329,366. Standard Telephones & Cables, Ltd., and Baker, J. H. E. Feb. 14, 1929. Automatic exchange systems.-In a by-path system, the switches used in the by-path circuits are of group selector type which move to a required group and then hunt in that group for a free by-path to the next switching stage. The switches in the speaking circuit are of the purely hunting type which hunt for a circuit selected from the by-path at the next switching stage. Each conversation switch is associated with a plurality of by-path circuits and in order to prevent cross connections a loop testing device is provided at each switching stage. The outlet groups from the by-path switches are of equal size, and if the number of by-path circuits in the group vary the unwanted terminal sets are permanently busied. The by-path switches have access to any free conversation switch associated therewith and the by-path test circuit may extend over parallel paths in all the conversation switches and be arranged to test non-selectable if all the associated conversation switches are busy. The general operation of the system is indicated in Fig. 1. A calling subscriber S is connected to a free by-path C1. Switch RM2 individual to C1 responds to the first digit and connects to a free by-path C2 having an individual switch RM2 that responds to the next digit. Meanwhile RM1 of C1 has found a free first selector which is connected over LF2, LF1 to the calling line and RM1 connected with C2 has found a free second selector S2. S1 hunts for S2 and in order to prevent cross connections the loop C1, C2, S2, S1, C1 is checked by M1. A similar loop is established to S3 and as the main circuit is completed the by-path is freed for further calls. RM2 of C3 responds to the tens and units digits and then steps home moving S3 in synchronism with itself, the bank connections of S3 being such that they are thus stepped onto the contact of the wanted line. Systems are also described in which he first two directively controlled conversation switches are backwardly hunting switches, in which provision is made for both incoming and outgoing junction calls and in which by-path switches are reached from more than one source (see Figs. 3 and 9). Connection to first by-path.-When the subscriber calls L (Fig. 2A) operates followed by ST and S in one or more by-paths (Fig. 3A) magnet RM1 drives its wipers rm11 - - rm104 in search of a free first group selector when T energizes over rm11 operating TT which releases T and connects magnet FM2 over rm13 and rm12 to drive FM2 in search of free first finder. When a free first finder is reached U and UU pull up stopping FM2 and enabling FM1 to hunt for the calling line. When the line is found, T reoperates over rm17 in series with CO releasing L, ST and S and operating K (Fig. 3A). Relay A pulls up over the calling loop energizing B which maintains UU and operated after S has released. Receipt of first digit train : testing of connection. Relay A responds to the first impulse train. At the first de-energization X operates and holds up when A again energizes completing a circuit for Y. Relay Y breaks the initial energizing circuit for X and when A releases the second time X de-energizes. When A pulls up Y de-energizes and the cycle is repeated X energizing on each odd impulse and de-energizing on each even impulse. As X energizes and de-energizes a circuit for RM2 is completed over rm24 and the wipers rm21 - - rm27 are moved over groups of contacts so that when the impulse train is completed rm24 is standing on the first terminal of the group dialled. C releases at the end of the train and E pulls up. RM2 now drives in search of a free line in the group selected when N operates over rm21 breaking the circuit RM2 and energizing H to extend the calling loop over rm26 and rm27 to the intermediate by-path. Relays A and B (Fig. 5) pull up relay B returning earth over rm21 to hold B (Fig. 3A). Magnet RM1 (Fig. 5) drives the switch in search of a free second selector and when a free circuit is reached K pulls up, operating BH and Q (Fig. 3A) and completing a circuit over rm102 and rm103 for SM1 which steps wiper sm11- - 14 in search of the selected switch. When the marked switch is reached a circuit is completed for relays M and N in series over rm22, (Fig. 3A), rm12 (Fig. 5), rm14 (Fig. 2A), and rm104 (Fig. 3A). M and N operate breaking the circuit of SM1 and G operates grounding the common start lead to MA (Fig. 8) which drives the common test circuit. When the test circuit reaches the calling by-paths MB operates and the test loop is extend by G over mm4. mm5 to MT. If the loop is complete MT pulls up operating TS releasing B and the first section of the by-path and enabling HS to operate over rm101, and extend the loop to the next by. path. If the connection is not true, MT will not operate and TF pulls up releasing G and the test equipment and stepping SM1 one step. TF releases and the original drive circuit for SM1 is established, enabling the hunt for the selected connection to be continued. Intermediate by-path. The calling loop is extended first over rm 26 and rm27 (Fig. 3A), and later over sm11 and sm12 Fig. 2A) to relay A, Fig. 5, and this relay responds to the next impulse train relays. X and Y operating as in the first selector to step RM2 to the required group and search for a free by-path to the. final selector is carried out as before. The final seelctor by-path hunts for a free connector and the loop over the main circuit is tested and established. Final by-path. Selection of wanted line; testing and ringing, metering. Relay A in the final by-path responds to the next impulse train, magnet RM2 stepping the switch wipers rm21 - - 24 over groups as marked by relay X as already described. At the end of the train E operates from rm23 and the next impulse train is operated by A direct to RM2 stepping the switch the required number ot steps. The selected final switch, which is standing off normal is driven home over rm19, rm103 and fm4. Relay L operates at the end of the last impulse train energizing V which allows Z to pull up. Z completes circuits from. RM2 and FM and these two magnets step together, Z and RM interacting, until RM is restored to normal. The lines are connected to the banks of FM in an inverse order so that the wipers now stand on the bank terminals of the line required. Z releases and V falls away slowly. During the slow release period V a testing circuit is established from fm3 over rm102 to relay H which pulls up if the line is free locking and holding relay V. Ringing is connected over relay F to the called line. If the previous by-paths have all released PF falls back so that when the called party replies F pulls up disconecting end releasing V. Relays BB and J operate over rm16 and rm17 switching the line through. Relay D energizes over the called loop releasing J and during the slow release period of J booster current is connected to the meter circuit to register the call. Line busy release. If the line tested is busy H does not energize but when V releases G pulls up from earth on the called subscribers test terminal (fm3, rm102). If PF has released denoting the completion of the talking circuit J pulls up over rm17 releasing the by-path and connecting AA in the calling loop. Busy tone, applied to AA from BT, is transmitted back to the calling subscriber. When the calling subscriber releases the connection, AA and J fall back releasing the switch train. Alternative, arrangement. In the alternative system shown in Fig. 3, the by-path switch C1 connected to S1 extends the call to by-path C2 which latter can also be taken by CII connected to S11. The arrangements in C2 that ensure that F2 extends the call to a selector which can extend the connection in the right direction and is also available for use from the calling loop are shown in Fig. 9. If Cl is the calling route relay J operates whilst JJ pulls up if the call matures from C11. Relays J and JJ connect different sets of terminals to test relay T. Groups of lines outgoing in the same direction are commoned. on the bank wiped by rm11and when the wipers of RM1 are being moved a circuit for relay T to stop the switch, is completed only when a free line marked on rm13 is connected through operated contacts of J or JJ to a group marked in rm11. Specification 295,421 is referred to.