US2222108A - Automatic telephone system - Google Patents

Description

Win19, 1940.. M L E ETAL 2,222,108

AUTOMATIC TELEPHONE SYSTEM Filed April 23, 1938 5 Sheets-Sheet l final v Ur! INVENTOR. MAX LAN GER E T HORN ATTORNEY.

1M. LANGER ETAL AUTOMATIC TELEPHONE SYSTEM Filed April 25,. 1938 5 Sheets-Sheet 2.

INVENTOR.

MAX LANGER ATTORNEY.

Nov. 1,

M. LANGER EETAL AUTOMAT I C TELEPHONE SYSTEM Filed April 25, 1938 5 sheets-sheet s E m m m 0 v H m A HORN MAX LANG-ER ERN mum Nov. 19, 1940. M. LANGER ET AL AUTOMATIC TELEPHONE. SYSTEM 5 Sheets-Sheet 5 Filed April 23, 1938 INVENTOR.

MAX LANGER BY ER T HORN ATTORNEY? Patented Nov. 19, 1940 UNITED STATES PATENT OFFICE AUTOMATIC TELEPHONE SYSTEM Application April 23, 1938, Serial No. 203,926 In Germany April 26, 1937 16 Claims.

The invention relates to combating disturbances affecting intelligibility in automatic telephone systems. I

The calls in such systems are established over switch wipers and relay contacts which in all systems are exposed to a certain amount of chat tering. When the contacts conduct substantial currents, such as feeding current or holding currents, for supervisory or release purposes, the chattering leads to undesirable crackling noises in the subscribers receivers because the chattering varies the resistance of the contacts and thus the strength of the current flowing over these contacts. It has been shown by experiments and experience that the variations in resistance are dependent upon the strength of the current. With large currents considerable variations in resistance arise and with small currents small variations.

On the other hand lengths of line in which no strong currents such as feeding or supervisory currents pass over the switch wipers and relay contacts, but only the weak speech currents, the undesirable phenomenon termed fading occurs. The fading occurs through the resistance at the contacts becoming very large and this frequently amounts almost to a complete break. The physical cause of this phenomenon has not been altogether satisfactorily explained. It is, however, assumed that the smallest chattering effect already diminishes the specific contact pressure, for example, at switch wipers, to such an extent that the highly polished layer at the contact points represents an insulating layer. Further conditions which assist this phenomenon are the accumulation of dust, oil, and moisture at the contacts. It is known that the blocking layer which is produced is broken down by the application of a potential of the order of 1-60 volts. In the known arrangements for applying the poten tial at the contacts the switching means such as condensers, which block the passage of direct current are bridged by high resistances which are termed sintering resistances.

Although the sintering resistances are now used to a large extent, up to the present the whole of the two effects, namely the cracking noises and fading, has not been removed. The invention resides in the new discovery that the known means for neutralizing the disturbances have not been used to a sufiicient extent at all the potential points of danger. According to the invention, therefore, all the currents which are liable to prejudice the intelligibility through their variations are only passed over parts of the speaking path which are free from contacts which are sensitive to chattering or over auxiliary leads while all contacts which are exposed to chattering have a high enough potential passed over them to break through the insulating layer produced at the contacts by the fading phenomenon.

The attached drawings represent embodiments of the invention:

Fig. 1 shows a circuit layout from which the feeding of the two subscribers connected in a call may be seen. The feeding no longer takes place as in known arrangements from the first group selector for the calling subscriber and from the final selector for the called subscriber, but the subscribers are fed over feeding bridges associated individually with the subscribers lines, while the wipers of the preselectors and of the final selectors are cut off by means of condensers. The feeding bridge relays are represented by R in the drawings. As may be seen from the circuit layout, the sintering circuits flow separately over one lead of the connecting line from the one pole of the feeding battery of one subscriber to the corresponding opposite pole of the feeding battery of the other subscriber while bridging the condensers over high resistances. The signalling and supervisory circuits which are required for the impulses and clearing signal transmission are not shown in this diagrammatic layout.

These circuits also must, of course, be so arranged that during the speaking condition only the speech currents and the sintering currents pass over the switch wipers and relay contacts disposed in the connection. Figs. 2-6 represent the solutions of this problem as well as disclosing the new manner of feeding the subscribers.

Fig. 2 shows an arrangement in which the impulsing and transmission of the clearing signal on the part of the calling subscriber is controlled over a special auxilary group selector. In contrast with this the clearing signal is transmitted by the called subscriber over the testing leads; in detail the figure shows the first preselector VWl, a first group selector GWI and a local-andtoll final selector LWI.

The mode of operation of the arrangement is as follows: When the calling subscriber Tnl takes up his receiver the feeding bridge relay Rel energizes over the closed subscribers loop: earth, winding II of relay Rvl the b-lead, the closed loop over subscriber Tnl, a-lead, winding I of relay Roi, battery and earth. At contact lrvl the following circuit is completed over the rotary magnet of the preselector: earth, interrupter RU, rotary magnet Dvl, contact 31ml and lrol,

wiper e of the preselector in the zero position, battery and earth. The preselector commences to rotate. After leaving the zero position the circuit for the rotary magnet is maintained over wiper e and the continuous contact segment. When the switch has found a free line, the following testing circuit is closed: earth, battery, wiper e of the preselector, the contact segment, contact lrvl, windings II and I of relay Pol, wiper c of the preselector, the winding of relay Cgl of the first group selector, the winding of relay Zgl, off-normal contact 2 lkgl and earth. In the first preselector thetesting relay Pol energizes and by opening contact 31ml brings the switch to rest. High resistance winding II of 5m is short-circuited at contact 21001 and the seized line is guarded against further seizure. Contacts Epvl and 6102)! switch the two speaking leads through to the first group selector.

