US2238235A - Automatic telephone system - Google Patents

Description

H. TATZL AUTOMATIC TELEPHONE SYSTEM April 15, 1941.

Filed Julye, 193s s sheets-sheet 1 m. QM/

INVENTOR. H A N S TA T Z ATTORNEY.

April 15, 1941., H. TATZL AUTOMATIC TELEPHONE SYSTEM A Filed July 9, 1938 3 Sheets-Sheet 2 April 15, 1941. H. TATZL AUTOMATIC TELEPHONE SYSTEM Filed July 9, 1938 3 Sheets-Sheet 5 ESN En w En INVENTORY HANS TATZL.

BY *7 i ATTORNEY.

P'atentecl `Apr. 15, 1941 UNITED" STATES PATENT OFFICE AUTOIWTIC TELEPHONE SYSTElVI Hans rllatzl, Berlin, Germany, assignor toSiemens & Halske Aktiengesellschaft, Wernerwerk, Siemensstadt, near Berlin, Germany Application July 9, 1938, Serial No. 218,370 In Germany `Iuly 14, 1937 2i) Claims.

ther it is also known to carry out' the stopping on the characterised speaking set by relays controlled over a particular wiper of the selector the speaking leads of the calling subscribers set. These known arrangements require a special wiper on the selector with free hunting for the stopping of the selector over which wiper the switch means for stopping the selector is controlled and furthermore in known arrangements with the use of speakingleads disturbing noises are produced by the extension over engaged sets.

The present invention concerns a special wiper" for stopping the selector in free selection on a characterised set and avoiding disturbances by the passage of the lines of a subscriber over an engaged connection which is attained according to the invention in that for the set on which the selector is to set itself in free hunting a ow of current is produced over the speaking leads bywhich the direction of the feeding current of an engaged set is reversed and in this way a relay situated between the speaking leads in the exchange which relay only energises in this definite direction, is excited on obtaining the set characterised by the current reversal and carries out the stopping of the selector.

Of particular importance is the arrangement according to the invention in cnice systems with local and exchange tramo in which after the characterising of the exchange selection a selector of the exchange line is set on the calling set and this selector is also set as line selector by the incoming exchange connections on the set desired by the exchange. In such systems a large number of conditions are to be fulfilled Awhich make necessary a control of the switch operations over the wipes of the selectors. In the arrangement according to the invention it is possible to fulll the particular conditions as for example enquiry, reversal, etc., also in systems in which the selector is equipped for internal connections and the exchange selector as up-and-around selector with three Wipers stepping over the contacts of the contact bank.

In Figs. l and 2 an embodiment of the invention is shown, illustrating an oice system with call-lnder and line selector for making internal connections on the exchange selector for making incoming and outgoing selections. The call finder, line selector and the exchange selector are considered as up-and-around selectors.

In Fig. 3 a group selector is also shown which is connected between the intermediate call finder and line selector in Fig. 1 `when a larger office system is concerned. If group selectors are present and if the number of neighbouring set subscribers which are authorised for exchange connections is greater than the capacity of an exchange selector a second exchange selector is connected parallel to the previous one.

Making of an exchange connection Should the subscriber Tnl in Fig. 1 wish to make an exchange connection the subscriber has iirst of all to lift the receiver, in this Way the relay R. is energised over: negative, winding I of relay R, contact It, loop of the set T'nl, contact 2t, positive. The relay R operates its contacts over contact 31'. The calling subscriber is characterised at the wiper c of the call finder AS. Over contact 4r the contact-row of the call iinder AS is characterised in which the calling set is included. Over contact 5r the following circuit is made: positive, resistance WI, contacts 51', Bt, lc, principal contact Bic, of the call iinder AS, relay RI, connecting point 5/6 contacts 911|, IIcZ,

principal contact Ilc of the line selector LW,

contact |21), winding III of the relay Al, negative. In the previous circuit the relays Rl and Al energise. The relay RI holds independently of contactiik over contact 3|rl. Through relay RI4 the vertical magnet H of the call nder AS receives current over: positive, contact l3c, Ilitl, I5l, IEu, shaft contact Ilw of the call nder AS, vertical magnet H, negative. The vertical magnet H lifts the call finder AS by one step. Through the vertical magnet H the contact I8. is also closed and with it relay TI over: positive, contacts |30, Ilih, winding I of relay Tl, negative. The relay TI opens the contact Ilitl so that the circuit of the vertical magnet H is interrupted again. The vertical magnet H opens the contact lith. Relay Tl releases and over contact Idil the circuit for the vertical magnet H and relay TI continues until the following circuit is closed again. The interplay between vertical magnet H and relay TI continues until the following circuit is over the contact level HS of the call finder AS: positive, contacts igt, 4r, topmost contact HS, contact 20H, winding II of relay U, negative. The relay U energises, opens the contact lBu and stops the call nder AS thereby on the contact row reached. By

' means of relay U the contact 2 lu is closed. There is now made the following circuit for the rotary magnet D of the call nder AS: positive, contacts I3c, Mci, 2| u, principal contact 22k of the call finder AS, rotary magnet D, negative. The

` rotary magnet D is energised and turns the call finder AS by one step in the contact bank which has been reached. Through the rotary magnet D the contact 23d is closed so the relay TI is energised over: positive, contacts 13o, 23d, winding I of relay Tl, negative. Relay TI opens at contact Illtl the circuit for the rotary magnet D. The rotary magnet D and the relay Tl act now as reciprocal interrupters until the following test circuit is made over the wiper c of the call finder AS: positive, contact 24M, winding I of relay C, winding I of relay U, wiper c of call iinder AS, contacts 31, winding II of relay R, contact 251' and 26t, relay T, negative. In the previous circuit the relay R holds, furthermore the relays C and T are energised and through the opposite winding I of relays U the relay U is brought to release. By means of relay C the Contact l3c is opened and the circuit of the rotary magnet D and the relay Tl opens. The call nder AS is stopped. Through relay C there follows the switching through at the Contact 2'Ic and 28e. Relay C interrupts the circuit for the relay Rl at the contact 'Ic. Relay Rl releases.

By the closing of contact 29el the calling subscriber is locked in known manner. In the circuit of the relay R the relay Al was energised. At the contact Sal, the short-circuit for the relay C2 is opened. Relay C2 energises.

For the relay C2 the following holding circuit is made: positive, winding II of relay Tl, connecting points 'I/8, winding II of relay C, contact 30o, connecting points 5/6, relay C2, contacts I l0c2, I I I x and I l2f, resistance W2, negative. In the previous circuit the relay Tl is not energised.

