor is shown in Figs. 5 and 8, the figures being arranged as shown in Fig. 12. The trunks are classified according to the signalling used and according to their transmission characteristics. The class of the incoming trunk is signalled from the trunk circuit to the operator's position and is transferred later to the marker 902 sized for the call but no indication is given to the operator. The condition of the channel is indicated by lamp signals and the types of call, e.g. a new call, a re-call or an unanswered call are indicated by both lamp signals and tone signals. When the operator ascertains the wanted number and sets it up on her key set, the associated sender 900 energizes a marker connector 901 to seize an idle marker 902 which 302, 303 cross-bars which together with a link-control circuit, Fig. 6, act as a finder to select a free channel, Fig. 4, to the operator's position. Each secondary contact group gives receives code registrations covering the distant exchange and the classes of the trunks concerned and extends the call. If the signals received by the marker determine that supervision is required, the marker operates a relay 114 in the incoming trunk circuit to lock the link but only the channel relay 411 is held in the channel circuit. Another relay 116 is operated if supervision is not required and the link and the channel relays are released, but either the calling party or the called party may seize a free channel to call the operator. Relays 114, 116 may be operated together by the operator to lock the link. Specification 642,455 is referred to. Link-control circuit. Circuits, relays and contacts relevant to only one trunk, one channel and one link-control circuit are shown in Figs. 2, 3. A pulse of A.C. on the ring- down trunk 135 operates relay 103 which releases relay 104 and is followed by trunk relay 202 and by relay 113. Ground is applied over conductor 320 to operate a link.start relay 311 common to a sub-group of trunks. A free link-control circuit is selected by a chain of relays 316, the selected one being followed by a multi-contact relay 317 to connect the control circuit. Free channels to the operator's position are marked by battery and ground on pairs of test conductors, the conductors 340, 341, for only the first channel, Fig. 4, being shown. The sub-group and the trunk are marked by relays 608, 600 in the link-control circuit, the corresponding sub-group relay 313 and select magnet 304 in the primary cross-bar are operated and the operation of the select magnet is checked by relay 601. Relay 312 disables the start relay 314. The class of signalling (in this case ringdown) is marked on relay 603 and relay 314 follows to connect the test wires 341 of suitable channels. A free channel is selected by a chain circuit, only one channel marking relay 605 being shown and the secondary select magnet 318 is operated. Test wire 340 is connected by relay 315, following the operation of 605. When the operation of select magnets 304, 315 has been checked in the link-control circuit by relays 601, 602 and when a second test of the operator's channel has been made over conductor 340 by relay 610 which operates 606, the hold magnets 305, 307 operate to switch the trunk through to the selected channel in which the channel relay 411 is operated over the cross-bars by the ground from 602 operating the hold magnets. The control circuit is released when the start ground is removed from conductor 320 later. Operator's position channel. Operation of relay 410 and guard lamp 409 is the first step in a sequence of relay operations building up the channel and position circuits. Relay 411 operates a busy lamp 407 and a second guard lamp 406. Relays 402-421 prevent seizure of more than one channel at a time and mark the position busy by releasing relays 401, 701, which are normally operated when an operator is in attendance. Relays 413, 403, 404 operate to extend the backward talking wires 323, 324 to the operator's position and the guard lamp 409 is changed from a steady to a flashing condition as an indication that a trunk has been connected. The forward talking circuit is blocked at 711 and relay 705 operates and prepares for sender start relay 704 to block the circuit again later. Relay 112 in the trunk circuit follows 403 and locks to remove ground from conductors 320 and thereby to release the link-control circuit. The hold magnets 305, 307 and the channel relay 411 are now held over 413. Class of trunk signals, position circuit. Relay 413 is followed by a position relay 702, class relay 709 and relays 710, 711. Relay 711 provides ground for a number of subsequent operations including the operation of relay 805. Class relay 709 connects the forward talking wires over conductors 715, 716 to separate pairs of marginal relays 1121, 1122 and 1123, 1124 and relay 710 blocks the forward talking circuit. The trunk wires themselves are separately grounded through resistances 218, 219 adjusted so as to give a desired indication on the marginal relays. The information is passed to the sender and marker, Fig. 9, over contacts of relays 1117-1120. Operation of any one marginal relay de-energizes slowto-release class relay 709, 706 operates, 710 duly falls back and the splitting relay 501 and the sender-start relay 704 follow. Relays 704 and 705 now operated together block the forward talking circuit. Position lamp and tone signals. A sleeveholding ground provided by trunk relay 202 is checked by relay 708 in series with a polarized relay 707. The polarized relay operates only if the connection is already established, which is now indicated by a lamp 1010 relay 800 being operated. Signals are identified by marginal relays 1014-1016 operated by battery through resistance 110 for a new call, or a recall from the calling operator, through 110 and 222 in parallel for a recall from a called operator and through 110 and 221 in parallel when no answer is received. The marginal relays control relays 1002, 1003, 1004, the first two of which along with 107 control indicating lamps 1010, 1011, 1012. A series of relays 1018-1021 started by ground from 711 operate along with 1013 to apply three pulses of tone relays 912, 911 and the route is marked by relays 906 or 907. The terminating area of the call is indicated by relay 913. Relay combinations for connections requiring supervision operate relay 806 and 807 over contacts of 908 and those requiring no supervision operate relay 809 and 808 over contacts of 909. The marker selects an outgoing trunk 280 to the wanted exchange 281, operates the trunk relay 289 and operates hold magnets 275, 276, &c. and select magnets 278, 277, &c. to connect the trunks. Select magnet 278 follows relays 915 and 801. A relay 251 signals the marker by operating 910 when the connection has been completed, the marker is released and the switches are held by the sleeve holding ground on 202 already referred to and by ground on conductor 329. In the outgoing trunk circuit, relay 290 operates and is followed by 512, 513, 514 in the position control circuit, but 512 immediately falls back. Lamp 406 is unaffected till the distant operator closes the loop and operates 288 to ground the ring conductor and therebv to short-circuit and extinguish the lamp 406. The same ground from 1022 to the talking wires over transformer 1005. Relay 1004 may cut off the tone after one or two pulses under control of the marginal relays. A call over a ringdown trunk operates at least one of the relays 1002, 1003 and establishes a circuit to the marker, the tone being transmitted only to the operator. If neither 1002 or 1003 is operated the tone is transmitted to the trunk conductors. Sender ; connection to outgoing trunk. Sender control relay 925 was operated as previously described by sender start relay 704 and was followed by relay 203. The operator ascertains the wanted number and sets it up on the key set. A free marker 902 is selected by a marker connector 901. Information concerning the wanted number, the routing and the areas of the trunks is transferred to the marker. High and low transmission characteristics of incoming trunks are marked on interrupts a timing circuit 107 previously started to measure an interval before signalling that a call is unanswered. Control of link from marker ; position release. As previously stated, conditions for supervision and for non-supervision are produced by operation of 806 and 809. The release of the marker is signalled by relay 926 and 814, and the position release relay 804 operates to lock either 806 or 809. Backward release relay 500 follows and locks. Relay 113 which is separately held over contacts of 804 and 500 falls back and the splitting relay 201 follows. If supervision is required, relay 114 now follows 806 and provides on conductor 328 a common holding ground for the link cross-bar hold magnets and the channel relay 411. The position is released ready for another call and only relay 411 remains operated in the channel circuit. If supervision is not required, relay 116 follows 809, but does not lock the link. Signalling circuits are established in each ca