US1888337A

US1888337A - Telephone system

Info

Publication number: US1888337A
Application number: US536342A
Authority: US
Inventors: John H Voss
Original assignee: Associated Electric Laboratories Inc
Current assignee: Associated Electric Laboratories Inc
Priority date: 1931-05-11
Filing date: 1931-05-11
Publication date: 1932-11-22
Anticipated expiration: 1949-11-22
Also published as: GB388752A; FR726873A

Description

Nov. 22, 1932. 5 1,888,337

TELEPHONE SYSTEM Filed May 11, 1931 8 Sheets-Sheet l ..Tul'm H. Uni:

Nov. 22, 1932. J. H. voss TELEPHONE SYSTEM Filed y 11. 1951 Il'LUIn HIP- V Jul-m H. Unili 8 Sheets-Sheet; 2

lllllL Nov. 22, 1932. J. H. voss TELEPHONE SYSTEM Filed May 11. 1931 8 Sheets-She et 3 -Inuen or NOV. 22, 1932. J 055 1,888,337-

TELEPHONE SYSTEM Filed May 11. 1951 8 Sheets-Sheet 4 PLY CONNEC7'0P Nov. 22, 1932.- J. H. voss 1,338,337

I TELEPHONE SYSTEM Filed May 11, 1951 8 Sheets-Sheet 5 FUSE 0005 CALL 5W.

T0 was cm SIGNAL -Inuen 0 x"- Juhn H. Uuss Nov. 22, 1932. J. H. voss TELEPHONE SYSTEM Filed May 11, 1931 8 Sheets-Sheet 6 Inuen ur Jul-m H. Uuss 1932- J. H. voss I 1,888,337 TELEPHONE SY STEM Filed May 11. 1951 8 Sheets-Shet 7 'Im' Jul-m H. Unis Patented Nov. 22, 1932 UNITED STATES JOHN E. VOSS, OI BERWYN, ILI LINCIS, ASSIGNOR; BY MESNE ASSIGNMEN'I'S, TO ASSO- CIATED ELECTRIC LABORATORIES, INC OF CHICAGO, ILLINOIS, A. CORPORATION OI DELAWARE TELEPHONE SYSTEM Application filed Kay 11,

This invention relates to telephone systems in general, but is concerned more particularly with small systems of 'the type commonly known as private automatic exchanges.

Private automatic exchanges are as a rule privately owned'by individuals or corporations' and are installed in factories or other establishments for providing local service between a small number of local telephone lines and for establishin lines to the main 0 cc of the telephone coma panys network.

It has been customary, heretofore, in systems of this type to provide an attendant operator or some special linefor completing all outgoing and incoming calls between the private automatic exchange and the main telephone ofiice. Accordingly,one of the objects' of the invention is to do away with the attendant operatorsposition or special line and to providemeans for enabling any one of a number of local subscribers lines to extend subscriber is informed of such transfer and.

releases from the connection. I V

A further object of the invention is toprovide holdin means whereby one trunk connection can e held while the local subscriber receives a local call, makes a local call, makes an outgoin trunk call, or; answers an'incoming trun call over the same telephone set. The local subscriber may, if desired, hold both trunk calls and return to each as desired or may transfer one of the trunk to another local subscriber.

A still further object of the invention is to provide guarding means for held trunk lines and for enabling only a certain subscriber to connections over trunk calls 1931. Serial No. 538,842.

be connected thereto when the trunk line is to be transferred to such subscribers line.

One of the features of the invention resides in the circuit arrangements whereby it is not necessary for a subscriber in making an out-" going trunk call or in answering an incoming trunk call to know which trunk to connect with. That is, a subscriber always. operates the same'key in accepting atransferred or answering an incoming trunk call on any trunk line and always operates the same outgoing key to make outgoing trunk calls re-,

ardless of the number of trunks available or such calls. This simplicity of o eration on the part of the subscribers is consi ered advantageous over the existing slmilar s stems because of the uniform operation per ormed by the subscriber.

Other features relating to signalling, supervision, code and conference calling and restricted service, are mainly improved circuit details and will, together with the ones named'above, be explained hereinafter, reference being had to the accompanying drawings which com rise Figs. 1 to 8, inclusive, and which showby means of the usual circuit ,dia rams a suificient amount of apparatus to enable the invention to be described and understood.

Fig. 1 illustrates two local subscribers lines S and S in which the line S can make and receive local calls only, while the line S can 'make and receive trunk calls, can transfer .trunk calls and can hold trunk calls while making or receiving local calls.

Fig. 2 illustrates a local subscribers lme S which can make and receive both local and trunk calls, can transfer trunk calls to other local lines, and can hold trunk calls while making, answering, or transfer-ring other trunk calls.

Fig. 3 illustrates the wipers and relay; of

- a finder in one of the finder-connector lin Fig.4'illustrates the wipers and relays of the connector in the corresponding finder connector link.

Fig. 5 illustrates a code-call finder switch.

Fig. 6 at TSS illustrates the transfer sequence switch, at BTR illustrates the busytone relays, at RIB illustrates the ring-1n- PATENT OFFICE terrupter relays, and at ASR illustrates the alarm-signalling relays.

' Figs. 7 and 8 illustrate the wipers of the trunk finder and relays of a two-way trunk circuit which is one of a number of trunk circuits connecting the local automatic exchange to a telephone exchange of the telephone eompany s network.

Referring now to Figs. 1 and 2 in detail, all subscribers, such as S, S", and S are .equipped with a standard and wellknown -monophne telephone set. Each set comprises a handset T and R, a calling device CD, a cradle and cradle spring CS, an induction coil IC, and the ringer bells RB. Each line has a local line circuit, the conductors of which'terminate in the local finder andconnector banks for completing local calls. Each local line circuit has a two-step cut-oil relay and a line relay, such as relays 7. 117, and 217, which is common to a groun of lines. Attached to the base of each standard mono.- phone set of subscribers lines, such as S and and S, is a sub-base. such as SB shown in the dotted rectangle. having a number of spring assemblies and keys for operating the same when making or receiving trunk calls. The sub-base of the subscribers lines, such as S have only two keys, an OUT key for initiating outgoing trunk calls, and an ANS key for answering incoming trunk calls; while the sub-base of subscribers lines, such as S, have an additional or HOLD key. A push-button is also provided in the sub-base for flashing an operator in a trunk call to a manual exchange or for cancelling the digits dialled if an error has been-made in dialling an automatic subscriber in either the local or a distant automatic exchange. Conductors extending from the spring assemblies controlled by these keys, extend to the trunk-line circuits and from there ter-' minate in the banks of the trunk finder. The

trunk-line circuits of subscribers lines, such dition. have/an individual holding circuit comprising four relays, one'of'which is also a two-step cut-ofi relay. Thev trunk-line circuits of S 'subscribers terminate in the banks of a trunk finder in the same manner as the S. subscribers trunk-hne c1rcu1ts,'and, in addi-- tion, terminate in an additional set of bank contacts in the trunk finder fo'renabling the subscribers lines, such as S to hold one trunk connection while talking over another.

