US1790091A - Private automatic exchange system - Google Patents

Description

Jan. 27, 1931. T. F. CROCKER iRIVATE AUTOMATIC EXCHANGE SYSTEM Filed Feb. '29, 1928 2 Sheets-Sheet 1 CG PAX In 1" 1.ren ur Thomas F Cracker Jan. 27, 1931.

T. F. CROCKER PRIVATE AUTOMATIC EXCHANGE SYSTEM Filed Feb. 29, 1928 2 Sheets-Sheet 2 Im Thomas F Cracker Patented Jan. 27, 1931 l PTA-TENT. oFF-Ica live 0,091...

magm s. cnocxnn, QECHICAGO; rumors; nssieivonmo nnsnnvn HOLDING coin .PANY OFBKANSASIZCITYQMISSOUBI, A conrone'r en or DELAWARE rnwnrnnuroiymric nxcnnncn SYSTEM Application filedlebruary 29, 1928. "Serial No. 258,108..

type having a cord-splitting key by means 7 of which the attendant, after setting up a connection to a called; subscribers line at the request of a calling; subscriber, mayconverse with thecalled subscriber without the calling subscriber hearingthe conversation. A par- 2 ticular feature of this invention is the provision of means in the cord; circuitwhich re- .quiresthe attendant to press. a special connect keybefore the actual talking circuit betweenthe called and the calling subscriber can be establishechthough sheinay converse.

with either separately before operating the special connect key.

Another feature of this invention is. that 1 should the attendant not care to converse -with the called subscriber separately before completing the talking connection abetween thecalling and the called subscriber, she may operate thevspecial con-nect key immediately so'that," the talking, connection between the calling and the called subscriber will be automatically completed as soon as the called subscriber answers.

Another; featu-reof this invention is that d after a connectionhas been set up and the 4 talking connection can again be completed with the first calling party, should the called party again remove his receiver, until the special connect key is again operatedattendant is. employed called party has hung up. his receiver, 110- Another feature of this invention resides in the arrangement for preventing the application. of ringing current on a 'busy line on a called line] after the called subscriber has answered, or for preventing any interference with the setting up of a connection,.shouldjthe attendantinsert theplugof her cord into a busy line jack andattempt to ring, or attempt to ring after a called subscriber hasanswered, or insert the plug of her'cord into a busy trunk 'j ack and attempt "to dial out.

Another feature of" the invention-is. the. f

manner in which aftrunk-lineextending toia maln exchange. .1s .-marked busy at the private automatic exchange when the trunk is' ta k'en.

for use atth'e main excl'iange Thetrunk is marked-busy by a high resistance relay atthe private automatic'exchange, which is COIlIlQCll-'. ed to one '.conduc,tor of the. trunk? line andtto' ground. This jrelayis energized whe the trunk is first seized in series withthe. ring;

cut-off relay ofpthe connector seizing the. trunk at themaineX hange, andis energized in series with the back-bridge relay" of the connector, if the trunk is h'elclfromthe main exchange'after it the'p'rivate automaticexchange. Y

has been 7 released at Theabove mentioned features and others 7 not particularly mentioned at this time will be described in detail in the following specification. with the 'aiol-of'the. accompanying.

drawingscomprising Figs. 1 and'fz. These drawings, whenFig. 2is placed to the right of Iilig. 1, illustrateby means of the usual Circuit diagrams a schematic layout 'o fa private automatic exchange systenrconnected by' means of two-way trunkli'nes with a main automatic exchange. i

The apparatus shown in Fig. 1, and that shown tothe left of the dotted vertical line in Fig2. is in the private automatic exchange," designated P. A. X;, while thatshown to the f right'of the dotted vertical linein Fig.2 2,

located in the main automatic. exchange, designated M. A. X 1 The trunk'compIisingc0n:-.

ductors, 201 and 202 isoneof the'trunk's connecting the P. A. X. with the M. A. X. At

theltl. A. X. this trunk terminates in the rotary l1nesw1tch LS and 1s accessible to a group of connectors, such'as connector C tical and rotar t e havin an automatic ror tary movement on the level in which the inter-otiicevtrunk 'lines are accessible A cone nector of this type is shown and described in the Jacobson Patent No. 1,349,381, granted August 10, 1920. In-the system chosento illustrate this. invention the main exchange, is assumed to be an automatic exchange, though the private automatic. exchange-is equally capable of being connected with a manual main exchange. 7 1

At the P. A. the trunk comprising conductors 201 and 202 terminates at the attend ants cabinet in the jack J and is provided with a call lanipll and a busyvisual VIS.