In the first group selector GWI the seizing relay Cgl energizes and also relay Zgl in series with it. Relay Zgl at contact 29zg| completes a circuit for transmitting the public exchange signal to the calling subscriber. This passes over earth, transformer AZ, rotary off-normal contact 30wgl, contact 292571, winding II of relay Agl and earth, and is thence transmitted inductively to winding III of relay Agl and from there back to the calling subscriber. Relay Cgl closes its contact 25cgl and holds up after the first vertical step of the switch. Furthermore, relay Cgl closes its contact Zllcgl and thereby prepares the impulsing circuit for the impulse receiving relay Ag I.

When the calling subscriber dials, the subscribers loop is opened and closed alternately. Each time the loop is interrupted the feeding bridge relay Rvl of the calling subscriber releases transitorily. When relay Rol releases the following impulse circuit is closed: earth, resistance Wil, contacts 91ml and Brol, wiper d of the first preselector, the d-lead, contact 20cgl, windings I and II of relay Vgl, winding I of the relay Agl, battery and earth. Relay Agl transmits the impulses to the first vertical magnet of the first group selector. The switch is raised on the level corresponding to the impulse train. On the first impulse relay Vgl is energized in series with relay Agl. Relay Vgl acts as a slow relay on account of the short-circuit across its winding I at contact 2| vgl. It holds up during the impulse train. At the end of the impulse train relays Agl and Vgl release. Relay Vgl switches over to the rotary magnet. The switch hunts freely over the selected level. Each time the rotary magnet draws up contact 28dgl is closed and relay Agl is energized and controls the rotary magnet circuit. When the switch has found a free line, the following testing circuit is closed: earth, contact dl'cgl, windings II and I of relay Pgl, wiper c of the first group selector, winding I of relay Cwl, ofi-normal contact 63kwl, battery and earth. The testing relay in the group selector brings the switch to rest, the high resistance windings II of relay Pgl is short-circuited at contact 4Upgl and the selected line is guarded against further seizure. The two speaking leads are switched through by the closing of contacts 3lpgl and 38pgl.

In the final selector LWI' the seizing relay Cwl energizes. Relay Cwl closes its contact BZcwl over which the relay holds up after the off-normal contact 63km! is opened after the first vertical step of the switch. The impulses for setting the final selector which are transmitted by the calling subscriber are transmitted from the first group selector to the final selector impulse receiving relay over the following path: earth, contacts 34agl, 3lpgl, wiper a of the first group selector, contact fifipwl, winding I of relay Awl, contact Blbwl, battery and earth. Relay Awl energizes impulsively and transmits the impulses to the vertical magnet of the final selector in a manner which is not shown. The switch is raised to the level corresponding to the impulses. At the end of this impulse train the operating circuit is switched over to the rotary magnet of the final selector in the known way by the aid of two slow relays Vwl and Uwl. The last impulse train is also transmitted to the impulse receiving relay Awl of the final selector by contact 34agl. The final selector switches round in the selected level on to the wanted subscribers line. After the switch has been set the relays Vwl and Uwl make delayed releases in succession. The following testing circuit is therefore closed before relay Uwl has released: earth, contacts 83cwl, Shaw], and 80'vwl, windings II and I of relay Pwl, winding II of the relay Cwl, wiper c of the final selector, contact 4T1) of the first preselector, wiper c of the first preselector in the zero position, windings I and II of relay Pvl, wiper e of the preselector in the zero position, battery and earth. In this circuit the two testing relays Pwl of the final selector and P2)! of the preselector energize. Relay Pwl in the final selector opens the rotary stepping circuit (not shown). Through the closing of contact l9pwl, the high resistance winding of the testing relay is short circuited and the seized line is guarded. Relay Pwl now closes the following circuit for relay Awl: earth, contact 651010! winding I of relay Awl, contact B'lbwl, battery and earth. Relays Aw! and Pwl complete a circuit for relay Vwl which is not shown and which at this stage of the connection serves as a ring-connecting relay. Relay Pwl moreover breaks the circuit for winding I of seizing relay Cwl at contact Gflpwl. Relay Cwl holds up however over its winding II which is in series with the testing relay Pwl in the testing circuit. Relay Bwl is energized over contact Blpwl over the testing lead from the first group selector: earth, contact Mlpgl, winding I of relay Pgl, wiper c of the first group selector, winding I of relay Bw l, contact Glpwl, 620ml, resistance Wi8, battery and earth. Relay BWI closes its contact 64bwl and thus locks up in the seizing circuit independent of relays Pwl and G'wi.

The ringing current of the called subscriber passes over the following path: earth, the ringing machine RM, contacts l ivwl and l5bwl, wiper a of the final selector, the closed loop at subscribers station T712, wiper b of the final selector, contacts lfibwl and 1211111! and earth. When the called subscriber replies his feeding bridge relay Rvl energizes over the closed loop. Relay Roi breaks the testing lead in the called partys preselector by opening contact 4rvl. Accordingly the testing relay Pwl releases in the final selector LWI. Relay Owl in series with this relay also releases. The testing relay in the called partys preselector on the other hand now holds up in the following local circuit: earth, resistance Wil, contacts 91ml and 'lrvl, wiper c of the preselector in the zero position, windings I and II of relay Vwl wiper e of preselector in the zero position, battery and earth.

