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Aug. 23, ‘19%. » R.‘ TAYLOR ETAL _ ZAQS?ZQ TELEPHONE SYSTEM Filed Aug. 19, 1943 YTI Y __U '2 Sheets-Sheét i‘ ' " \ INVENTORS 5 v REGINALD TAYLOR eeone; THOMAS BAKER ATTORNEY ' Au‘ 3» i» - R. TAYLOR ETAL. ,5, TELEPHONE SYSTEM -Filéd_ Aug. 19, 1945 I ‘I E 2 Sheets-Sheet 2 > t 3 ' lNVENTORS REGINALD TAYLOR GEORGE TH OM§$ KER Patented Aug. 13, 1946 2,405,620 UNITED STATES PATENT OFFICE. 2,405,620 TELEPHONE SYSTEM Reginald Taylor and George Thomas Baker, Liverpool, England, assignors, by mesne assign ments, to Automatic Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application August 19, 1943, Serial No. 499,194 In Great Britain September 17, 1942 18 Claims. 1 The present invention relates to telephone or like systems and is more particularly concerned with systems in which it is desired to effect the setting of automatic switches or the like by trains of impulses transmitted over considerable distances. ( Cl. 179-27) 2 which serves to e?ect the operation of a grid controlled gas discharge tube which is adapted to be intermittently operated subsequently in re sponse to the transmission of numerical im pulses. According to another feature of the invention, in a telephone or like system having signals transmitted over a. junction by the intermittent control is e?ected by repeatedly interrupting the completion of a direct current circuit thereover, ?ow of direct current over the control circuit, dif 10 the equipment at the incoming end includes a ?culties are experienced as the length of line in grid-controlled gas discharge tube which oper creases owing to the distorting effect produced by ates in response to each numerical impulse to the inductance and capacity of the line. Another control the repetition of that impulse to an au di?iculty which arises under long line dialling tomatic switch and also operates in response to a conditions is due to the impulse distortion pro 15 signal indicating that impulse sending is com duced by speech transmission condensers in asso pleted to introduce speaking conditions in the ciation with impedances which are connected to incoming equipment. the speech conductors. The removal of these The invention will be better understood from reactive elements from the line prior to ‘and dur the following description of one method of carry ing dialling out has hitherto involved consider 20 ing it into effect which should be taken in con able circuit complexity as well as impulse storage junction with the accompanying drawings com and regeneration. The chief object of the in prising Figs. 1 and 2, which should be arranged vention is to provide improved arrangements with Fig. 1 on the left. Fig. 1 shows the equip enabling direct current impulsing to be used ' ment associated with an operator's sleeve control satisfactorily over very long lines by taking suit position at the outgoing end of a long junction able steps to minimise the distortion which would line, while Fig. 2 shows the corresponding receiv otherwise be produced. In arrangements of the type normally em ployed in automatic telephone systems in which According to one feature of the invention, in a telephone or like system having‘ connections set ing equipment at the incoming end. The general arrangements are similar in some respects to those described in our application Se up over a junction in response to impulses trans- 30 rial No. 446,900, ?led June 13, 1942, where another mitted from an operator’s position, at the in coming end of the junction the circuits are nor mally conditioned for the reception of impulses solution of the general problem is disclosed, this however involving the use of mechanical regen erator at the. receiving end which is not neces sary with the arrangements according to the in an operation performed by the operator on the 35 vention. The setting up of a normal call will completion of impulse sending. and speaking conditions are set up in response to According to another feature of the invention, in a telephone or like system having connections set up over a junction in response to numerical impulses transmitted from an operator’s posi tion, each impulse is transmitted by the mo mentary completion of a direct current circuit ?rst be traced through briefly. When the line in question is free, relay SLR, Fig. 1, will not be operated and will co-operate with similar relays associated with other lines to complete a chain circuit for giving a visual indi cation of the next line to be used. If this is the line shown, when the operator inserts her over the junction while the establishment of cur plug to seize the line, no circuits are completed rent ?ow over the junction in the opposite direc over the line conductors but battery applied to tion in response to an operation performed by‘ 45 the sleeve of the jack J operates relay SR which the operator on the completion of impulse send in turn at its contacts. srl energises its relief ing serves to introduce speaking conditions in the relay SS. Thereupon at