There exists now the following feeding circuit for the calling set Tnl: negative, winding I of relay AI, contacts 32ml, 49g, 3532i, 36pl, connecting points 2/l, contacts 3'Itl, 2'Ic, wiper a of the call nder AS, winding I of relay X, subscribers loops of the speaking set Tnl, winding II, of relay X, Wiper b of the call iinder AS, contacts 28C, 38H, connecting points, 3/4 contact 39pl, winding II of relay AI, contact 42ml, -positive.

When the subscriber now sends the rst impulse series out the interruptions by means of the dial are transferred to the set on the feeding bridge relay Al, Relay Al of itself transfers the impulses .to the vertical magnet I-I of the line selector LW` The circuit for the vertical magnet I-I of the line selector excites over: positive, contact dlp, winding I of relay V, contact 42c2, 431ml, 44u2, shaft contact 45u), contact 46m, 41p, vertical magnet H, negative. series the relay V is energised. At the end of the rst impulse series the relay V releases so relay U2 is energised over: positive, contacts dlp, p, principal contact SIIC, of the line selector LW, contact 52c2, winding I of relay U2, shift contact 45111, contact 41m, 47p, vertical magnet H, negative. In the above circuit the vertical magnet H is not operated- Through relay U2 the contacts 44122 is opened and the contact 53u2 closed. The subscriber now sends out the second impulse series for setting the line selector LW on a denite contact on the row of contacts that has been reached. The impulses are again undertaken by the relay Al and now transferred through the contact 43al on to the rotatry magnet D of the line selector LW. The circuit for the rotary magnet D is as follows: positive, contact dlp, winding I of relay V, contacts 4202, 43al, 53u2, rotary magnet D, negative. After the first rottary step of the line selector LW the contact 45u: is opened and so the circuit over the winding I of relay U2 is interrupted, The relay U2 holds over: positive, contact SM2, 55o, winding II of relay U2 negative. At the end of the impulse the relay V rst releases and puts back Through the impulse its contacts to their rest position. The contact 55o is opened, the relay U2 holds for a short time as a result of its release delay and the following circuit is thereby made: positive, contact E61/.2, 51e, windings I and II of relay P, contact 5802, relay PI, wiper c of line selector LW, conductor 59, contact llal, winding I of relay F connecting points l0/Il, top contact Blrk, relay HSA, negative. Line 59 is not multipled, Over the line 59 a relay F arranged on the line selector LW alone is energised. According to the previous description the subscriber of the calling set 'I'nl selects digits for setting the line selector LW The arrangement can also be so constituted that after lifting the line selector LW to a deiinite row of contacts the contact, on which the line selector 59 is connected, is automatically selected.

In the circuit of the wiper c of the line selector LW the relays Pl, F and HSII are energised. The relay PI closes the contact GZpl so Ithat the circuit over wiper c of the line selector LW is maintained independent of `the contacts 56u2, 51o and 58c2, The relay U2 restores. The relay PI opens 'the contacts 63pl, 64pl, 36pl, 3510I and closes the contacts Gpl, Spl. In the circuit over the wiper c, the relay is also energised so that now the following circuit is made: positive, contact 29ml, winding II of relays AI, line Ii'I, contact 63, wiper b of line selector LW, contacts 691, pl, S'Itl, 21e, wiper a of the call nder AS, winding I of relay X, subscribers loop of subscribers line Tnl, winding II of relay X, wiper b of the call nder AS, contacts 38H, 66pl, 10f, Wiper a, of

`the line selector LW, line II, relay Anl, negative, By means of the relay F the current direction over the speaking leads of the calling set 'Inl is thus reversed.

The relay Anl is, therefore, energised by sending out of the exchange characterising digit. The

Arelay Anl closes vthe contact 'Manl so that the following circuit is made: positive, contact 14am, resistance W3, contacts cl, 76713, 11b, winding I of relay R2, negative. The relay R3, situated over the contact 181V parallel to relay R2, is not energised in view of the previously connected resistance W3. By means of relay R2 the contact 8I1-2 is opened and the contacts 8!r2 and 19r2 are closed. By means of relay R2 the vertical magnet I-Il of the exchange selector AW is energised, and actually over: positive, contacts 8r2, $2122, 830s, NT2, 88ul, shaft contact 89u22, shaft contact Sllwl, contact 9Ir3, vertical magnet HI,-

negative. The vertical magnet HI causes the lifting of the selector AW on to the first contact row- By means of the vertical magnet HI, the contact 92M is closed so that the relay V2 is energised: positive, contact 92M, windings II and I of relay V2, contact 89s, resistance W4, negative. Relay V2 opens the Contact 82112 and interrupts thereby the circuit for the Vertical magnet HL The Vertical magnet HI sets back the contact 9271A into its rest position again so that the circuit for the relay V2 is interrupted. The relay V2 closes the contact 82112 so that the circuit for the vertical magnet HI is made again. The interplay between the vertical magnet HI and the relay V2 acting as, mutual interrupters continues until over the Contact level 93hs of the exchange selector AW the contact characterised by contact 9417134 of `the relay HS4 is energised and the relay Ul is operated. The circuit for the relay UI extends over: positive contact 94hs4, contact 95, bank contact 93hs, contact 96r2, windings I and II of relay UI, negative. The relay UI sh0rtl AW, negative.

circuits at contact 9lu| .the Winding I of relay Ul so that the relay Ul has a delayed release. Relay Ul interrupts at the contact 88m the circuit of the vertical magnet I-Il. Over the contact 90ul, the following circuit over rotary magnet Dl is made'. positive, contacts 8012, 82112, 8312s, 1922, 9011A, 85c3, B604, rest contact 8'l1cl of the wiper set over al/bl/cl of the exchange selector AW, rotary magnet Dl of the exchange selector The rotary magnet DI only connected the Wipers al,.bl and cl from contact to contact. By means of the rotary magnet Dl the contact lldl is closed so that the relay V2 is energised over: positive, contact 88dl, winding II and I of relay V2, contact 89s, resistance W4, negative The relay V2 energises and opens at the contact 82122 the circuit of the rotary magnet Dl. The rotary magnet Dl sets back the contact Blldl to its rest position. The relay V2 restores and there is closed again over contact 82122 the u circuit for the rotary magnet Dl. The interplay between rotary magnet Dl and vrelay V2 continues until the following test circuit is made, positive, contact 40ml, winding II of relay Al, contact 98, wiper b of line selector LW, contact 69j, tpl, connecting points 2/l, contacts 3'ltl, 2'lc, winding I of relay X, line 90, wiper al of the exchange selector AW, contacts |0003, ll12, rectifier GR, winding I of relay C4 wiper bl of the exchange selector AW, line |02, winding II of relay X, wiper b of the call nder AS, contacts 23C, 30H, connecting points 3/4, contact lpl, lili, wiper a of the line selector LW, line 1l, relay Anl; negative. The relay C4 of the lexchange selector energises in the previous circuit 5` as a result of the current reversal after the carrying out of the digit selection for outgoing exchange connections. The exchange selector AW is not stopped on an existing connection since at the existing connection an oppositely directed current flows over the speaking lead by which the relay C4 cannot be energised through the rectifier GR. No potential is situated on the moving wiper of the exchange selector AW so that a disturbing noise due to ruiming over an A,