The local line circuitsof the S subscribers" lines also terminate in two places in the local finder banks for enabling such subscribers to transfer a second trunk call to another 251, 252.256, 257. and 258.

local subscriber, while the first trunk call is being held. All the subscribers lines also terminate in the finder banks of the code call switch shown in Fig. 5.

The mechanical constructlon of the spring assemblies and keys in the sub-bases,diagrammatically illustrated in Figs. 1 and 2, are

S, are somewhat similar, the key and spring assemblies in the sub-base of Fig. 2 only will be described. lVhen the hand telephone set is removed from the cradle, springs 25 automatically open and springs -1 automatically close. If either the OUT key or the ANS key is depressed, the bar 281 slides ina horizontal direction. and by means of

bushings

262, 2 3, and 264, operate springs Springs 955 are operated only when the ANS key is depressed. -A mechanical lock (not shown) is provided for preventing either the OUT or ANS key, from being operated when the other is operated. The catch bar'26O is op eratcd responsive to the removal of the handset from the cradle to position itself so that when springs 2'2 and 255 are operated; the

same will be mechanically locked until the handset is replaced on the cradle irrespective of the subsequent operations of the keys. The HOLD key controls only springs 259. The three keys ANS, OUT, and HOLD are keys or levers which aremoved oil center and remain in such positions until moved by hand back to normal or until the handset is replaced on the cradle which automatically returns the keys totheir normal positions.

The push-buttonPB', controlling springs 250 is of the self-restoring type and operates the springs '250 only as long as the1push-button is depressed.

The finder switches, .on'e'of which-is shown in Fig. 3, are mechanically similar to the Ill) well-known rotary lineswitch having no normal position. while the circuits thereof have been altered inaccordance with the present invent-ion.

The-start relays SR, shown at the bottom of Fig. 3, are arranged to start alternate finders included in Jthe group of finder-connector links in the "manner to be more 'fully described hereinafter.

The connector switch. shown in Fig". 4 is mechanically similar to the finder, but instead of automatically hunting for a marked potential as the finder does, the connector is directly controlled, in part, by impulses to a certain set of contacts and is thereafter automatically rotated to normal position after termination of a call. Near the lower left-hand corner of Fig. 4, the chain start circuit through the remaining finder connector links is diagrammaticzilly illustrated in the dotted rectangle.

Near the lower center portion of Fig. 4, a

dial-tone relay 453 for generating dial tone is common to all of the finder connector links. To the right center of Fig. 4, the dotted rectangle illustrates the connections for enabling local subscribers to make conference calls. The resistance shown'in the dotted rectangle is connected to only one of the switch banks and is not multipled, while the

conductors

462 and 47 2 extending to relay 468 are common and are multipled between the banks of the various connector switches.

The code-call finder switch shown in Fig. 5, is mechanically similar to the finder of the finder connector link and also hunts for a marked potential. The rela s associated with this switch are for enab ing a calling subscriber to send out ringing current impulses over the code-call circuit in accordance with dialling impulses. The code-call quired. The code-call switch is accessible from the banks of the connector switch.

The transfer sequence switch TSS shown in Fig. 6 is mechanically similar to the aforementloned rota lineswitch, being, however, of a small capacity and permits onl one call at a time to be transferred. The usy-tone relays, BTR, comprise relays when operated for generating abusy tone,'while the ringing interrupter relays, RIR, comprise relays for generating interrupted ringing current. Relay.621 is equipped with a vibrating armature 626 which vibrates between its upper and lower contacts when the relay 621 is deenergized. The alarm signalling relays ASR are provided for transmitting alarm signals over a two-conductor trunk to the main telephone exchange when trouble occurs.

In Figs. 7 andS, one of a number of twoway trunk circuits is shown connecting the local prlvate automatic exchan e with one of the telephone companys exc anges which may be either manual or automatic. The trunk circuits terminate in the private automatlc exchange in trunk relay groups and finder switches for connecting theftrunk's to the local subscribers lines. The trunk finders are mechanically similar to the local.

finders while the circuits of the relay groups, however, are modified to ermit two-way working. At the bottom of. *ig. 7, relay 768. shown in the dotted rectangle, is provided for sequentially taking the trunk finders into use. The dotted rectangle shown to the lower right of Fig. 8 diagrammatically illustrates a portion of the chain circuit extending through the trunk circuits. The correspondingly numbered armatures illustrate similar armatures on the succeeding trunk circuits. Relay 833, shown near the upper portion of Fig. 8, is the incoming signalling relay is common to all of the trunk circuits, and

,to an automatic exchange, a

shorting the winding of relay 802 is pro-' which which when operated, closes the circuit for operatin the incoming trunk signals. When the trun circuit, shown in Fig. 8, extends,

vided, while this strap is cut when the trunk circuit extends to a manual exchange.

In the finders shown in Figs. 3, 5, and 7, relays 309, 508, and 712'are polarized relays of the type disclosed in Patent No. 1,673,884, issued June 19,1928, to H. C. Pye. The core of these switching relays are provided with tted strap L two windings wh ch are separated by a magneticspacin member, a projection of which extends bac and forms a back stop against which the armature of the'relay rests. The winding on the heel end of the relay is the magnetlzing soil, and the flux produced thereby traversing the path including this magnetic iron back stop, which is in shunt of the air gap, and no amount of flux strength in this magnetizing coil will cause the armature to operate. A very small current in the.

' second operating winding of the relay, when in the same direction as the magnetizing winding, will cause the relay to o erate. The use. of these relays as testing re ays permits the callin line to bemarked in the finder banks wit a potential through relatively high resistanceand impedance and prevents undesired disturbances in the searching cir-.

cuits. All the relays of this type have been shown in the drawings with lugs extending from their fcenter portions.

Although a plurality of batteries are shown connected to the Various relays in the drawings, it will be understood that there is but one main battery and that the battery-' feed leads to the various switches are fused separately as indicated by the letters MB in a circle in the various sheets of drawings.

