388,935. Automatic exchange systems. GENERAL ELECTRIC CO., Ltd., Magnet House, Kingsway, London, HAMILTON, R. M. and MOSS, B. F., c/o General Electric Co., Stoke, Coventry. March 3, 1932, Nos. 6413 and 9127. [Class 40 (iv).] In a system including local switching centres where exchange trunks terminate in switches acting alternatively as line finders and connectors, the local switch steps simultaneously with a switch at the exchange by mutual interaction, the latter switch serving on calls from the switching centre to control metering and various discriminating effects and on calls to the switching centre to determine the extent of movement in accordance with the final digit which is registered on the line switch associated with the selected trunk. The second selector (assuming a 4-digit system) responds to the second and third digits of the wanted number and then hunts for a trunk to the required switching centre, and subsequently supplies ringing and feed currents. Where a switching centre accommodates 11-20 subscribers, either of two third digits produces the same numerical effect at the second selector, and a discriminating signal is sent forward to cause selection in the first or second group of ten in response to the final digit. Non-numerical switching, Figs. 1 and 2. In response to the operation of line relay J when a call is initiated, relay S associated with an idle trunk energizes and connects up relay C, whereupon magnet LF pulls up, followed by relay SS and sends an impulse over wire 5 to relay AL which connects up relays AC, AL and repeats the impulse to magnet MF. The latter on pulling up now sends back an impulse over wire 4 to relay A which releases magnet LF. This interaction continues so that the metering switch MF steps in synchronism with the finder LF until the calling line is reached, when relay D energizes in series with cutoff relay K and connects up switching relay HA. The common starting circuit is extended to relay S of the next idle trunk. On the relapse of relay AC, relay AL holds over the loop in series with relay H, and the usual hunting circuit is completed for magnet SF and test relay AK, which energizes and switches through when an idle first selector is found. Relay AB energizes and marks the trunk busy by grounding wire 9. Second selector, Fig. 3. When seized by the first selector, line and release relays P, B energize, and two trains of impulses are repeated to the vertical and rotary magnets V, R in parallel with dialling relay CC, change-over being effected by relay EB. The selector has two sets of wipers, and five groups of 4 trunks are connected to terminals 2 .. 11 in each level. If the third digit is odd, relay Q energizes over wiper W7, sends an impulse to the magnet R to bring the wipers into engagement with the first pair of trunks of the required group and operates discriminating relay SA. If the third digit is even, the wipers are already standing on the first pair of trunks. On the relapse of relay CC, the two trunks are tested in turn by relays HC, HB, relay GA energizing in the interim; if both are busy, relay GA falls back when relay EB releases and the magnet R reoperates, being disconnected by relay U which also connects up relay EB again. A fresh pair of trunks are tested, and it will be assumed that switching relay HB operates. Relay T energizes over wires 7, 4 in series with relay A, Fig. 1, and connects up relay EA which closes an impulse-repeating circuit over wire 8. Relay SA if operated releases and sends a booster impulse over wire 9 to energize marginal relay N, Fig. 2, in series with relay AD ; otherwise the latter alone energizes. At the switching centre, relay A connects up relay D and busies the trunk by extending the common starting circuit. Final digit. Impulses are repeated by relay P over wire 8 to magnet SF and relay AG and in parallel to dialling relay CC which connects up relays KA, U. Relay AG on operating connects up relays AE, AB and on falling back at the end of the train releases relay AD, thereby connecting up relay AC which disconnects relay A, Fig. 1, so that relay E pulls up. Magnet MF operates and sends an impulse over wire 4 which energizes relays A, C, SS and magnet LF, which on pulling up sends back an impulse over wire 5 to energize relay AA and release magnet MF. The switches MF, LF therefore step automatically in synchronism until wiper MF4 reaches a terminal marked over wiper SF5. If, however, relay N is operated, it remains locked in series with relay AC over wiper MF5 until half the bank has been traversed and holds open the marking circuit, so that stepping continues until wiper MF4 reaches the marked terminal in the second half of its bank. Relay AD energizes in the marking circuit and disconnects relay AC which opens the stepping circuit and releases relays A, C, D, E, SS. Busy test and ringing. The switch LF is now standing on the wanted line, and on the relapse of relay C, relay T is connected over wires 7, 4 and relay G to test 'wiper LF3. If the line is busy, relay T does not operate, and relay RC, which was held over wire 8 while relay AC was energized, on falling back disconnects switching relay HB or HC to release the trunk and connects up relay BB to send back a busy signal. If the line is idle, relay T pulls up, connects up relay RB which holds relay RC, connects in its low-resistance winding to operate relays G, K and then releases consequent on the relapse of relay SS. Switching relay HA energizes and ringing current is applied over ring-trip relay F, wires 7, 4 and relay H. On reply, relay F energizes followed by back-bridge relay DA, which reverses the feed through line relay P and sends back a booster impulse over wire 12 pending the relapse of relay U to actuate the calling party's meter over the metering switch MF. Relay H energizes and connects up relay D. Release. Assuming the called party hangs up first, relays H, DA fall back and lamp CSH lights. Relays Z, C operate and relay HA holds over wires 4, 7 until the calling party hangs up, when line relay P falls back. The operated switches then return to normal. Disabling a subscriber's line. This is done by connecting the corresponding terminal of bank MF3 to terminal NU. If the subscriber calls, the switch MF is positioned and relay AK energizes immediately over this connection, preventing the switch SF from hunting and sending back NU tone. Coin-box subscribers. The corresponding terminals of bank MF2 are connected to terminal CB. When the switch SF moves off-normal after the switch MF has been positioned, relay NB energizes over wiper SF3, connects up relay AG and advances the switch SF automatically to the second half of its bank. An idle first selector is then found and a discriminating signal is sent forward over wiper SF5 and wire 13 due to the operation of relay AG to enable NU tone to be given when the selector is raised to certain levels. The switch SF is stepped to the second half of its bank since the first half of bank SF5 is employed for marking purposes. Number unobtainable. If a subscriber's line is out of use, the corresponding marking wire from bank SF5 is connected to terminal 17 or 18 according as it is in the first or second group of ten. If, however, corresponding lines in the two groups are both out of use, the marking wire is connected to terminal 19. When the number of such a line is dialled, relay NB energizes in series with relay AD, holds relay T over wire 7 and connects up relay RC over wire 8, whereupon relay RB operates and supplies NU tone. Operator hold. If the calling party hangs up before the operator clears, relays H, D fall back and relay Z connects the holding circuit of relay HA to wire 4. The operator may recall the subscriber by applying ringing current. Current supply. The battery at a switching centre may be charged from the exchange over an idle trunk or locally from rectified A.C. supply. Alternatively, the rectified supply may be employed directly by the apparatus. According to the second Provisional Specification, each switching centre may have 6 trunks which are graded over a number of second selectors.