engaged set is avoided.

The relay C4 lof the exchange selector AW energises and opens the contact 8604 and closes contact |0304. In this way the exchange selector AW is stopped and the winding II of relay C4 is short-circuited so that the relay C4 is made l/2 contacts pl, 691,, wiper b of the line selector LW, contact 60, line 6l, the winding II of the relay Al is short-circuited. The relay Al restores so that relay C2 is short-circuited over contact 90H. The relay C2 sets back its Contact into its rest position. Following circuit for the rotary magnet D of the line selector LW is made: positive, contacts 4|p, 5011, principal contact Elk, contacts |0902, l0tu2, rotary magnet D, negative. The rotary magnet D closes the contact |01, so that relay U2 is energised over: positive, 4lp, |0812, rest contact |09lc, cont-act |01d, winding II of relay U2. Relay U2 opens the contact lllluZ so that the circuit of the rotary magnet D is interrupted. Relay D opens the contact lld, so relay U2 is released, Over conta-ct l00u2 the circuit of the rotary magnet D is closed again. The interplay between the rotary magnet D and rel-ay U2 continues until the rest contacts 5llc and l091c are opened in the rest position of the line selector T, negative.

LW, By short-circuiting of the relay C2 over contact 9al the relay C2 releases, opens the holding circuit for the relay C over contact ll0c2. In this Way it releases the relay C. The rotary magnet D of Athe call find-er AS receives current over: positive, contacts l3c, l4tl, ll3rl, raised contact 221C, rotary magnet D, negative. By means of the rotary magnet D the contact 23d is closed and thereupon relay Tl energises. Relay Tl opens at contact |4t| the circuit of the rotary magnet D. The rotary magnet D causes the release of the relay Tl by opening the contact 23d. The interplay between relay T and rotary magnet D continues until the principal contact 22k is opened in .the rest position of the call finder AS.

By the release of the line selector LW Ithe circuit over wiper c of the line selector LW is also interrupted so that the relays Pl, F and I-IS4 travel back to their rest position. By the release of the line selector LW the circuit for the relay C4 and Anl is also interrupted, The switch means included in the local aggregate are thus set back again to their rest positions. By the release of the relay Anl the circuit of the relay R2 is interrupted at the contact 14am. The co-ntact 96M the circuit for the relay Ul is opened. The relays R2 and Ul set back the contacts to the rest position. If the locking of the calling set by the opening of the contact 29e is removed i the relay C3 of the selector AW energizes before the release of the relay C4 over positive, contact ll4c4, winding II of relay C3, contact llum, 266t1', Wiper cl of the exchange selector AW, line l I6, winding II of the relay R, Contact 251', relay Relay C3 holds the 4previous circuit independently of contact ||4c4 over contact ll'lc3, the relay C3 opens the contact ||0c3 and closes the contacts ll8c3 and ll9c3 so that the lines of the exchange selector AW are switched through. The following feeding circuit for the calling station Tnl now exists over: negative, winding I of rel-ay S, winding I of relay X2, contacts le, ll8c3, wiper al of the exchange selector AW, line 99, loop of the set Tcl, line |02, wiper bl of the selector AW, contact |l9c3, l2 le, winding II of relay X2, winding II of the relay S, positive, The relay S is energized. The relay X2 is considered as differential relay, By means of relay S the switching through of the exchange line over contact |225)` results. The relay S opens 89s and closes l23s. The relay VI is energized over: positive, contact |235 relay Vl, resist-ance W4, negative. By the energizing of the relay Vl the contact `l5c| is opened and the contact l24vl is closed. A further calling impulse for making anoutgoing exchange connection is transferred thereby onto the impulse relays R2 and R3 of -another exchange line. By means of relay Vl the contact lcl is opened and the contact l20vl is closed. Relay B is energized over: positive, contacts |2'l'11s, |281c2, |26v'l, relay B, negative. The relay B holds independently of the contacts l2'lvs, l282, and l20vl, over contact |2911.

The calling subscriber can now operate his dial for the purpose of setting the selectors which are reached over the exchange line AL and in this way causes the feeding bridge relay S to release impulsively. The impulses are transferred through contact |225l on to the exchange line AL. By relay Vl over contact lvl the circuit for the relay V2 is prepared. When therefore the impulses are sent out, by the first release of the relay S the short-circuit for the relay V2 is removed over contact l23s and the relay V2 is energized. The relay V2 remains energized during the impulse series and short-circuits the windings III and IV of the transformer Ue over contact I3Ic2.

If a larger system is concerned a group selector GW is inserted between the call finder and the line selector LW in Fig. 1 according to Fig. 3. The connections result in that the connecting point I in Fig. 1 is connected to the connecting point l in Fig. 3, the connecting point 2 in Fig. l to connecting point 2 in Fig. 3, the connecting point 3 in Fig. 1 to connecting point 3 in Fig. 3, the connecting point 4 in Fig. 1 to connecting point 4 in Fig. 3, the connecting point 5 in Fig. 1 to connecting point 5 in Fig. 3, the connecting point 6 in Fig. 1 to connecting point 6 in Fig. 3, the connecting point 'l in Fig. 1 to connecting point 'I in Fig. 3, the connecting point Il in Fig. 1 to connecting point ll in Fig. 3. The connection between l0 and Il in Fig. 1 is interrupted. The relay Anl which is connected to the line selector LW is connected to the group selector GW in Fig. 3.