contacts ssl a circuit is incoming equipment. A further feature of the invention is that in a telephone or like system having signals trans mitted over a junction by the intermittent com pletion of a direct current circuit thereover, the equipment at the incoming end is initially pre pared for operation by the momentary comple tion of a direct current circuit over the junction ' completed for relay SLR which alters the visual idle indicator circuits so as to mark the next line as free, While at contacts ss2 ?icker earth over lead It! is connected up over the low resistance upper winding of relay SE to ?ash the operator’s supervisory lamp, thereby indicating that the equipment is ready for further operation. Relay SR remains operated under this condition and in 2,405,620 3 fact remains operated until the operator subse quently removes her plug from the jack, When the operator throws thedial key, a high resistance is included in the sleeve circuit and resistance battery is connected to both line con ductors so that relay RR is operated after a short interval. Relay A does not operate under this condition owing to high resistance of the resists . ance battery feed to line. Relay RR in operate “ 4 (on the grid of the tube GTA and triggering this tube into operation, whereupon relay PD is ener gised. ‘This relay then at contacts poll connects the discharged condenser QH in series with its winding whereupon the tube is at once extin guished by the substitution of negative for posi tive potential and as the condenser becomes charged the current falls until relay PD can no longer remain operated. With this arrangement the period of operation of relay PD may be read ing, at contacts rrl brings up‘ relay RP which ily adjusted by suitable choice of the value of at contacts rpé and M25 completes a battery feed condenser QI-I. Relay PD also at contacts 11:12 ing circuit over the junction including resistances completes a circuit for relay KO which looks up YK, YL, YH and YJ, contacts mrB and mrl, over its contacts kol, completes a, loop forward windings of relay RS, inductance Tl which with condenser QD and resistance YG forms a low 15 to the incoming ?rst selector at its contacts 7e02, prepares circuits at contacts 7e04, R05 and bot pass ?lter, and the primary winding of trans for relays PS, PB and KN and at contacts 7:03 former T2, Fig. 2, at the distant exchange. Re completes a circuit for relay CD. Relay CD is lay RS operates and at contacts rsl completes a slow-to-operate and consequently contacts cd2 circuit for relay DT to the earthed contacts ss3, do not complete a circuit for relay KN at this while at contacts 1'32 it holds relay vRP independ 20 time since relay PD will'have released before ently of relay RR. Relay DT in operating, at relay CD, operates. No further changes occur contacts dtl and (1253 completes locking circuits until the operator commences to dial. for itself and for relay RP and at contacts c2152 When the dial is moved off-normal, the bat disconnects the flicker earth from the jack sleeve and prepares a circuit for relay MR. This relay 25 tery connection to the T and R lines at the oper ator’s position is removed and these line con cannot operate owing tothe high resistance in ductors are left directly connected together over troduced in the sleeve circuit at the operator’s the dial springs. When these springs first open, position, but the resistance of relay MR is sum cient to extinguish the operator's supervisory . relay A releases and at contacts 0.! removes the lamp. Relay DT also at contacts dill shuntsvout 30 shunt across the battery connected to the junc tion line circuit so that line current again ?ows. relay RR and at contacts dt5 connects battery This has the effect of reducing the negative po via resistance YE to the other winding of relay tential on the grid of tube GTB and since its A which now operates. Relays A and .DT to gether at contacts a1 and CH6 shunt. the battery... anode circuit is now completed by the operation connection to the junction so that current falls 35 of relay KO, it strikes and operates relay PS. rDhis in turn at contacts psi brings up relay PB to zero in the winding of the distant transformer which at contacts pbl connects relay P2 to the anode of tube GTA in place of relay PD, at con The tubes GTA and GTB in the incoming equip tacts pbZ locks up, at contacts @173 releases relay ‘ ment are gas discharge tubes of the type in which. _ a grid controls the striking of the tube but‘ there 40 PS and extinguishes tube GTB, at contacts p124 repeats the impulse by opening the circuit to the after exercises no further control, the tube being ?rst selector, and at contacts pbS transfers relay extinguished only by a change in the anode cir CD to the P conductor extending back from this cuit. These tubes are preferably of the hot cath switch so that it is held up by the guard earth ode argon-?lled type. It will be noted that a circuit is normally completed for the ?laments 45 which was connected thereto when the loop was