US2249067A - Telephone system - Google Patents

Description

July 15, 1941. P. w. wADswoRTH 2,249,067

' TELEPHONE sYTEM Filed June 22, 1940 gnnd'" L y MARC/NAL afm] DIAL

TONE

. s f l l]- lNl/ENTOR @I i By Rwmaoswomw O v-n diez ATTORAuj/ Patented July 15, 1941 TELEEVHONE SYSTEM Paul W. Wadsworth, Flushing, N. Y., assignor to .Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June Z2, 1940, Serial No. 341,827

7 claims. (cl. 17e-18)' Y This invention relates to'telephone systems and particularly to systems compri-sing subscriber controlled automatic switches for establishing connections between calling andcalled lines.

Objects of the invention are an Vincrease'in the eXibility, adaptability and efficiency of group selector switches and the trunking arrangements 1 of telephone systems comprising such switches. t

This invention is a selector switch comprising Vgroups oi terminals, including one or more groups in which some of the terminals are connected to trunks in one group of trunks and in which some of the terminals are connected to trunks in another groupof trunks, one brush being provided for testing the trunks in the one group and another brush being provided for testing the trunks inthe other group, switching from one to thev other of the test brushes being effected by the actuation of shaft-operated springs upon selection of a particular one of said groups of terminals wherebyA the shaft and brushesare restored tonormal to cause the operation of a brush switching relay. v

A feature of the inventionis a selector of the above-described character comprising a brush switching relay which is partially operated by the energization ofV an operating winding in series with the release magnet when the shaft reaches normal, the complete operation being effected by 4closure of a circuit through a locking winding and contact.

A clearv and complete understanding of the invention will be facilitated by considering a system and selector in which the invention is embodied, one such system and selector being shown schematically in the drawing which forms a part of this specification. l

The drawing, which consists of a single figure, shows schematically a calling subscribers station A and line Ll, a line-Ender switch LF-I, a selector S-l, outgoing trunk circuits OT-I and (JT-2, connector switches C-l and C-2 and a ca-lled subscribers lineLZ and station B.

While only one selector is shown in a connection between a calling liney and a called line, it is understood that intermediate selectors may be provided, the total number of switches required for establishing connections between calling and called lines depending upon the number of lines and the number of digits in the telephone numbers assigned to the subscriber.

The subscribers stations A and B may be of known type, each comprising a dial or other impulse sender for` controlling the operation of selector switches. Each of the line-finder, selector and |connector switches may be of the y wellknown two-motion step-by-step type and reference may be had to pages 53 to 65, inclusive of the second edition of Automatic Telephony by Smith' and Campbell for a description ofthe structure ofv such switches and their operation when used as selectors vand connectors. Reference may be had to the patent to R. L. Stokely No. 1,799,654 granted April 7, 1931 for a description of the operation of such switches when used as line finders. Each-of the line-finder andconnector switches is represented in the drawing by a set of brushes and a single setV of "terminals, Tall loperating magnets, relays and other associated apparatus being omitted.

The selector S--L which is shown in detail, comprises a'set of brushes 3l to 34 inclusive, a bank of terminals consisting of ten levels,n each -level consisting of ten sets of four terminals each and only one level being shown, a vertical-stepping magnet 21, va rotary-stepping magnet 28, a release magnet 29,` vertical off-normal springs 35, shaft-operated springs 36 ladjusted to be operated when the brushes have been raised to a particular level, 11th rotary step springs 31, a

line relay 2l, release relay 22change-over relay 23, cut-through relay 24, rotary-stepping and test-relay 25, and test brush switching relay 26.

Although the selector S-l is shown as being a first selector connected directly to a line finder LF--L it is, of course, to be understood that the invention Yis applicable to selectors other than first selectors, and that there may be one or more selectors in the connection between the calling line and the-selector S-I. Two groups of trunks 4are connected to the terminals 40 to 49, inclusive,

of the level shown in the drawing. A first group of trunks connect the odd-numbered terminal sets of the level shown to'connector switches, including the switches C'--l and C-2. A second group of trunks connect the even-numbered terminal sets of outgoing trunk circuits and interoce trunks, including Vtrunk circuits Of1`l and OT-Z and trunks T-| and'T-Z. The trunks in one group are tested by brush 33 andthe trunks in the other group are tested by brush v34, rthe switching relay 26 "being .provided for transferring from brush V33 to brush 34. As shown, the odd-numbered terminals are tested by brush 34; and the terminals of these setsy which are engageable by brush 33 are perma-f nently connected to ground so as to prevent selection of any odd-numbered set when brush 33 is being used to test for an idle trunk. The

even-numbered terminals are tested by brush 33; and the terminals of these sets which are engageable by brush 34 are permanently connected to ground so as to prevent selection of any evennumbered set when brush 34 is being used to test for an idle trunk.

