US2294500A

US2294500A - Telephone system

Info

Publication number: US2294500A
Application number: US361365A
Authority: US
Inventors: Hovland Henry
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1940-10-16
Filing date: 1940-10-16
Publication date: 1942-09-01
Anticipated expiration: 1959-09-01

Description

Sept. 1, 1942. H. HOVLAND 2,294,500

TELEPHONE SYSTEM Filed Oct. 16, 1940 10 Sheets-Sheet 1 'FIG. 4 FIG. 7

FIG.

NMO du amt. I

sue 3 A TTORNEV Sept. 1, 1942. HOVLAND 2,294,560

TELEPHONE SYSTEM flii iiii a in 0, n

Filed Oct. 16, 1940 10 Sheets-Sheet 2 TONE INI /ENTOR H HUI/LAND AT TORNEV FIG. 2

MARGINAL Sept. *1, 1942. H. HOVLTAND I 5 0.

TELEPHONE SYSTEM Filed Oct. 16, 194p IO SheetS-Sheet 3 E ga 2 k E] 2 u a g 9.

INVENTOR H. Hal LAND 4 7 TORNE 1 Fer Sept; 1, .1942. I H. HOVLAND TELEPHONE SYSTEM Filed Oct. 16, 1940 10 sheets-sheet 4 IINI/ENITOR HHOVLAND A T TORNEV Sept. 1, 1942. H. HOVLAND 2,294,500

TELEPHONE SYSTEM Filed Oct; 16, 1940 10 Sheets-Sheet 5 H a at F El 2 3 Q Q m x 53; '0 0 w z INVENTOR H. HOVLAND A TTORNEV Patented Sept. 1, 1942 UNITED ST TELEPHONE SYSTEM Application October 16, 1940, Serial No. 361,365

(Cl. 179-l8) 16 Claims.

provement in the supervisory facilities of such systems.

It is usual in automatic telephone systems comprising-a main and a branch office to provide switching selector repeaters in the branch ofiice forselecting a-' trunk to the main office, for re-* peating all dial impulses over the selected trunk to the main office on calls to subscribers in the main office or other offices of the system, and for repeatingdial impulses over the selected trunk to the main oflice on calls to subscribers in the branch oflice until as many digits have been dialed as are needed to determine that the called subscriber is located in the branch office, the

trunk to the main office being released as soon as these digits have been dialed and the succeding trains of dial impulses being eliective' to complete the desired connection through switches in the branch office. Among the'patents disclosing branch omce systems and switching selector repeaters are the patent to E. J acobsen No. 1,537,306,'May 12,1925, and patents to H. Hovland No. 1,799,796, April '7, 1931, and No. 2,025,389, December '24, 1935.

According to the invention, an improved switching selector repeater is provided in a branch office of an automatic telephone system, the selector repeater having various new and novel features. One of the features is a circuit arrangement whereby the selector switch of a branch office selector repeater absorbs the last digit of the branch office code if this digit is a particular numerical value, or one of certain numericalvalues, and immediately hunts for an idle trunk in the corresponding level of its bank; but does not absorb the last digit of the branch ofice code, if the last digit is some other numerical value. This arrangement enables the connection of trunks to different orders of local switches in different levels of the bankof the selector of a selector repeater, thereby minimizing the numafterrelease of the sele'cted'trunk'to the main office on a call to a subscriber in :the branch office.

Another feature is a branch oflice selector/repeater .arranged to automatically release the selected trunk to the main office in case. nofdial impulses are received within a predeterminedinterval of time, such for instance as twenty-four seconds.

A further feature is-a branch ofiiceselector repeater arranged I to release the line finder through which it is connected to a calling: line within a predetermined interval, for instance two minutes, after the selected trunk to the main oflice is released due to a permanent signalcondition.

Another feature is a branch office-selector repeater arranged to operate a'line lock-outrelay on calls from lines equipped with such relays in case nodial impulses are receivedfrom the. calling line within a predetermined intervalyof. time, for instance two minutes.

Another feature is a branch officeselectorrepeater arranged to deny to restricted service-subscribers access to main office trunks and'aaccess to trunks in any desired level of the selector, .the restricted service subscribers lines being connected to one or more particular subgroups .in the banks of the line finders throughwhich the lines are connected to selector repeaters.

Another feature is a branch office selector" repeater arranged to route calls to the mainoffice through the non-numerical trunking switch and to route calls toanother central o'ffice througha level of the numerical selector switch in response to the dialing of two digits.

Another feature is a branch omceselector repeater arranged to route calls to an operatorin response to. the dialing of either an ltXicode or an Xll code, the letter X in each case representing a-digit of any numerical value.

'I'hese'and other features of the-invention are embodied in the telephone system represented schematically in the drawings which form a part of this specification; and a clear and. complete understanding of the invention-will be facilitated by considering the system and the selectorrepeater circuits'shown in the drawings. 'The invention'is, however, not limitedin itsapplication to the particular system shown but is generally applicableto any system comprising automatic main'and branch-ofiices.

--Referring to the drawings,

Fig. 1 shows in a branch ofiice subscribers stations A and B, 'lines Li and L2, linecircuits switches when used as line finders. line finder, selector and connector switches LF,

LCI and LCZ, a line finder LF and a connector BCI;

Figs. 2 and 3 show a branch ofiice selector repeater SRI;

Figs. 4 and 5 show a branch office selector re- 5 Fig. 6 shows permanent signal timing apparatus TA and miscellaneous alarm and register apparatus MA in the branch omce;

Fig. 7 shows an intermediate local selector BS3,

a service code selector SCS, connectors BC? and B03, and subscribers stations X and Y, all in the branch office;

Figs. 8, 9 and 10 show various trunking and numbering schemes which may obtain in a main and branch ofiice system; and

Figs. 11 and 12 show the relative positions in which Figs. 1 to 7 are to be placed to form operative arrangements.

eThe subscribers stations are of the type emcircuits LCI and L02 and the line-finder circuit .LF are similar to those disclosed in the patent to T. L. Dimond No. 2,210,068 granted Aug. 6, 1940. The line circuit LCI includes a line relay M and a permanent signal lock-out relay I6. The line circuit LCZ is similar to the line circuit LCi except that it is not provided with a permanent signal lock-out relay.

The line-finder, selecter and connector switches are of the well-known two-motion st-ep-by-stern. type. Reference may be had to pages 53 to 65, inclusive, of .the second edition of Automatic Telephony by Smith and Campbell for a description of the structure of such switches and their operation when used as selectors and connectors. Reference may be had to the patent to '40 R. L. Stokeley, No. 1,799,654, granted April '7, 1931, and to the aforementioned patent to T. L. Dimond for .a description of the operation of such Each of the SCS, BCI, B02 and B03 i represented in Figs. 1 and 7 by a set of brushes and one, two, or three sets of terminals, all magnets, relays and other associated apparatus being omitted; except that.

the line finder ICE is further represented by three 3U sets of contact springs 25, 26 and 28 and release .magnet 29. The numerical selector switches BS! and BS2 of the selector repeaters SRI and SR2 and the branch office selector BS3 are shown in.

detail. The bank of each of selectors BSI and BS2 is represented by four sets of terminals.