When in a system with group selectors the calling subscriber wishes to make a connection with an exchange line then after lifting the receiver and switching the call finder AS of the group selector GW on a desired contact row the group selector GW can be reached from a dennite group of subscribers and the dividing of the subscribers results in the first place in a group of subscribers which are connected at the wiper cl/bl/cl of the exchange selector AW (Fig. 2) and are reached over the contact series l, 3, 5, etc., of the group selector, while the other groups of subscribers are connected to the wipers a2/b2/c2 of the exchange selector AW and are reached over the directed contact row 2, 4, 6, etc., of the group selector. When the subscriber who is connected over the wipers al /bl/cl of the exchange selector AW wishes to make an exchange connection then after the setting of the calling and operating of the relay C in Fig. 1, the following circuit is made: positive, winding II of relay TI, connecting point 'I, winding II of relay C, contact 35C, connecting point 5, in Figs. l and 2;.contact l35a2, 01T normal contact I3'Ik of the group selector GW', winding III of relay A2, negative. In the previously mentioned circuit the relay C in Fig. 1 holds, furthermore the relays CI and Al in Fig. 3 are energized. The relay TI does not operate. The calling subcriber receives feeding current over: rest contact l33102, |391D2, and windings I and II of relay A2. The relay A2 opens the short-circuit for relay C2 at the contact If36a2. The exchange characterizing is done by means of the relay A2 and transferred to the vertical magnet of the group selector GW which is not shown. After the first stepping the off-normal contact |3176 is opened, relay C in Fig. 1 and relay Cl in Fig. 3 are energized over contact lilcl. After the lifting of the group selector GW onto a denite contact row the group selector GW rotates in known manner. When the contact is reached on which the relay Anl is connected relays P2 and Bl in Fig. 3 are energized over: positive, contacts I4lcl, windings II and I of relay P2, wiper c of the group selector GW, line I33, contacts lSfIbl, l35a1il, winding I of relay BI, negative. Relay P2 operates. Over the wiper c of the group selector GW in Fig. 3 the following circuit is now made: positive, winding( II of relay Tl, connecting point l in Figs. 1 and 3, contact l32p2, winding I of relay P2, wiper c of group selector GW, line |33, relay Anl, con= tact l43bl, l44anl, resistance W5, negative. Relay Anl actually has the contact I35anl opened and thus disconnects the winding I of the relay Bl, the relay BI holds over: positive, winding 1I of relay BI, contact lllabl, wiper a of the group selector GW, contact M6122, connecting point I in Figs. 3 and 1, contact 3'Itl, 21o, wiper a of the call finder AS, winding I of relay X, loop of the set Tril, winding I of relay X, Wiper b of call nder AS, contact 28e, 33H, connecting point 3 in Figs. 1 and 3, contact I4'I1D2, wiper b of group selector GW, contact l48abl, resistance W6, negative. 'Ihe current passes over the previously mentioned circuit over the speaking leads in a reverse direction to the feeding current of the existing connection. By means of this current reversal over the speaking leads the calling subscriber who has selected the exchange characterizing digit is repeatedly characterized. The contact l49anl, is closed by means of the relay Anl Over the bank contact ISGhS of the group selector GW, positive potential is applied to the connecting point l I in Figs. 3 and l so that over the direction contact Elric of the call finder AS the contact level relay H84 is energized by means of which it is characterized that the calling subscriber is connected in the fourth contact row of the call nder AS and of the selector AW wipers al/bl/cl.

By means of relay Anl in Fig. 3 the contact Mani in Fig. 2 is closed, thus relay R2 energizes. Relay R2 brings about in the previously described rnanner rst of all the operation of the vertical magnet Hl of the exchange selector (wiper set al to cl). Next the rotary magnet DI is operated for setting on the contact of the contact row which has been reached. When the exchange selector AW reaches the contact of the calling set relay C4 is energized by the current reversal over the speaking leads. There follows in the same way as has already been described the stopping oi the selector AW, switching through etc. By transitory earthing of the line 99 after the operation by the relay C4 of the contact |6404, relay BI in Fig. 3 is short-circuited, the relay BI releases and opens the circuit for the relay P2 at contact l43bl. The relay P2 restores and opens the contact I5Ip2 the holding circuit for the relay C in Fig. 1. Relay C restores and effects the release of the call nder AS and over the Wiper cl of the exchange selector AW the locking circuit becomes effective.

By the release of relay P2 in Fig. 3 the group selector is set back into its rest position in known manner.

The stopping of the exchange selector AW on a calling subscriber who wishes to make an exchange connection results therefore, as follows from the above, by the current reversal over the speaking leads. However, by the reversal of the exchange selector AW after the making of a homing enquiry connection Ifrom the calling subscriber to the called subscriber the characterization of the called subscriber also results by the current reversal over the speaking leads.

If a connection exists between the set Tnl and the exchange line AL, the differential relay X2 is energized by the transitory operation of the key Tal. Through contact I55cv2, relay Q is energized over: positive, contact IZ'IUS, l55r2, 2h32) winding I of relay Q, thermal relay Thi, negative. Relay Q opens the short-circuit over the winding II of relay Q by means of its winding 1 of relay YI, contact |59as, negativel Relay ZI operates, opens the contact |5'Iz| and closes the contact I6Ixl. There exists now the following circuit for the relay ZI: positive, contact |29b, I5MJI, IyI, winding I of relay ZI, contact IGIZI, winding I1 of relay ZI, contact |59as, negative. The contact IIiZzI and |632| are opened by means of relay ZI and contacts Hillel and |85z| are closed. There follows the switching of the exchange line AL onto the en- A,

quiry line IBG/|61. Over Contact I68e| and resistance WI the exchange connection is held. By switching on to enquiry line |65/ |67 the loop is made. In this way the call relay, not shown, of the enquiry line IGS/|61 is energized and thus brings about the setting of the call nder AS of the enquiry line IGS/|61. After the setting of the call iinder the impulse series for setting the selector upon the enquiry path is sent out from the calling subscriber. are taken up b-y relay S and transferred over the contact |223 as to the enquiry line IE6/|61.

1f group and two exchange selectors connected in parallel are used in this system the rst im- The impulses pulse series by means of which the setting of the group selector results also transfers to the discriminating device consisting of relays 1, 1I, III and IV (Fig. 2). By means of this discriminating device it is characterized Whether the connecting set calling in an enquiry is in an even l,

or odd contact row of the group selector and thus is connected at the contacts of the wipers a2/b2/c2 of the AI/bI/cI/ of the exchange selector AW.

rl'he setting of the group selector results in the manner already described.