extended forward. of the tubes in series in a circuit including re When the impulse springs again close, relay A sistances YN, Y0 and YR. The grids of the reoperates and battery is again shunted off the tubes are connected to separate windings of T2. , , ' . line. Tube GTA is now triggered into action transformerT2 in such manner that one tube with the result that relay PZ operates and at 60 is caused to strike by a fall in the line‘ current contacts p22 effects the release of relay PB. Re and the other tube by a rise in the line current. lay PB again closes the loop to the ?rst selector The purpose of the arrangement shown whereby the negative bias of the tube GTA is derived by at contacts 12b4, transfers relay CD to its orig inal circuit at contacts M25, and at contacts pbl way of resistance YN which equals the hot re and p173 again prepares circuits for relays PD sistance of the ?lament of either tube GTA or and PS, tube GTA being extinguished and relay GTB is to ensure that the standing negative grid PZ releasing on the opening of the contacts pbl. bias on each tube with respect to its ?lament In response to the next break, relay PS is again and therefore the striking voltage will be sub operated and brings up relay PB and when the stantially the same in spite of the fact that the loop is again closed relay PZ operates and brings 60 ?laments being in series are at slightly different down PB. This operation continues during the potentials. The purpose of the condensers QF di?erent trains of impulses which are according and QG and associated resistances YM and Y? ly repeated to the ?rst and subsequent selectors and the position at the end of dialling is that . operation of relay RP at the outgoing end pro 65 only relays K0 and CD remain operated in the incoming equipment, relay KO being locked over duces a momentary voltage in the secondary contacts ski! and 7001 and relay CD being held windings of transformer T2, the effect of which locally over contacts 7e03, p126 and 7on3. isto reduce the negative bias on the grid of the The operator now restores the dialling key, tube GTB but it does not strike at this time owing whereuponv the high resistance introduced into 70 to the fact that its anode circuit is not yet closed. the sleeve circuit at her position is short-circuited Shortly thereafter when relays A and DT oper will be dealt with later. The initial closing of the line circuit on the ate, the current in the line circuit is again re duced to zero and a pulse is then produced in and this causes the operation of relay MR. This relay thereupon locks up over contacts mrZ to battery via resistance YB, prepares answering the ‘opposite direction by the transformer T2. This has the eifect of reducing the negative bias 75 supervisory circuits at contacts mr3, mrll and mr5, 5 2,405,620 opens the circuit of relay DT at contacts mrl, reverses the battery connections to the junction at contacts mr? and mi‘? and opens the shunt at contacts mr8 so that current ?ow over the junction now takes place but in the reverse direc tion to that previously flowing. Consequently, at the distant end, the impulse generated by transformer T2 at the distant ‘ex change is in the same direction as that previously produced by cessation of current flow over the line, that is to say, in the sense to cause the op eration of tube GTA. As a result relay PD is again operated and since relay CD is energised at this time, a circuit is now completed for relay KN from earth over contacts cd2 and lco?. Relay KN locks up at contacts .‘cnl dependent upon relay KO, opens the circuit of relay CD at contacts 7:713 and at contacts 7on2 completes a shunt across the speaking leads by way of re sistance YS to prevent the release of the estab lished connection during the changeover opera tion. On the release of relay CD after its slow period, relay SK is operated over contacts ccZ'l and 1on3 and thereupon locks up over contacts 0031, p126 and $703 to the P conductor. At contacts ski and slc2 it disconnects the primary winding of the transformer T2 and switches the line through, while at contacts sled it opens the circuit of relay KO. This relay releases after its slow period and in turn releases relay KN. The speaking leads are then switched straight through at the incoming relay set and the only relay remaining 6 for through supervision purposes. The operator now releases her cord circuit speaking key so as to cause a through circuit to be provided in the cord circuit between the calling and called parties, and vconversation now proceeds. At the end of conversation if the called party hangs up ?rst, relay D will release and at contacts (ll will re-light the operator’s supervisory lamp, while at contacts d2 the resistance battery poten tial will be removed from the R conductor so as to give through supervision to a distant originat ing operator in cases where the incoming call has arrived over a junction. The sleeve control op erator may now wait for clearing supervision 15 from the originating exchange and when