Having described the equipment and apparatus, a detailed description of the operation will now be given. For this purpose, it will be assumed that subscriber S desires'to call subscriber S. In order to call subscriber S, subscriber S will remove the handset from the cradle and dial

digits

4 and 2 correspondrelay 7, armature 4, conductor 1, impulse "sfrrings of the calling device CD, transmitter u. per ring of the cradle springs CS, mid e win ing of the induction coil IC, con-- ductor 2, armature 6, and through the upper; winding of relay 3to battery. Relay 3 en ergizes and at. this time operates only its armature 5 to mark the line S as calling in the banks of the finder over conductor 11. Common line rela 7, upon energizing, at armature 8 groun the common start conductor 9 which extends to start relay 390 in Fig. 3. Relay 390, u on energizing, at arnia ture 394 closes an 0 vious circuit for relay 391-, and the latter relay at armature 396 prepares a circuit for energizing relay 392 which relay at this time is short circuited. At armature 395, relay 390 prepares a circuit for short circuiting relay 391, and at armature 393, grounds start conductor 377 in the finder chain circuit. The start relays 390 to 392, inclusive, are so arranged that when common conductor 9 is first grounded, relays 390 and 391 operate. Relay 391 locks to armature 398 and at armature 396 preparesthe circuits to relay 392, when ground is removed from conductor 9, relay 390 releases, while relay 391 is maintained energized in series with relay 392, which now energizes since the short circuit has'been removed. Relay 392 at armature 397 switches the start circuit from conductor 377 to conductor .378. When conductor 9 is again grounded, relay 390 grounds the start conductor 378 instead of conductor 377. Armature 395 closes a short circuit around relay 391 which tie-- energizes, while relay 392 is maintained operated. When ground is removed from conductor 9. relays 390 and 392 deenergize with the result that all of the relays are now in the normal position. The alternate grounding of

conductors

377 and 378 causes successive finder connector links to be taken into use in response to successive calls.

\Vhen conductor 377 is grounded, a circuit may be traced for energizing start relay 310 ofthe finder as follows: from grounded conductor 37 7 armature and its resting contact, armature 331 and its resting contact, and

through the winding of start relay 310 to battery. .lelay 310, which is fairly slow to operate and release, at armatures 319 and 320prepares the circuits for testing relay 309. The circuit through the lower winding of test relay 309 merely energizes the 'electro-polarizcd winding to prepare the test relay for operation when .the test wiper 303 encounters a calling potential. At armature 321', start relay 310 completes an energizing circuit for stepping relay 308 by way of armature 318 and its resting contact and interrupter springs 316 of the'niotor magnet 307. Stepping relay 308, upon energizing, closes a circuit by way of armatures 321 and 318 for energizing the motor magnet 307. Motor magnet 307 positions its pawl preparatory to stepping the wipers ofthe finder and at armature 316 opens the circuit of stepping relay 308. Relay 308, upon 'deenergizing, at armature 317 opens;,the circuit of a stepping I magnet 307, whereupon the magnet. 307 deenergizes and stepsthe wipers otthe finder one step. At armature 316 motor magnet .307, upon-dcenergizing, reestablishes the circuit for stepping relay .308 with the result that the stepping relay energizes and again completes the circuit for energizing the motor magnet 307. This interaction between the stepping magnet 307 and the stepping relay 308 continues until test wiper 303 encounters a calling potential, in this case a negative battery potential. When test wiper 303 reaches the calling line, a circuit may be traced for energizing relay 309 in series with relay 3 as follows: from ground by way of armature 319, upper operating winding of test relay 309,'test wiper 303 in engagement with the fourth bank contact, conductor 11, armature 5, and through the lower winding of relay 3 to battery. Relay 3 is energized through its second step, thereby operating armature's 4 and 6 to disconnect the common line relay 7 and to disconnect the upper winding of relay 3 from line conductor 2. Common line relay 7 deencrgizes and at armature 8 removes ground from the common start conductor 9 with the result that the start relays SR in Fig. 3 operate in the same manner as previously described.

Returning now to the finder shown in Fig. 3, test relay 309, upon energizing, at armature 318 closes a circuit for energizing relay 312 over an obvious circuit and also opens the circuit extending to the stepping relay 398 and stepping magnet 307 The finder, therefore, comes to rest in engagement with the wipers 301 to 306, inclusive, in en agewent with their fourth bank contacts. clay 312, upon energizing, at armature 32 9 opens a point in the circuit of alarm relay 650; at armature 331 opens a point in the circuit of start relay 310 at its resting contact, and at its working contact closes a. point in the chain circuibto the next finder connector link. $tart relay 310 does not immediately deenergize on account of the co per slug, diagrammatically illustrated by t 1e upper black portion, which renders the same slow-to release as well as slow-to-energize. At armature 327, relay 312 grounds test wiper 303, thereby grounding conductor 11 and making the line S busy; and at armature 328 prepares a locking circuit'for relay 311. At

armatur'cs

334 and 332 relay 312 close'sthe circuit for the line relay 407 for the connector as follows from ground through the lower winding of line relay 407, conductor 375, armature 332, armature 326, wiper 306 in engagement with the fourth bank contact, line conductor '2' over the callin' loop of subscriber S to line conductor 1, o armature 333, armature 334, conductor 363 to the upper'windingof line relay 407 to battery. Line relay407, upon energizing, at. armature 416 completes a circuit for energizing slow-to-release relay 4-08 over an obvious circuit; Relay 408 at armature 421 grounds conductor 367 before relay 310 fully deenergizes in order to hold relay312 in operated position. At armature 418, release relay 408 prepares a circuit for switching relay 414;

urth bank contact and wiper 301,

and also closes a circuit for energizing relay r- 413 by way, of armature 444 and its resting contact. Relay 413 at armature 437 prepares a circuit for the series relay 409; at armature 438 prepares a circuit for relay 410; at armature 439 places another ground connection on .conductor- 367, at armature 449 removes ground from the multipled bank contacts and conductor 466, at armature 440 completes a circuit for energizing the differentiallywound dial-tonerelay 453 which now operates in the manner of a buzzer to generate dial tone, and at armature 441 closes a circuit for connecting dial tone to the calling subscribers line. The dial-tone circuit may be traced as follows: from. armature 454 of the dial-tone relay 453, conductor 456, normal-position bank contact and wiper 406, armature 441', resting contact of armature 436 and said armature, armature 429, through the two condensers in series, conductor 375 from whence the dial tone may be traced over the loop circuit previously traced.

In response to the dial tone, subscriber S will then operate the calling device CD in accordance with the number of the desired subscriber, in. this case subscriber S or number 42. The operation of the calling device for the first digit interrupts the line circuit of line relay 407 four times, thereby causing line relay 407 to deenergize four times. Release relay 408 as illustrated in the drawings has a copper sleeve around its core, thereby making the same slow-to-release, and inthis case, release relay 408 does not deenergize during the dialling period. Uponthe first de-energization of line relay 407, a. circuit may be traced for energizing stepping magnet 415 as follows from ground by way of the normally closed springs controlled by armature 416, armature 420, armature 434, armature 446, and through the Windingof motor magnet- 415 to magnet. Motor magnet 415 energizes and operates its pawl preparator to stepping the wipers'of the connector. branch of this circuit extends from armature 446, by way .of

armatures

437 and 417 for energizing'slow-to-release series relay 409.