The terminals H and 2' represent the first levels in the bank of selectors BSI and BS2; terminals 12 and 2'12 represent levels connected directly to connectors; terminal 13 represents a m level of the bank of selector BSI connected to third selectors BS3; terminals 213 represent a level of the bank of selector BS2 connected to trunks to another Office; and terminals 14 and 2'! represent the tenth levels of the banks of U5 selectors BS1 and BS2.

The selector repeater SRI shown in Figs. 2 and 3 includes a trunk hunting switch TSI comprising brushes 9!, 92 and S3 and a stepping magnet 90, a numerical selector switch BSI. comping magnet 15, rotary stepping magnet 16 and release magnet 11. The selector repeater SR] further includes a line relay 41, impulse repeating relay 45, switching relay 40 and other control relays as hereinafter described. Two terminal strips 18 and 19 are provided for interconnecting the segments of commutator 6'6 and the windings and contacts of control relays to 80, inclusive, 88 and 89 as required for any particular trunking arrangement and numbering scheme. There are sixteen terminals in each of the terminal strips 18 and 19, which terminals are referred to in the detailed description as being numbered from 1 to 16, beginning at the bottom of each strip.

The system represented in Figs. 1, 2, 3, 6 and 7 conforms to the trunking plan shown in Fig. 8. Referring to Fig. 8, the trunking arrangement is designed to provide for a total of 9,700 fivedigit subscribers numbers in a main office and a branch office. Numbers from 80000 to 81999 and 84000 to 89999 are assigned to themain office; numbers from 82000 to 82999 and 83300 to 83999 are assigned to the branch oifice; and it requires a maximum of two digits to determine whether a called number is located in the branch or in the main ofiice. The first digit of the main and branch ofiice numbers could be any desired digit other than 0 to 1. The digit 0 is used for calling an operator and the digit 1 is employed for special service codes comprising three digits, the first two of which are the digits 11.

In the branch office, the subscribers lines terminate in line-finder switches, such as LF, and in the banks of connector switches, such as BC. The line finders are used to establish connections with lines when calling, and the connectors with lines when called. The station A represents a calling station and the stations X and Y represent called stations. Each line finder is permanently associated with a selector repeater SR which includes a numerical selector switch BS and a non-numerical trunk hunting switch TS. The terminals in the banks of the trunk hunting switches connect to trunks such as TI leading to the main oflice and terminating in incoming selectors such as ISI. The terminals of the first level of the bank of the selector switches BS of the selector repeaters SR are connected to service code selectors. The terminals of the second level are connected to third selectors such as BS3; the terminals of the third to ninth levels are connected to connector switches; and the terminals of the tenth or zero level of the bank are connected to trunks leading to an operators position.

In the main ofiice, the subscribers lines are in like manner terminated in the banks of linefinder switches (none being shown) and in the banks of connectors such as C. The station Z represents a subscribers station to which the connector C has access. In addition to the incoming first selectors such as ISI for extending calls originated by subscribers in the branch office, local first selectors (not shown) are provided for use in extending calls originated by subscribers in the main office. The first selectors, both incoming and local, have access to the same or similar groups of trunks. Terminals of the first levels of the incoming selectors such as ISI are connected to auxiliary first selectors for absorbing a preliminary impulse. The terminals of levels 2 to "I, inclusive, and level, 9 may be connected to trunks leading to other ofiices. The terminals in level 8 are connected to second selectors such as S2 and the terminals -and fifth digits being 'the dialing of the in-the-tenth or zerolevelof the incoming first selectors are not used. The terminals in the secondland third levels of the second selectors S2 are connected totrunks leading to incoming selectors the branch office for use-in completing calls from .main oilice subscribers to sub- -seribers in the branch 'o'ffice. the'otherlevels of the second selectors arecon- -nected to third selectors such as S3. ,The terininals of alllevels of the third selectors S3 are -connected -to connector switches C having The terminals in access to subscribers lines.

:When the line-finder switch LF associated with the selector repeater SRI of Figs. 2 and 3 starts to hunt for acalling line, the trunk huntiing s'witchTSi starts to hunt foran idle trunk =to the main ofiice. When the calling subscriber dialsa first digit, the dial impulses are repeated overthe trunk to the main office and the brushes of selectorBSI are advancedto a corresponding level. If all. ofthe trunks to the main oflice are busy, the selector BSI responds to the first digit and if the calledstation is in the branch office the call will be completed; but if the called station is not. in the branch ofilce, a busy tone is transmitted to the calling station. If the first digit is .0, the selector BSI hunts in the tenth level for .an idle trunk to an operator; and the trunk to the main ofiice is released. If the first *digit is l, the brushes of selector BS5 are stepped up to the first level and are then restored to nor- -mal; if the next digit is any other than 1, the

operation is the same as if no preliminary digit had been'received; but if the second digit is also the digit 1, the brushes are again stepped up to' the first level and rotated therein to select an idle service code selector SCS. If the first digit dialed is any other than'O, l or 8, the brushes of selector 'BSl are advanced to the corresponding level, and remain in this position without trunk hunting while succeeding trains of dial impulses are'repeated over the trunk to the. main office to control the completion of the desired connection. If the first digit dialed is 8,'the called station is either in the branch or in the main ofiice; and the brushes are advanced to level 8, and are then restored to normal. When the second digit is dialed, the impulses are repeated over the trunk to the main oifice and the brushes are again advanced to the corresponding level. If the second digit is any other than 2 or 3, the called station is in the main office, the brushes of selector BSI are held opposite the selected level without trunk hunting, and succeeding trains of dial impulses are repeated over the trunk' to the main omce to control the selector and connector switches through which connection with the called station in the main ofiice is completed, If the second digit is the digit 2, the trunk to the main oflice is released and the brushes are advanced in the second level to select an idle selector BS3, the impulses created by the dialing of the third, fourth transmitted through the brushes of selector ES! to control the operation of the selector B53 and a connector B03 to complete connection with the called station. If the second digit is the digit 3, the brushes of selector BSI are again returned to normal and the trunk to the main orfice is released. When the third digit is dialed the brushes of selector BSI are advanced to the corresponding one of levels 3, 4, 5, -I5, I,-=8 or 9 and advanced in this level to select an'idle connector B02, the impulses created by fourth andfifth digits being repeated: through the brushes of-selector BSI to tion is a restricted: service station, the actuation ofiithe springsx25 :of. the line finder so. indicates;

and, if thefirst digit'dialed is any other than 0, lior 8, the brushes" of selectorBSI are advanced in the corresponding level to. the 11th rotary position and a busy toneis transmitted to the calling station. In some trunking arrangements in which the selector repeater. SRI may be used, trunks to another omcemay :be connected to' a level of the selector BS5 and, when a restricted service subscriber calls this other ofiice, the operation of normalpost springs '19 efiectsthe advance of the brushes to the 11th rotary position to effect the transmission of. a busy signal to the calling station.