The cord CC is one of a plurality of cord circuits provided for the attendant for completing calls from the M. A. X. for subscribers at the P. A. This cord circuit terminates in the plugs P and Pand is provided with a connect and'ringing key RK, and a cord key CK for associating the operators telephone set TS and the common equipment comprising the telephone set relays TSR, the cord splitting key ,GSK, the dial key DK, and

the dial D with this particular cord.

One of the private automatic exchange subscribers lines, namely, the line of substation A, is shown, terminating in the lineswitch LS and having the individual line jack J The lineswitch LS may be of the'usual rotary type like the rotary lineswitch LS This lineswitch has access with other similar lineswitches to a group of selectors suchas the selector S. The selector S may be of the usual Strowger vertical and rotary type, and

has access to a number of groups of connectors, such as connector C.

5 In one particular level, the selectors such as selector S have access to the M. A. 'X. and in another level to key-ending trunks extending to the attendants position. A In the banks of the connectors access is had'to the individ u al subscriberslines.

:With this general description ofithe'efeatu'res of the invention and-the apparatus in;

volved in mi'nd,'a further understanding of the invention may best be gained "from a detailed description of the operations involved in completing-connections. For this purpose it will first be assumed that asubscribenat the M. A. X. wishes to converse with the subscriber at substation A. It will be assumed that the calling subscriber has extended the connection to. the connector G which has been operated to'seizethe trunk to the P. A. X. via the branch comprising conductors the trunk busy to the selectors.

separating'contacts 211 and 218 in the usual manner. 7

After relay 245 of connector C is operated, a circuit is closed for the busy guard relay 110, at'the P. A. X. which includescontacts 101, conductor 201, conductor 201, and the upper Winding of the ring cut-ofi'relay 250 of the connector (3?. The resistance ofjrelay 110 is sufli'ciently' high to prevent a suffi'cient' fiow of current in this circuit to cause the operation of the ring cut-ofl' relay 250 at this time. Relay 110 atthe P. A. X. operates, however, and closesthelcircuit for the busy visual VIS at contacts 111 to mark this trunk busy at the attendants position and'also closes a circuit'for relay 115 at contacts112g Relay 11'?) operates and at contacts 116 grounds the release conductor 206 to mark Relay 245 of connector C in operating, also causes ringing current'to be intermittently applied to'the trunk conductors, causing the operation of the ringing-response re lay 106 bridged across the trunk conductors at the P. A. X. Relay 106, in closing contacts 107, closes a locking circuit for its lower winding and also closes a'circuit for the call lamp L The lighting of lamp L apprises the attendant of the incoming call.

To answer the call, the attendant inserts the answering plug of an idle cord, such as plug P of cord circuit CC; into jack J As aresult, relay 30 and-relay operate in series. Relay 100, in operating, disconnects relays and 106 from the upper talking conductor at'contacts 101, closes an auxiliary circuit for busy visual VIS at contacts 103, at the same bility of relay/106 being operated'and locking at the end of the call when the ringingbridge is again placed across the trunk conductors upon the removal or plug P fromj ack J Relay 110 restores as soon as it is discon' nected from the uppertrunk conductor. opening the initial-circuit of the visual VIS and the circuit of relay 115. Relayrestores atterwhich the trunk is marked busy by the presence of ground through the low-resistance winding'of relay 30 via contacts 117.

Relay 41 also operates when the plug P is inserted in the jack J from ringing current passing through its lower winding. This relay, in operating, closes a locking circuit for its upper winding at contacts 42, at thesametiineclosinga circuit for'the supervisory l'am'piL- r v Aft-er inserting the plug P into jack J? the attendant operates the cord key CK. iAs

aresult, relayl24 operates overthe circuit in cluding contacts 82 and 81'. This relay con- 32being closed. The operation of key CK also closes a circuit for relay35 in series with relay 50, .the circuit includi11g contacts-83.