she withdraws her plug from the jack J the sleeve relay SR will be released and will bring about the release of relays SS and MR. Relay MR in re leasing at contacts mr5 opens the loop via relays 20 D and I to the outgoing junction thereby initiat ing the release of the automatic switch train in the distant exchange. When this has been ef fected earth is removed from the P conductor to allow relay SK to release, whereupon both the 25 outgoing and incoming equipment is ready for further use. In case the Wanted subscriber is found to be busy, busy tone and ilash is returned over the speaking leads from the ?nal selector and relay I 30 ?ashes in response to the busy ?ash signals, and at contacts i2 ?ashes the sleeve control position operator’s battery viasupervisory resistance lamp, YF iswhile intermittentlyv at contactsap operated is relay SK which is locked up to the P conductor. plied to the R conductor so as to bring about the At the outgoing end, relay DT, in releasing in 35 flashing of the calling supervisory lamp of the response to the operation of relay MR, at con originating A operator in cases where the call has tacts diZ further prepares a circuit for giving arrived over a junction. The calling party is called party supervision to the operator. At con then advised and the connection is taken down, tacts (its the short-circuit is removed from relay whereupon release of the outgoing and incoming RR, which however cannot re-operate since the 40 equipments shown is effected in the manner al battery connection to the T and R lines was broken when the dial key was restored. At con tacts dt3 one looking circuit for relay RP is opened and the continued holding of this relay is now dependent on relay RS. When relay SK at the incoming end operates and initiates the release of relay KO, during the slow release of relay KO there will be no loop holding circuit for relay RS at the outgoing end, and this relay will thereupon release and in turn bring down relay RP. At contacts we and M05 relays D and I are now connected across the outgoing loop and on release of relay KO at the incoming end relay I will operate in series with the battery feeding relay of the ?nal selector at the distant exchange. Relay I in operating at contacts 2'! energise the polarising winding of relay D, at contacts 12 ex tends full earth back via the upper low resistance winding of relay SR to the sleeve circuit so as to ready described. The case must also be considered in which the desired call is not set up as just described, but is prematurely released by the sleeve control op~ eratc-r withdrawing the plug before completing dialling. In this case relays SR and SS are re leased by the disconnection of the sleeve circuit, while relay DT which is held from the earthed contacts 883 commences to release slowly. Relay A also releases but relay RP remains held during the release time of relay DT over contacts dtt and mi. On the release of relay A, current is again allowed to ilcw out through relay RS and over the outgoing loop to the transformer T2 so that relay re-operates and maintains the circuit of relay RP independently of relay DT. The cur rent flow over the loop produces an impulse in transformer T2 in such direction as tocause tube GTE to strike at the beginning of a normal light the operator’s supervisory lamp and indicate 60 dialled impulse, whereupon relay PS operates and that connection has been made with the called party but that he has not yet answered the call, while at contacts 13 the battery connection via resistance YF and the right hand winding of relay in turn brings up relay PB. Relay; PB looks over its contact pbfé and at its contacts gobs opens the loop to the selector train. Since this condition persists, the automatic switch train is released A is removed from the R conductor, this arrange 65 in due course earth is removed from the P ment being used as will be later described for conductor. Relay CD has been previously held through supervision purposes cases where the up over this circuit via contacts @735 and E03 and originating call has arrived over an incoming it will now commence to release slowly and when junction. it falls away a circuit is completed over contacts When the called party replies, battery reversal and cell for relay SK. Relay SK in operating takes place over the line from the distant ?nal at contacts ské opens the holding circuit of relay selector, whereupon relay D operates and at con KC and when this relay releases after its slow tacts dl extinguishes the operator’s supervisory lamp and at contacts d2 restores the connection of resistance battery potential to the R conductor period it brings down relay PB at contacts 7e05, whereupon the circuit for relay SK is opened at contacts and it also releases. 