The series of discriminating relays consisting of relays I to IV is also controlled by the first impulse series for setting the group selector GW of the enquiry path. The iirst release of the relay S by the impulse series brings up the relay I over: positive, contacts |9221, |93s, |84IV, |9511, relay I, negative. Furthermore, the relay III is `energized over: positive, contacts |92zI, |368, IQ'IIV, relay I1, negative. The relay III operates independently of the contacts |965 and IQ'IIV over contact IBBIII. By means of relay I the contact |991 is closed so that the relay II is energized over: positive, contacts IQZZI, |991, relay II contact |9511, relay I, negative. The relay II opens contact |9511 and closes contact N011. At this point of time contact |938 is opened again and Contact 2|J|s closed. The relays I and II hold over: positive, contact ISZzI, |991, relay II, contact ISIIIV, 20|s, relay I, negative. ing circuit: positive, contacts I92z| relay II, contact 20H11, relay II, negative. When the second release of relay S occurs thecontact 20|s is opened and the contact I93s closed. The circuit for relay I is thus interrupted, relay I restores and opens the contact |991, so that the relay II also sets back its contacts into the rest position. Only relay III of the series is now energized. At thevend of the second interruption the contact |933 is opened and contact 2|||s For the relay 1I there exists the followclosed, but however, an energizing of relays I or II does not come about since contact |991 is opened. When the third interruption of the relays S comes about by means of the third impulse then by the release of the relay S over contact |538 relay I is energized again and by closing its contact |991 operates the relay II. As follows from the above the relays I, 1I, 111 are energized by odd impulses while only relay III is energized by even impulses. If at the end of the rst impulse series the relay V2 restores, the relay IV is energized over, positive, contacts lvl, 2ll2r2, 203mm 204172, 205111, relay IV, negative. The relay 1V holds over: positive, contacts |92zl, ZEIISIV, relay IV, negative. Relay IV opens the contact |941V and closes contact ZB'IIV so that a further influencing of relays I and II by means of the following impulse is pre- Vented.

1f a subscriber is concerned who is connected to an odd contact row of the group selector GW, the relays I and I1 are energized and characterized so that the subscriber called in enquiry is connected on the wipers al to CI of the exchange selector AW.` By operation of the relays I and II contact 2081 is opened and contact 2091 is closed. Further the Contact 2|01V is also closed.

Over the closed Contact 2691 the energizing of the relay R2 is prepared and thus alsothe setting of the exchange selector AW and the set of Wipers al/bI/.CL If, however, a called subscriber is concerned who is connected in an even contact row, the relays I and II are rested so that over Contact 2081 the energization of the relay R3 and thus the setting of the selector with its wipers a2/b2/c2 is prepared.

By the setting of the line selector LW on the speaking set over the wiper of the line selector LW only the relay P is energized. The relay P operated by the free set locks the switch by closing the contact |5829. Over the 5-second switch 5sac, contacts llu, |102, I'IIp, relay M, negative, the relay M is energized at 5 second intervals and sets calling current on the speaking lead by opening contact |'I2m and closing contact |3m. The calling circuit extends from the calling current source R., contact |13m, winding I of relay Y, winding I of relay X, contacts IMp, tpl, |151, wiper a of the line selector LW, bell of the set T112, wiper b` of the line switch LW, contacts I'If, lpl, I'I'Ip, winding II of relay X, contact Ham, positive. If the subscriber on the set TnZ, replies the relay Y is energized in the calling paths. The relay Y closes the circuit for the relay Z over: positive, winding 1 of relay Z, contacts IIy, IBDaI, negative. Relay Z opens at contact Illia the circuit for the relay M. The set T1L2 receives the feeding current circuit over contact I'I2nI.

in the line selector LW |8|2 and |822. The switching through of the selector LW is maintained independently of contact Iy over contact |832.

Should the subscriber of the set Tn2 calling in enquiry wish to take up a conversation with 4the exchange then the subscriber of the set T112 By means of the relay Z there follows the switching through extends over positive, winding III of relay Y, winding II of relay V, contact ISIr, |6811, resistance W'I, negative. Relay XI holds indeendently of contact I84x over contact Ii. By opening of contact 0x the winding II of relay F is connected. The relay F does not energize. Later, however, should the subscribers set T112 release the key Ta2 the differential relay X restores. At the contact I8'Irc, the circuit for the relay V is interrupted. This delay relay V holds the contact in its working position for an additional short time. During this time the upper speaking lead is interrupted at the contacts 48o, 35ml and ISIx and the relay Q of the exchange line thus releases.

The relay UM is energized by means of relay Q: positive, contacts 2| lao, 2 I2q, 2I3yl, winding I of relay UM, negative. The relay UM operates and closes the contacts 261mm. The following circuit is made: negative, resistance W8, contact 26mm, line |61, wiper b of the call nder AS, Contact 28o, 38H, connecting point 3 in Figs. 1 and 3, contact I4Ip2, wiper b of the group selector C-W, connecting point 4 in Figs. 3 and l, contact 39pI, winding II of relay AI, winding II of relay F, positive. In this circuit the relay F energizes. There exists nOw the following feeding circuit for the set T112: negative, contact IIZm, winding I of relay Y, winding I of relay X, contact I'I4p, BSpI, 69j, wiper b of line selector LW loop of the set T112, wiper a of the line selector LW, contact 7Bf, 64M, I'I'Ip, winding II of relay X, contact |8912 positive. Over: positive, contact 268m, 2I8f banlk contact ack of the line selectors LW, relay HSA, negative, the relay I-ISII is energized and characterizes the called set which is connected in the fourth contact row of the line selector LW and also in the fourth contact row of the exchange selector AW.

By the energizing of the relay UM, since contact 2|4q is open, the circuit for the relay C3 is interrupted at contact IISum. Relay C` opens, at contact |I8c3 and I I9c3, the circuit for the relay S. Relay S restores. The relay UM now holds over: positive, contact |9221, ISGS, 2|5IV, 2|6um, winding II of relay Um, common relay AB, negative. Common relay AB is energized and opens contact 2| lab the energizing circuit of the windings I of all relays UM.

In this way at no other exchange line can the relay UM be energized and thus no other relay F on another line selector can come to operate by a reversing impulse and undertake the characterizing for the set by current reversals.

The relay AB closes the contact 2I1ab so that the winding I of the relay F is short-circuited, in this way a current reversal is prevented from occurring in a line selector after the characterizing digit; selection so that the controlled exchange selector AW can in no case be set on another subscriber than the one the characterizing digit has selected for the outgoing exchange connections.