At armature 423, series relay 409' closes a' shunt circuit around armature spring 434 controlled by busy relay 412; at armature 422 prepares a point in the circuit for busy relay. and at armature 424 and its workingc a I 1 contacts closes a CII'Cllli} for energizing relay 410 as follows: from ground by way of armature 424, armature 348, wiper 404 in normal po it on, conductor 369, armature 322 which by this time has closed, conductor 368, normally closed spring controlled by armature 428 through the resistance and upper win ing of relay 410 to battery. Relay 410 at a mature 425 opens a point in the test circuit;

at armature 426 prepares a point in the circuit for relay 314, at armature 427 prepares a point in the automatic stepping circuit for the connector switch in case the digit; 4 is dialled; at armature 428 completes a locking circuit for itself from closed armature 424;

and at armature 429 disconnects dial-tone from the calling loop.

When line relay 407 energizes after the first interruption or impulse, the circuit of release relay 408 isagain closed and the cir- "net 415 and series cuit through the motor ma eries relay 409 is relay 409 is interrupted.

slow to release and also maintains its armatures attracted during impulsing. The motor Tnagnet 415, however, releases. its armature, thereby causing thewipers 401 to 406, inclusive, to take one step; Since four impulses are being sent, the wipers 401 to- 406,

inclusive, will take four steps and come to rest 'deenergization of series relay 409 is the establishment of an automatic steppingwircuit for magnet 415 as follows: from ground by way of contacts 467,'wiper 404, armature 438, armature 424 and its resting contact, armature 427, armature 447, interrupter springs 448 and through the winding of magnet 415 to battery. The motor magnet 415 operates in a buzzerlike manner until wiper 404 no longer encounters a grounded contact, the first ungrounded contact being the fourteenth contact accessible to wiper 404. Relay 410 also deenergizes when wiper 404 reaches its fourteenth contact.

Before proceeding with the dialling operation of the second digit, it may be advisable to describe the operations which take place in case the digit 3 were dialled instead of the digit 4. In this case, the wipers 401 to 406, inclusive, will come to rest on their third set of contacts. Wiper 404 does not, therefore, encounter ground with the result that relay 410 deenergizes when its circuit is opened by relay 409. The wipers are not automatically 'impulses are sent out over the calling loop to operate line relay 407 as before. The series relay 409 and motor magnet 415 operate in the same manner as described, but relay 410 does not energize because its original energizing circuit was opened when wiper 404 moved off normal. In response to the operation of the motor magnet 415, the wipers 401 to 405, inclusive, are operated into engagement with their sixteenth bank contacts terminating the conductors associated with theline of subscriber S.

In case the line is busy, there will be a ground potential on conductor 110, and a circuit for energizin busy relay 412 before series relay 409 deenergizes may be traced as follows: from grounded conductor 110, wiper 403, normally closed springs controlled by armature 443, armature 426, armature 422 and its working vcontact and through the winding of busy relay 412 to battery. b'hortly after the last dialled impulse, relay 409 deenergizcs and at the make-before-break .5 rings of armature 422 completes a locking circuit for the busy relay 412 as follows: from ground by way of armature 418, armature 433 and, its working-contact, normally closed springs controlled by armature 422 and t rough the winding of busy relay 412 to battery. 7 I

Atarmature 435, busy relay grounds the busy and generator-start conductor 371, thereby energizing relay 615 over the common conductor 360. Relayr615 at armature 626 starts the ringing'vibrator by energizing the difl'erential relay 640 through its lefthand winding which operates the vibratory 641 to the left to close'a circuit through the right-hand winding in opposition to the left hand winding. Relay 640, being diiferentially wound, releases its vibrator 641, thereby causing the vibrator to swing to the right and open a circuit extending through the right-hand winding. The vibrator 641, therefore, vibrates and closes alternate circuits through windings 642 and 6430f the induction coil to set up current flow in op poslte directions.

This current flow is inoedun the winding 644 which transmits pulsating or ringing current out over the common ringing conductor 457 when relay 6201s energlzed. At armature 624 relay 615 closes a circuit for energizing relay 620 over armature 637 and its-resting contact. Relay 620 at armature 6 closes a circuit for energizing relay 621 by way of armature 639. Relay, 621 attracts the vibrating armature 636 to close its lower contact-and thereby close a circuit for energizing slow-acting relay 622 by way of armature 634. After an.

interval, relay 622 energizes, and at armature 637 opens the circuit of slow-tiY-releasc relay 620 and at armature638 closes a circuit for energizing slow-to-release relay 623.

Relay 623 at armature 639 opens the circuit of relay 621 which deenergizes and releases lt S vibratlng armature 636. Armature 636 vibrates between its upper and lower contacts to hold rela 622 energized for a .pre-

determined peri after which relay 622 de grounded and connected to the source of ringing current at coil 644.

Relay 615, upon energizing, at armature 625 completes a circuit for energizing relay 616 and busy interrupter relay 618. Relay 618 is difierentially wound and operates like a buzzer to open and close springs 630. Re-

lay 616 at armature 627 opens the short circuit from around relay 617, thereby causing the energizatlon of the latter relay. Relay 617 at armature 629 closes the busy-tone generated by relay 618 to the common busy-tone conductor 459. At armature 628, relay 617 closes a short circuit around relay 616 which deenergizes after an interval and reestablishes the short circuit around relay 617 which also deenergizes after an interval. lVhen relay 617 deenergizes, the tone is dis connected from conductor 459 at armature 629, and at armature 629 the short circuit around relay 616 is removed and relay 616 reenergizes to again Ereenergize relay 617. This cycle of operations continues as long as ground is connected to conductor 360.

Returning now to the connector switch in Fig. 4, and responsive to the energization of the busy relay 412, busy-tone may be traced from conductor 459 over the working contact of armature 436 and said armature, armature 429 and over the calling loop. In response to the busy tone, the calling subscriber will replace the handset on the cradle and the'finder connector link in Fi 3 and 4'will be released in a manner to subsequently described.

