393,652. Telegraph exchange systems. CREED & CO., Ltd. and GRIFFITH, R. G., Telegraph House, Croydon. Dec. 11, 1931, No. 34389. [Class 40 (iii).] In a telegraph exchange circuit, each subscriber's line has a single jack (or equivalent terminal connection) which serves both as an answering jack and a calling jack, means being provided to maintain the same direction of signalling current over the line in either case. To this end, a relay associated with the line jack is operated when a subscriber calls (but not when he is called) to reverse the direction of current over his line. The signalling circuit established between two lines includes one strand only of the cord circuit. The invention is described as applied to a printing telegraph system in which a subscriber dials the number of the wanted line to an exchange indicator and receives on his printer typed information as to the progress of the call. Fig. 1 shows the substation apparatus, Fig. 2 the line equipment at the exchange, Fig. 3 the circuits individual to a cord, Fig. 4 the circuits of an operator's position common to a number of cords, and Fig. 5 the auxiliary signalling apparatus common to the whole exchange, Figs. 3, 4, and 5 being delimited by chain lines. Calling the exchange and operation of number indicator. By depressing his calling key CFK, a subscriber earths both his lines through a resistance Rl, thereby energizing his line relay A which looks up and causes relay AA to light the calling lamp CL. The insertion of the answering plug AP energizes relays B, CO, C. (The operator whose plug first establishes the "ring" " connection seizes the call other operators receive an alarm signal as described later.) Relay C energizes relay MS which starts the exchange code transmitter CT. Relay B completes a holding circuit for relay AA, disconnects lamp CL and relay A, and establishes a signalling circuit from battery SB, over the subscriber's loop and printer relay, and the impulse relay D, Fig. 4. This current energizes the dialling indicator SI and causes the printer relay to start the teleprinter motor. Relay D energizes the usual slowrelease relay E which prepares a circuit through the change-over relay F for the magnets DMI .. DM4 controlling the switches NS1 .. NS4 of the lamp indicator. By operating his dial DU, the subscriber sets the switches NS1 .. NS4, the relay F controlling the usual digit switch DS. The circuit of the " hundreds " amp, say, is initially completed over the arm W3 of the-digit switch DS in its second position and subsequently over the arm W2 of the tens switch NS3 in its off-normal position. The operator inserts the calling plug CP in the line jack of the wanted subscriber. Operation if wanted line is free. If the line is disengaged, the relay H energizes over the sleeve of the plug in series with the relay B of the called line. Relay H lights the supervisory lamp SL. As relay AA of the called line is not energized, relay B completes a circuit from ground at aa2 over the subscriber's loop, the tip of the calling plug, relay J, and resistance R5 to battery. This current starts the teleprinter motor of the called subscriber. Relay J causes relays K, L, M to energize in succession. Relay L locks up, and disconnects the number indicator at contacts l5, l6. Relay M locks up, disconnects the supervisory lamp SL, and prepares a circuit for the pick-up relay N, which is energized when the contacts PUC of the exchange code transmitter are next closed. Relay N lights lamp TL at the exchange and connects the code contacts TT of the code transmitter to the calling line, causing the letters THRU to be printed on the teleprinter. At the end of the cycle, contacts TOC energize a relay O, which locks up, disconnects relay N, and completes the connection between the two subscribers through the tips of the answering and calling plugs. A " who are you? " signal is transmitted to the called line from the code contacts WHORU. During the connection, relays H, J .. M remain energized. Operation if wanted line is engaged. If the calling plug is inserted in the jack of a busy line, relay H is not energized, but a relay R, energized over the sleeve of the calling plug, closes a local circuit for relay J, thereby energizing relays K .. M in succession as before. The relay R energizes a relay S which lights the " engaged " lamp EL at the exchange and substitutes the code contacts BT for the code contacts TT so that when the pick-up relay N is energized, the word BUSY is printed at the calling station. At the end of the cycle, relay 0 is energized as before, opening the circuits of relays J, N, S. Relay J de-energizes relay K which causes a relay P to energize the supervisory lamp SL and a position pilot lamp PPL, which glow until both plugs are withdrawn. If a subscriber has more than one line, an operator may test them in succession by inserting the calling plug, before notifying the calling party that the number is engaged. To this end, the operator depresses " engaged test kev " ETK, which interrupts the circuit of relay J,.so that the code BUSY is not transmitted, but the relays R, S control the " engaged " lamp EL as before. The operator may also transmit the code BUSY by inserting the calling plug in a special busy jack. Disconnection. The depression of the clearing key CK at either station de-energizes relay J which causes relay K to energize relay P which lights lamps SL, PPL as in the preceding paragraph. These glow until both plugs are withdrawn. To prevent a subscriber holding the line engaged after he has finished signalling, the teleprinter, after an idle line period of, say, three minutes, opens the line circuit automatically to give a clearing signal. Position alarm. An alarm TB on each position indicates to an operator when she should withdraw a plug for any reason except at the end of a connection, for example (1) when several operators answer an incoming call, (2) when the answering plug is inserted in the jack of a subscriber who has been called or has not initiated a call, and (3) when the calling plug is inserted in the jack of a calling subscriber. In example (1), the relay E of the first operator answering the call opens the circuit of relay V, but at other positions, relay V energizes over the sleeve of the answering plug and causes relay T to energize relay U, which disconnects relay V and causes the alarm TB to sound until the plug is withdrawn. Special operating features. A subscriber may clear his call before the operator answers (or after misdialling) by depressing his clearing key CK. If the wanted line is out of order, the operator inserts the calling plug in " out of order " jack OJ, thereby energizing relay W but not relay H. Relay W energizes relay R. and also relay . X which lights the # out of order " lamp OL and substitutes the code contacts FT for the'contacts TT so that when the pick-up relay N is energized, the word FAULTY is printed at the calling station. Dummy jacks DJ1, DJ2, representing a calling and called line, are provided for testing doubtful cords. If the number indicator is out of order, the operator may by depressing her Re-dial key ReDK effect the transmission of a code signal from ReDT requesting the calling subscriber to re-dial the wanted number. This is controlled by relay Y. Similarly by depressing key ReCK the operator may effect the transmission of a code signal from Re CT requesting the subscriber to recall the exchange. This is. controlled by relay YY. Connection through two exchanges. If the number displayed on the indicator shows that. a connection on another exchange is desired, the operator depresses her " trunking key " which holds the number indicator by maintaining the circuit of the impulse relay D. She inserts the calling plug CP in the jack of a. junction to the second exchange and, when that exchange replies, dials the wanted number on her dialling unit EDU. The relay Q prevents the transmission of the letters THRU from the first exchange to the calling line in such circumstances, but a similar signal is sent back from the second exchange when the connection is completed. Specification 355,788 is referred to.