If group selectors are used in the system the contact 2mal) is opened by the relay AB in Figl 3 and the contact 2|9ab is closed. When now another subscriber stops the group selector by the exchange characterizing digits, the relay Anl is operated. Over contact 2I9ab and 226mm, however, the relay ABI is energized and in this way the contacts |45ab| and |48ab| are opened and contacts 22Iab| and 222ab| closed. Thus the current reversal is avoided so that the reversing exchange selector AW in Fig. 2 can in no case switch this on a calling subscriber who has selected the characterizing digit for outgoing exchange selections. In the group selector according to Fig. 3, however, the relay BI is maintained energized in spite of the current reversal. When, therefore, after the reversal of the exchange selector in Fig. 2 the common relay ABI restores, contact IIQdoI in Fig, 3 is also opened and thus relay ABI is released. Relay ABI has put back the contacts in their rest position and there is then made possible a testing of an exchange selector on the subscriber who has selected the characterizing digit for outgoing connections.

By the release of the relay S after the coming into effect of reversing impulses relay VI is shortcircuited over Contact |398, relay VI sets back the contacts in their rest position. By the opening of the contact IEIIUI the relays YI and ZI are brought to release.

After the release of the relay C3 the rotary magnet DI of the exchange selector AW receives current over: positive, off-normal contact 223lcI, contacts 82122, 2321s, 8|r2, 85e, 85c3, 8404, principal contact 'IkI, rotary magnet DI, negative. The rotary magnet DI operates, steps the arms AI/b-I/cI of the exchange selector one step forward and closes contact dl. In this way relay V2 is energized. Relay V2 opens the Contact 881.22 so that the circuit of the rotary magnet DI is interrupted. The interplay between rotary magnet DI and relay V2 which interrupt each other follows until the exchange selector AW has reached its rest position in which the principal contacts 223m and 8'IkI are opened. If the selector is with its wiper aI /bI /cI in their rest position then the shaft contacts are also set back to their rest position.

In the following it should bo assumed that the subscriber calling in enquiry is a subscriber who is reached over an odd Contact row of the group selector GW. In this case the circuit for relay R2 is prepared over contact 2891. Vv'hen, however, the exchange selector AW has reached the rest position the following circuit for the relay R2 is made: positive, contacts |9224, |96s, 2|5IV, 2 IGum, ZIIJIV, winding II of relay R2, shaft contacts 89u22, .'ilwI, contact SITS, vertical magnet HI, negative. In the above circuit the relay R2 is energized. The following circuit for the vertical magnet HI is now made: positive, contact 8012, 82112, 830s, T912 88M, Contact 9910, SUwI. contact BITS, vertical magnet HI, negative. The vertical magnet I-II works together with relay V2 (over contact 92M) as an interrupter until the following circuit is made by contact characterizing the relay HSII associated with the bank contact of the relay UI positive, contact 94h54, bank contact 93m, contact 9612, windings I and II of relay UI, negative. Relay UI closes contact 9'IuI so that the relay UI has a delayed release. At contact 88M the circuit of the rotary magnet HI is interrupted. By means of relay UI the contact 98u| is closed so that the rotary magnet DI now works together with the relay V2 as mutual interrupters until the following circuit for the relay C4 of the exchange selector AW is made: positive, contact |8912 winding II of relay X, contact I'I'Ip, iflpl, 7Bf, wiper a of line selector which is set on the called set T112, line 224, wiper al of the exchange selector AW, contact Illlc3, I r2, rectier GR, winding I of relay C4, wiper 4bl of the exchange selector AW, line 225, wiper b of line selector LW, contact Gef, GSpI, |7412, winding I of relay X, winding I of relay Y, contact II2m, negative. In the preceding circuit the relay C4 operates, so that by the opening of the contact 8604 the rotary magnet DI is stopped. The relay CII effects repeatedly the transitory earthing of the lead 224 vso that the relay Y in the line selector is short-circuited and released. Relay Y interrupts the circuit over winding I of the relay Z by opening the contact |19y. Relay Z still holds over winding II. There however follows the short-circuit of the relay C2 over contacts 226e and 2271;. Relay C2 restores and brings about in the described manner the release of the line selector LW of the group selector GW and of the call finder AS which has set itself on the enquiring line. After theV release of the relay P' in the line selector LW the locking circuit over wiper CI of the exchange selector LW is made, in which the relay C3 is energized. The relay C3 causes the switching through of the exchange lead at the contacts I I 8c3 and ||9c3. The relay S is energized again and over contact |23s the relay VI. At the contact |968 the circuit of the relay UM and the circuit of the common relay AB is brought to release. Thus the selector called in enquiry is connected by the reversal of the exchange selector AW with the exchange lead AL.

The series of leads I, II, III and IV is also used to characterize the exchange selector over which the subscriber desired by the exchange is obtained by incoming exchange connections. B-y incoming exchange calls the relay AR is energized over the rest contact 228111. Over positive, resistance W9, contact 229m" winding I of relay H3, negative, the relay H3 is energized. Relay H3 holds over: positive, contacts |2'|11s, |2B2, |2511I, 284713, winding II of relay H3, negative. At the same time the calling lamp ALI is lit up over: positive, contacts 236m, 23H1, lamp ALI, negative.

The operator takes up the receiver and operates the exchange key connected with the calling exchange line. The relay AS is energized. The relay AS holds over contact 233cm. By the opening of contacts 243as and 235as and the closing of contacts 23Gas and 23'Ias the apparatus BA is connected over contacts 238113 and 239113 to the calling exchange line. The operator can discover the desired local subscriber from the calling subscriber. The operator operates the connecting key ZIIIIVT. The relay VS is energized over contact 24hrs. Relay VS holds independently of the key VT over contact 242115'. Relay VS opens contact 238115' and 2350s and closes the contacts 243118 and 21mm. Over the contact 245as and 246as the apparatus BA is connected to the connection side of the exchange line and energizes the relay S. The relay S brings about the energizing of the relay VI by closing contact |233. By opening contact |2511I and closing contact I2Ii11| the holding circuit of the relay H3 is interrupted and the relay B is energized. Relay B holds over contact I29l1. The operator now sends the rst impulse series for the desired subscriber. The first impulse series is taken up by the relay S and transferred to the series of relays I, II, III, IV and the relay I is energized by the first interruption over positive, contacts 241118, |935, IMIV, |9511, relay I,` negative. Relay I closes the contact |991 so that the relay II is also operated. After the release of relays S the holding circuit for relay I is made over: contact Is so that relay II is maintained energized over contact |991. If by the second interruption the relay S is brought to release the holding circuit for the relay I and thus for the relay II is removed. The relays I and II remain in theirrest position for the even impulse series. At the end of the first impulse series the relays I, II and III are energized by odd impulses While by Cil even impulse series the relays I and II are in their rest positions.