The short-circuit around relay 35 is opened by theoperation of relay BOat contacts 83. Relay 35, in operating, opensthe circuit of lamp L and the holding circuit of. relay 41 atcontacts 38, and connects the common conductors 74 and 75 to the tip and ring conductors ofplug P. A ring-tripbridge has now been closed across'the trunk conductors for causing the operation of ring cut-off relay 250 in the connector C2,v which extends from the tip of plug P, contacts 36, conductor 7 5,

1 upper normally closed contacts of keys DK and CSK,.c0nductor 71', contacts 87 and 27, the upper right hand. winding of repeating coil R contacts 25, and 32, the lower right hand Winding of repeating coil.R,= contacts 28 and 88, conductor 72, lowerinormally closed contacts of keys CSK and DK, conductor 7 4, contacts 39, to the ring of plug P. Relay 250 therefore operates and trips-the ringing inthe-usual manner and closes points in the talking connection. The back-bridge 255 then operates over thetwo trunkronductors and the bridge across the tip andring of plug P with the usual results.

Relay 50, in operating in series with'relay' 35, opens a point in the receiver circuit at contacts 51 and closes a circuit for relay 55 at contacts 52. Relay 55 operates and closes a pointin the receiver circuit atcontacts 56,- closes a'holding circuit for itself at contacts 57, and disconnects relay '50. Shortly after its circuit is opened, relay 5'0 deenergizes andby clos1ngcontacts51 closest-he receiver 011- cuit. Thereafter relay 55 is held energized in serieswithrelay 35-. By so dclayingthe closureiofthe receiver circuit. until some time after the ring is tripped and the current over the trunk conductors has become steady, annoying clicks in the receiver are prevented. The'attendant may now converse withthe calling-subscriber at. the mainautomatic eX- change to learn the name of the subscriber with whom connection is desired. After obtaining this information, the attendant touches the tip of the plugPitothe sleeve of jack J (having assumed that the subscriber at substation A is the called subscriber) to test the line for the busy or idle condition in the usual manner. Finding the line idle, the

' attendant-inserts plug P into jack J. Relay D 2 operates inseries with. the-.cutoff :rela-y '97 the circuit of the of the lineswitch LS, closingpointsin the:

contacts 5.

After inserting plug .jack J, the 555,,

tendantoperates key RK'tothe right .topro-Q talking circuit at. contacts 4-an'd 6, and closing.

supervisory lamp Lat] jectringing current out on the calledsub fy scribers line. contacts 14 and .19, and contactsf4and 6 5 When the subscriber. at substation. A

answers by removing his receiver, acircuit is closed for battery feed relay 10 Relay 10, in operating,.opens the circuit oflamp L at contacts 11 and prepares a circuit for re:-' 7, I

lay 15 at contacts 12. The extinguishment of lamp L notifies the attendant that the called subscriber has answered.

Before completing the talkingconnection V with the calling subscriber, the operator may common cordsplitting key CSK to theright.

This operation disconnects .thecommon leads 74 and 75 from the operators talking; conductors 76 and 77, and at contacts 84 places a holding bridge including resistance 85 across the tip and ring conductors. Operating key CSK to the right also opens the circuit of relay'24 atcontacts 81. Relay. 24,..in deenergizing, connects the common talking. leads 71 and 72 on the tip and ring conduc-- tors of plugP at contacts-26 and-29 via con tacts on the ringing key RK. The attendant may now speak with the subscriber at substation A' without the calling party hearing. the conversation. If the subscriber at substation A does not wish to speak with thecalling subscriber, the attendant by again re- The ringing circuit includes.

key OK to disassociate her'head'set and. common apparatus from the cordcircuit-CC.