2,405,620 Meanwhile at the outgoing end the operation of relay It? with relay SS normal has the effect that at contacts m2 the busying battery via resistance YA maintains relay SLR and at contacts m3 bat. tery via resistance Y0 and contacts ss5 is applied to the jack sleeve for engaged test purposes in case this should be required. When relay SK at the incoming end operates, contacts skl and ski? open the holding circuit for relay RS at the outgoing '8 said trunk line at such times thereby to char acterize said trunk line as busy. 3. In a telephone system wherein call-directing impulses are transmitted from an operator’s posi tion in one exchange over a two-conductor trunk line to another exchange, and wherein said other exchange has numerical switching equipment, cir cuits in said other exchange normally conditioned to receive said impulses from said one exchange end, and since relay RP is dependent upon relay 10 and to control said equipment in accordance therewith, and means in said other exchange con RS,‘ this relay also releases and the outgoing trolled by an operation performed at said op equipment is completely restored to normal. At erator’s position after said impulses have been the incoming end when relay SK releases after sent for preparing circuits in said other exchange having released relays K0 and PB in turn, this for the transmission of speech. equipment also is ready for further use. 4. In a telephone system, two exchanges inter It will be understood that if for any reason connected by a trunk line, an operator’s position the automatic switches do not release so that in one of said exchanges, means under the control earth is maintained on the P conductor, relay CD of said position operated to transmit direct cur will remain held, relay SK will not be operated, rent impulses of a certain polarity over said trunk and current will continue to ?ow over the loop to lineto the other of said exchanges and then op maintain relay RS and in turn relay RP at the erated to transmit a direct current impulse of the outgoing end. Relay RP then maintains relay opposite polarity over said trunk line to said other SLR operated so as to prevent the line being in exchange, means in said other exchange for re cheated as idle, while the engaged test battery is ceiving the impulses of said certain polarity and maintained on the jack sleeve. Moreover, at con directing the extension of a connection in ac tacts 7113 the circuit for relay SR is held open so cordance therewith, said connection including a that if the operator should re-insert the plug normally disabled speech circuit, and means in after only a momentary withdrawal, this relay said other exchange controlled by the impulse of cannot re-operate until the equipment at the dis said opposite polarity for enabling said speech tant end has released and permitted relay RP to fall away. With regard to the paired condensers and re sistances QF, YM and QG, ‘YP which are asso circuit. ‘ - 5. In a telephone system, two exchanges inter connected by a trunk line, an operator’s position in one of said exchanges, means under the control ciated respectively with the tubes GTA and GTB, these serve to integrate the effect of impulses from 3: of said position operated to transmit numerical impulses over said trunk line to the other of said the windings of transformer T2 so that satis exchanges, a pair of grid controlled gas discharge factory operation from non-steep-fronted re tubes in said other exchange associated with said ceived signal waves can be obtained without in trunk line and ?red alternately by said impulses, volving false operation due to parasitic line volt_ ages of equivalent instantaneous value. These 40 means controlled by said tubes for repeating said impulses thereby to~ control the extension of a arrangements are dealt with more fully in the connection in accordance therewith, said connec speci?cation previously referred to. tion including a normally disabled speech circuit, It will be appreciated that the impulsing ar means responsive to an operation performed at rangement shown whereby battery is shunted off said position after the transmission of said im the line is preferable to the more useful method pulses for again ?ring the tube last ?red by said of opening the line as it permits the rise and fall impulses, and means controlled by said tube for of current to follow substantially similar curves thereupon enabling said speech circuit. and thus enables the two gas discharge tubes to 6. In a telephone system, two exchanges inter operate under substantially identical conditions. connected by a trunk line, means in one of said It will be seen also that with the arrangement exchanges operated at will to complete a direct shown the ratio of the impulses supplied to the current circuit over said trunk line, an interrupter automatic switches is dependent on the ratio of in said circuit at said one exchange then oper the dialled impulsesrbut that this ratio is not ap ated to transmit impulses over said trunk line, a preciably distorted since the factors which pro grid controlled gas discharge tube associated with duce distortion with relay repetition are not op said trunk line in the other of said exchanges, erative on the improved circuit. . What we claim as