371,861. Telegraph exchange systems. SIEMENS & HALSKE AKT.-GES., Siemensstadt, Berlin. Jan. 23, 1931, No. 2304. Convention date, Jan. 25, 1930. [Class 40 (iii).] A telegraph exchange system is so arranged that the connection between two subscribers cannot be disturbed by the simultaneous transmission of marking impulses in both directions. Fig. 1 shows a system in which the exchange operations are effected over an earth-return circuit while message signals pass over a metallic circuit. This system suffers from the above defect, and according to the invention (e.g. as shown in Fig. 4) transmission in one direction inhibits transmission in the other. The exchange may be of the kind described in Specifications 306,406, [Class 40 (iii), Telegraphs, Electric], and 349,124. Idle position, Fig. 1. At the exchange the armatures er1, sr1, er2, sr2, Fig. 1, which are respectively individual to the stations T1, T2, rest in the " false resting position z, which becomes the marking position when the lines are working. The windings of relay ER1 oppose one another, but winding 1 overpowers winding 2. Relay U1 is energized, and motor A1 is consequently out of action. Calling. Calling is effected over an earth circuit by reversal of key ET1, which causes winding 1 of input relay ER1 to lose current since it is now in series with rectifier Vt. Under control of winding 2, armature er1 passes to contact t which is the spacing contact for messages. Relay X1 is consequently energized and starts a preselector in the apparatus VS. This hunts for a group selector which, when found, energizes the sending-on winding SR1 so that armature sr1 moves to its t contact, cutting out the calling battery SB1 and cutting in the oppositely polarized message battery TB1. The direction of the current in relay ER1 is thus reversed, but winding 1 overpowers winding 2 owing to its larger number of turns, and keeps armature er1 on its t contact. Since the polarity of the battery has been reversed, rectifier Vt now permits the slow-release ralay VI to take current, relay U1 is cut out at v1 and motor A1 is started up. Dialling is effected by means of contact NS1 and operates in known manner to select station T2. Message circuit. During a message from T1 to T2 the armature sr1 rests on its spacing or t contact. Messages therefore go over a metallic circuit. Armature er1 responds to the transmitter S1 and rests on its t or spacing contact when S1 is closed, since winding 1 overpowers the opposed winding 2 ; but armature er1 moves to its marking contact z when S1 is opened, since winding 1 is then cut off. The movements of armature er1 control the sending- on relay SR2 and are thus passed on to station T2. Line balances N1, N2 are provided. Release. When the release key AT1 is opened for a sufficiently long time, slow-release relay X1 de-energizes and effects release of the exchange connection in known manner. Slowrelease relay V1 also releases and, through relay U1, stops the motor A1. Effect of two simultaneous marking signals. If while subscriber T1 is sending a mark (so that armature er1 is at z) subscriber T2 simultaneously sends a mark before armature sr2 has moved, then contact er2 also moves to its z contact. When, therefore, armatures sr2, sr1 move to their z contacts, current is cut off in the subscribers' lines, and the armatures fail to be restored to their spacing contacts t at the end of the impulses. The invention provides means to prevent this from happening. Thus, relay SRI, Fig. 2 (not shown) and Fig. 4, or relay ER1, Fig. 3 (not shown), may be provided with extra windings for this purpose. The winding SR<1>1 in series with fuse F, Fig. 4, is cut in by relay X and so long as armature er1 is on its marking contact z this winding prevents any marking signal sent by T2 from influencing relay ER1. Alternatively, Fig. 3 (not shown), relay ER1 may have an additional winding shunted upon battery SB1 and armature sr1, for the same purpose; or, Fig. 5 (not shown), winding 2 of relay ER1 may take the place of the additional winding. The additional winding may be dispensed with, its function being performed by one of the windings shown in Fig. 1. System in which calling circuit is metallic. Two opposite rectifiers Vt, Vs, Fig. 4, may be provided at the subscriber's station so that, in the idle position shown, current flows from battery SB1 through the metallic circuit a, b, relay ER1, and subscriber's motor-control relay U1; whereas when the current-direction is reversed by passage of armature sr1 from its z to its t contact, motor-control relay U1 is cut off by rectifier Vs and current then passes through the telegraph apparatus S1, M1, battery SB1 being permanently cut off at y by the slowrelease relay Y. When, therefore, a break-in signal applied to SR1 conspires with a marking signal applied to ER1, so that armature sr1 tends to move to its z contact while armature er1 is on its z contact, the additional winding SR1 overpowers winding SR1 and prevents the break-in signal from taking effect. In another arrangement, Fig. 5 (not shown), the armature y, Fig. 4, when attracted feeds winding 2 of relay ER1, and winding SR1 then becomes unnecessary. Safety contacts r (not shown) and x1 may be operated by the automatic exchange apparatus.