When the selector repeater SRI is connected to acalling station, it starts the operation ofa common permanent signaltiming apparatus TA, If all of the subscribers lines are arranged for permanentsignal lock-out, and if no dial impulses are received within twenty-four seconds, the line finder is released, the trunk to the main ofiice is released, and the permanent signal lock-out relay in the calling subscribers line circuit is operated to .-'prevent connection of the line to another selector repeater. If some but not all subscribers lines are arranged for permanent signal lock-out,

the trunk to the main ofiice is released if no dial impulses are received within twenty-four seconds after connection of a calling line to the selector repeater; and, after a-further interval of two minutes, the line finder is released and the permanent signal lock-out relay of the calling line .is operated in case the call is from a line including such a relay. If the calling line does not include a lock-out relay,-the line will again be connected-t0 .a selector repeater. If none of the lines is arranged for permanent signal lock-out, and if no dial impulses are received within twenty-four seconds after a calling line is connected to the selector repeater, the trunk to the main oifice is released; but the selector repeater SRI is held until a further predetermined interval has elapsed; whereupon a permanent signal alarm is operated as hereinafter described in connection with a permanent'signal condition existing on a line connected to selector repeater The timing apparatus TA comprises means for measuringintervals of from two to four minutes, means for measuring intervals of from twenty to thirty minutes, means for measuring intervals of approximatelytwenty-four seconds,

andmeans for releasing selector repeaters to which are connected lines on which permanent calling conditions exist.

The means for measuring intervals of two to four minutes comprises relays I2I, I22 and I23. When these relays are not in use, ground is connected through back contacts of relays I2I and I23 to conductor I25. When ground is connected to start conductor I28, relay I2I operates, disconnecting ground from conductor I25. The operation of relay I2I causes the operation of relay I22, from two to four minutes later. The operation. of relay I22-causes-the operation of relay I23. Theoperation of relay I23 causes the release of-relay -I2I'and connects ground to conductor I24. The release of relay I2I causes the release of relay I22; the release of relay I22 causes the release of relay I23; and the release of relay I23 disconnects ground from conductor I24, connects ground to conductor I25, and connects relay I2I to the start conductor I20. This cycle of operations is repeated as long as ground is connected to start conductor I20. Relay I2I is slow in operating to insure the connection of ground to conductor I25 for a required interval of time, and relay I23 is slow in releasing to insure the disconnection of ground from conductor I24 for a required interval of time.

The means for measuring intervals of twenty to thirty minutes comprises relays II, I52 and I53 which, upon connection to ground to start conductor I50, operate to connect ground to conductor I55 for a short interval once every twenty to thirty minutes and to disconnect ground from conductor I54 for a short interval once every twenty to thirty minutes.

The means for measuring intervals of approximately twenty-four seconds comprises timing relays I3I, I32 and I33 and a start relay I34. Upon connection of ground to start conductor I30, relay I34 operates closing a circuit for operating relay I3I. Relay I3I closes a circuit for operating relay I35 to disconnect battery from conductor I03 and other similar conductors. Relay I3I also closes the circuit for operating relay I32; and, twenty-four to twenty-eight seconds later,

relay I32 operates. Relay I32 closes a circuit for operating relay I33. Relay I33 closes a holding circuit for relay I35, causes the release of relay I34, and connects ground to conductor I05. The release of relay I34 causes the successive release of relays I3I, I32, I33 and I35. The release of relay I33 reconnects start relay I34 to conductor I30; and the above-described cycle of operations is repeated as long as ground is connected to conductor I30. By the connection of ground to conductor I05, the release of main ofiice trunks is effected in selector repeaters connected to lines on which a permanent signal condition exists.

The means for releasing selector repeaters connected to lines in a, permanent calling condition comprises relays I42 to I45, inclusive, the operation of which is hereinafter described in detail.

Referring now to the detailed circuits disclosed in Figs. 1, 2, 3, 6 and 7 arranged according to Fig. 11, assume that a call is originated at station A. Upon removal of the receiver by the calling subscriber, a circuit is closed for operating the line relay I4 of line circuit LCI. This circuit is traced from battery through the middle and upper windings of relay I4, inner upper back contact of relay I4, conductor I2 of line LI, through the telephone instrument at station A, conductor II of line LI, inner lower back contact and lower winding of relay I4 to ground. Relay I4 is thereby energized sufiiciently to close its outer front contacts without afiecting its remaining contacts. At its upper front contact, relay I4 closes a circuit including the inner lower back contact of relay I6, the outer upper back contact of relay I4, and conductor I5 for operating the start relay I8, relay I8 being thereupon effective to start the operation of an idle line finder LE to search for the calling line as described in detail in the aforementioned patent to T. L. Dimond. At its outer lower front contact, relay I4 closes a connection from its middle winding, through back contacts of relay I5 and conductor I3, to the test terminals of the sets of terminals in the banks of the line-finder switches having access to the line LI. As soon as a line finder LF starts searching for the calling line, ground potential is connected to conductor 33, thence through the upper back contact of relay 85, a, lower back contact of relay 51 and the outer back contact of relay 50 to the winding of relay 55, thereby operating relay 55 of the selector repeater SRI to start the trunk hunting operation of switch TSI as hereinafter described. When the brushes of the line finder LF connect with the terminals 2| to which the calling line is connected, ground potential is connected through the lower brush to conductor I3 to fully energize the middle winding of relay I4, and a circuit is closed for operating the line relay 4'! of the associated selector repeater SRI. The energization of the middle winding of relay I4 upon connection of ground potential to conductor I3 completely operates relay I 4, thereby disconnecting the line conductors II and I2 from the windings of relay I 4 and disconnecting ground potential from conductor I5 so as to release start relay 48 if there is at this time no other calling line in this same line finder group awaiting extension to a selector repeater. The circuit for i operating the line relay 4'! is traced from battery through the lower winding of relay 41, the upper back contact of relay 44, a lower back contact of relay 43, conductor 32, the middle brush of line finder LF, conductor I2, through station A, conductor II, upper brush of line finder LF, conductor 3|, an upper back contact of relay 40, inner lower back contact of relay 44, upper winding of relay 4?, upper back contact of the 11th rotary step springs 58, inner lower back contact 5 of relay 43, inner lower back contact of relay 57,

through the primary winding of the dial tone transformer 53 to ground. Relay 47 closes circuits traced from ground through a lower back contact of relay 40 and the lower front contact of relay 4? for operating relays 42 and 53, the circuit for operating relay 53 further including the lower back contact of release magnet 11 and the back contact of the vertical off-normal springs 57. At its outer upper front contact, relay 42 connects ground to conductor 33 to hold the line finder LF' and relay I4 in usual manner, and to maintain the operation of relay 55 in case the switch TS! has not completed its trunk hunting operation. Relay 42 also closes a circuit for operating the start relay 39 of the ringing and tone supply apparatus RA.