In the final selector relay Awl releases after the release of relay Pwl. The release of relays Pwi and Cwl also caused the interruption of the circuit for the ring-connecting relay Vwl. After the release of relay Pwl earth is applied to the a-lead over: earth, contact BBbwl, winding I of relay Aw l, contact fifipwl, and the a-lead as a signal that the wanted party has replied. After relay Cwl has released metering potential is applied to the b-lead over: earth, battery, winding II of relay Awl, contacts 69bwl and metal and the b-lead.

The twosubscribers can now converse together.

When at the end of the call the called party replaces his receiver, his feeding bridge relay Rvl releases. The release of relay Rul causes the testing lead to be switched through to the final selector over contact Moi. Over this circuit the testing relay Pol on the one hand holds up in the called subscribers preselector and on the otherhand the testing relay Pwl in the final selector energizes in this circuit over: earth, battery, wiper e of the preselector, windings I and II of relay Pol, wiper c of the preselector, contact 41ml, wiper c of the final selector, contact Howl, winding I of relay Pwl, contact lflbwl and. earth. It is not possible for relay Cwl to energize over its winding II in series with the testing relay because this winding is finally short-circuited over contact How! on the release of the relay. Relay P112! in the final selector removes the earth from the a-lead by opening contact SEp-wl. By this means in long distance working the called subscribers clearing signal is indicated at the long distance operators position.

If now the calling subscriber also replaces his receiver his feeding bridge relay Rvl likewise releases. After relay Roi is released the following circuit is completed for relay Ag! in the first group selector GWI: earth, resistance Wil, compacts 91ml and final, wiper d of the preselector VWI, contact Zllcgl, windings I and II of relay Vgl, winding I of relay Agl, battery and earth. Relay Agl energizes over a prolonged period in this circuit. At contact Z'Zagl the seizing relay Cgl is short circuited for a similar period and deenergizes. Before it does, so, however, the call, is metered. A long earth'ing impulse is transmitted to the final selector LWI over the a-lead by contact Magi when relay Agl energizes and this causes switching in the final selector so that in the case of local traffic earth potential is again applied to the a-lead. Through this potential, winding III .of Pgl of the first group selector is energized. Thus relay Pgi does not release in spite of relay Cgl having short-circuited its winding I at contact 3909!. The connection to the succeeding switch is maintained over: earth, contacts lflpgl and EQcgl and the connection to the preceding selector over earth contact ZGpgl and resistance WiZ and Wi3. After relay Cal has released the metering relay Zgl energizes over: earth, the winding II of Zgl, contacts 36cgl and 38pgl, wiper b of the group selector, contacts w! and fillbwi, winding II of relay Awl, battery and earth. Relay Zgl of the first group selector energizes and by closing its contact 212g! shortcircuits: the resistancesWi2 and Wi3, and thereby increases the current over the c-lead whereby the meter ZVI in the preselector of the calling subscriber is energized. Relay Zgl shlort-circuits winding- III of relay Pgl at contact 3320!. Relay Pgl releases. Consequently both the seizing circuit to the calling partys preselector and also: the testing circuit to the final selector are broken. In the preselector VWI relay Pol releases. The selector steps around over its wiper e, the contact segment and contact 31ml until it reaches the zero position.

On account of the breaking of the test lead from. the first group selector relay Bwl in the final. selector releases. Relay Bwl first breaks the testing circuit to the called partys preselector at contact l8bwl. In addition it disconnects the metering. potential from the b-lead and also by opening contacts 152m)! and 'lfibwl it disconnects both the speaking leads and finally completes a circuit (not shown) for the rotary magnet of the final selector over which this is restored to normal. When the testing circuit to the preselector of the called party is broken, the test relay Pwl in the final selector also releases as does the testing relay Pol in the called partys preselector.

For the sake of completeness it will be mentioned at this point that the final selector LWI is also adapted for trunk working for on the establishment of trunk calls which in the normal way are set up over a trunk group selector which is multipled to the first group selector GWI which is shown, it is possible in the event of the wanted subscriber being engaged in a local call to establish connection to the engaged line by means of the relay Bwl. The connection to the engaged line is effected by the application of potential to the b-lead at the trunk operators position, whereupon the following circuit is completed. Potential applied to the 1)- lead at the trunk operators position, contact llcwl, winding II of relay Bwl and earth. The connection takes place at contact l5bwl and lfibwl The permanent energizing of relay Bwl after the busy connecting impulse has terminated, is prevented in that after the closing of contact filibwl, winding I of this relay is disposed in a branch circuit in parallel with the seizing relay Cwl over resistance WM and contacts GZcw-I and iill pwl (relay Pwl is not yet energized) over which circuit winding I of relay Dwl does not receive suificient current to maintain its armature attracted. Moreover, winding I .of relay Bwl is arranged as a counter winding II.

The sintering circuit for the contacts passes separately over the two speaking leads, namely from earth in the final selector over contact Giibwl, winding I of relay Awl, contact GBpwl and the a-lead in both directions to potential at the two feeding batteries. The sintering current for the b-lead passes over earth, bat- ,tery, winding II of relay Awl, contacts 69bwl and lllcwl in both directions to the earthed pole of the feeding batteries of the two subscribers. The condensers disposed in the speaking leads are shunted by high resistances in the usual way.