Operating'key RK. momentarily to the. left closes a circuit for relay 20.. Relay 20 energizes and closes a holding circuit for ie I "lay. 24 at contacts 23, closes a point in talking connection at contacts 22, and closes aholding circuitfor itself inseries with relay 15 at contacts 21.v Relay'15 operates and'at contacts 16 and 18 completes theftalkingcircuit, and byseparating contacts 14 and. 19 disconnects theoperating springs of the ring:- ingkeyRKifrom conductors 95 and 96*. The calling and called subscribers mayfn'ow cons verse with each other. restored, the talking circuit extends through the springs of keys CSK and DK. When key CK. is: restored, the. circuit of; relay; 35

Before key or; is

and relay 5:5 is opened at con'tacts831 Relay 55vinrestoring, places the telephoneset-relays TSRin their normal condition. Relay 35, in. ,deenergizing, connects the talking conductors 93 and 9a to the tip and ring otplug P directly, thereby. completing thetalkingcircuit' between the calling and the called subscriber completed, she may do so by again asso.ciat-' ing/the'common equipment including key CSK with the cord circuit CC by operating the cord kcv GK, and then operating key CSK to the right or left depending 'on which party she wishesto speak with. Throwing key CSKto the left disconnects the common talking conductors 71 and 72 from theopen ators talking conductors 76 and 77, and also opens points in the holding circuit across the tip and'ring ofplug P extending through the springs of key CSK whenever key CK is operated, but'asubstitute one is provided through the secondary windings ofthe operators induction coil when condenser 78 is short-circuited at contacts 86.

At the termination of the conversation both subscribers will replace their receivers. Responsive to the replacement of the receiver at substation A, relay restores thereby opening the circuit of relays and at contacts 12, and closing the circuit of super-' v'sory lamp L at contacts 11. Relay 15, in deenergizing, again opens the talking circuit at contacts 16 and 18. Relay 20, in deenergia ing, opens the holding circuit of'relay 24. Relay 2% deenergizes and at contacts opens the direct current bridge across the tip and ring conductors of P thereby opening the circuit of back-bridge relay 255 of connector C at the main automatic exchange. The release of back-bridge relay 255 in the well known manner causes the release of the connect-or C2, providing .thecalling subscriber has previously replaced his receiver If the calling party, however, does not replace his receiver, the connector C as well as the other automatic switches usedin the connection, will not be released until he does so, as is well known I v, I

Should-the called subscriber again remove his receiver .after he has hung up, even though the plugs P and P are in jacks J and J and the calling subscriber is still holding the'connection' to the P. A. X. no talking connection can again be completed since relays 155ml 20 are de'energize'd. Of course, if the attendant again operated keyRK to the right thetalking connection would be completed as previously described. The lighted condition of the lamp L after conversation has taken place indicatesto the attendant-that the conversation is terminated.

She will then remove the-plugs P and- 'P of 100 in deenergizing' again places the trunk to the'niain exchange in normal condition. If,

. however, the calling subscriber is still hold ing the connection, relay llOwill again oper-. ate, this time in serieswith the upper wind ing of back-bridge relay 255 of co-nnector'C Relay 110 will again function to operate the busy .visual'VlS, and mar; .thetrunk busy to-the selectors. 1 2" As noted above, the trunk is marked idle at the M. A. X. as soon as both the'calling and the called partyreplace their receivers. "For I this reason a second call can be extendedfrom themai-n automatic exchtngeto the private automatic'exchange over the trunk comprising conductors 201 and 202 before the attendant has removedplugs P and P from jacks J and J after the termination of acall. such a case the connector seizing the trunk causes the operation of relay 110 and projects ringing current over the trunk conductors as previously described. Again relay 41 responds, closing the circuit for lamp L and a holding circuit for its upper winding at contacts 42. The lighting of the lamp L at this time notifies the attendant that a second call has come in over the trunk, while the li hted condition of lainp L indicates to her that the previous connection has been released. It shouldbe particularly noted that ringing current over the trunk conductors 'at thistime cannot resignal the-subscriber at substation A in whose jack the plug P rests, since'the talking connection to the subscriber at substation A was opened upon the releaseof relay 15. The attendant may now answer the second call inthe regular manner. The presence'of plug 'P in jack J has no effect. After learning the wants of the calling subscriber she may remove the plugP from jack J' and insert it into the proper jack andcall the calledsubscriber as previously described.

