301,115. Pollak, J. E., (Associated Telephone & Telegraph Co.). Aug. 26, 1927. Automatic and semi-automatic exchange systems.-A private branch exchange limited to 100 lines employs plunger-type line switches and a single marking connector for selecting the line switch of the called subscriber, which then makes connection with the same trunk as the calling subscriber's line switch and the connector is released, feed current being supplied to both subscribers through a relay individual to the trunk. In the case of an outgoing call initiated by a privileged subscriber, connection is made to a junction in a similar manner, and the subscriber then controls the main exchange switches. Incoming calls are set up by the P.B.X. operator, who also deals with outgoing calls when required. A subscriber absent from his telephone may be called by means of a common bell circuit, and the connection is then established over a link circuit which terminates at each end in a line switch. Local call. When the calling subscriber A removes his receiver, line relay 203 energizes and connects up winding 205 of the line switch LS1, which plunges after a delay imposed by a slow-toenergize relay 201. Relay 203 disconnects wire 202 from the succeeding line switches in order to prevent double connections. Line relay 305 of the connector energizes and connects up relays 306, 307. Winding 204 of the line switch now operates and disconnects line relay 203, and ground is removed from wire 202 to prevent further calls while the connector is in use. Dialling tone is transmitted over wire 362 from the tone generator 368. Relay 367 energizes and connects up interrupting relays 365, 366, which deliver impulses to the magnet 350 of a, timing switch by which the connector is released after 4¢ seconds if no impulses are received. The vertical and rotary magnets 317, 319 respond to dialled impulses, and test relay 31C is connected to the test wiper 323 by relay 311. If the wanted line is busy, relay 310 energizes and locks, and when relay 311 falls back ground is extended over a contact of interrupting relay 365 to the tone generator 368 to transmit an inter. bittent busy tone. If the wanted line is idle, switching relay 312 energizes when relay 311 falls back and disconnects the tone generator 368. Relay 367 is unlocked by relay 311 at the beginning of the rotary movement, and relay 364 interacts with the magnet 350 to advance the timing device rapidly to normal. When this is reached and relay 311 has fallen back, relay 367 reoperates to re-start the timing switch, and ringing current from the generator RT is sent out over contact 356, consisting of a short preliminary ring followed by a long ring. Ring-cut-off relay 309 energizes over the loop if the called party replies during the non-ringing period, or else it energizes automatically over contact 353 in position 5 of the timing switch. Relay 309 connects up relay 304 and over wiper 324 causes the called party's line switch to make connection with the trunk 226-229 associated with the calling line, since the master switch MS has not advanced. Cutoff relay 218 associated with the trunk energizes, initiating the release of connector, and connecting up feed relay 217, which locks over the calling and called lines and maintains the line switches operated. When the connection is released, the line switches come at once under the control of the master switch MS opposite the trunk preselected at that time. Master switch MS. When relay 307 falls back in the connector, a circuit is closed over wires 223, 225 and test wiper 231 for start relay 233, which locks relay 304 to keep wire 202 disconnected and connects up relay 234, thereby unlocking the segment and allowing the U-spring 237 to advance the master switch. Relays 233, 234 fall back when an idle trunk is reached, and stop the switch. Relay 304 then relapses and closes the line switch operating circuit over wire 202. If the master switch is standing on the first trunk, trip relay 235 energizes in parallel with relay 234 and operates solenoid 236 to restore the switch to the position shown. Standstill release. If no impulses are received at the connector, ring-cut-off relay 309 energizes over contact 354 in position 9 of the timing switch and affects release by operation of cut-off relay 218. The master switch advances as before to the next idle trunk. Outgoing call established automatically. The subscriber dials the digit 0, followed by the desired main exchange number. The connector is raised to the 10th level, on which shaft springs 313 are closed, and relay 303 energizes over wire 202 and the fourth line switch spring and locks in series with relay 301. Relay 310 is now connected up over springs 314 when change-over relay 308 falls back, and acts as a stepping relay in conjunction with the rotary magnet 319 to hunt for