The next impulseseries is now transferred by an odd rst impulse series over: positive, contacts 241215, |963, 2|5IV, 24mm, 88ul, shaft contact 891112, Illwll, contact M13, vertical magnet HI. negative, to the vertical magnet HI. The wipers cI/hI/cI of the exchange selector AW are lifted to a definite contact row. If at the end of the second impulse series the relay V2 restores, the relay UI is energized over: positive, off-normal contact 223m, contacts 82111, 248118, shaft contact 2501112, 25|wI, winding II of relay UI, negative. The contact Etui is opened and the contact 98u| is closed. The next impulse series is transferred to the rotary magnet DI. After the first ten steps the circuit for the winding III of the relay UI is indeed opened at shaft contact 25IwI, the relay UI is maintained energized, however, over: positive, principal contacts 223Vcl, 252112, 255ml, Winding III of relay UI, negative, during the last impulse series. At the end of the last impulse series the relay V2 first releases and relay E is energized over: positive, principal contact 223lcI, Contact 82112, 248115, 286111I, 259m, winding I of relay E, negative. Parallel to' relay El the relay H3 is energized. The relays E and H3 hold independently of the contact 259m over contact 258e. A short while after the release of the relay V2 the relay VI also releases. Over positive, contact 25er-s, 255h3, winding I and II of relay C3, contact IIBum, 265151, wiper cI of the exchange selector AW, line I It, Winding II of relay R, contact 2te, relay R, negative, a test circuit is made in which the relays C3, R and T operate by a free subscribers set. Relay C3 closes the contacts I |3113, I |903, relay T interrupts the calling circuit of the speaking set at contacts It and 2t. Relays R and T and C3 are maintained energized independently of contact Zt over contact 25T,

By means of relay C3 of the exchange selector AW the call circuit is closed over the calling current source, winding II of the relay E1, contact 255e, H803, Wiper al of the exchange selector AW, line 99, bell of the set Tnl, line |02, Wiper bl, of the exchange selector AW, contact |I9c3, 221e, positive. When the subscriber replies the reverse winding II of relay E` becomes eifectivein the calling circuit. Relay E restores. At the contact 258e the hold circuit for the winding I of the relay E is interrupted, since the relay UI is already released after the release of the relay V2 and the holding circuit is opened at the contact 25Std. The operator hangs up the receiver, thereby interrupts the holding circuit for the relays AS and VS, the exchange line being switched through at the contacts 234118/235015. By the release of the relay E the feeding circuit for the calling set now extends over the relay S so that relay S remains energized.

The circuit for the relay R3 is closed by even impulse series over: positive, contacts 282115, 28H, 2BIIIV, winding I of relay R3, negative, when at the end of the first the relay V 2 has released and relay IV has been energized. By means of relay R3 the contact 9Ir3 is opened and the contact 2%13 is closed. The next impulse series is transferred to the vertical magnet H2 and thus lifts the wipers a2/b2/c2 of the exchange selector AW to a definite contact row. Ofi-normal contacts 285lc2 is closed so that the next impulse series effects the rotary magnet D2.

The end of the connection and the hanging up over contact 89s so that this relay also sets back its contacts to their rest position. Over positive, contact |2803, |282, |25vl, 260e, 2Mb, winding I of relay Q, relay Rhl, negative, the relay Thl is energized after a short interval. The relay Thl by its operation closes its contact 262ml so that the relay B is short-circuited and brought to release. By the release of relay S, the relay V2 is transitorily energized over contact 89s before the release of the relay VI and thus makes the circuit for the relay TR over: positive, contact 26317, 264m, 265122, relay TR, negative. The relay TR opens the contact 26612" so that the relay C2 restores. Relay C3 closes the contact 8503 and the rotary magnet DI and the relay V2 act as mutual interrupters until the selector AW has reached its rest position in which the oit-normal contact 223k| is open.

The selectors which have been used are thus set back in their rest positions.

What is claimed is:

1. In a switching system, a relay, a plurality of lines each having two conductors, means for connecting a potential across the conductors of any one of said lines, a switch operative responsive to the connection of said potential across said one line to connect said relay across the conductors of said lines successively, said potential efective to operate said relay responsive to the connection of the relay across the conductors of said one line, and means including said relay for terminating the operation of said switch responsive to said operation of the relay.

2. In a switching system, a plurality of lines each having two conductors, means for connecting diierent potentials across the conductors of different ones of said lines, a switch, a testing device in said switch, means for operating said switch to connect said device across the conductors of said lines successively, said device responsive to only a predetermined one of said diierent potentials, and means controlled by said device for halting operation of said switch responsive to the operation of the device.

3. In a switching system, a plurality of lines each having two conductors, means for connecting a potential of predetermined polarity across the conductors of diierent ones of said lines at diiferent times, a plurality of switches, means for operating a switch to hunt for and extend a connection to each of said lines when said potential is connected thereacross, a polarized relay in each switch connected across the conductors of said lines successively during the hunting of that switch for a line having said potential connected thereacross, said relay operated responsive to the connection of same across the conductors of the line having the potential connected thereacross and effective to halt the operation of said switch, and means including said relay for then disconnecting said potential from the line and connecting thereto a potential of the opposite polarity, thereby to prevent subsequently operated ones of said switches from being halted on that line.

4. In a switching system, a plurality of lines each having two conductors, a potential of predetermined polarity connected across each of said lines, means operated to disconnect said potential from any one of said lines and to connect thereacross potential of the opposite polarity, a switch then operative to hunt for said one line, a polarized device connected across the conductors of each of said lines in turn during the hunting of said switch, said device responsive only to said potential of the oppositepolarity and effective to cause said switch to seize said one line.

5. A switching system as claimed in claim 4, wherein said device comprises a bridge containing a relay and a rectifier connected in series.