Fig. 3 shows a further embodiment of the invention. The first preselector is denoted by VWZ and the first group selector by GWZ. The final selector used in the final selector LWI shown in Fig. 2. In this embodiment in contrast to the embodiment according to Fig. 2, a separate auxiliary lead between the first preselector and the first group selector is dispensed with. The transmission of the impulses in this case is by means of alternating current, for example ringing current. Also the calling subscribers clearing signal is transmitted from thefirst preselector to the group selector by means of alternating current. Signals are received in the first group selector by an alternating current relay which is disposed in a bridge across the speaking leads and is guarded against direct current by condensers. The two subscribers are fed as in all the embodiments of the invention over feeding bridges associated individually with the subscribers lines. By this arrangement the sections of the connecting path containing relay and switch contacts which are exposed to vibrations are kept clear of large currents such as the feeding currents or supervisory currents.

The mode of operation of the arrangement is as follows: When the calling subscriber takes up his receiver his feeding bridge relay R112 energizes over the closed subscribers loop. The rotary magnet of the preselector is switched on over contact i2r112; the preselector is in operation. As soon as it finds a free line, the testing relay P112 energizes over the testing wiper c of the preselector and the seizing relay Cg2 of the first group selector which is also disposed in the testing circuit energizes over: earth, battery, wiper d of the selector VWZ, the contact segment, contact l2r112, windings II and I of relay P112, wiper c of the preselector VNZ, the winding of relay CgZ, the winding of relay Z92, off-normal contact 25kg2 and earth. Relay P112 in the preselector brings the preselector to rest and switches the speaking leads through to the group selector.

Relay Q02 in the group selector GWZ switches through the a-lead at contact 2002 and at contact 26cg2 a holding circuit is established for relay cg2, which is independent of the ofi-normal contact. Finally, through the closing of contact 40cg2 and the opening of contact 38092, the testing circuit to the final selector is prepared.

When the calling subscriber dials the wanted number, his feeding bridge relay R112 releases impulsively. When relay R112 releases, alternating current is applied to the speaking leads and the impulse receiving relay Jg2, in the first group selector is energized over: earth, resistance W1'5, contacts M112 and 910112, wiper b of the preselector, the alternating current relay J92, contact 200 2, wiper a of the preselector, contacts 811112 and H112, the source WQ of alternating current and earth. Relay J 92 in the group selector GWZ energizes in response to these impulses and repeats them to relay A92 over: earth, windings I and II of relay V92, contact 281512, winding I of relay A92, battery and earth. In this circuit the relays V92 and A92 energize. Relay VgZ short-circuits its winding in the usual way over its contact 2111g2. It consequently works as a slow relay and holds up throughout the impulse train. Relay Ag2 repeats the impulses to the vertical magnet of the group selector. At the end of this impulse train relay V92 releases and switches on the rotary magnet of the group selector. The selector rotates over the level which has been selected. As soon as it finds a free line the testing relay P92 energizes: contact 40cg2, the windings of relay PgZ, wiper c of the group selector, winding I of relay Cwi, the off-normal contact 637cwl, battery and earth. In the test circuit, the seizing relay 01111 of the final selector also energizes.

The further switching operations take place in the same way as has been described in the preceding embodiment.

. The signal for taking up of the receiver by the called subscriber is controlled over the testing lead between the preselector of the called party and the final selector as is also the signal for the replacement of the receiver.

When at the end of a call, the calling party hangs up his feeding bridge relay R112 releases. The speaking leads between the calling subscriber and the first group selector are broken through the opening of contacts 21112 and 31112. Alternating current is applied to the leads of the line of the first group selector GWZ over the contacts [T112 and 41-112. The alternating current relay Jg2 energizes over a prolonged period and short-circuits the seizing relay CgZ over its contact 24ag2 for a corresponding time. Relay C92 is caused to release. After the metering has been carried out as described in the preceding example, the first group selector releases and therefore also the first preselector in the final selector.

Fig. 4 shows a third embodiment of the invention in connection with the establishment of a connection comprising a first preselector VW3, a first group selector GW3 and a local and final trunk selector LW3. This embodiment is similar to the preceding one in so far as a separate auxiliary lead for impulsing and supervising the calling party between the first preselector and the first group selector is dispensed with. The impulse and supervision of the calling subscriber also takes place over the speaking leads in this arrangement, not by means of alternating current and alternating current receiving relays but by the aid of the charging and discharging currents of condensers by means of which serially arranged polarized bridge relays are operated. In this arrangement also all sections of the line which at the contacts are exposed to vibration, are kept free of feeding and supervisory currents.

The mode of operation of the arrangement is as follows: When the calling party Tnl takes up his receiver the feeding bridge relay R113 is energized over the closed subscribers loop. At contact lr113 the rotary magnet D113 of the preselector is operated over: earth, battery, wiper d of the preselector, contacts H113 and 311113, the rotary magnet D113, interrupter RU and earth. The preselector is set in operation and searches for a free line. As soon as this is found the testing relay P113 energizes over the test wiper c of the preselector. Relay P113 brings the switch to rest and guards the seized line against further seizure. Finally the two speaking leads are switched through to the first group selector by contacts 511113 and 611113.