Assuming now that line S was idle when -.called', instead of busy, the busy relay 412 would not be energized, and when relay 409 deenergizes after the last impulse, a circuit may be traced for energizing switching relay 414 inseries with the two-ste 113 as follows: from ground y way of armature 418, armature 433 and its resting contact, lower winding of switching relay' 414, resting contact and armature 422 and said armature, armature 425, normally closed springs controlled by armature 443, wiper 403 in engagement with the bank contact terminating conductor 110, conductor to Fig. 1, and through the lower windin of two step cut-ofi relay 113 to battery. Witching relay 414 operates its'armature 445 to complete a circuit from grounded armature 419 cut-01f relay .armature 450 grounds conductor in order to fully operate the relay and to ductor 102, through the substation circuit to complete a locking circuit for' itself. At conductor 101, push-button springs 150, con armature 443, relay 414 grounds the test ductor 121, spring 156 and its resting contact,

wiper 403 to busy the calledline and to mainconductor 131, wiper 401, armature 442,

tain the cut-off relay 113 of the called line 'armature 430 and its working contact, and in operated position. At armature 444 and through the fuppenwinding of back-bridge its restingcontact relay 414 opens the cirrelay 410 to battery. Back-bridge relay 410 cuit of relay 413 which now deenergizes and .feeds talking battery to the called subscriber at its working contact prepares a circuit for over the circuit just traced. Relay 410, upon locking relay 314. At armature 446, relay energizing, at armature 429 disconnects ring- 414 opens the point in the impulsing circuit; back tone; at armatures 428 and 427 preat armature 447 opens a point in the autopares circuits for maintaining relay 410 in matic restoring "circuit of the connector, at operated position during the automatic re- 1 to start storation of the connector to normal; and the busy and ringing interrupter relay as preat armature 426 completes a circuit or enviously described; at armature 451 closes a ergizing relay 314 as followszfrom ground by ring-back circuit to the calling Subscriber way of armature 419, armature-444 and its 7 by way of conductor 457, resting contact of working contact, armature 426, conductor armature 432 and said armature, armature 366 and through the winding of relay 314 451. restingcontact of armature 436 and said to battery. At armature 341, relay 314 comarmature, armature 429 and thence over the pletes a locking circuit for itself over concalling loop; and at

armatures

442 and 452 ductor 365 independent of relay 410; at connects ringing current to the called-line. armature 342 connects the upper winding of Relay 413, upon deenergizing, at armatures relay 313 in series with-

wipers

302 and 402 439' and 449 prepares circuit for automatifor a. purpose to be more fully described here cally restoring the connector switch to its inafter in connection with the transfer of a normal position, and at armature 40, opens call; at armature 343 removes ground -for the circuit to the dial-tone relay 453. The generator start conductor 360; and at armaringing circuit for ringing the called subtune 335 completes the talking circuit over scriber may be traced as follows: from ringthe heavy black lines between subscribers S ing interrupter lead 457, resting contact of and S. Relays 312 and 314 in the finder armature 432 and said armature, armature switch, and relays 407, 408, 410, 411, and 452, wiper 406 in engagement with the bank 414 in the connector are energized during contact terminating conductor 132', conductor nversatlon- I 132, through the condenser andringer bell After conversation and responsive to the RB of substation S, conductor 131, wiper called subscriber replacing his handset on the 401, armature 442, armature 430 and its restcradle, relay 0 d energ zes. esponsive to ing contact and through the lower winding the calling subscriber replaclng his handset of ring cut-01f relay 411 to battery. on.the cradle, line relay 407 deenerglzes, and

When cut-01f relay 113 energizes in series at armature 416 opens the circuit of release with switching relay 414, relay 113 operates relay 408 which deenerglzes after a short 1nall its armatures to disconnect the common terval. At armature 419, relay 408 removes line relay 117 at armature 114; at armature holding ground from

relays

411, 414, and 314 116 disconnects the upper winding of relay which now deenergize; at armature 421 re- 113 from conductor 132 and at armature 115 moves ground from conductor 367', thereby connects conductor 110 to conductor 111 to deenergizing'hold relay 312, and at the restbusy the line in the banks of the local finder ing contact of armature 421, closes a c1rcu1t and code-call finderfor automatically restoring the connector When subscriber S removes the handset wipers to normal position. This circuit may from the cradle in ,response to the operation be traced as follows: from ground at armaof the ringer, bells RB, ring cut-01f relay 411 ture 449, or from ground at the multiply is energized over the calledsubscribers loop connected contacts 467, wiper 404, normally and said relay at armature 431 completes a closed springs controlledby armature 439, locking circuit for itself. At armatures 430 armature 421 and its resting contact. armaand 432, relay 411 disconnects the ringing ture 447, interrupter springs 448 and through is current from the line conductors and eo'ithe winding of motor magnet 415 to battery. nects through the talking conductors indi- The motor magnet operates in the manner of cated in heavy black lines. A circuit may a buzzer to step the wipers 401 to 406, inclunow be traced for the back-bridge or batsive, until thewiper 404 loses ground which tery-feed relay 410 as follows: from ground it does when it reaches its normal position or through the lower winding of relay 410. the position shown'engagedby the wiper working contact of armature 432 and said in the drawings. The deenergization of rearmature, armature 452, wiper 406, conduclays 312, 314,411, and 414 and the returntor 13218561132, contact of spring 151- and to-normal of the connector wipers places this said spring in the silb-base SB, Fig. 1, con vfinder-connector link in the condition shown relay 716.

.

tures

331 and 331, in the various finder-com nector links would be operated with the result that no circuit iscompleted to a start relay, suchas relay 310 and, therefore, no finder connector link would be seized and the calling subscriber would not receive dial tone, thereby indicating such a busy condition.

Incoming calls It will now be assumed that an incoming trunk call is received over the conductors 840 and 841 from the distant exhange. 'Whcn an incoming call is received over conductors 840 and 841, relays 806 and 807 operate. Relay 806 is a very high wound relay and opcrates in response to either the ringing current or during the silent eriod, while relaylay807 at armature 827 completes a locking circuit. for itself by way of

armatures

819 and 747; at armature 828and its resting con tact opens its original energizing circuit and bridges the AC signalling relay 808 across the trunk conductors'840 and 841; at armature 829icompletes a circuit for energizing relay 710 over conductor 759; and at armature 830, pre ares a point in the incoming start circuit or the trunk finder. Relay 808 operates in response to ringing current and at armature 831 closes a circuit for signalling relay 833 whichrelay, at armature'834, completes a circuit for operating the incoming trunk signals over conductor 835. Relay 804, upon energizing, at armature 820 completes a circuit for energizing at relay 803; at armature 819 completes the locking cir cuit for ring-u 'relay 807; at