If the attendant, after receiving an incoming call fromthe main automatic exchange knows that the called subscriber is waiting for a call from this particular calling sub scriber, it is unnecessary for her to first speak with the called subscriber. She may call the called subscriber by operating the ringing key BK to the right (having set up the connection to the called line by inserting the plug Pinto'the j ack; J) and immediately op- "erate the key BK to the left momentarily before the called subscriber answers. Theoperationfof key RK to the left prepares the cord circuit CO in such a way that the talk ing connection willfbe established as soon as the called subscriber remove his receiver. This operation closes the circuit for relay 20 which again 'operates and closes a locking circuit at contacts 21,-th1s;t1me over a circuit including contacts .13., 17, and 5, relaylO not being energized because the called subscriber has not yet answered. After'operatingwthe l-tey Elite the left nionientarily,f the attendant may disassociate 'her common: equipment including her "telephone set itroin the; cord circuit'GC'by restoring key GK and attend to other call s. dVhen the :called subscriber answers, relay again energizes 'and byclosing contacts: :12 again causes "the operationz'of-relay 15in series with relay 20, With the results aspreviously described In this system it .is also possible for any :of the'subscribersisuch as thesubscriber .at substation A to extend a call to the automaticjex changeover Zone of theinterconnect-ing trunk lines, these trunks being accessible in apartlCll1!JI l8'V.l IiIl/thGfbilIlkS of the selectors.

Forexample," when a call is extended from.

the vsubstation-A.-Via the. individual .lineswitch LS and the .selector .S tothe trunk line comprising conductors .201. and 202 by Way of the branchfcoinprising conductors204 to 206, the 'lineswitch LS operates inthe usual Way and extendsithe connection to a selector in the main fautoma'tic exchange. When the trunk line is seized, ground potential is placed onrelease conductor 206 thereby closing a circuit "for rel'ay lOO. Relay F operates and' disconnects the 'ringingbridge from 1 across the trunk} conduoto'rsiat contacts 101 and. :Jand closes acir'cuitifor thebusy 7 visual lSgati'contacts 103; "Re'lay. also operates in series with the trunk conductor 201 and at c0ntactsii126-fcl'oses a'circuit for release relaylZO. iR'elay 120 directly grounds release con j uctor 106 at contacts 121- to main tain the automatic switches in the P. A. X. operated "and imaintainv this trunk marked busy. The subscriber"at 'substation .A, by

dialing the proper "number controls the setting up of theconnectioninthe usual manner. When the subscribers replace their receivers at the termination of the call, the operated automatic switches'at the P. and the .60 I the lamp L? indicates to the attendant that someiin-iorm'ation is wanted. Toanswer the call the attendant operates key K .As a result VreIa yZQGO is operated over the circuit including 7 contacts 28i. -lhe lamp L. as ex:

tinguished; Atcontacts 282, a circuit is closed :for relay 6510f the (telephone set rela s w'IS-R. Relay-65, inenergizing, connects t e telephone circuit to the common conductors 291,292, and; 293; At contacts "68, aci1fcuit causesfthe energization-of-relay 155, with the results previously described. 7 y In the systemdisclosed, an attendant may also [set up =.a; so-called' tail-to-tail connection; that is, should she be asked by one of the P. 'subscribersgto set up a connec tion for him-to the M. A.'X. she'may insert the plug P into-the jack of'an idle trunk extending to the main automatic exchange, suc'ha-s the jack J2 for example, then operate the corresponding cord key CKgthen operate-the common dial key vDK, and-then dial the desired number. Inserting the plug P i'ntojack J againcloses a-circ'uit, for relays i 30-and-'-100- in series. These relaysoperate, performing the same circuit changes as previously described; The operation'of keyGK closes a circuit of relay-.35 which operates and connects conductors 74'and 75 'to. the

tip andring of plug P. :Operatingthe-dial key DKbrides the dial D across conductors 1 74 and 75.