new and desire to secure by said tube operated initially by the completion of said circuit and reoperated by each of said im Letters Patent is: pulses, equipment conditioned by said initial op 1. In a telephone system, two exchanges inter eration of said tube and thereafter controlled by 60 connected by a trunk line, an operator’s position the reoperations of said tube to direct the exten in one of said exchanges, means operated under sion of a connection in accordance with said im the control of said position to transmit call di pulses. recting impulses over said trunk line to the other 7. In a telephone system, two exchanges inter of said exchanges, circuits in said other exchange normally conditioned to receive said impulses and 65 connected by a trunk line, means in one of said exchanges operated to transmit direct current im to control the extension of a connection in ac pulses over said trunk line to the other of said cordance therewith, said connection including a exchanges, a gas discharge tube associated with normally disabled speech circuit, and means for said trunk line in the other of said exchanges and enabling said speech circuit responsive to an op eration performed at said position after the trans 70 operated intermittently by said impulses, means controlled by said tube for directing the extension mission of said call directing impulses. of a connection in accordance with said impulses, 2. In a telephone system as claimed in claim 1, said connection including a normally disabled wherein said connection is maintained at times speech circuit, means in said one exchange oper after said trunk line has been released at said po. ated after said impulses have been transmitted 75 sition, a relay in said one exchange operated over 2,405,620 10 to transmit a further'impulse over said trunk 1ine, and means operated by said tube responsive to said further impulse to enable said speech circuit. 8. In a telephone system, two exchanges intere connected by a trunk line, means in one of said exchanges operated at will to complete a direct necting said receiving equipment from said trunk line and connecting to said trunk line the ex tended speech path. 14. In a telephone system, a line, means for current circuit over said trunk line, an interrupter in said circuit at said one exchange then oper ated to transmit impulses over said trunk line, a pair of gas discharge tubes in the other of said 10 exchanges, means whereby one of said tubes but not the other operates responsive to the comple tion of said circuit and said two tubes thereafter operate respectively at the beginning and end of each impulse, equipment conditioned by the ini 15 tial operation of said one tube and thereafter con trolled conjointly by said two tubes to direct the extension of a connection in accordance with said impulses. \ 9. In a system as claimed in claim 8, wherein each of said tubes has a grid and a cathode, a cir cuit for heating said cathodes including both said cathodes in series, and means for normally apply ing to the two grids a slightly different bias to compensate for the di?erence in mean potentials of said cathodes, whereby said tubes require sig nals of substantially equal strength to be applied ‘ impressing direct current impulses upon said line, impulse receiving equipment normally connected to said line, said equipment including a ?rst gas discharge tube operated responsive to the begin ning of each impulse and a second gas discharge tube operated responsive to the end of each im pulse, meanscontrolled conjointly by said tubes to direct the extension of a speech path, means operated after the last of said impulses to impress upon said line such a potential as to reoperate said second tube without ?rst reoperating said ?rst tube, and means thereupon controlled by said second tube to disconnect said impulse receiving equipment from said line and to connect said speech path to said line. 15. In a telephone system, two exchanges inter connected by a trunk line, means in one of said exchanges operated to transmit direct current impulses of a certain polarity over said trunk line and then operated to transmit a direct current impulse of the opposite polarity over said trunk line, a gas discharge tube in the other of said exchanges, means whereby said tube operates in to their respective grids to bring about the opera termittently responsive to the impulses of said tion of said tubes. certain polarity and causes a connection to be 10. In a telephone system, a line, means for 30 extended in accordance with said impulses, said impressing direct current impulses upon said line, connection including a disabled speech circuit, a pair of grid controlled gas discharge tubes so associated with said line that one of said tubes op erates responsive to the beginning of each impulse and the other operates responsive to the end of ‘ each impulse, means controlled conjointly by said and means whereby said tube operates responsive to the impulse of said opposite polarity thereby to enable said speech circuit. 