armatures

821 and 822 comp etc points in the chain circuits; and at armature 818 completes a circuit for energizing slow-to-operate relay 650 which relay, however, does not have time to energize'be fore its circuit is opened at armature 816 by relay 803. At armature 814, relay 803 disconnects relay 802 from talking conductor 754 and connects the trunk conductor 840 in place thereof; atarmature 816, relay 803 prepares a locking-circuit for itself and at armature 817 prepares a locking circuit for car It will be assumed that subscriber S answers the incoming trunk call. To do this, subscriber S operates the ANS key and removes the handset from the cradle. In response to the operation of the answer key, springs 151, 152, 155, and 156 are operated in the manner previously described, and a circuit may, therefore, be traced from ground through springs 155, normally closed springs controlled by armature 143 and throughthe winding of'relay 141 to battery. Relay 141 at armature 147 prepares a locking circuit for itself; at armature 146 and its working contact completes a circuit for the common incoming line relay 161; and at armature 148 disconnects the outgoing test conductor 167 and connects up the incoming test conductor 166. A. circuit may now be traced for

relays

161 and 140 as follows:

from ground through the winding of the common incoming line relay 161, working contact of armature 146 and said armature,

armature 142, working contact of springs 151, conductor 102 over the subscribers loop to conductor 101, springs 150,'conductor'121, spring 156 and its working contact, armature 145, and through the upper winding of two-step cut-01f relay 140 to battery. Cutoil relay 140 operates in its first step only to close armature 144 to connect a negative battery potential from the lower winding of relay 140 to conductor 166 which terminates in the test bank contact accessibleto test wiper 703 of the trunk finder. Common line relay161 at armature 163, grounds the com-' mon incoming start conductor 165 to complete acircuit by way of conductor. 765, ar-

mature 830 and its working contact, conductor 7 63, armature 745 and its resting contact, armature 736 and its resting contact and through the winding of slow-to-operate start relay 711 to battery. Start relay 711 at armaturel 726 completes an energizing circuitthrough the upper winding of relay 709 which energizes-and locks up through its lowerwinding; at armature 727 prepares the operating circuit for three-pole test relay 712, and at armature 728 completes a circuit for the lower electro-polarizing winding of relay 712 which relay, as previously described, will not operate until a circuit is completed through its operating winding in the proper direction. At armature 729, relay ,711, completes a circuit for energizing stepping relay 708 as follows: from ground by way of armature 729. armature 730 and its resting contact, winding of relay 708 and through the interrupter springs 717 of the motor magnet 709 to battery. Stepping relay 708 at armature 718 completes a circuit for energizing the motor magnet 707 and the motor magnet, upon energizing, at its 7 interrupter springs 717 interrupts the circuit of the stepping relay 708 which now deenergizes. When stepping relay 708 is fully deenergiz'ed, said relay at armature 718 opens the circuit of the motor magnet 707 with the result that motor magnet 707 steps the switch wipers 701 to 706, inclusive,

and at interrupter springs 717 reestablishes the circuit for stepping relay 708. This interaction between stepping relay 708 and motor magnet 707 continues until test wiper 703 encounters a calling potential, in this case negative-battery potential. When wiper 703 engages the bank contact terminating conductor 166 circuit may be traced for ener gizing the two-step cut-0E relay 140 in series with the operating winding of the threepole relay712 as follows: from ground by way of armature 727, upper operating winding of'test relay 712, armature 742 and its resting contact, armature 723 and its working contact, test wiper 703,.conductor 166, working contact of armature 148 of said armature, armature 144, and through the lower winding of relay 140 to battery. Twostep relay 140 energizes through its second step .over this circuit to disconnect thelcom mon-line relay 161 at armature 142; to comill plete the locking circuit for relay 141 and to disconnect the upperwinding of'relay 140 from talking conductor 172. Common line relay 161 deenergizes and at armature 163 removes ground from the incoming start conductor 165. In the trunk finder, the threepole test relay 712- energizes over the abovet-ra'ced circuit and at armature 730 and its resting contact opens the stepping circuit, and at its working contact completes a circuit for energizing relay 713 from grounded armature 729. At armature 732, relay 713 grounds the test wiper 703 to busy line S to other trunk finders; at armature 733 pre pares a locking circuit for itself; at armature 735 opens a point in the circuit of the slow-to-operate start relay which maintains its armatures attracted for ashort time thereafter and prepares a point in the chain cir cuit to the succeeding trunk finder; and at

armatures

731 and 736 completesthe'talking circuit between subscriber S and the main exchange as well as completing a circuit for energizing relay 801. The circuit for energizing relay 801 may be traced as follows: from ground applied to' conductor 840 at the main exchange, armature 814 and its working contact, conductor 754, armature 731, wiper 701, conductor 171, through the sub-station circuit and sub-base keys over the circuit previously traced to conductor 172, wiper 706, armature 736, conductor 766, winding of relay 801, resting contact and armature 813 and said armature, and over trunk conductor 841 to battery. At armature 810, relay 801 completes an energizing circuit for slow-to-release relay 805. At annature 82 4, relay 805 opens the circuit of relay deenergizes a short interval after relay 806 7 deenergizes and at armature 819 opens the locking circuit of ring-up relay 807; at armature 820 opens the original energizing circuit 6f relay 803 which is now locked up over armature 823. When relay 716 in Fig. 7 energizes, said relay at armature 747 opens the, locking circuit of relay 807 whereupon the latter relay deenergizes and opens-the c rcuit of relay 808. It should be mentioned at this time, however, that in response to the operation of the switching-through relay 713 in Fig. 7, a bridge including the subscribers loop was placed across the trunk conductors 840 and 841 to automatically cut-0H the ringing current. Relay 808, upon deenergizing, at armature 831 opens the circuit of the com mon signalling rela-y833 with the result that the incoming signals are no longer operated in response to this trunk call. At armature 748, relay 716 prepares a, circuit for relay 715 which will be effective only incase the trunk call should be transferred to another local subscriber as will be more fully described hereinafter. At armature 749, relay 716 completes a locking circuit for itself over conduct-or 758 and armature 817; at arma ture 750 completes a. locking circuit for switch-through relay from grounded armature 750, resting contact of armature 743 the calling subscriber over the heavy talking conductors and both the calling and called subscribers receive their talking battery from the main' exchange. The calling and called subscribers may now converse as desired.