manner. This selector and .otlierautomatic l ines-witchLS operates andfe'x-' tends-the-connection to a'selectoringtheusual j switches in theinain "automatic exchange re: spond to the "mani ulations of the-dial; D in accordance Withthe call-ed'subscribers inums I berjto extend the connection tol the called After setting up *the connection to the called subscriber the attendant may call the A. X. {subscriber who requested theconline.

nection in the same manner-as 'though'the call had beenfinitiated .by the called-subscriber rat themain automatic exchange. In this case, she may-also operate the key BK mo- 'mentarily tothe left immediately and disassociate the common equipment from the cord circuit by restoring rkey' CK before swered,-or.she'mayiconverse "with the P; A. X. subscriberafte'r he answers without the other'party hearing the conversation the same as previouslydescribed. I V

-Aparticular characteristic of-this inven tion is'that should an attendant attempt to extend a call to. a P. A. X.. sulbscriber already busy in another connection'or engaged in theprocessbf setting up a connectio'n, the plugging in of' 'the plug of'acord i-ntohis line j aokwill .not interfere with the exist ing connection or with the setting'up ofthe other connection; For example, should the attendant linsert plug .P into jack J. (ignore ing the busy test while the subscriber at sub; station. A isjengaged .insetting up automatic V switches inf'the P; A. X., relay 2 could not operate vsince direct groundpotential would be present gathesleeve Qt J. Inthi either the calling -or called partyhas an- 70 is also closed for fire-lay 50 whichenergizes and case lamp 'L would not light whichwould furtherindica-te to the attendant that the line wasbusy'. Evenifshe also ignored this indication and attempted to ring'by-operating key RK' to-the left, no ringing-current could be projected out on the connection by reason of the open condition of contacts 4 and 6.

-It is also'impossible for the attendant to project ringingcurrent on the called line I after-the talking connection between thecallingand called subscriber has been completed. 'When relay -15 0perates and completes the talking-connection, the ringing leads are disconnected from conductors 95 and 96 at contacts 14 and 19. Thereafter any operation of key BK is ineffective.

' Furthermore, the attendantcan not interfere with theYsetting-upofa connection to the main exchange by the calling P. A. X. subscriber by inserting plug P into the jack of the'trunkextending to the main exchange over whichthe connection is being set up. For example, should the attendant insert plug P into jack J while the subscriber at substation A is engaged in the process of setting up a connection to the main automat-i0 exchange overthe trunk comprising conductors 201- and 202, no harm would resu'lt; Since ground potential would be present'on jack J relay '30 could not operate in series with relay 100. WVith relay 30 renaining in a 'deenergized position relay would remainshort-circuited at contacts so that throwing cord key GK and the dialling key DK would not bridge thedial' D across the tip and ring of plu-giPl By relay 35 remaining in a deenergized condi ti'on the tip and ring of plug P remain connected to conductors 93 and 94c. Throw-- ing key CK would, however, cause the operation of relay 2a and the operation of relaysand as previously described. The attendantis head set would then be r bridged across the tipand ring of plug'P so that she would be advised of her error by hearing clicks produced by the dialling operation or byhea'ring the conversation. v r

' This completes the description of the op eration of the invention. Whatis considered new and is -desired" t o have protected by Letters Patent is specifically stated in the appended claims. f

'jVVhat is claimed is:

p 1. In a'telephone'system, a line,a cord cir relay in said cord, means operable at will for operating said self-locking relay, a second relay responsive to the answering of the call, and means for connecting said sections jointly controlledby said self-locking relay and said secondrelay.

p -3. Ina telephone system, aline, a cord circuit comprising two normally disconnected sections, means for connecting'said cord circuit to said'line-to extend a call, a key, means responsiveto a.momentary operation of said key after the subscriber at the station on. the line has removed his receiver to answer the call for connecting said sections, and means responsive to the replacement of the receiver for deactuating said last means so that-it cannot again be operated should the receiver again be removed until'saidkey'is again oper ate 14:- Ina telephone system a. line, acord circuit-comprising 'two normally disconnected sections, means for connecting said cord circuit to said line to extend a call, a key, and means responsive to the answering of the call providing said keyYis-operated before thecall is answered, and: responsive to the operation of said key providing the'c'all is V answered before the key is operated, for connecting said sections.