The aforementioned operation of relay 55 connects the winding and interrupter contact of stepping magnet 93 of trunk hunting switch TSI, through the outer upper front contact of relay 55, inner back contact of relay 59, and .through the test brush 93 of switch PS5, to the test conductor of the trunk connected to the terminals with which the brushes of switch TSI are then in engagement. Relay 55 also closes a circuit from ground through its outer lower front contact and the winding of relay 55, in series with the interrupter contact and winding of stepping magnet 33. If the trunk connected to the terminals then engaged by the brushes of switch TSI is busy, the test conductor is connected to ground thereby preventing the operation of relay 50 and operating magnet 93. Magnet opens its own operating circuit, and its release advances the brushes of switch TSI to the next set of terminals. The operation and release of magnet 90 is repeated to advance the brushes step by step until an unguarded set of brushes is encountered. If the trunk, connected to the terminals with which the brushes of switch TSI is in engagement when relay 55 operates, is idle, orassoon,

thereafter as the brushes are advanced. into engagement with a set of terminals, connected to an idle trunk, relay 50is operatively'energized in" series withthe stepping magnet 96.]. Since'the stepping magnet 9|] is marginal, it, does not operate inseries withrelay-Efl; and, therefore, no further advance of the. brushes". occurs.

ground at a front contact of relay 42.. With ,both' of relays 50 and 41operated,a-circuitiszclosed for operating relay 455 main ofiic'e is connected through-brushfll, the

upper front contact ofv relay. 45,.theouterrlower front contact of relay 53; through" resistor 5.4, to battery; and the lower: conductor. ofithe selected trunk'T is connectedfthrough:brush.92.,'the

inner upper front contact of relay 56,, the lower front/contact ofrelay 45, the outer upperfront contact f relay 53, through resistor 5 l toground. The connection of battery and ground to the conductors of the selected trunk'Tto the main office effects the operation of the linerelay (not shown) of the incoming selector associated with the trunk T in the main-office to preparethe incoming selector'for response to dial impulses.-

Dial tone is transmitted'through the; windings of transformer 5B,and the upper winding of relay 41, to thecalling stationtoindicate thatthegsubscriber may start dialing. When the. subscriber dials the first digit of a called ,subscribers' number, relay 4'! is alternately released: and reoperated as many times as there are:units;in the digit dialed. The release of relay 4?, in-response to the first impulse, connects groundthroughits back contact, the outer lower front contact of relay 42, lower back contacts. of: relays:49 and43, to the winding of relay 5i and to :the winding of;

the vertical stepping magnet;.l'5.. Relay 5! operates, locks to the ground on conductor. 33; disconnects its winding from the impulse contact of relay 41, disconnectsv the dial tone transformer from the :upper winding ;.of :relay; 4?, and; connects ground potential directly to this: winding. In case the calling, subscriber startsto dial before a trunk to the-main'office has been selected, thev operation of relay 5'! causes :the release of'relay 55, thereby stoppingthe operationrof trunk'hunting switchrTSl: The-operationof vertical stepping-magnet l5 raises the brushes 6!, 62, 63 and 64 up to the first level of theterminalbank; the

vertical-off-normal springs fil'being actuated'as' soon as the shaft is moved out of itsnormal position. Withvertical off-normal springs 61 actuated, the winding of therelay 53 is disconnected; from-the lower front contact of relay; 4! and is connected through'the middle front'contact of' springs 6l and-a lower back contact ofrelay 44,

in parallel with the vertical stepping magnet 75 under the control of the lower back contact of relayAl. When relay 4'! reoperates at the end of the first impulse, magnet l5 releases; but relay 53.is-a.slow-to-release relay and it -remains-operated until all of the impulses in the train createdby-the dialing of the first digit have been received. Relay 42.. is also slow in releasing and remains operated during the response-of relay;

Relay 50 connects ground-to test brush .93to. guard "the selected trunkto the main officeagainst seizurei With relay; 45 operated, the upper conductor of the selected: trunk T to the stepping magnet 15, thereby advancing the brushes GI, 62, Hand 64 to the level corresponding to the digit'dialed. Each releaseof relay41- further causes the release of :relay 45 to disconnect battery and ground from the conductors of the trunk to the-main office and to thereby repeat the'dial impulses to the line relay of the incoming selector inthe main office. At-the.end-

of the train of impulses corresponding-tothe first digit, relay 4? remains operatedthereby causing a The release of relay-53- therelease of relay 53. disconnects battery and ground from theconductors of the trunk:to the-main officeand closesa bridge across these conductors, this bridge beingtraced from brush 9| through the uppercontact of relay 45, outerlower'back contact ofrelay 53; lower winding of relay 44, upper back contact-of relay 53, upper track contact of relay 48, inner upperfront contact of relay53, to brush-92 :of The lower winding of relay 44- is:

switch TSI. thereby energized in series with the linerelay of a second selector in the main ofiice, the @current through the windingof relay 44 being inthe nonoperating direction.

The normal post springs 69. areadjusted-tobe actuated on all levels; and the actuation of these springs, as soon as the brushesare stepped up to the first level of the bank, closesa circuit for operating relay 81. This circuit includessprings 69, the back contactlofxrotary;stepping magnet 16; the upper back-contact of relay 49, the inner lower front contact of relay 53',- and theinner upper'back contact of relay 43; Relay 81' looks .through its lower front contact;.the middle upper back contact of relay 43; the inner lower back contactof relay 45', conductor 33, to groundat a:

front contact of relay 42. RelayBl preparesfor the operation of the rotary stepping magnet [6 in case trunk hunting is requiredon the level selected'inresponse tothe first train of impulses.

Assume first that the call is to an operator, .the first digit dialed being the digit 0. In this case, the brushes of selector BS! arepadvancedv to the tenth or new level; and, when-relay 53 releases, a circuit'is closed for operating relay 86. This circuit is traced from the winding of relay 86, through the lower back .contactof relay 8!, the tenth terminal of commutator 66, brush the lower back contact of relay 86; upper back'contact of releasemagnet ll, inner lower backpcontact of relay 53,- to ground at the inner upper:

back contact of relay 43. Relay 83 closes a lock.-

ing'circuit through its inner upper front contact.

before its operating circuit'is opened at'its lower back contact, the locking circuit includingback contacts of relays 43 and 40, conductor 33, to.

ground at a front contact-of'relay 42. Relay 86 closes a circuit for operating-the rotary stepping magnet'16'; through the Winding of magnet 15, the outer lower front contact of relay 86-,- back contacts of relays 83" and Min parallel, the upper front contact of relay 81, upper'back contact of release magnet inner lower back contact of .re--

lay 53, to ground at a back contact of relay 43. The operation of magnet '56 steps the brushes 6|, 62, 63 and 64 into engagement with the firstset of terminals in the tenth level and opens the locking circuit of relay 81. Relay ill releases, thereby causing the release of" magnet 16; If

the trunk'connected to the first set of terminals:

Each release andreoperationof' relay 4'! in response'to-succeeding.impulses of the train created by the dialing of the This circuit is traced from battery is busy, the test brush 63 encounters a ground potential which is extended through the lowermost back contact of relay 45, the middle lower back contact of relay 43, upper back contact of relay 49, back contact of rotary stepping magnet 16, springs 69, to the winding of relay 81, thereby causing the reoperation of relay 81. Relay 81 again closes the circuit for operating the stepping magnet 16; and the brushes are advanced into engagement with the next set of terminals. Relay B1 and magnet 16 are thus effective to continue the advance of the brushes until a set of terminals, connected to an idle trunk circuit OT, is encountered. If all the terminals in the level test busy, the brushes are advanced to the 11th rotary position in which contact springs 68 are actuated to connect the busy tone source BT of appartus MA to the upper winding of relay 41, thereby transmitting an all-trunks-busy tone to the calling subscriber. If and when an idle set of terminals is encountered, there is no ground potential connected to the terminal engaged by test brush 63 and relay 49 is operated, the circuit for operating relay 49 is traced from battery through the winding of relay 81, springs 69, back t,

contact of rotary stepping magnet 13, winding of relay 49, inner lower front contact of relay 86, lower back contact of the 11th rotary step springs 68, to ground at a front contact of relay 42. Re-

lay 81 is marginal and does not operate in series 3 with the winding of relay 49. The operation of relay 49 opens the short circuit around resistor 31, thereby connecting this resistor in series with the winding of the vertical stepping magnet 15, to prevent the further operation of magnet 15. Relay 49 closes a circuit from ground at the middle upper front contact of relay 42, through the upper back contact of relay 83 and the lower front contact of relay 43, for operatively energizing the upper winding of relay 43 in series with the winding of relay 4!. The operation of relay 4| may momentarily close a circuit for energizing relay 43; but relay 43 is slow in operating and, as soon as relay 46 operates, the circuit for energizing relay 43 is opened. Relay 4O locks through its lower winding to ground on conductor 33, disconnects the windings of relay 41 from conductors 3| and 32, and connects conductors 3|, 32, 33 and 34 through brushes 6!, 62, 63 and 64 to the selected trunk circuit OT. The operation of relay causes the release of

relays

41, 86, 42, 45, 49, 55, 56 and 4|. The calling line is thus connected through conductors 3! and 32, brushes El and 52 and terminals 14, to the trunk circuit OT which is effective in usual manner to connect a holding ground potential to the ter minal engaged by test brush 63, thence through brush 63 to conductor 33, to hold relays 43 and 51 of the selector repeater, to hold the line finder LF, and to hold the combined line and cut-off relay [4 of the calling line. Reference may be had to the patent of R. B. Buchanan et al., No. 2,233,282, granted Feb. 25, 1941, for description of a trunk circuit associated with a trunk to an operator. A class of calling subscriber indication is given in a circuit including the level contacts 26 of line finder LF', conductor 34, a front contact of relay 40, outer lower back contact of relay 83, brush 64 and the terminal with which it is in engagement. One class of calling line is indicated by the connection of ground through a resistance 21 to conductor 34 as shown in Fig. 1; another class by the connection of ground directly to conductor 34; and a third class by the lack of connection of ground to conductor 34.

The release of relays 45 and 50 opens the loop across the conductors of the trunk to the main office, thereby efiecting the release of the associated incoming selector in the main office. The main oifice trunk is not released until after the selector repeater has found an idle trunk to the operator or until the brushes have advanced to the 11th rotary step position if all of the trunks are busy. Since the operators trunks are connected to the tenth level of the selectors of branch omce selector repeaters, the terminals in the tenth level of the incoming selectors associated in the main oifice with trunks from the branch office are not connected to operators trunks; but these incoming selectors trunk hunt in the tenth level and therefore the test terminals in this level should be connected to ground to prevent the selection of any of these terminals. Otherwise the incoming selector might immediately begin to release before the selector in the branch office had found an idle trunk to the operator; and the line relay of the incoming selector might then be reoperated during release and thereby jamb the wipers.

When the call is released by the calling subscriber and by the operator, the holding ground potential is disconnected from the terminal engaged by test brush 63, thereby causing the release of

relays

40 and 51 of the selector repeater SBA, and the release of the combined line and cut-off relay E4. The release of relay 40 closes a circuit including back contacts of relays 40, 41 and 42 for operating the release magnet 11 of selector BS! and the release magnet 29 of line finlder LF. The brushes of the selector BSI and line finder LF are thereby restored to normal. When the shaft of selector BS1 reaches normal, the contact springs 61 are restored to normal, causing the release of magnet 11; and when the shaft of line finder LF reaches normal, the contact springs 2-8 are restored to normal, releasing magnet 29.

Assum next that the first digit dialed is a digit other than 0, 1 or 8, the called subscriber being located in some ofi'ice other than the main or branch offices, and the call being completed over a trunk to the main office. When relay 53 releases after the reoperation of relay 41 at the end of the last impulse in the train, it closes a circuit for operating relay 43. If the first digit is 4, 5, 6, '1 or 9, this circuit is traced from battery through the winding of relay 43, upper back contact of relay 88, terminal I3 and one of

terminals

4, 5, 3, 1 and 9 of terminal strip 19, the

segment of commutator 66 with which brush 55 is in engagement, the lower back contact of relay 83, back contact of release magnet 11, inner lower :back contact of relay 53, to ground at a back contact of relay 43. If the first digit is 2, this (circuit includes the upper back contact of relay 88, the outer lower back contact of relay 84, ter- .minals l4 and 2 of terminal strip 19, and segment 2 of commutator 65. If the first digit is 3, this circuit includes the upper back contact of relay 88, terminal 13 of terminal strip 19, terminal l6 of terminal strip 18, lower back contact of relay 62, terminal l5 of terminal strip 18, terminal 3 of terminal strip 19, and segment 3 of commutator 66. Relay 43 locks through its upper front contact, the inner lower back contact of relay 4!], conductor 33, to ground at a front contact of relay 42, this locking circuit being closed before the operating circuit is opened. The operation of relay 43 causes the release of the rotary stepping relay 81, opens the short circuit around resistor 31 so as to prevent further operation of the vertical stepping magnet 15, and disconnects ground from theconductor leading to the winding of the start relay 39 of the ringing and tone apparatus RA.

If, on a call to be completed through the main office, all of the trunks to the main office are busy upon seizure of the selector repeater SRI, relay 55 is released due to the operation of relay 51; and, when relay 43 operates, busy tone is transmitted to the calling subscriber. To this end, the upper winding of relay 4'! isconnected through the upper \back contact of the 11th rotary step springs 68, the lower front contact of relay 43, the inner back contact of relay 55, conductor. [09, through the winding of relay H2 and the primary winding of the busy transformer BT to ground in the common miscellaneous alarm and register apparatus MA. Relay. H2 operates, closing a circuit for operating relay H3; .and relay H3 closes a circuit for operating the over-flow register H4. When the calling subscriber releases, relays M, 5, 51, a2 and 43 are released. The release of relays 41 and 42' closes a circuit for operating the release magnet 11, thereby restoring the brushes of selector BS1 to normal.

