336,627. Standard Telephones & Cables, Ltd., (Stevens, C. E., and Mamor, A.). July 17, 1929. Semi-automatic exchange systems.-A toll line T operated by A.C. impulses and signals is normally reserved for one-way traffic from a toll to an automatic exchange, but when a nightkey NK is thrown at the automatic exchange the line becomes available, for both-way traffic, and a toll call delivered by a local switch to the trunk LT instead of arriving at the position LOP of a local recording operator is diverted to the line T and is handled entirely at the toll exchange. Thus' at night the automatic exchange mav be unattended. Call from toll to automatic exchange. When the operator plugs into the junction, relays Tr, Br, Er are energized followed by Ar, which energizes Cr and releases Br, Er, so that an A.C. impulse is sent to line by Er. This is repeated by Rar, Rbr, to Brr1, Fig. 2, to kick off the sequence switch of the selector, TIGS, whereby a circuit is closed which, if the night key is thrown, energizes Rdr, Fig. 1, to busy the toll line to calls in the reverse direction. The sequence switch goes to position 3, in which a free register is picked up, and then on again to 5. These movements energize first So1r, Fig. 1, and then Btr, which cuts out So,r in favour of So2r, and Rcr is momentarily energized to send back an A.C. impulse. This energizes Rr, Dr, Hr (not Kr which is marginal) and a lamp CL (which was lit by Tr on plugging-in) is flashed by an interrupter FL1. Hr remains locked to continue the flash signal, and when the impulse ends Jr is energized. Dialling. In response to this signal the operator throws key DK and dials. Relay Er is energized by the dial off-normal springs and Sr is controlled by the dial to send A.C. impulses, which are repeated by Rbr and Irr, Fig. 2, to the register. The operator then restores DK. In due course the selector has its trip spindle set in position 6, hunts for a free outlet in position 7, goes on to 10¢, in which the register is connected to subsequent selectors, and when selection is completed goes on to the ringing and busy-signal position 12. Relay Btr is now de-energized and cuts back So1r in place of So2r, and Rcr is momentarily energized to send back an A.C. impulse. This energizes Rr, Dr to short-circuit Hr and energize Kr (which locks) and at the end of the impulse Jr falls back. The lamp CL is thus disconnected from FL1 and glows steadily. Busy signal. If the subscriber is busy, Cs1r, Fig. 2, is energized from the final selector and de-energizes Bcr, and an interrupter BFI is connected to Btr, Fig. 1, so that a stream of A.C. impulses is sent back, alternately energizing and shorting Hr, to flash the lamp CL at a distinctive rate. Answering and clearing signals. If the subscriber is free and answers, the selector goes to the talking position 13, and Btr is energized by the supervisory relay Csr and sends back an A.C. impulse to lock up Hr and extinguish CL. When the subscriber hangs up, another A.C. impulse is sent back, Hr is shorted and CL lights again. Re-ringing. The operator can re-ring the hungup subscriber by throwing key RK to energize Sr (intermittently) and Er, thus sending out a stream of A.C. impulses which energize Bcr (for the duration of the stream) and Rrr (at each impulse). Relay Rsr is energized and locks under control of Bcr (i.e. as long as RK is thrown) and re-rings the line. Release. When the operator unplugs the junction, Tr, Ar release successively and Br, Er pull up, the latter holding until Cr, Br have fallen back. An A.C. impulse is thus sent to line long enough to release the sluggish relay Prr, which initiates the release of the selectors. Premature releases; false calls. Release may be effected at any stage by de-energizing Prr; in the course of such release Pgr is energized and locks up. Or, in the case of a false call, Pgr is energized from the register and initiates the release. Pgr connects Ncr under control of Rbr so that if a new call is received during release Ncr locks up and eventually moves the sequence switch out of position 1 to deal with the new call, which would otherwise be lost, since the calling current is transient. Back release. If the dotted connection dc, Fig. 2, is provided, when the called subscriber hangs up the sequence switch goes to position 15 to release the succeeding selectors, and there waits until the toll operator's A.C. disconnect signal arrives, so that this signal shall not have the effect of a false call. Night call from automatic to toll exchange. The night key NK being thrown, if the local recording trunk LT, Fig. 1, is picked up by a second group selector, 'Tar, Rhr, Rcr are energized followed bv Tqr which de-energizes Tar, Rcr, and an A.C. impulse is sent over the toll line, energizing Rr, Dr, Fr (which locks) and lighting CL. The toll operator plugs in, Tr, Gr are energized, Fr is unlocked and CL extinguished. Plugging also results, as described before, in the transmission of an A.C. impulse which is now repeated to Rlr, which locks, and at the end of the impulse Rfr is energized and connects the local recording trunk to the toll junction. When the operator takes down the plug, an A.C. impulse is again sent over the junction and energizes Rgr, which effects disconnection by unlocking Rbr, Rfr and, when the impulse ends, Rhr. Release is completed when the automatic subscriber clears.