In the first group selector GW3, the seizing relay C93 also energizes in the testing circuit: earth, battery, wiper d of the first preselector, contact l1113, the windings of the testing relay P113, wiper c of the preselector VW3, the windings of relays C93 and Vg3, off-normal contact I2kg3 and earth. Relay Vg3 connects the public exchange signal to the calling subscriber in a manner which is not shown here. Relay Cg3 prepares the testing circuit by closing its contact l8cg3. Relay C93 holds up over contact Icg3 after the switch has made its first vertical step. The polarized relay Jg3 is connected across the leads of the line over contact llcg3. The condenser C03 is therefore charged over the following circuit: earth, the winding of relay R113, contact 611113, wiper b of the first preselector VW3, winding I of relay Jg3, condenser C03, contact Ilcg3, wiper a. of the first preselector VW3, contacts 512113 and 41113, the choke-coil Dru, battery and earth. The polarized relay M3 is so adjusted that it does not switch over its armawanted subscriberis line.

ture in response to this charging impulse. It may be mentioned at this point that beyond the polarized relay there is arranged a blocking circuit in the speaking leads comprising two condensers in each lead and a choke coil which prevents the charging and discharging impulses from being extended.

When the calling subscriber dials. the wanted.

number, the subscribers loop is alternately opened and closed. When the subscribers loop is opened the circuit for the bridge relay R03 of the calling subscriber is broken. The fdisconnect impulse of relay R03 operates to discharge the condenser 003, the discharge current flowing over the windings of relay R03. A fresh charging of the condenser (relay R03 releases) is prevented by the opening of the a-lead at contact 41123. When the subscribers loop is closed relay R03 is energized again. Contact 41 3 switches the a-lead through again and the condenser is charged afresh. These operations are repeated alternately throughout the course of the dialling. In response to the discharge impulse over the condenser 003 when the loop is broken for the first time, relay J03 switches over its contacts. When the loopis closed it restores its contacts to normal. When relay J03 has 0perated its armature the following circuit is closed: earth, winding II of relay V03, contact 19193, the winding of relay A93, battery and earth. In this circuit the relays V93 and A03 energize. Relay VH3 holds up during the impulsing because it is slow to release. Relay A93 repeats the impulses to the vertical magnet of the group selector. After this impulse train, the operating circuit is transferred to the rotary magnet of the group selector by relay V93. The rotary magnet searches freely for a free line. As soon as such a line is found, relays P93 and C03 energize .in the testing circuit: earth, contact l8cg3, windings II and I of the testing relay Pg3, wiper c of the first group selector, winding I of the relay Cw3 in the final selector, contact 33010.3, off-normal contact 3070103, battery and earth. The testing relay in the group selector brings the switch to rest in the known way, guards the seized line against further seizure and switches through the speaking leads.

The seizing relay C103 in the final selector .LW3 opens its contact 330103 and closes its contact 34010 3. The high resistance winding II of relay C103 is thereby connected in the seizing circuit. The relay holds up in this circuit independently of the off-normal contact of the final selector. I

When the calling subscriber transmits the impulses for setting the final selector, his feeding bridge relay R03 energizes and deenergizes impulsively. In response to this the polarized receiving relay J93 alternately moves its contact into the operated and rest positions. Consequently relay A93 is operated impulsively in the first group selector. Relay A03 transmits the impulses to the impulse receiving relay Aw3 of the final selector over the following circuit: earth contacts 206103 and M1703, wiper a of the first group selector, contact 3311103, winding I of relay Aw3, battery and earth. Relay A103 repeats the impulses to the vertical magnet of the final selector. The switch is raised to the desired level. The last impulse train is similarly transmitted to the receiving relay Aw3 of the final selector. This repeats the impulses to the rotary magnet and the final selector is stepped to the If the wanted subscriberis free the following testing circuit is completed during the release period of slow relay Uw3 which is not shown: earth, contacts 5lcw3, 521.0103 and 530103, windings II and I of relay P203, wiper c of the final selector, wiper c of the preselector of the called party in the zero position, windings I and II of relay P03, Wiper d of the preselector in the zero position, battery and earth. Relay P03 in the preselector opens its contact 3pv3and thereby prevents this switch from operating. The test relay P023 of the final selector brings the switch to rest, guards the seized line against further seizure by short-circuiting high resistance winding II at contact 4612103 and switches through the speaking leads to the called line over its contacts 5429103 and 5521203.

In the final selector there is disposed the winding I of; a polarized relay Jw3 across the speaking leads in series with a condenser C1 3. After the speaking leads have been switched through to the called line the condenser C2 3 receives a charging impulse over the called party's feeding bridgezearth, winding of relay R03, wiper b of the final selector, contact 5510103, relay Jw3, condenser C193, contacts 530103 and 5 lpw3, wiper a of the final selector, choke-coil Dr0, battery and earth. Relay Jw3 is soadiusted as not to energize in response to this impulse. Consequently a circuit is closed after the energization of relay Pw3 over a winding of relay VW3, which now-serves. as the ring-contacting relay: earth, contacts 35iw3, 3811703 and 5911103, winding of relay VW3, interrupter LU, battery and earth. Relay VW3 applies ringing current to'the. speaking leads over its contacts 5'l0w3 and 600w'3.

When the called subscriber replies in-response to the ringing current, the subscribers loop is closed when he takes up his receiver, and the called partys feeding bridge relay R03 is energized. Through the closing of the called subscrib-ers loop the potential over the line is altered and this acts as a discharging impulse over the condenser 0:03. In response to this discharging impulse relay Jw3 switches over its armature.

