222,246. Siemens Bros. & Co., Ltd., and Humphries, H. E. July 16, 1923. Automatic and semi-automatic exchange systems; metering ssytems.-In district office systems in which local calls are effected without using trunks to the main office, and in which the determination that a call is local occurs after two or more digits, a subscriber on calling gets access to a trunk to the main office and to a discriminating switch which acts as or controls a local group selector, and is released and made available for other calls as soon as it is determined that a call is not local. The switch responds to dialled impulses and is released after each digit until it is determined that the call is a local one. The switch may then hunt for a local selector in the level to which it was set by the final discriminating digit, or may be arranged to release after that digit and the next and act as a group selector on the following digit. In the forms in which the discriminating switch does not act as a selector it is made available for further calls as soon as it has started the hunting of a local selector. The discriminator may be modified in a number of ways for dealing with particular conditions of numbering, but in every case has a wiper cooperating with a row of terminals which are marked during each discriminating train of impulses in accordance with the value or values in local numbers of the digit being received. Control arrangements may be provided for busying 2nd line switches giving access only to repeaters with no idle trunks or no idle discriminators available to them and for preventing 1st line switches from starting if all the trunks in a division are busy. In case, however, there are idle trunks that cannot be associated with idle discriminators, the controlling circuits are modified so that calls, whether main office or local, can go over these trunks. The subscriber's meter is arranged to be operated once for local calls, but for distant calls may be put under control of main office apparatus which operates it more than once. The subscriber is prevented from getting dialling tone until he is associated with a discriminator, and from getting busv tone from the main office until it is certain that the call is not a local one. First system, Figs. 1 - - 8, in which the discriminating switch acts as a local selector. As soon as the repeater, Fig. 1, is picked up a switch DF starts up in search of an idle discriminator the finding of which starts a switch JF in search of a main office trunk. Repeater. Relay A is first energized over its winding 1 followed by relays V, E, and is then put under the control of the subscriber over its windings 2, 3. Magnet DD drives the switch DF under control of relay TD. When an idle discriminator is found relay TD short-circuits its winding 1, connects battery over a<2> to the b-wire to the discriminator, and switches in the driving magnet DJ of the switch JF. When an idle trunk is found, relay TJ short-circuits its winding 1 and switches the talking wires through to the main office so that dialling tone can be received by the subscriber. Impulses are repeated over the trunk at contact a" and to the discriminator at contact a<2>. The first relapse of relav A energizes relay B, which locks up, connects relay C to the a-wire to the discriminator and connects the b-wire to the impulse circuit independently of contact c<3>. If the call is external the discriminator on receiving the determining digit releases relay TD, which releases relay B by short-circuiting it and breaks the impulse circuit to the discriminator. If it is local the discriminator returns a momentary ground over the a-wire to energize relay C, which locks up, releases relay TJ, B, and extends the a- and b-wires so that the remaining digits are repeated over contact a<3> and the main office switches are released. Relay B is provided to keep the a-wire to central office open, to prevent supervisory signals from reaching the calling party until the route for the call has been determined. Discriminating seelctor. The switch shown in Fig. 3 provides for three-digit discrimination and is arranged for a district office serving lines of which the first three digits are 532 - - 538. Connection. is made to a manual operator if the first digit is O. The switch is of Strowger-type but has an extra wiper MW which partakes of the vertical movement only and co-operates with ten marking terminals MC. When the switch is seized, relay N energizes over the incoming c-wire in series with relav TD at the repeater which connects battery to the b-wire so that relays AA, D energize. Dialled impulses are repeated by relay AA to the vertical magnet VM. The change-over of off-normal springs k<3> make relay D dependent on a back contact of relav AA and energizes relay Q, k<3>, n<3>, t<1>, s<1>, r<1>, Q(1), r<2>, s<2>. Relay Q marks the proper terminal MC, in this case the fifth, by connecting it to relay EE. If now or at a