If, on a call to be completed through the main ofiice, an idle trunk to the main ofiice is selected prior to the dialing of the first digit, relay 55 does not release when relay 51 operates, and the brushes of selector BS! remain opposite the selected level without trunk hunting therein. The succeeding trains of impulses transmitted over the calling line are repeated over the trunk T' to operate switches in the main and called offices to complete the desired connection. The talking connection includes back contacts of relay 40, front contacts of relay 51, front contacts of

relays

51, 45 and 50, and condensers 59wand 60. When the connection is released by the call-v ing subscriber, the release of relay causes the release of relays 42, 51 and 45; and the release of relay 42 causes the release of

relays

43, 55, 50 and 14, the selector BS! and line finder LF being restored to normal in the manner above described.

If the calling subscriber is a restricted service subscriber and the springs 25 of the line finder LF are actuated, the operation of relay Elcloses a circuit for operatively energizing the lower winding of relay 38. If then the first digit dialed indicates that a calling subscriber is dated in an office other than the main or the branch ofiice, the release of relay 58, after relay 41 reoperates at the end of the train of impulses, closes a circuit for operating relay 89. This circuit is traced from battery through the winding and back contact of relay 8%), outerwpper front contact of relay 88, terminals-of terminal strips 19 and '58 as commutator 65 with which brush 65- is-in eng-agement, lower back contact of relayBfi, upper back contact of release magnet ll, inner lower back contact of relay 53, to ground at a back contact of relay 43. Since relay 88 has been operated, relay G3 is normal. Relay 89 looks through its outer lower front contact, back contacts of relays 13 and 1-6, conductor 33, to ground at a front contact ofrel-ay 52. Relay 8%] closes a circuit "for operating relay 86 and connects ground through its upper front contact to test brush 63 to simulate a busy condition on all test terminals engaged by; this brush. The rotary step-ping magnet it is thereby operated to adabove described, segment of vented. When the calling subscriber releases the connection, the selector repeater and line finder are restored to normal as above described.

Assume next that the first digit dialed is the digit 8. In this case the called line is either in the main or branch office, but it cannot be definitely determined whether it is in the one or the other until the second digit has been dialed. When relay 53 releases after relay 41 operates at the end of the last impulse of the train, itcloses a circuit for operating relay 84. This circuit is traced from battery through the winding of relay 84, inner upper back contact of relay 82, terminals l5 and 8 of terminal strip 19, terminal 8 of the vertical commutator 65, brush 65, thence to ground at the back contact of relay 43 as hereinbe'fore described. Relay 331010565 a locking. cir-. cuit through its inner lower front contact, winding of relay.82, through back contacts of relays 43 and; 4c, conductor 33, to ground at a front contact of relay 42'. Relay 82 is, however, shortcircuited through the commutator brush 65 and does not operate until the operationof release magnet 11' opens this short circuit. Relay 84 closes a. circuit foroperating the release magnet 11. This circuit includes the lower front contact of springs 67, and the outer upper back contact of relay 82. Th v release magnetof theli'ne' finder is not energized at this time because'the circuit for operating this magnet is not closed until the cut-through relayv (not shown) of the line finder. is released. Magnet 1'! looks through the lower front contact of springs 61, the front contact of magnet ll, inner lower back contact of relay 53, to ground at a back contact of relay 43; causes the return of the brushes of selector SI to normal; and opens the operating circuit of relay 3%. When its operating circuit is opened, relay 85 1s held operated in series with the winding of relay 82, and relay 82 operates in a circuit through back contactsof relays 43 and 40, com ductor 33, to ground at a front contact of relay 42. When the shaft reaches normal, relay 31 releases, the vertrcaloff-normal springs 61 are restored to normal, and the lowerwinding of relay 85 is operatively energized in series with the release magnet 77; This circuit includes the middle lower front contact of relay 51, the lower winding and inner lower back contact of.relay. 35, the. front contact of magnet ll, thence as above described, to ground at a backcontact of relay as. Relay 35 locks through its upper winding and upper front contact under control of relay 12, the operating circuit being opened at its inner lower back contact to release the magnet TI. The aforementioned operation of relay S2 in series with the windingof relay'fi l disconnects terminal 8 of commutator from the winding of relay M and connects this terminal to terminal ll. of terminal strip l9 thence, through terminal E3 of terminal strip 19 and the upper back Contact of relay 88, to the winding of relay 23. The impulses created by the dialing of the second digit cause the reoperaticn of the selector BSE; and .these impulses are repeated over themain office trunk to operate a sccondselector in the main'ofii'ce. If the second digit is any digit other than 2 or 3, the called line is in the main ofirce; and the release of relay 53, after the last impulse of the train, closes a circuit for operating relay 43; or, if the call is from a restricted service subscriber and relay 88 has been operated, the release of relay 53 closes a circuit for operating relay 89 to effect the transmission of a busy tone to the calling station. The circuit for operating relay 43 is traced through the upper back contact of relay 88, terminal I3 of terminal strip 18, through the segment of commutator 56 with which the brush 65 is in engagement, lower back contact of relay 86, back contact of release magnet 11, through the inner lower back contact of relay 53, to ground at a back contact of relay 43. Relay 43 closes its locking circuit before its operating circuit is opened, inserts resistor 31 in series with the vertical stepping magnet to prevent further operation of this magnet, releases relay 81, and disconnects ground from the start lead of the ringing and tone apparatus RA. The brushes of the selector remain opposite the selected level and the succeeding trains of dial impulses received from the calling line are repeated over the main office trunk to operate selector and connector switches in the main office to complete the connection to the called subscribers line. The talking connection includes condensers 59 and 8H and the connection is under the control of the calling station, release being effected as above described.

If the second digit dialed after absorption of a first digit 8 is the digit 2, the release of relay 53, after the last impulse of the train, closes the above described circuit for operating relay 86; and relay 85 causes the advance of the brushes of selector BS! in the second level to hunt for an idle trunk leading to a third selector BS3. When an unguarded set of terminals are engaged by the brushes, relay 49 operates but if all tercrninals are guarded, the brushes are advanced to the 11th rotary position and a busy tone is transmitted to the calling station. If relay 49 operates, it closes the circuit for operating relays 40 and 4|. When relay 40 operates and looks as hereinbefore described, it causes the release of relays 83, 41, 42, 45, 58 and 55. The release of relay 50 opens the bridge across the trunk to the main office to efiect the release of the switches in the main oilice.