Relay 3103 is now energized in the selector LW3 over: earth, contact 369103, winding II of relay B 103, battery and earth. At the same time the slow relay MW3 is energizedflover: earth, contact 667103, the winding of relay MW3, battery and earth. Relay B203 locks up in the following circuit: earth, winding I of relay B203, contacts Mbwt and 43pw3, battery and earth. Relay B103 breaks the circuit for the ring connecting relay at contact 5319103. Furthermore metering potential is applied to the b lead over: earth, battery, winding I of relay A003, contacts 3929103 and mps/"3 and the b-lead. Finally for the purpose of sintering the contacts earth is applied to the a-lead over: earth, the high resistance Ws,

contact 52329103, winding II of relay A103, contact 3113103 and the a-lead.

The two subscribers can now converse together. The called party replaces his receiver at the end of the call. A circuit for the feeding bridge relay R03 of the called party is broken. The break impulse operates to reverse the polarity of the condenser 0:03 in the final selector. The

charging impulse is in such a direction that the in that after the armature of relay J w3 has been restored the following circuit for a second winding of relay Jw3 is completed over contact 60mw3: earth, contacts 357'w3 and 60mw3, winding II of relay Jw3, off-normal contact 3lkw3,

battery and earth. The second winding of the relay is arranged so that it holds the armature of the polarised relay in the rest position. The locking circuit for winding II of relay Jw3 remains closed until relay MW3 has released. In the meantime the counter-impulse has died down. The direct connection of earth to the alead after the called party has replaced his receiver over contact 35a'w3, 58pw3, winding II of relay Aw3 and contact 3'lb'w3 gives the clearing signal at the trunk operators position in the case of trunk calls.

When the calling party also replaces his receiver his feeding bridge relay R123 releases. This reverses potential over the line and results in a discharge impulse at the condenser C03 of the first group selector GW3. Relay J 93 consequently switches over its armature and closes contact l9y'w3. Relay A93 in the first group selector is energized for a prolonged period and short-circuits the seizing relay Cg3 for a corresponding period over its contact l3ag3. Relay Cg3 releases.

By opening its contact l5cg3, relay Cg3 initiates the release of the preselector VW3 of the calling subscriber. By short-circuiting winding I of the testing relay Pg3 at contact 6lcg3 this relay is caused to de-energize and opens the testing circuit to the final selector at contact 62pg3. In the final selector the seizing relay Cw'3 releases in consequence. Relay Cw-3 by opening its contact 5|cw3 releases the testing relay Pw3. Relay Pw3 brings about the release of the final selector when it releases.

Trunk calls can also be set up over the final selector LW3. These pass from the trunk operators position in the usual way over a trunk group selector connected in multiple with the first group selector. The switching operations up to the setting of the final selector on the wanted subscribers line are the same as in the case of a local call. If the called subscriber is engaged in a local call, the trunk operator can connect to the engaged line by applying potential at the trunk position to the b-lead, whereby relay B103 of the final selector is energized in the following circuit: the potential applied to the b-lead at the trunk operator's position, contact p103, winding I of relay B103 and earth. The operator is connected to the busy subscriber over contacts 4811103 and 49bw3. The trunk operator is enabled to break down the call by applying earth to the a-lead after establishing connection with the engaged subscriber and thereby energizing relay Jw3: earth on the a-lead, contact 38bw3, winding I of relay A103, battery and earth. Relay Aw3 connects direct earth to the speaking leads on the one hand over contacts 460L103 and (M3 and on the other hand connects earth over contact 50aw3 and a low resistance of the testing relay T3 to the testing lead of the called subscriber. If the wanted subscriber is the calling subscriber in the existing low-rated call the condenser C03 is discharged over the earth connected to the a-lead and to the b-lead.

At the same time the feeding bridge relay R03 releases. Relay J93 of the first group selector switches over its contacts and energizes relay Ag3 for a prolonged period over contact 19193. Relay A93 short-circuits relay Cg3. This releases and initiates the releasing of the existing local call. Relay J93 is restored to normal 10- cally over a second winding: earth, contacts 23093, 240.93, winding II of relay Jg3, battery and earth.

If the subscriber who is wanted for the trunk call is the'called party in the existing local call, the testing relay P203 of the final selector in the local connection is short-circuited by the application of earth over contact 5Jaw3 in the final selector seized by the trunk operator. releases and brings about the disconnection of the local call.

It should be mentioned at this point that the trunk knock down" can of course be brought about in a similar manner to that in the present embodiment in the other embodiments of the invention. Thus in the embodiment l, the feeding bridge relay of the subscriber engaged in a local call is deenergized and the release of the first group selector seized in the local call is brought about over the d-lead.

Fig. 5 shows a further embodiment of the invention. The first preselector is indicated by VW and the first group selector GW4. Only those parts of the local and trunk final selector indicated by LW4 which concern the invention are shown. Both the subscribers are fed as in all the embodiments over feeding bridges associated with the subscribers lines. The impulsing and clearing signals of the calling subscriber are transmitted over the two leads of the calling line by direct current while the clearing signal from the called subscriber is transmitted over one of the two speaking leads. All sections of the line which contain contacts which are liable to vibrations are protected from disturbing currents by the position of special condensers Col and C112 between the subscribers feeding bridge and the multiple field of the switches.

The mode of operation of the arrangement is as follows: When the calling subscriber takes up his receiver his feeding bridge relay R114 energizes. The rotary magnet of the preselector is switched on over contact lrv l. The preselector is set in operation and searches for a free line..

When such a line has been found, the testing relay PM of the preselector and the seizing relay CQd of the first group selector energize in the testing circuit. In the testing circuit relay V94 energizes in series with relay C914 and transmits the exchange signal to the calling subscriber. Moreover, the impulse receiving relay A274 is connected to the 'a-lead over contacts l3cg4 and l2vy4.