In response to the replacement of the handset by the subscriber S the ANS key and spring'assemblies shown in the sub-base SB are automatically restored to normal or the position shown in the drawings to open the circuit through relay 801. Relay 801, upon deenergi'zing at armature 810 opens the 'circuit of relay 805 which deenergizes and at armature 823 opens the locking circuit of relay 803 and at armature 825 opens a point in the original energizing circuit of relay 716. After a short interval, relay 803 deenergizes and at armature 814 disconnects the incoming trunk conductor 840 from the talking conductor 754; atarmature 816 opens a point in its own locking circuit; and at armature 817 opens the locking circuit of relay 716. At armature 750 relay 716, upon .deenergizing, opens the locking circuit of Outgoing calls It will now be assumed that subscribers desires to call a subscriber in the main automaticor main manual exchange over one of the trunk circuits, such as shown in Figs. 7 and 8. Subscriber S thereupon operates the OUT keycin the sub-base SB, thereby operating the bar 261 in av horizontal direction to operate

springs

251, 252, 256, and 258; In

response to the removal of the handset from the cradle, springs 253 are opened, springs 254 are closed, and the locking bar 260 is operated to lock spring 252 in operated position. A circuit may now be traced from ground through the common outgoing line relay 260, terminal 203 connected to conductor 204, resting contact of armature 246 and said armature, armature 242, resting contact of armature 291 and said armature, working contact and springs 251 and said, spring, conductor 202 through the calling subscribers loop a to conductor 201, push-button springs 250, conductor 221, spring 256 and spring 258, armature293 and its resting contact, armature 297 and its resting contact, and through the upper winding of two-step relay 240 to battery. Two-step relay 240 energizes and operates only armature 244 in its first step at this time to connect the calling potential extending from the lower winding of two-step relay 2-10 to the outgoing test bank contacts accessible to wipers 702 of the trunk finders such as shown in Fig. 7. The common outgoing line relay energizes over the above-traced circuit and at armature 262 grounds the. common outgoing start conduc tor 164 to complete a circuit for the start relay ofthe first idle trunk finder.

For the purpose of describing the start chain circuit of the trunk finders, it will be assumed that the first trunk finder in the chain is busy with an incoming call and that the second trunk finder is busy in an outgoing call, and, therefore, when common start conductor 164 is grounded, the start circuit will extend by way of armature 722 and its resting contact, conductor 764, armature 822 and its resting contact, conductor 763, arma ture 745 and its resting contact, armature 735 and its working contact, conductor 762 to the succeeding trunk finder, armature 722 and its working contact, and conductor 7 62 to the next succeedingor third trunk finder.

In case thiscall should'come in before relay 804 in a busy-trunk finder has deenergized in -response to the deenergization of relay 806,

the chain circuit is taken through armature 822 and its working contact to the succeeding trunk finder. In case thetrunk finder may be busy in a transfer call, the start circuit is completed by way of armature 745 and its working contact instead of its resting contact. In case all of the trunk finders are busy, the start circuit extends through the chain circuit of all of the trunk finders in the manner previously described to conductor 671, thereby completing a circuit for relay 619 to initiate the operationof the busytone relays 616, 617, and 618 as previously described. At armature 632 all-trunks-busy relay 619 closes the busy-tone circuit as fol.- lows: from vibrating armature 630, interrupter armature 629, armature 632, the condenser and conductor 106, through Fig. 5 to Fig.2, conductor 206, and through the wind ing of common outgoing line relay 260 and over the calling subscribers loop circuit. The calling sul scriber may either replace his receivcr or waituntil an idle trunk circuit is available. It is believed advisable at this time to describe another branch of the chain circuit which has been provided to insure the successive seizure of trunk circuits when making outgoing calls. Armature 7 21, in each of the trunk finders, connects ground to conductor 760 whenever such trunk finder is taken into use. Conductor 760 of the first trunk finder extends to terminal such as terminal 7 53 in the succeeding trunk finder and so forth through the chain. As each trunk is taken into use for an outgoing call, the relay 7 09 is energized and locked, and when the last trunk finder is taken into use in an outgoing call, a circuit may be traced from armature 721 of the last trunk finder for energizing slow-to-operate relay 7 68 over conductor 767. Relay 768 at armature 769 opens the locking circuit of all relays 7 09 in the various trunk finders to cause their release so that the sequence of seizing the trunk circuits on succeeding calls will start all over. again. When the relay 709 in the last trunk c rcuit is deenergized, the relay 7 68 will also be energized afterian interval and again prepare the locking circuits for the relays, such as relay 7 09, in the trunk circuits. It will now be assumed that the trunk finder shown in Fig. 7 is the first idle trunk finder, and, therefore. when the common outgoing start conductor is grounded, a circuit may be traced for the start relay 711 over armature 722 and its resting contact, conductor'764, armature S22 and its resting contact, conductor 763, armature 745 and its resting contact, armature 735 and its resting contact, and through the winding of start relay 711 to battery. Start relay 711 at armature 726 completes a circuit for energization relay 7 09 which now locks up over its armature 720. and at

armaturcs

727, 728. and 729 prepares the test circuit through the three-pole test relay 712 and completes the stepping circuit for the stepping relay 708 and motor magnet 707 in the same manner as described for an incoming call. In response to the ope'rationof the stepping relay 708 and motor magnet 7 07, the wipers 701 to 706, inclusive, of the trunk finder are operated step-by-step in the manner previously described until the outgoing test wiper 702 encounters the negative battery calling potential applied to conductor 267. When test wiper 702 encounters the bank contact terminating conductor 267, a circuit may be traced for energizing the three-pole switching relay 712 in series with the two-step relay 240 as follows: from ground at armature 727 through the upper operating winding of relay 712, armature 742 and its resting contact, armature 723 and its resting contact, wiper 702, conductor 267, resting contact of armature 248 and said armature, armature 244 and through the lower winding of relay240 to battery. Two-step relay 240 energizes and operates its armatures through their second step to disconnect the common line relay 260 at armature 242, and to disconnect the upper winding of relay 240 from the lower talking conductor 272. In the trunk finder the three pole test relay 712 is energized over the abovetraeed circuit, andat armature 730 opens the stepping circuit at its resting contact, and at its working contact completes a circuit for energizing. switch-through relay 713. At

armature 732, relay 713 short circuits'the upper operating winding of relay 712 and also connects ground to test wiper 702 to make the'line S busy. At

armatures

731 and 736 connects the calling subscribers loop circuit to

conductors

754 and 766 over the following circuit: conductor 754, armature 731, wifper 701, conductor 271, restingcontact o armature 291 and said armature,

working contact of spring 251 and said spring,

conduct-or 202, over the calling substation circuit to conductor 201, push-button springs 250, conductor 221, springs 256 and 258, armature 293 and its resting contact, conductor 272, wiper 706, and through'armature 736 to conductor 766.

