329,626. Standard Telephones & Cables, Ltd., (Polinkowsky, L., and From, 0. C.). Feb. 14, 1929. Semi-automatic exchange systems. - A rural network in which calls are set up by an operator at a main exchange over a junction which is subsequently released comprises offices arranged in tandem formation in which a number of offices are directly connected to the same tandem office. Circuit changes at the rural offices such as for re-call ringing, reception of digits, ringing, completion of connection and toll break-down, are effected by impulses sent over one wire of the junction or junctions, the digits being sent over the other wire. As described, the system comprises a main exchange A, Fig. 1 (Prov.), having an operators position with provision for extension of incoming calls from the rural offices to a manual exchange M1 or M2 if the position is vacated. A rural office F and a tandem office D are connected directly to A, further offices C, E being connected to office D. In these offices which are of 10-line capacity, the connections are set up entirely by relays, all junctions except those in the most direct path being released, and conversation taking place over a link circuit in the office which is nearest the main exchange. The system also comprises a larger capacity step-bystep tandem office B having outlying offices G - - J single and tandem, similar to offices C - - F. The office B comprises a junction circuit, Fig. 1 (Comp), control and marking circuit, Fig. 2 (Comp), and a link circuit LF1, LS1, Fig. 3 (Comp), which is used for calls within the subsystem B, G - - J in which one of the parties is in office B. The junction circuit, Fig. 1 (Comp), comprise finders FS1 for extending calls from junction a, b, at the foot of the Figure, from outlying offices to the junction at the top of the Figure, to the main exchange. The control and marking circuit marks a group terminal of LS1 in accordance with the first digit but repeats the second digit directly to the switch. For calls in coming to the sub-system, the link circuit is not used, but switch FS1 is operated by the control and marking circuit. The capacity of office B may be increased by using pairs of successively hunting switches in place of single switches FS1, LF1, and associating an additional switch with LS1. Subscribers' line relays lock up when a call is made so that a calling party may replace his receiver and await re-call. The batteries at the various offices may be changed over junctions from the main exchange. Provision is made for dealing with faulty junctions. Calls from rural offices C - - F. Offices such as C, E, F connected directly or over tandem office D to the main exchange A are organized as shown in Figs. 6 and 7, and the tandem office as shown in Figs. 4 and 5, a junction a b incoming from an office C or E being shown at the lower right hand side of Fig. 5. When a subscriber S at any of these offices operates his magneto, line relay Lr pulls, disconnects itself from the line, and locks up a common relay Lsr which connects ground from a contact of Hbr over Br and contacts of Jcr and Rcr1 to the b wire of an outgoing junction. In the case of offices D, F. this completes the circuit of junction lamp JL, Fig. 3 (Prov.), and supervisory relay Svr at the main exchange A. In the case of offices C, E the grounding of the b wire of the junction to the tandem office, operates relay Jlr, Fig. 5, which connects ground over Br, to the b wire of the junction to the main exchange in a similar manner. The operator, whose cord circuit is shown in Fig. 2 (Prov.), plugs into the junction and relay Cor, Fig. 3 (Prov.), operating over the sleeve in series with lamp SL1, connects ground from the cord circuit to the a wire to operate relay Ar, Fig. 4 or 6. Lamp JL is extinguished. Relay Ar operates Hr in turn so that ground over Br is removed from the b wire. The right-hand back contacts of the line relay Lr, and also of the junction relays Jlr in the case of a tandem office, are connected in a chain circuit over leads " a," " b," " c," so that the first operated line or junction relay connects ground from a contact of Chr over contacts Ar, pgr, Hbr to the associated cut-off relay Cor which locks up to ground over a front contact of Hbr. Relay Cor connected battery from a back contact of Hbr to prevent the release of Lr before Hbr operated which would have transferred the ground to the cut-off relay associated with a subsequent line relay which happened to be operated. Subscriber's line is now connected through to retard Asr and relay Bsr so that Bsr pulls up and operates Sr which completes the speaking connection to the operator and connects ground from a contact of Hbr over Br to the b wire of the junction to operate Csr, Fig. 2 (Prov.), which dims lamp SL1. Calling party not on line; operator rings up. In this case lamp SL1 remains bright and the operator momentarily throws dialling key D to prepare the ringing circuit. The resulting