' 55in a telephone system,'a cuit' comprising two normallydisconnected sections,-means for connecting said cord circuit to :saidrlineto extend a call, means for signalling the subscriber on saidpline, key

springs, and means responsive: a momen-' taryoperat1onofsa1d key spr ngs after the call is answered for connecting said sections and .for disabling saidr'signalling means.

-6. In'acord'circuit comprising two normally disconnected sections, a ringing key and leads; extending from said key to the talking strands of the, cord-circuit, a relay for connecting said two sections;.and contacts son-said relay for disconnecting said leads from said talkingstrands zpi 7, ,In 'a telephone systein,; -a cord circuit having a.-fir st and, asecond plug, a pair of talking conductors having two branches, means for connecting the first branch-,to'the first plug, means for disconnecting the first branch from the pair of talking conductors, and meansresponsive to said .last means for connecting said second branchto said second plug. I V

- 8. In a telephone system, a cord circuit,a pair of "talking conductorsf having two branches, a key in said cord, connecting means responsive tothe operation of said key undercertain conditions for connecting the-first branch to one end-of said cord cirline, a cord cir- 1 cuit, means for maintaining said connecting I i nveogom cuit, providing :said connecting n1eans are maintained IDQP'GTa tWQ.

- 9. in a telephone system, I a i plug ending cord cirouit, a jack ending {tr-unk, a pair of talking "conductorg apair-aof control conducltorsa key, and means responsive to :the oper' ation of said key after the pl-ug of said :cord

i'siinserted into fsaid j a'c'kfor-"connectingsaid control conductors to -sai'd plu g, providing said trunk idle, and for connecting said;

talking conductors to s'aitl plug, providing the trunk is busy.

10. In a telephone system, a first exchange, a second exchange, an interconnecting trunk line, a switch at said second exchange having access to said trunk, a ring cut-ofi" relay inv said switch, a busy guard relay at said firstv exchange connected to one conductor of said trunk line and to ground, and energized inseries with said ring cut-ofl relay responsive to the seizure of said trunk line by said switch in the extension of a call to said first exchange, means cont-rolled by said busy guard relay for marking said trunk line busy at said first exchange, a' ringing bridge including a ringing relay across the conductors of said trunk line at said first exchange, means in said switch for projecting ringing current on the trunk conductors to operate said ringing relay to signal an attendant at said first exchange, and means responsive to the answering of the call for disconnecting said busy guard relay, for opening said ringing bridge, and for operating said ring outoff rela r 11. n a telephone system, a first exchange, a second exchange, a connector at said second exchange, ,an interconnecting trunk line accessible to said connector at said second exchange, a ring cut-ofi relay and a back-bridge relay in said connector, a busy guard relay at said first exchange connected vto a conductor of said trunk line and energized in series with said ring cut-ofl' relay responsive to the seizure of sald trunk by sald v connector to extend a call to said first exchange, means for disconnecting said busy guard relay from said conductor and for operating said ring cut-01f relay when the call is answered and for reconnecting said busy guard relay to said conductor at the termination of the call, said busy guard relay being energized in series with said back bridge relay upon its reconnection to said conductor, and means controlled by said busy guard relay for marking said trunk line busy at said first exchange.

12.'In a telephone system, a first ex change, a second exchange, an interconnecting trunk line, one conductor of said trunk being normally connected to ground at said second exchange, a busy guard relay at said first exchange connected to said one conductor and to ground, an automatic switch at said second exchange having access to said ;t run k line, a battery having one'poleconnect- "ed to ground, a ring'icu't -ofi relay in said switchconnected to theungrounded pole of said battery, me'ansresponsiveto the seizure of saidtrunk'by' 'saidswitch in'theextension q seriesconn QtedacrOSSthe conductors of 'said -trt1=nk lin e, .-means for extending a conne'c-t-ion over said trunk'line and forprojectin-g rings ing cu-rrent over the conductorsof said trunk line to operate-said relay, -.rneans for dis'conmeeting said bridge -fromsaid con ducto-rs aud torshort circuitingasaid condenser fwhen saidlbri dg \V across said-conductors,atthe-termin-ation oifthecall.