When the calling party dials the feeding bridge relay R114 releases impulsively. Consequently the following circuit is completed for the impulse receiving relay AM f the first group selector: earth, the winding of relay Ayd, contacts l3cg4 and 121294, wiper a of the first preselector, contacts 21004, 5TD4 and 311 W per b of the first preselector, winding I and II of relay V94, battery and earth. In this circuit relays A124 and Vgd energize. RelayVglholdsup throughout the impulse train. Relay A114 repeats the impulses of the vertical magnet to the group selector. After this impulse train the switch rotates freely and searches for a free line. When a free line has been f und the testing relay PM energizes in the first roup selector and the seizing relay Cwd in the final selector. Relay Cw4 prepares the setting circuit and also the testing circuit.

The further impulse trains from the calling subscriber are taken up by relay A94 and are extended as earth impulses over contact lllag4 to This relay the final selector. The final selector LW4 is raised to a certain level and stepped round. If the final selector LW4 finds the wanted subscribers line free the following testing circuit is completed. Earth, contacts 2001124, 2|uw4 and 2211104, the windings of the test relay P204, wiper .c of the final selector, wiper c of the preselector wt in the zero position, the windings of the test relay PM, wiper d of the preselector VW4 in the zero position, battery and earth. In this circuit the two test relays energize. The testing relay of the final selector carries out its own functions. The testing relay P114 in the preselector applies earth to the b-lead leading to the final selector, whereby a relay X104 in the final selector is energized over: earth, wiper a of the preselector VWG in the zero position, contacts 21104, Brad and 3PM, wiper b of the preselector vw l in the zero position, wiper b of the final selector LW4, the Winding of relay Xwfl, battery and earth. Relay Xwil applies earth directly to the a.-lead over contacts 2411104 and 2511104 which indicates at the trunk operators position the termination of the selection of a trunk call.

When the called subscriber replies his feeding bridge relay Ro l energizes. Through the opening of contact 5W4 earth is disconnected from the b-lead to the final selector. Relay Xw4 releases; accordingly the earth is disconnected again from the a-lead at contact 24rw4 and this gives the signal at the trunk operators position that the called party has replied. Earthis applied to the I'm-lead over contact 257M114 and the high resistance Ws for the purpose of sintering the contacts.

When the called subscriber hangs up at the end of the call, his feeding bridge relay Ro l releases on account of the interruption of the loop. Earth is applied again to the b-lead of the final selector over contact 51174. Relay Xwd energizes again and gives the clearing signal at the trunk operators position.

When the calling subscriber also hangs up his feeding bridge relay R214 releases. A loop is set up in consequence to the first group selector in the manner already described over: earth, the winding of relay Ag4, contacts I 3cg4, and Hpg l, wiper a of the first selector VW4, contacts 210114, fired and 3:004, wiper b of the first preselector, windings I and II of relay VGA battery and earth. In this circuit the two relays A94 and FM in the first group selector energize permanently. Relay A94 short-circuits the relay C94 over contact leap/4. Relay C04 releases and promotes the release of all the switches in the manner which has frequently been described before.

Fig. 6 shows details of a further embodiment of the invention, namely the circuit of a call finder serving as the preselecting device.

The feeding is again over a feeding bridge Rs associated individually with the subscribers line. Moreover, a relay Ts is associated with each subscribers line. When the calling subscriber takes up his receiver the feeding bridge relay R8 is energized and switches over contact lrs. A starting relay is operated in the known way and sets the call finder in operation and in addition relay Rs prepares the testing circuit by closing contact are. When the call finder has found the calling line its testing relay Ps energizes in the following circuit: earth, windings I and II of relay Ps, wiper c of the call finder, contacts 215s and 3rs, winding I of relay Ts, battery and earth. In this circuit relays P3 and Ts energize. Relay Ps brings the call finder to rest and switches the line through to the first group selector. Relay Ts locks up in the following circuit: earth, contacts his and Ears, winding II of relay Ts, battery and earth. After relay Ts has energized the testing circuit is broken at contact Etc. The testing relay P's of the call finder holds up, however, in the following local circuit: earth, contact l'ps, the low resistance winding II of the testing relay Ps, contact lips, resistance battery and earth. Another call finder is thereby prevented from testing on the calling line and also a final selector.

When the calling subscriber who has established a local call over the call finder As is wanted from the trunk exchange earth is applied to the c-lead in the final selector which is set by the trunk operator on the subscriber"s line. This is represented as being connected over a key contact Fa in the figure for the sake of simplicity. If the trunk operator desires to break down the connection. the testing relay Ps of the call finder is short circuited over: earth, key contact Fa, contact 8138, wiper c of the call finder, winding II of the testing relay Ps, contact fps and earth. The

relay Ps releases and promotes the release of the call finder and thus of the wanted subscriber.

What is claimed is:

1. In a telephone system, an exchange, a plurality of subscribers lines terminating at the exchange, a plurality of feeding bridges at the exchange each individual to one of said lines and connected to the terminus of that line, each of said bridges containing a source of direct current to supply talking current to the associated line, switching means for completing a. connection between any of said lines for the transmission of speech currents therebetween, said switching means including variably operable contacts over which said speech currents pass, and means for enabling the bridges connected to the lines between which said connection has been completed to supply to said connection current of sulficient value from said source to break down the contact resistance of said contacts.

