437,591. Automatic exchange systems. TELEFONAKTIEBOLAGET, L. M. ERICSSON, 33, Kungsgatan, Stockholm. March 22, 1934, No. 9032. Convention date, March 23, 1933. [Class 40 (iv)] In a system comprising a number of tandem offices T1 ... T7 each serving a group of local exchanges All ... A13 &c., each tandem office is characterized by a single-digit number which is repeatedly sent to extend a call from one tandem office to the next until the wanted office is reached when a signal is sent back to initiate the transmission of the localexchange number and the numerical digits. The arrangement may be extended to provide also for the repetition of the local-exchange number where certain local exchanges are only connected to the associated tandem office indirectly. In the system shown, in establishing a call between exchanges A11, A54, say, only the local-exchange digits (54) and the numerical digits are dialled (in addition to the digit 0 which indicates an outgoing call), and a register-controller, Fig. 4, at the outgoing tandem office T1 effects by translation the transmission of the tandem-office digit (6) characterizing office T6 and a routing digit (4) for selecting at T6 the wanted local exchange A54. In systems not employing registercontrollers, the tandem-office digit is dialled by the calling subscriber and is automatically repeated as often as required, and a buzzer signal is sent back to inform the subscriber when the local-exchange and numerical digits are to be dialled. The system shown also includes arrangements for transmitting from the calling tandem office to the wanted local exchange an indication of the class of call (ordinary or toll), and from the wanted exchange to the register-controller at the outgoing end an indication of the number of numerical digits to be expected. Connection to registercontroller at tandem office T1. When subscriber P1, Fig. 2, at exchange All, Fig. 2, dials 0 the local register RE11 energizes the starting relay R11 of a finder FS1 associated with an idle two-way junction F11 to tandem office T1. When the calling line is found, relays R12, R13, R14, R15 operate completing an outgoing loop over the junction in which R20, Fig. 3, operates and energizes R21, R22. Relays R23 in idle register-controllers operate to drive associated finders RS1 in search of the calling junction and when this is reached, relays R24, R25 operate releasing R20, R21. The four remaining digits (5421) are then repeated by relays R13, R26 in successive positions of sequence switch SO1 to registers MV1, MV2, MV3, MV4, the circuit for MV2 including a wiper of MV1 since there is a different register MV2 for each pair of settings of MV1. Extension of connection to wanted exchange A54. After the two local-exchange digits have been received, magnet KVM3 is operated over normal contacts 49, KV1 in position 1 and SO1 in position 4 to drive switch KV3 to position 2 in which an interrupter circuit over relay R29 and off-normal contacts 50 is closed. At each impulse switch KV3 is advanced one step and relay R29 transmits an impulse over circuit 53 to magnet RVM1 of switch RV1. After six impulses representing the digit characterizing office T6, relays R30, R32 operate over a jumper wire 54 at frame RK, opening the impulse circuit, driving KV1 to position 2, and causing KV3 to return to normal over interrupter 61, relay R30 then being shortcircuited. The switch RV1 energizes start relay GR1 for the finders associated with idle two-way junctions F1 leading to tandem office T2, and when the calling circuit is found, relays R35, R37 operate, completing a loop to relay R61, Fig. 5. Relays R62, R63 operate causing switch RV2 to move to position 2 in which the driving interrupter is connected to magnet RVM2 over relay R64 which transmits an impulse back over the junction by shunting resistance M2. Relay R38 responds operating R39 which repeats the impulse to relay R40 in the register-controller causing the successive operation of R41, R51 and the closure of a new starting circuit over contacts 115, 116, for KV3. The digit 6 is now sent out again in the same manner and is repeated by relays R50, R61, R62 to magnet RVM2, Fig. 5. At the end of the digit the relapse of R73 completes a circuit for start relay GR2 which acts similarly to GR1 to extend the call over a two-way junction F2. The connection would similarly be extended through any more intermediate tandem offices, the digit 6 being sent out as many times as required in response each time to the transmission back of a single impulse. At the terminating tandem office T6, switch RV2 in response to the digit 6 effects the transmission back (by connections similar to those at positions 6 and 7 in Fig. 5) of a signal of two impulses. In the register-controller, relays R40, R41, R51 respond as before to the first impulse, and the second impulse is operative over contacts 93, 141 to move switch KV2 to its second position so that a translated digit 4 necessary for reaching the wanted exchange A54 from the office T6 is sent out under the control of jumper 143. Completion of connection ; indicating class of call. In exchange A54, Fig. 6, relays R82, R84, R85 operate causing relay R86 to start local finders AS. When the calling junction is found, relays R87, R88 operate and the switch RV4 moves to position 6 sending out in positions 2 to 5 four impulses to signal the number of numerical digits (2) in the subscriber's number. These impulses are repeated back to the register-controller, the first driving switch KV2 to position 3 and the other three moving KV1 to position 5. The class-indicating digit of 1 or 2 impulses as determined by the operation or not of relay IR is now sent out, relay IR being operated over a different wiper of switch RS1 for junctions from a toll exchange such as T7. This digit is repeated to switch RV4 and in the case of a toll call, a signal is sent to the local register RE54 over circuit 204. With KV2 in positions 4 and 5, the numerical impulses are sent out under the control of wiring between the lower bank of KV3 and the banks of the numerical registers MV3, MV4, and in position 6 of KV2, with KV1 in position 5, a circuit is completed for switching-through relay R45 which releases the register-controller. If the wanted subscriber is busy, relay R96 is energized causing switch RV4 to return to normal and during this movement R89 sends back a number of impulses which are repeated to relay R46 and are sufficient to actuate relay R44 to connect up busy-tone SU. If the wanted subscriber is idle, relay R93 is energized to switch through to the called loop, the resistance of which is such that the relay in T6 corresponding to R78 operates and causes the corresponding operation of R78, R38 and R16. Irregular operation. At the first operation of relay R30, Fig. 4, relays R42, R43 operate, the latter disconnecting the operating circuit of R42 but holding it operated for an interval by a pendulum armature P. If for any reason, for example, due to all junctions F1 being busy, relay R42 relapses before R43 is released by R41, a circuit is completed for R33, Fig. 3, which energizes R34 to connect up busy-tone SU. Specification 420,607 is referred to.