1 145. In a stelephone system, a 'trunkline, a bridge includinga relay and acondenser in seriesgconnected across-the conductorsof said 7 x-trun-k line, 'meansz-foi ext ending a connection over. sardtrunkq hneand for pro ectrng ring- 'l ng current overrthe conductors-of said trunk 4100 disconnectingnsaid :bridge when-the call is line vto operate .said relay a second-relay 'fo-r answered-and for reconnecting c saidbridge the-call r-is i rll w-ered and for reconnecting across said conductors atthe termination of rlllle I call, and contacts on @said second relay for short-circuiting said condenser when the calliis answered ..t.ordischarge the same to elimi-nate t-he ,poss'ibilityvof said first relay being operated by 1a idischarge otsai'd .con-

denserwhenfthe bridge is reconnected across said conductors. a. f: A 1-15, a telephone syst,en1,\.a cord circuit for answering and exten dingcalls, apa-ir oi":

talking conductors, a.-'double-throw, cord- I splitting-ikeyffor associating said rtalking conductor s'with either endof said cord 1 circuit to the exclusion of the-other, means responsive tothe answering of a call :Eor connecting a direct current bridge across the conductors of one end corJd, said bridge -includ=ing normally closed contacts onesaidjkey, other contactslonn'said key for providing a substitute direct current bridge including said talkingeconductors in series when-said first bridgefis' opened bythe opera- -tionof said key fin one direction, a f'resistance, --andho.ther contacts on sai'cl key for -p'rovi dings a substitute ".(llIBCl', current :bri-dge including isa-i'd resistance .when said first bridge isopened by theoperation of said key inthe other direction. 1

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fa trunk accessible tosaid cord circuit, a pair of dialling conductors, a key, -mean's controlled by said key-for connecting said con- (Motors to the cord circuit, provided said trunk is idle whenconnected with, and means effective in case the-trunk is busy for rendering said means non-responsive to said key.

'17. In a telephone system, a cord circuit, a trunk accessible to said cord circuit,'a pair of dialling'conductors, a key, means controlled by said key for connecting said conductors to the cord circuit, provided the trunk is idle when connected with, meanseiiective in case. the trunk is busy lforrendering said means non-responsive to saidkey, and means for extending a connection to a called line,

a ringing key fortransmitting ringing. current overthe called 'line,aa,frelay controlled by the called subscribenaa second relay con- 192s.,- I v THOMAS r. onoo controlled by said key for notifying the op- V erator of the busy condition of said trunk,'-

":18. In a telephone system, two-way trunk, aconnectorswitch for'connecting with g said trunk at one end, nieans'in said connectorjior ringing over the trunk. means in said connector for establ'lshlng a'direct currentcircuit over only one conductor of the trunk'andior maintaining the same while the ringing operation-is proceeding, and a busying relay at the distant end of the trunk controlled over said direct currentcircuit.

19. In a telephone system,atwo-w ay trunk line,a connector for seizing said trunk at one end, two circuits established by said con necto'r on seizing the trunk line, one circuit including both conductors of thetrunk in series and a source of ringing current and the other circuit including only one conductor of the trunk and ground, a ring-cu't-ofi relay in said connector included in both said circuits, and busying means at the distant end I of the trunkcontrolled over. said grounded v circuit. 'r I 20. Ina telephone system, a two-way trunk line, a connector for seizing said trunk line I at onev end. means in the connectorfor connecting a ringing current source to one-conductor of said trunk line, a ring cut-oii relay in said connector connected'between the'other ccnducgor of the trunk and the exchange batteryin readinessto cut off the ringing cur-rent when the call is answered, and husying means at the otherend of the trunk line operated by current supplied over the said other trunk line conductor through the" winding o f said ring cut-ofi relay. A i c a a 21. In a telephone system,.a cord circuit for connecting a calling and a called line, a double-throwdzey in said cord circuit, means responsive to the operation of said key in one direction for transmitting ringing current over the called line, andmeans responsive to the operation of said key in the otherdirection after the calledsubscriber has answered for rendering said-key ineffective in case it is reop'erate'd' ingsaid first direct-ion.

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