Upon selection of a set of terminals connected to a branch ofiice third selector BS3 and operation of relay 48, a circuit is closed for operating the line relay I1I of selector BS3. This circuit is, traced from battery through the lower winding of relay I1I, inner lower back contact of relay I14, brush 62, a front contact of relay 48, conductor 32, middle brush of line finder LF, conductor I2, through the calling subscribers station, conductor I I, upper brush of line finder LF, conductor 3i, 2. front contact of relay 48, brush BI, inner upper back contact of relay I14, upper winding of relay I1I, to ground at a back contact of the 11th rotary step springs I86. Relay I1I closes a circuit for operating relay I12; and relay I12 connects ground, through conductor I18, to the terminal engaged by test brush 63, thereby maintaining a ground potential on conductor 33 to hold relays 48 and 51, to hold the line finder LF, and to hold the line and cut-ofi relay I4. When the calling subscriber dials the third digit of the called subscribers number, relay IN is alternately released and reoperated in response to each impulse. The release of relay I1I in response to the first impulse closes a cir-' lcuit, which includes back contacts of relays I14 and IN, the lower front contact of relay I12, the winding of relay I13 and the winding of the vertical stepping magnet I11, for operating relay I13 and magnet I11. The operation of magnet I11 steps the brushes up to the first level of the terminal bank. When the shaft moves out of normal position, the vertical off-normal springs I are actuated, thereby closing a circuit from ground at the upper contact of relay I12 through the front contact of relay I13 and upper contact of springs I85, for operating relay I15. Relay I15 looks through the back contact of rotary stepping magnet I18, to ground at the outer upper back contact of relay I14. When relay I1I releases at the end of the first impulse, magnet I11 releases. Relay I13 is slow in releasing and remains operated until after relay I1I has reoperated at the end of the last impulse of the train. Relay I12 is slow in releasing and remains operated during the response of relay I1I to dial impulses. Each succeeding release of relay I1I causes the reoperation of stepping magnet I11, the brushes being thereby raised step by step to the level corresponding to the digit dialed. When relay I13 releases after the reoperation of relay I1I at the end of the last impulse of the train, it closes a circuit including the lower front contact of relay I15 for operating the rotary step- .ping magnet I18. The operation of magnet I18 advances the brushes into engagement with the first set of terminals in the selected level. Magnet I18 opens the locking circuit of relay I15; and the release of relay I15 opens the operating circuit of magnet I18. If the connected switch connected to the first set of terminals is busy, the test brush I83 encounters ground potential which is extended through the lowermost back contact of relay I14, back contact of magnet I18, and. the upper contact of springs I85 to reoperate relay I15. Relay I15 again closes the circuit for operating magnet I18, the brushes being thereby advanced to the next set of terminals. Relay I15 and magnet I 18 are in this manner operated and released to advance the brushes until an unguarded set of terminals is encountered; whereupon relay I14 is operated in a circuit from battery through the winding of relay I15, upper contact of spring I85, back contact of magnet I18, winding of relay I14, lower back contact of the 11th rotary step springs I 85, to ground at the upper front contact of relay I12. Relay I15 is marginal and does not reoperate in series with relay I14. Relay I14 disconnects the windings of relay I1I from the calling line and extends the connection from the calling line through its front contacts and through brushes NH and I82 to the line relay (not shown) of the connector switch BC3 to which the selected set of terminals is connected.

The dialing of the last two digits of the called subscribers number controls the operation of the connector BC3 in usual manner to establish connection with the called line. If the called line is idle, ringing current is transmitted to signal the called station in usual manner. After relays HI and I 12 of selector BS3 release, the connection is maintained under the control of the calling subscriber, holding ground potential in the connector BC3 being connected to the terminal engaged by test brush I83 to hold relay I14 of selector BS3, relays 40 and 51 of the selector repeater SRI, to hold the line finder LF,

and to hold the line and cut-off relay [4 in usual manner.

When the connection is released by the calling subscriber, is holding ground potential is disconnected, causing the release of relays I14, 40, 51 and I4. The release of relay I14 closes a circuit for operating the release magnet I19, the brushes of selector BS3 being thereby restored to normal. The release of relay 4!] closes a circuit including back contacts of relays 40, 41 and 42 for operating the release magnet 11 of the selector BSI and the release magnet 29 of the line finder LF. The brushes of the selector and the brushes of the line finder are thereby restored to normal, the circuits for operating their release magnets being opened at their vertical off-normal springs 61 and 28 when the shafts reach normal. When the called subscriber also releases the connection, the connector BCS is restored to normal in usual manner.

Assume now that the next digit dialed, after absorption of a first digit 8, is the digit 3; in

which case, the brushes of the selector B SI are advanced to the third level. The release of relay 53 at the end of the last impulse of the train, closes a circuit for operating relays 48 and 83 in parallel. This circuit is traced from the winding of relay 48,through terminals l2 and I3 of terminal strip 18, front Contact of relay 84, thence in parallel with the Winding of relay 83 through terminals II and 14 of terminal strip 18, the outer lower front contact of relay 82, terminal I5 of terminal strip 18, terminal 3 of terminal strip 19, segment 3 of commutator 6E, brush 65, lower back contact of relay 83, back contact of release magnet 11, inner lower back contact of relay 53, to ground at a back contact of relay 43. The operation of relay 48 causes the release of relays 55 and 50, thereby releasing the trunk to the main office and the operated selector switches therein. A locking circuit for relays 48 and 83 is closed through the inner lower front contact of relay 83, the upper front contact of springs 61, back contacts of relays 43 and 40, to ground at a front contact of relay 42. Relay 83 closes a circuit for operating relay 8E. Relay 86 looks under the control of relays 43 and 49, and closes a circuit for operating the release magnet 11, this circuit being traced from the winding of magnet 11 through the lower front contact of springs 61, outer upper front contact of relay 82, inner upper front contact of relay 83, outer upper front contact of relay 8%, through back contacts of relays 43 and 4G, to ground at a front contact of relay 42. The operation of magnet 11 restores the brushes to normal, the return of contact springs 61 to their normal position being effective to release relays 43 and 83. When the third digit of the called subscribers number is dialed, this digit being one of digits 3 to 9, inclusive, the brushes of the selector BS! are reoperated to select a corresponding level. Relay 81 reoperates as soon as contact 69 closes; and, relay 86 being operated and locked as above described, the release of relay 53 at the end of the last impulse of the train closes the circuit for operating rotary stepping magnet 16, thereby advancing the brushes into engagement with the first set of terminals inthe selected level. If

the first set of terminals is guarded against seizure, the brushes are further advanced until an unguarded set of terminals is encountered; whereupon relays 49, 4| and 4!] are operated as hereinbefore described to seize the terminals and connect the calling line through brushes 61, I52

and 63 to a connector switch BC2 having access to the called line. The dialing of the last two digits of the called subscribers number effects the operation of the connector switch BCZ to.