The operation of the trunk finder is slightly difi'erent dependent upon whether the trunk conductors 840 and 841 extend to a manual or an automatic exchange. In either case, ground is disconnected from the trunk conductor 840 at the main exchange, while the conductor 841 extends to a signalling relay and negative battery in case of a manual exchange, or extends through the line'relay to battery of an incoming lineswitch in case the main exchange is an automatic exchange.

in addition, relay 802 in Fig. 8 is permanently short circuited in case the trunk conductors 840 and 841 extend to an automatic exchange, while the short circuit is removed from around said relay'in case the trunk conductors extend to a manual exchange. It

i will firsthe assumedthat the trunk conductor extends to a manual exchan e and, therefore, a circuit may be traced rom ground through the winding of relay 802, through the normally closed springs controlled armature 814, conductor 754, and from thence 10 over the calling loop to conductor 766-,winding of relay 801, resting contact of armature 813 and said armature, conductor 841, and through the incoming signalling relay at the main manual exchange to battery.

Relays

801, 802, and the signalling relay in the manual'exchange operate over the above-traced circuit and the signalling relay in the manual exchange operates a signal in the well-known manner to indicate an incoming call to the so manual operator thereat. Relay 801 at armature 810 completes a circuit for energizing relay 805 and relay 802 at armature 811 also completes a circuit for energizing relay 805, and at armature 812 connects negative bat- 35 terv to the lower terminal of relay 801 to maintain this relay energized, and at armature 813 connects ground to conductor 841 to maintain the incoming signal relay, operclosing a circuit extending by way of the normally closed springs controlled by armature 816, armature 823, and through the winding of relay 803 to battery for energizing the latter relay. Relay 803 at armature 816 completes a locking circuit for itself and also completes, at armature 817, the locking circuit for relay .716. At armature 814 relay 803 opens the circuit of relay 802- and prepares a point in the talking circuit between the subscriber S and the manual operator. A short period thereafter, relay 802 deenerizes, and at armature 813 completes the talkmg circuit over the heavy talking conductors. The calling subscriber S informs the manual operator of the-subscriber desired,- and the manual operator completes the connection in the well-known manner.

It will now be assumed that the trunk conductors 840 and 841 extend to an automatic exchange and, therefore, the operation of the switch-through relay 713 completes a circuit from ground over the short circuit of relay 802, normally closed springs controlled by armature 814, and from thence over the calling subscribers loop circuit by way of

conductors

754 and 766, winding of relay 801,

resting contact of armature 813 and said ar- 1 mature, and over conductor 841 through the winding of the line relay in the incoming lineswitch to negative battery. The line relay of the incoming lineswiteh in the automatic exchange operates in the well-known manner to seize an idle trunk at which time dial tone is transmitted back over conductors 840 and 841 to the calling subscriber. In addition, ground is connected to trunk conductor 840, thereby causing the energization of relay 803 which now energizes and causes the cnergization of relay 716 over circuits previously traced. Since relay 802 is not operated on calls to automatic exchanges, a direct metallic loop circuit is completed over conductor 8-10, armature 814 and its working contact, conductor st over the previouslytraced subscribers loop circuit to conductor 766, through the winding of relay 801, resting contact of armature 813 and said armature, and over the trunk conductor 8-11 to the main exchange In response to the receipt of dial tone, subscriber S dials the telephone number of the desired automatic subscriber, whereupon the switches in the automatic eX- change are operated in the well-known manner to complete the desired connection. Relay 801 intermittently operates, but since relay 805 is slow-to-release on account of the copper sleeve around its core, said relay is maintained energized throughout the impulsing period. \Vhen the called subscriber answers, talking battery is fed to both the calling and called subscribers from the main exa chan e. \Vhen the subscribers hang up the circuit through relay 801 is opened and said relay at armature 810 opens the circuit of relay 805, and the latter relay, upon deen'ergizing, at armature 823 opens the circuit of relay 803. Relay 803 after an interval deenergizes and, at armature 817 opens the locking circuit of relay 716 which now deenergizes and opens the locking circuit of switchthrough relay 713. Relay 713 deenergizes and the trunk finder is placed in condition to receive another outgoing trunk call as soon as relay 709 dccncrgizes when the last trunk in the exchange has been taken into use.

Subscriber S may originate an outgoing call in the same manner as described for subscriber S and subscriber S may answer an incoming trunk call in the manner similar to that described for subscriber S, and since the equipment and operation are somewhat similar, it is believed that a detailed description of such operations may be clearly understood without describing the same in detail.

H old'hzg trawl" circuits In order to describe how subscribers equipped with sub-bases, such as SB, can hold a trunk call while either making or answering a local call, it will first be assumed that sub scriber S has answered an incoming trunk call and that the substation circuit of S is,

therefore, connected to the trunk circuit by the operation of the answer key. It will further be assumed that during the time subscriber S is conversing over the trunk cir cuit, that another local subscriber calls subscriber S over one of the finder connector links and, therefore, operates the ringer bells RB associated with the line of subscriber S. Since there are no multiple connections between the local finder and the trunk finder for guarding the local subscriber against intrusion when such local subscriber is engaged in a trunk call, the connector switch is operated to cause the operation of the ringer bells RB in the same manner as previously described. In response to the ringer bells, subscriber S informs the main-exchange subscriber to wait for a short period of time and then releases the answer key, whereupon the bar 161 is operated to the position shown in the drawings, thereby releasing springs 151 and 156,

while

springs

152 and 155 are maintained locked in operated position by the bar 160. In response to the restoration of springs 151 and 156, the subscribers substation circuit is directly connected across the local line conductors 131.and,132 to cause the operation- 01 the ring-cut-oli relay in the connector and to complete the talking circuit between the calling local subscriber and subscriber S. As soon as the answer key was restored to its normal position, a holding circuit was closed over the talking

conductors

171 and 172 as follows: conductor 171, locking spring 152. spring 154:, springs 157 and 158 which close .before spring 158 is disengaged from spring 156, toconducto r 172. Subscriber S may return to the trunk connection by operating either the OUT or the ANS key. The bar 161 is thereupon moved in a horizontal direction and again completes the connection between the trunk circuit and the substation circuit. The operation of either key automatically releases the local connection.

In case subscriber S desires to originate a local connection and still maintain the connection over the trunk circuit, the main exchange subscriber will again-be told to wait awhile and then subscriber S restores the operated key to normal position. In re- 1 sponse thereto, a circuit may now be traced from common line relay 117 by way of armature 114, conductor 131, resting contact of spring 156 and said spring, conductor 121,

push-button springs 150, conductor 101 over 1 the substation circuit to conductor 102, spring 151 and its resting contact, conductor 132, armature 116 and through the upper winding of two-step cut-off relay 213 to battery. T wo-step relay 113 operates only in its first step to operate armature 115 to connect negative battery calling potential over conductor 111 to the banks of the local finders. Linc relay 117, upon energizing over the abovetraced circuit at armature 118 grounds the