successive operation and relapse of relays Psr, Pr sends two impluses over the a wire to relay Ar, Fig. 4. The first relapse of Ar connects ground to Dkr and, at the end of the impulse, Dkr1 operates in series with Dkr to transfer the operating circuit to Rcr. Relays Rcr, Rcr1 are similarly operated at the end of the second impulse, and respectively connect the a and b junction wires to subscriber's line which is rung by throwing the key R. Relays Psr, Pr operate as before when R is thrown but this is without effect since the a wire is grounded over R. When, however, the calling party replies and R is restored, Psr, Pr fall back and send an impulse over the a wire, and Ar responds to operate Olr, Olr1. Relays Dkr, Dkr1, Rcr, Rcr1 fall back. Relay Bsr operates over subscriber's loop, and the resulting operation of Sr releases Olr, Olr1. Connection between subscribers on tandem office D, Figs. 4 and 5. If the calling party is on a tandem office and requires a local connection, the operator throws key D and dials the wanted number over the calling junction. The operation of D sends an impulse over the a wire as described in the preceding paragraph, and Dkr, Dkr1 energize. Relay Dkr extends ground from the outer right contact of Hbr, over contacts of Sr, Jcr, Lbr to relay Tlr which locks up and connects ground from Lbr (outer left) to a lead common to outer right contacts of all relays Cor the operated one of which extends the ground to the local connecting relays Acr, Ccr. If the local connecting circuit is idle, battery from Lbr (outer right) operates Ccr which transfers the calling line to the circuit over Gcr, Lcr, in which Lcr operates and connects ground from Bdr (outer right) to hold Ccr and operate Lbr which connects battery to the connecting relays Acr on the answering side and disconnects the original operating ground for relays Acr, Ccr. Relay Cor is released. When relay Dkr operated, ground over resistance A was connected over Br to the junction b wire, and Dkr1 disconnected Bsr which released Sr to clear the junction for dialling. Impulses over the b wire are repeated by relay Br to the counting chains Scr, Tcr. The first impulse operates Scr1 which, at the end of the impulse, locks up in series with Tcrl to ground at Dkr1 (left outer), and subsequent impulses operate succeeding pairs which release preceding pairs of counting relays. The operation of the last pair transfers the pulsing lead to the first pair and operates Adr which holds over Adrl, and when the last pair are released by re-operation of the first pair, the shunt about Adrl is opened so that this relay energizes. Relays Adr, Adr1 change over the marking wires if the digit is greater than five. Slow-to-release relay Dsr, connected to the pulsing lead, falls back at the end of the digit and connects ground from the contact of the operated relay Scr over relay Btr and the inner right contact of the operated relay Tcr and one of the leads 5 - - g to the cut-off relay of the wanted line. This ground also branches over contacts of Btr to slow-to-operate relay Otr so that if the line is busy and Btr does not operate, Otr pulls up and connects the self-interrupting winding of relay Tr to ground at Dkr1 (outer left) and the other winding of Tr across the junction, to give the busy signal. If Btr operates, it locks up to ground at the Scr relay and connects this ground direct to the cut-off realy Cor of the wanted party who is thereby connected to the junction. When the operator restores the dialling key, an impulse over the junction a wire operates relays Rcr, Rcr1 as previously described to prepare the ringing circuit. The operator throws the listening key and intermittently operates the ringing key. On restoration of keys R, L, the impulse over the a wing operates Olr, Olrl which release Dkr, Dkr1, Rcr, Rcrl. Relay Olr1 connects ground over contacts of Olr,Tlr to the common lead to the outer right contacts of the cut-off relays Cor so that the Acr relay associated with the operated relay Cor of the wanted, party connects his line to the answering side (Gar, Lar) of the local circuit. Relay Lar connects ground from Bdr (outer right) to lock Acr which opens the original circuit for Cor but this relay remains held to ground at Hbr (outer right) until Tlr is released. Called party fails to reply. When relays Olr, Olr1 have operated after the restoration of the ringing and listening keys, a speaking connection is completed from the condensers in the junction to the calling side of the condensers in the local circuit so that the calling party may be informed. Local connecting circuit busy. Relay Lbr is operated so that the preparatory impulse immediately prior to dialling operates Lor instead of Tlr, the calling party is therefore, not connected to the local circuit, and Otr pulls up to give the busy signal, as described in the preceding paragraph, and open the ringing circuit. The operator throws the listening key L and, on