2. In a telephone system, an exchange, a plurality of subscribers linesterminating at the exchange, a plurality of feeding bridges at the M exchange each individual to one of said lines and connected to the terminus of that line to supply talking current thereto, switching apparatus having variably operable contacts, and means for controlling said apparatus to complete a connection between any of said lines over said contacts, the potential of said feeding bridges being thereby applied to the ends of said connection and effective to break down the contact resistance of said contacts.

3. A telephone system as claimed in claim 2, wherein said connection comprises a twoconductor path for the transmission of speech currents, wherein said switching apparatus includes a plurality of switches for extending both said path and a control path, and wherein said last means controls certain of said switches over said control path thereby to extend said connection.

4. In a telephone system, an exchange, a plurality of subscribers lines terminating at the exchange, a plurality of feeding bridges at the exchange each connected to the terminus of a line to supply talking current thereto, switching means for completing a connection between any of said lines for the transmission of speech currents therebetween, said switching means including variably operable contacts over which said speech currents pass, and means for applying potential to said connection continuously during the transmission of speech thereover to break down the contact resistance of said contacts.

5. In a telephone system, an exchange, a plurality of subscribers lines terminating at the exchange, a plurality of feeding bridges at the exchange each connected to the terminus of a line to supply talking current thereto, switching means for completing a connection between any of said lines for the transmission of speech currents therebetween, said switching means including variably operable contacts over which said speech currents pass, and means for causing said contacts to be traversed by current continuously during the transmission of speech thereover, said current being of suflicient value to substantially eliminate the contact resistance of said contacts.

6. In a telephone system, a plurality of telephone substations, means for completing a speech transmission path between said substations, a plurality of feeding bridges each individual to one of said substations and having a source of direct current potential for supplying talking current to that one substation, each bridge supplying said talking current to its associated station over a portion of said path which includes no variably operable contacts, variably operable contacts included in another portion of said path, and means enabling said bridges also to supply to said other portion of said path potential of sufficient value to break down the contact resistance of said contacts.

7. In a telephone system, a plurality of telephone substations, means for completing a speech transmission path between said substations, a plurality of feeding bridges each individual to one of said substations and supplying talking current to that one substation over a portion of said path which includes no variably operable contacts, variably operable contacts included in another portion of said path, condensers separating said last portion of said path from said portions which include no variably operable contacts, and means enabling said bridges also to supply to said last portion of said path potential of sufficient value to break down the contact resistance of said contacts.

8. In a telephone system, a plurality of subscribers lines each having a feeding bridge for supplying talking current thereto, switching apparatus having variably operable contacts and means for completing a two-conductor connection between any of said lines over said contacts, the potential of the feeding bridges of the lines between which said connection is completed thereby being applied to the ends of said connection and being so poled that substantially no current flows over either conductor of said connection, and means for applying to said connection another current feeding bridge so poled with respect to the bridges applied to the ends of said connection that current of sufficient value to break down the contact resistance of said contacts flows over said conductors.

9. In an automatic telephone system, a subscribers station and a line therefor, a relay having a winding bridged across said line in series with a source of current, switching apparatus having variably operable contacts, means at said station for controlling said relay over said line thereby to control said apparatus to extend a connection from said line over said contacts, and means for bridging said relay winding across certain of said contacts in series with said source thereby to break down the contact resistance of said certain contacts.

10. Inan automatic telephone system, a subscribers station and a line therefor, a relay bridged across said line in series with a source of current, switching apparatus having variably operable contacts, means at said station for controlling said relay over said line thereby to control said apparatus to extend a connection from said line over said contacts, means for bridging said relay across the extended connection in series with said source, and means including said bridge of said relay and said source across the extended connection for causing said contacts over which said connection is extended to be traversed by current of sufficient value to substantially eliminate the contact resistance thereof.

11. A telephone system as claimed in claim 10, having a source of alternating current, wherein said relay, responsive to said control over said line, transmits to said apparatus impulses of alternating current from said last source to control said apparatus to extend said connection.

12. A telephone system as claimed in claim 10, wherein said apparatus includes a condenser, and wherein said relay, responsive to said control over said line, transmits to said condenser impulses of charging current to control the extension of said connection by said apparatus.

13. In a telephone system, a plurality of subscriber lines, a plurality of relays each bridged across one of said lines in series with a source of current, switching apparatus having variably operable contacts, means for controlling any of said relays over its associated line to transmit impulses of current to said apparatus, a condenser in said apparatus charged by said impulses and effective in accordance with said charging impulses control said apparatus to ex- 14. In a telephone system as claimed in claim 13, means for controlling, over said last line, the relay associated therewith thereby to transmit impulses to said apparatus, and impulse responsive means for controlling said apparatus, said last means including a condenser charged and discharged under a control of said last relay.

15. In a telephone system wherein the circuit for the transmission of speech between subscribers stations extends over one or more variably operable contacts, the method of preventing said contacts from adversely influencing said transmission of speech comprising dividing said circuit into a plurality of serially related sections so chosen that the end-sections include no variably operable contacts, impressing potential upon said end-sections to provide talking current for the subscribers stations which terminate said end-sections, and utilizing said potential to break down the contact resistance of variably operable contacts included in the section adjacent to each endsection.

16 In a telephone system wherein the circuit for the transmission of speech between subscribers stations extends overone or morevariably operable contacts, the method of preventing said contacts from adversely influencing said transmission of speech comprising dividing said circuit into a plurality of serially related sections so chosen that the sections which terminate at the subscribers stations include none of said con tacts, and, during the transmission of speech over said circuit, causing each section to be traversed by direct current which does not traverse any other section.

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