an idle junction. If all the junctions are in use, springs 314 operate on the 11th step and lock relay 310, and a busy signal is sent back when relay 311 falls back. Springs " 30 and 00 " operate and connect direct ground to wire 222 to energize feed relay 217 in case the calling subscriber does not hang up until the connector has released. If an idle junction is found, switch-through occurs and busy lamp 153, Fig. 1, lights, while ring-cut-off relay 309 energizes at once and effects the operation of the line switch LS2 associated with the junction. Wire 222 is now grounded through resistance 320, so that though cut-off relay 218 energizes, feed relay 217 does not. Line relay 101 of the line switch LS in which the junction TL, Fig. 1, terminates energizes in series with relay 114, which connects up release relay 115, and the main exchange switches operate in the usual manner to complete the connection. Graded service. In the case of subscribers entitled to make main exchange calls, the fourth line switch spring is grounded. If a non- privileged subscriber dials O, there is no direct ground on wire 220 for operating the rotary magnet 319, and consequently a busy signal is given. Incoming call. When the junction TL, Fig. 1, is seized by a main exchange connector, relay 111 energizes during the battery period of ringing current, but resistance 110 prevents the ring-cutoff relay from operating. Ringing current energizes relay 112, which locks and connects up relay 113. Calling lamp 152 lights. The P.B.X. operator removes her receiver, and throws key RCK momentarily to the right, whereupon relay 117 energizes and locks over guard relay 401. The operator's set is now connected to the junction TL by way of wires 130, 131 and to the line switch LS2 by way of wires 133, 134. The ring- cut-off relay of the connector operates, and the operator then converses with calling subscriber. Relays 114, 115, 116, 118 pull up. To set up the call desired, she throws dialling key DI, disconnecting wires 130, 131 and grounding wire 132 to operate line relay 209 and cause the line switch LS2 to plunge, and dials into the connector. An error in dialling may be rectified by means of key RIK, which opens the loop and energizes relay 119 to restore the line switch LS2. The fourth spring of the line switch LS2 is connected to battery in order to operate relays 303, 301, and so prevent the energization of feed relay 217, feed current being supplied at the main exchange. Relays 114, 115 fall back when the called party hangs up, and lamp 151 lights to give a clearing signal. The operator releases by throwing key RCK to the left, when relay 120 disconnects release relay 118, and unlocks the local line switches. Outgoing call set up by operator. The subscriber dials the number of one of the information trunks, and a calling lamp is lighted in parallel with the line switch cut-off winding. The operator presses the corresponding key AK1 to energize switching relay 403, which locks over guard relay 401, ascertains the connection desired and hangs up momentarily to release the connection. She then seizes an idle junction TL by throwing key RCK to the right, whereupon relay 117 completes the loop over wires 130, 131, and sets up the required connection with dial key DO operated. The calling party is recalled with dial key DI operated, the connection being held meanwhile by bridged impedance 402. General call for a subscriber absent from his telephone. On obtaining no reply, the calling subscriber releases and calls the number 30. Springs " 30 and 00 " operate to energize ring- cut-off relay 309 and to prevent feed relay 217 from energizing, and line switch LS4, Fig. 5, operates. In the code selector CS, relay 501, 502 pull up, and the rotary magnet steps wiper 507 in response to the dialled code digit of the wanted subscriber. Relay 503 falls back after the last impulse and starts the motor M to apply the code signal to the common bell circuit. The called subscriber replies by dialling the number 39 at any telephone, thus becoming connected to the line switch LS3, Fig. 5. Ringing current from the connector energizes relay 504 to open the code ringing circuit and restore wiper 507. The cut-off and feed relays energize to provide talking current for the called party, that for the calling party being obtained over relay 501. Arrangement of connector bank. If the capacity of the P.B.X. is limited to 50 lines, the two pairs of wipers 322, 325 and 323, 324 are spaced to cooperate with the two halves of the bank, and the tens digits 2-6 are preferably used to avoid danger from an accidental impulse ; thus the digit 2 would raise the wipers to the 1st and 6th levels. For outgoing calls, springs 313 are arranged to operate after 6 steps, even though the digit 0 is dialled, to stop the vertical movement and initiate hunting.