later stage the wiper MW comes to rest on an unmarked terminal, the relapse of relay D at the end of the train releases relay N, which energizes the release magnet MM and, by opening the incoming c-wire, releases relay TD at the repeater, with the result explained in the description of the repeater. On the other hand, if wiper MW comes to rest on a marked terminal relay EE energizes and operates the release magnet but holds up relay N and remains locked upon the release of relay D. When the contact k<3> changes back to normal relay D reenergizes and releases relay EE so that relay R which has hitherto been short-circuited energizes s<2>, r<2>, Q(1), R(1), q<1>, n<4> to mark the terminal appropriate to the second digit, and removes a short-circuit from its winding 2. On the next change over of contacts k<3> at the beginning of the second digit, windings Q(1), R(1) are short-circuited and relay Q relapses but relay R holds up over its winding 2. When wiper MW reaches the marked terminal, relay EE energizes, and the release magnet operates as before. This time the release of relay EE allows relay S to energize s<2>, R(2), r<2>, S(1), and at the beginning of the third digit relay R is short-circuited but relay S holds up to mark the appropriate terminals, viz. 2 - - 8. This time, relay S being energized, the energization of relay EE connects ground to the incoming a-wire to operate the relay C at the repeater and release the main office switches. The relapse of relay D takes the ground off and energizes the rotary magnet RM in an interrupter circuit under control of test relay T so that the switch hunts in the selected level in the same wav as an ordinary group selector. Relay T on energizing, in addition to its ordinary functions, releases relay S. Calls for a manual operator. \Vhen the digit O is dialled, relay S energizes in series with relay EE over an extra winding 3 so that the switch hunts in the tenth level and a signal is sent to release the main office switches. Relay Q is held up in a circuit EE, s<5>, Q(2), q<2>. Modifications for two-digit discrimination. The switch shown in Fig. 3 may be modified by omitting contact r<3> and bridging the spring r<1> and its back contact. In this case, as soon as relay R energizes when the switch reaches normal after the first digit, relay Q is short-circuited and relav S energizes s<2>, R(2), r<2>, S(1), q<1>, n<4>. At the beginning of the second digit relay R is shortcircuited and the remaining operations are the same as after the third digit in the case just described. The Specification describes in detail a switch for two-digit discrimination in which the relay R is omitted. Relays Q, S have the same functions as in the arrangement shown in Fig. 3. Modifications for a sub-office serving groups with different second digits. (1) In the case where two groups beginning, for example 126 - - 129 and 131 - - 134, are served the switch shown in Fig. 3 is provided with an additional relay which energizes in series with relay EE when the second digit has the higher value and causes relay S to mark terminals 1 - - 4 instead of terminals 6 - - 9. (2) In case the office serves (say) lines 28100 - - 28899 and 29100 - - 29899, it is arranged that for one group the discriminator acts as a second selector and for the other as a third selector. To effect this a relay is provided which energizes in series with relay EE if the second digit is 9, prevents the release of the switch and energizes relay S so that a third selector is picked up in readiness for the third digit. In the case of the other group the extra relay has no effect. Numbers beginning 289 but not 299 may be used for outside calls. (3) In another case the local numbers are in two groups which have different second digits but the same third digit, and if the call is for one group the switch is given an extra vertical step after the end of the third digit to enable it to extend connections to this as well as to the other group. As shown, Fig. 5, a slow release relay L is provided in addition to the relays Q, R, S, and the marking is for lines beginning 253, 263. Calls for the former group and external calls operate as described in connection with Fig. 3, but if the second digit is 6, relay L energizes in series with relay EE, and locks up. Release of relay R at the beginning of the third digit puts relay L under the control of relay D, and when at the end of the digit relay D falls back, relay L is released. Before it falls back, however, it makes a circuit VM, l<2>, s<4>, n<2>, k<2>, d<2> t<5> so that the vertical magnet is given another step. Adding lines to a 1000-line sub-office which is already full, Fig. 7. Instead of changing from three to two-digit discrimination and adding a group selector stage, the first terminals in certain levels of a switch similar to that shown in Fig. 3 are connected to simple forwardly hunting switches which give access to connectors serving the additional line