6. In a switching system, a relay, a plurality of lines each having two conductors, a potential of predetermined polarity normally connected across each of said lines, means operated to disconnect said potential from any one of said lines and to connect thereacross a potential of the opposite polarity, a switch operative responsive to said operation of said last means to connect said relay across the conductors of said lines successively, said potential of the opposite polarity effectve to operate said relay responsive to the connection thereof across the conductors of said one line, and means including said relay for halting the operation of said switch responsive to said operation of the relay.

7. In a switching system, a relay, a plurality of lines each having two conductors, a potential of predetermined polarity normally connected across each of said lines, means operated to disconnect said potential from any one of said lines and to connect thereacross a potential of the opposite polarity, a switch operative responsive to said operation of said last means to connect said relay across the conductors of said lines successively, said potential of the opposite polarity effective to operate said relay responsive to the connection thereof across the conductors of said one line, and means including said relay for restoring the potential across said one line to normal responsive to said operation of the relay.

8. In a telephone system, a primary speech path, a plurality of secondary speech paths, means for impressing a potential upon any one of said secondary speech paths, a switch terminating said primary speech path thereafter operative to search for said one secondary speech path and to connect thereto said primary speech path, a discriminating device in said switch connected to each secondary speech path in turn during said hunting of said switch, said potential eiective to operate said device responsive to the connection of said device to said one speech path, and means controlled by said device for halting the operation of said switch responsive to said operation of the device.

9. In a telephone system, a plurality of speech paths, a potential of predetermined polarity impressed on each one of said speech paths engaged in a call, means operated to initiate a call over any other one of said speech paths, a potential of the opposite polarity impressed upon said last speech path responsive to said operation of said means, a switch operative to search for said last speech path to extend a connection therefrom, a discriminating device in said switch connected to said plurality of speech paths successively during said search and responsive only to said potential of the opposite polarity to cause said switch to seize said last speech path.

10. In a telephone system, a primary speech path, a plurality of secondary speech paths, a potential of predetermined polarity normally impressed upon each of said secondary paths, means operated to remove said potential from any one of said secondary paths and to impress thereupon a potential of the opposite polarity, a switch terminating said primary path thereafter operative to search for said one secondary path and to connect thereto said primary path, a relay in said switch connected to said secondary paths successively during said hunting of the switch, said potential of the opposite polarity only effective to operate said relay responsive to the connection of said relay to said one secondary path, and means including said relay for terminating the operation of said switch responsive to said operation of the relay.

11. A telephone system as claimed in claim 10, wherein the operation of said switch is initiated by said operation of said rst means.

12. In a telephone system, a primary speech path, a plurality of secondary speech paths, a potential of predetermined polarity normally impressed upon each of said secondary paths, means operated to remove said potential from any one of said secondary paths and to impress thereupon a potential of the opposite polarity, a switch terminating said primary path thereafter operative to search for said one secondary path and to connect thereto said primary path, a relay in said switch connected to said secondary paths successively during said hunting `of the switch, said potential of the opposite polarity only effective to operate said relay responsive to the connection of said relay to said one secondary path, and means including said relay for terminating the operation of said switch and for restoring the potential across said one line to normal responsive to said operation of the relay.

13. In a telephone exchange, subscribers lines, switching apparatus, means for seizing said apparatus over any of said lines to extend either a local or an outgoing call, means in said apparatus for normally feeding talking current over the calling line in a particular direction, and means operated over the calling line only if said apparatus is seized to extend an outgoing call to cause said current to be fed over the calling line in the opposite direction.

14. In a telephone exchange, subscribers lines, a special line, an automatic switch having access to said lines, means for seizing said switch over any of said subscribers lines and for operating same either to extend `a call to any other one of said subscribers lines or to connect with said special line, a battery feed bridge in said switch supplying current in a particular direction to the calling line if the call is extended to another of said subscribers lines, means for rendering said bridge ineffective if said special line is connected with, and a battery feed bridge in said special line effective if said special line is connected with to supply current in the opposite direction to said calling line.

15. In a telephone system, an exchange, subscribers lines terminating in the exchange, a potential of predetermined polarity connected across each of said lines when that line is engaged in a call, a trunk outgoing from the exchange, means operated by a subscriber over any o-ne of said lines' to initiate an outgoing call, means for connecting a potential of the opposite polarity across said one line responsive to said operation of said last means, a switch terminating said trunk line operative to hunt for said one line and to connect thereto said trunk line, a discriminating device in said switch connected across each of said lines in turn during said hunting of the switch, said potential of the opposite polarity effective to operate said device responsive to the connection of said device to said one line, and means controlled by said device for terminating the operation of said switch responsive to said operation of said device.

16. A telephone system as claimed in claim 15, wherein said rst means comprises an automatic switch having access to said subscribers lines.

17. In a telephone system, a calling line having a particular potential connected across the line conductors thereof, a local switch train including a local switch, means responsive to a call from said line for extending a connection therefrom to s-aid switch, another switch having access to said line, means for operating said local switch over said line in response to the transmission of La particular digit, means responsive to said operation of said switch to change the potential across said calling line and cause said other switch to hunt for said line, said other switch responsive to said changed potential to seize said line.

18. In a telephone system, a line having a particular potential connected across the line conductors thereof, a local switch, a iinder switch responsive to a call from said line for extending a connection therefrom to said local switch, means for operating said local switch over said line in response to the transmission of a particular digit, another switch having access to said line, means operated over a wiper of said finder switch responsive to said operation of said local switch and eiective both to change the potential across said calling line and to cause said other switch to hunt for said line, said other switch responsive to said changed potential to seize said line.

19. In a telephone system, a plurality of lines, a plurality of local switch trains common to said lines and each including a local switch, another switch having access to said lines, means responsive to a call from any one of said lines for extending a connection therefrom to one oi said local switches, a particular potential connected across the line conductors of the calling line, means for operating said one switch over the calling line in response to a particular digit, means individual to said one switch and operated over a wiper thereof responsive to said operation of said switch to change the potential across said calling line and cause said other switch to hunt for said line, said other switch responsive to said changed potential to seize the calling line.

20. In a telephone system, a plurality cf groups of lines, a plurality of switches each having access to the lines of one of said groups, a local switch train including a local switch, means responsive to a call from one of said lines for extending a connection therefrom to said local switch, a particular potential connected across the line conductors of the calling line, means for operating said local switch over said calling line in response to the transmission of a particular digit, means responsive to said operation of said local switch to change the potential across said calling line and cause that one of said plurality of switches having access to said calling line to hunt for said calling line, said one switch responsive to said changed potential to seize said calling line.

HANS TATZL.

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