'inding of release magnet 11, lower front contact of the vertical off-normal springs 61, lowermost back contact of relay 85, outer upper back contact of relay 80, terminal I of commutator 66 and brush 65, lower back contact of relay 86, upper back contact of release magnet 11, inner lower back contact of relay 53 to ground at a back contact of relay 43. Release magnet 11 looks through the lower front contact of the vertical off-normal springs 61 and upper front contact of magnet 11, to ground at the back contact of relay 43. The operation of magnet 11 causes the brushes to be restored to normal; and, when the shaft reaches normal, the opening of the lower contact of springs 61 causes the operative energization of the lower winding of relay in series with the winding of the release magnet 11. The circuit for operating relay 85 is traced from battery through the Winding of the release magnet 11, middle lower front contact of relay 51, lower winding and inner lower back contact of relay 85, front contact of release magnet 11, to ground as above mentioned at a back contact of relay 43. The operative energization of the lower winding of relay 85. effects the closing of a locking circuit through the upper winding and inner upper front contact of relay 85 before the circuit through its lower winding is opened at its inner lower back contact. This locking circuit is traced from battery through the upper winding and inner upper front contact of relay 85, to ground at an upper front contact of relay 42. The opening of the circuit through the lower winding of relay 85 causes the release of magnet 11. The dialing of the second digit l causes the reoperation of the selector BSl and the repeating of the impulses over the trunk to the main olfice. The brushes of selector BS4 are stepped up to the first level of the bank vand, when relay 53 releases after the reoperation of relay 41, it closes a circuit for operating relay 83. This circuit is traced from battery through the winding of relay 83, terminals H and H] of terminal strip 18, lower front contact of relay 85, outer upper back contact of relay 80, terminal I of commutator 66 and brush 65, lower back contact of relay 86, upper back contact of release magnet 11, inner lower back contact of relay 53, to ground at a back contact of relay 43. Relay 83 locks through its inner lower front contact, the upper front contact of springs 61, through back contacts of relays 43 and 40, conductor 33, to ground at a front contact of relay 42. Relay 83 closes a circuit for operating relay 86, this circuit including the middle upper front contact of relay 83, and the outer upper back contact of relay 86. Relay 8% closes its. locking circuit as hereinbefore traced before its operating circuit is opened, and closes an operating circuit for the rotary stepping magnet I3. The brushes of selector BSI are thereupon advanced in the first level of the bank until an idle set of terminals is engaged. If all of the trunks connected to terminals in the first level are busy, the brushes are advanced to the llth rotary position in which the actuation of springs 68 effects the release of relays 5i] and 55 so as to release the trunk to the main office, and effects the transmission of a busy tone to the calling subscriber by connecting the upper winding of the line relay 41 to the busy tone transformer ET in the common miscellaneous alarm and register apparatus MA.

Since the impulses from the digits 11 are repeated over the trunk to the main ofiice, the incoming selector in the main ofiice hunts in the first level of its bank to select a service code selector, and the selected service code selector through brush I64 of selector SCS and brush 65' of selector BSI to cause the operation of relays 40 and M. Relay 56 locks as hereinbefore described, causes the release of relay H, and connects the calling line to the trunk OTI. The holding ground potential connected to brush I63 holds the selector SCS, holds

relays

33 and 51, holds the line finder LF, and holds relay I4. When both the calling subscriber and called operator release, the selector SCS, selector repeater SRI and line finder LF are restored to normal.

hunts in the first level while the selector BSI is hunting for an idle service code selector in the branch office. If the service code selector in the main office encounters an all-trunks-busy condition, a busy tone in transmitted from the main office to the calling subscriber in the branch ofiice, but this does not interfere with the operationof the selected service code selector in the branch oifice; and, if the service code selector in the branch office is operated responsive to a third digit to complete a connection with a special service trunk as hereinafter described, the trunk to the main oifice is released.

Upon selection of a unguarded set of terminals 'II in level I of selector BSI, relay 49 is operated as above described. Since relay 83' is operated, the operation of relay 59 closes a-connection from the winding of relay II, through the upper windingof relay 46, the inner lower front contact of relay I9, and outer upper front contact of relay 83, to brush 84 of selector SI, so that operation of relays 4B and II will be controlled according to the selection made by the service code selector SCS in response to the third digit dialed. The service code selector SCS is similar to the branch office selector BS3. The bank of selector SCS is represented by two sets of terminals, the upper set representing the tenth level having recording completing trunks to a long distance operator connected thereto, and the lower set representing a level which has no trunks connected thereto, selection of this level indicating that the call is to be completed through the main office. Since relay 43 is not operated, relay 4'!- remains connected to the calling line and the operation of relay 49 closes a bridge across the brushes GI and 62 for operating the line relay (not shown) of the service code selector SCS. This bridge is traced from brush SI, through the inner upper front contact of relay 41, and outer upper front contact of relay 69, to brush B2. The impulses created by the dialing of the third digit are thus repeated by relay 41 to control the group selecting operation of selector SCS. The brushes of selector SCS are thereupon advanced in the selected level to hunt for an unguarded set of terminals.

Assume first that the third digit dialed is the digit 0, in which case selector 808 trunk hunts in the tenth level and selects an idle recording completing trunk OTI in this level. The bridge across the brushes 6| and 62 is thereupon extended through brushes NH and I32 to the selected trunk circuit which operates in usual manner to connect a. holding ground potential to the terminal engaged by the test brush I63 and this ground potential is also connected If the third digit dialed is one which effects the selection of a level having no trunks connected thereto, the brushes of selector SCS are advanced into engagement with the first set of terminals in the level and the brush I64 engages a terminal which is connected through a resistor I35 to ground, thereby causing the operation of relay 4!. Relay 40 is marginal and does not operate. The operation of relay II causes the operation of relay 43. Relay 43 looks under control of relays 4t and 42. The operation of relay 43 causes the release of relays 83, i3, 83 and II.

he release of relay 49 opens the bridge across brushes BI and 62 thereby causing the release of the service code selector SCS, the connection over the trunk to the main office being maintained under the control of relays II and 42. When the calling subscribed releases, the selector repeater SRI and line finder LF are restored to normal as hereinbefore described.

The selector repeater SRI may be used in a trunking arrangement and numbering plan such that a group of trunks to another ofiice is con nected to one of the levels of selector BSI, or a level of this selector may represent a route which can be obtained only by calling the operator. On a call from a restricted service subscribers station, relay 83 is operated in the manner above described; and upon selection of the level in question, normal post springs III are actuated to cause the advance of the brushes to the 11th rotary position to actuate springs 68 and thereby transmit a busy tone to the calling station.

When the line LI is connected through line finder LP to selector repeater SRI as above described, the operation of relay 42 connects ground through its inner lower front contact, upper winding of relay 38, outer upper back contact of relay 51, and lower back contact of relay 88 to conductor I03 leading to the timing apparatus TA. If at that time relay I35 is normal or as soon thereafter as relay I35 releases, conductor I03 is connected to battery and relay 88 operates. Relay 88 looks in a circuit from battery through the winding of relay III in the miscellaneous alarm and register apparatus MA, conductor I04, outer lower front contact of relay 88, outer upper back contact of relay 57, upper winding of relay 88, to ground at a front contact of relay 42. Relay III, operated by the current in this circuit, closes circuits for operating relays II! and I44 and for immediately operating start relay I34 if relay I33 is then normal, or for operating relay I34 as soon as relay I33 releases. Relay II'I connects ground to conductor I20 to operate timing relays I2I, I22 and I23. The operation of start relay I 34 causes the successive operation of relays I 3|, I32 and I33. The operation of relay I3I causes the operation of relay I35, and relay I35 disconnects battery from conductor H33- and other like conductors. The operation of relay I33, approximately twenty-four seconds after the operation of relay