231,237. Siemens Bros. & Co., Ltd., and Humphries, H. E. Dec. 28, 1923. Automatic exchange systems.-In a group-selector circuit employing only three control relays, an impulse relay, a change-over relay and a test and switching relay, the holding of the connection over preceding switches is effected before and after impulsing by the impulse relay and during impulsing by the change-over relay. This necessitates that the change-over relay, in addition to remaining energized during the reception of impulses and releasing at the end thereof owing to the continuous energization of the impulse relay, must also release if the impulse relay remains continuously de-energized owing to premature release by the calling subscriber. This may be effected without extra relays by making the maintenance of the circuit of the change-over relay dependent either upon the fact that the selector is in motion or upon the movement of an armature of the impulse relay between its front and back contacts as distinguished from its remaining steadily in either of these positions. According to one arrangement, Fig. 2, a winding of the change-over relay includes back and front make-before-break contacts of the vertical magnet. In the system shown in this Figure, the change-over relay also acts as a stepping relay to control the hunting operation. In a second arrangement, Fig. 3, the movement of an armature of the impulse relay between its back and front contacts alternately charges a condenser and discharges it through a winding of the change-over relay. In a third arrangement, a winding of the change-over relay is in series with the vertical magnet over a back contact of the impulse relay and is short-circuited over contacts of the magnet at the end of the stroke so that if the impulse relay remains de-energized due to premature release whilst impulsing, the changeover relay will remain short-circuited and after a pause will fall back and release the connection. In other arrangements the circuit for the changeover relay may include back and front makebefore-break contacts of the impulse relay, or contacts operated by the movement of the vertical detent of the selector. A more detailed description of the first two arrangements is given in the succeeding paragraphs.. Provision is made for transmitting a busy signal to the calling subscriber if the selector fails to find an idle trunk in the selected level. First arrangement, Fig. 2. The impulse relay A, energizing in a circuit including its winding 1, the a- and b-wires, winding 2, normal contacts k<1> and a winding of a dialling-tone transformer, grounds the incoming c-wire and completes a circuit over normal contacts k<2> for winding 3 of the change-over relay C. Relay C short-circuits its winding 2 to render it slow-releasing. Impulses are repeated to the vertical magnet VM over back contact a<1>, the incoming c-wire being now grounded over contact c<1>. During the vertical movement, winding 3 of relay C is intermittently energized over make-and-break contacts vm<1>, vm<2>, of the magnet VM but at the end of the digit relav C relapses and completes a circuit d<3>, c<3>, a<3>, k<3> for the rotary magnet RM. At the end of its stroke, contacts rm of the magnet connect the test relav D in series with winding 3 of relay C. but if the first line is busy relay D is shortcircuited over the grounded test wiper C‹ and relay C energizes partially to open its contact c<3>. The rotary magnet is de-energized to release relay C and the switch takes another step. When an idle trunk is found, relay C does not energize but relay D energizes to stop the rotation and to switch through the connection. relay A remaining energized for a short interval owing to the short-circuiting of its winding 3 to maintain the grounding of the c-wire until ground is sent back from the succeeding switch. The relapse of relay D at the end of the conversation completes the circuit of the release magnet MM. If all outgoing trunks are engaged, relay D energizes as for an idle line and completes a circuit over contact d<1>, wiper C‹, and the last test ,terminal, for winding 1 of relay C which energizes. Relay A is maintained energized over winding 1, wiper A<0>, the a- and b-wires, contact c<5>. winding 2, and contact a<2>, whilst busy tone is transmitted over wiper B‹ to the calling subscriber. When the subscriber hangs up, relay C relapses and the release of relay D restores the selector as before. Second arrangement. In the arrangement shown in Fig. 3, the current-feeding bridge is at the group selector, whilst idle outgoing trunks are marked by the connection of battery potential. The impulse relay A connects battery to the cwire through a resistance to hold up preceding switches and completes a circuit for winding 2 of the slow-release change-over relay C. During the reception of impulses, a condenser K is alternately charged over a back contact a<5> and discharged over the front contact through winding 1 of relay C. At the end of the impulses, relay C releases and completes an interrupter circuit for the rotary magnet RM. When an idle trunk is reached, relav D operates over winding 1, locks up over winding 2 and directly grounds the test wiper C‹ to mark the succeeding switch busy. Further impulses are repeated by relay A at contact a<3>. When the calling subscriber hangs up at the end of the conversation, relay D releases and release magnet MM energizes over back contact a<2>. If release occurs before dialling, the vertical magnet takes one step, the circuits of relay C are opened at a<5>, k<2> and the magnet MM operates as before. Similarly, if release occurs during impulsing, the continued opening of front ccntact a<5> releases relay C. Trouble signal. The continued lighting of a lamp in circuit with a back contact of the test relay and a front contact of the change-over relay may indicate a permanent loop.