16. In a telephone system, two exchanges inter tubes to direct the extension of a connection in connected by a trunk line, means in one of said exchanges operated at will to complete a direct accordance with said impulses, said connection current circuit over said trunk line, an inter including a disabled speech circuit, means oper rupter in said circuit at said one exchange then ated after the last of said impulses to impress 4.0 operated to transmit impulses over said trunk line, upon said line such a potential as to reoperate a pair of gas discharge tubes in the other of said said other tube without ?rst reoperating said one exchanges, means whereby one of said tubes op erates responsive to the completion of said cir tube, and means thereupon controlled by said other tube to enable said speech circuit. cuit and thereafter operates responsive to the 11. A telephone system as claimed in claim 10, end of each impulse, means controlled by the wherein said tubes are inductively connected to initial operation of said one tube for conditioning said line so that any particular change of current the other of said tubes for operation whereby in said line causes potentials of opposite polarity said other tube thereafter operates responsive to to be impressed simultaneously upon the grids of the beginning of each impulse, and equipment said two tubes. conditioned by the initial operation of said one ' 50 12. In a telephone system as claimed in claim 10, wherein said one tube has a normally open anode circuit and said other tube has a normally tube and thereafter controlled conjointly by said two tubes to direct the extension of a connection in accordance with said impulses. 17. In a telephone system, two exchanges inter closed anode circuit, means for controlling said other tube over said line before said impulses are 55 connected by a trunk line, means in one of said exchanges operated at will to complete a direct impressed upon said line, thereby to cause said current circuit over said trunk line, an interrupter other tube to energize its anode circuit, and means in said circuit at said one exchange then operated controlled by said energization of the anode cir to transmit impulses over said trunk line, a trans cuit of said other tube for causing the anode cir cuit of said one tube to be closed, thereby to pre 60 former in the other of said exchanges having its pare said one tube to respond to the beginning primary winding connected to said trunk line, a of each impulse. grid-controlled gas discharge tube having its grid connected to a secondary winding of said trans 13. In a telephone system, two exchanges inter former so that said tube is brought to its critical connected by a trunk line, said trunk line com- I prising not more than two conductors, an oper 65 grid potential responsive to the completion of ator’s position in one of said exchanges, means operated under the control of said position to transmit call directing impulses over said trunk said circuit and also responsive to the end of each impulse, another grid controlled gas discharge tube having its grid connected to a secondary winding of said transformer so that said other line, impulse receiving equipment in the other of said exchanges normally connected to said trunk 70 tube is brought to its critical grid potential re sponsive to the beginning of each impulse, each line for responding to said impulses and causing tube effective to operate when brought to its criti a speech path to be extended in accordance there cal grid potential, equipment conditioned by the with, and means operated responsive to an opera tion performed at said position after the trans operation of said ?rst tube responsive to the ini mission of said call directing impulses for discon 75 tial completion of said circuit and thereafter con trolled conjointly by said two tubes to direct the 2,405,620" 11 extension of a connection in accordance with said 12 ing a discharged condenser and, in series there~ with, a, source of potential so poled with respect ' to'said ?rst source as to aid said ?rst source in 18. In combination, a circuit including in series charging said condenser through the winding of a source of potential, the winding of a relay and relay, the condenser charging current hence a, gas discharge tube, said tube normally prevent C1 said maintaining said relay operated for an interval ing said source from causing current to ?ow over after the discharge through said tube has ceased, said circuit, means for momentarily applying to and said relay eifective to restore to normal when said tube such a potential as to initiate the dis the condenser charging current ceases to ?ow due charge thereof, thereby to cause current to flow over said circuit and operate said relay, a circuit 10 to the condenser becoming charged. connected by said relay upon operation in bridge REGINALD TAYLOR. of the discharge path through said tube thereby GEORGE THOMAS BAICER. to extinguish said tube, said bridge circuit includ impulses.