The number of trunks in each of the two groups may be varied as desired and it is, of course, not necessary that the groups be connected to alternate sets of terminals although such an arrangement tends to equalize the average time required for trunk hunting in each of the two groups. Although one group is shown as an interofce group of trunks and the other as a local group of trunks leading directly to connector switches, each of thetwo groups of trunks connected to terminals in one level may be of any desired character. Furthermore, the local group of trunks may terminate in selector switches instead of leading directly to connector switches, the number of intermediate selectors required depending upon the number of digits in the subscribers numbers. E'a-ch of the other levels of the bank, with the exception of the particular level with which the contact Asprings 36 are operatively associated, are arranged in like kmanner to give access to Atwo groups of trunks. The operation of springs 36,

Aupon selection of this Vparticular level in response to the first train of dialimpulses incoming to the selector S- effects the restoration of the brushes to normal and the operation of the brush switching relay 26 to connect test Vbrush 34 to the test relay 25 in place of the normally connected test brush 33, all as hereinafter fully described. I 1

The trunk circuits OfI' and OT-Z and the other outgoing trunk circuits in` the same group may be similar to the outgoing repeater circuit shown in Fig, Gland described on pages '70 to 72 inclusive of the aforementioned second edition of Automatic Telephony. e I

Assume that a call is originated at station A, and that a line nder LF-IY is operated in usual manner to connect the calling line L| with a rst selector S-|. Upon connectionof` a calling line with selector S-I a circuit is closed from battery through the lower winding of relay 2|, a lower vback contact of relay 24, conductor I2, brush 2, of line finder LF,-|, through line L| and the telephone set and dial at station A, brush I of line finder LF-|, conductor ||an upper back contact of relay 24, upper winding'of relay 2|, normally closed upper contact of the eleventh rotary; step springs 3l, a back contact ofA relay 26, and the'right winding of the dial tone source 30, to ground. Relay 2| is operated by the current in this circuit, thereby closing a circuit for operating relay 22. ductor I3 to hold the line finder LF-l and the cut-off relay (not shown) of the line Ll in the usual manner.

No further operationV takes place until the calling subscriber dials the number of the subscrib' ers station with which a connection is desired. When the iirst digit is dialedthe impulse contacts of the dial alternately open and close the operating circuit of relay 2| as many times as there are units in the digit dialed. The release of relay 2|, in response to the first dial impulse, closes a circuit from ground throughy the back Contact of relay 2|, a frontvcontact of relay 22, aback contact of relay 24, winding of relay 23 and winding of vertical-stepping magnet 2l tobattery. Relay 23 and magnet 21 are operated Relay 22 connects ground to con- -relay 22.

by the current in this circuit. The operation of magnet 2l steps the shaft up one step so that the 'crushes 3| to 34 are aligned with the rst level of terminals in the terminal bank. The actuation or tne vertical oir-normal springs 35 when the shait moves out of normal position closes a circuit from battery through the winding of relay 25, upper Contact or" springs 35, iront contact of relay 23, to ground at a front contact of relay 22. Relay 25 locks, independent of relay 23 in a circuit including the back contact of the rotary-stepping magnet 28, lower front contact of relay 25, back contact of release magnet 29, lower back contact of the eleventh rotary step springs 3l, to ground at a front contact of relay '22. The reoperation of relay 2|, at the end of the iirst impulse, opens the circuit through the windings of relay 23 and magnet 2l and the magnet 2 'immediately releases; but relay 23 is slow in releasing and remains operated, during the responseof relay Y2|, until Yall of the impulses in the train created by the dialing ofthe first digit havefbeen received. Since'relay 22 is slow in releasing itfdoes not release during the response of relay 2| ,to-dial impulses. Each release and reoperation of'relay 2 I, in response to succeeding impulses-of the train, causes a corresponding operation and release of magnet 21 thereby stepping the brushes into alignment with the level corresponding to the digit dialed.

When relay 23 releases after relay 2| has reoperated following receipt of -the last impulse of the rst digit, and the selected level is a level other than the one with which springs 36 are operatively associated, it closes a circuit from battery through 4the winding of the rotary-stepping magnet l28, normally closed contact of springs 36, Yupper front contact of relay 25, back contact of relay'23, to ground at a front contact of relay 22. I-'he operation of magnet 28 steps the brushes 3| to 34, inclusive, into engagement with the first set of terminals -in the selected level. When magnet 28 operates, it opens the locking circuit of relay 2-5; and the release of relay 25 opens the operating circuit of magnet 28. The release of magnet 28 closes a circuit 'path from the winding of relay 25', through the A branch clrcuitpath may be traced from the back contact of magnet 28 through'the Winding of relay 24, back contact of release magnet 29, lower back Contact of Isprings 31, to ground at Since the trunkk connected to the first setiof terminals is a local trunk leading to a connector switch C-l, the -terminal engaged by brush 33 is permanently connected to ground. Relay 24 is short-circuited by 'this ground and does not operate; but relay 25 reoperates, and again locks under. control of magnet '28. The reoperation of relay 25 closes the circuit for operating magnet 28, thereby stepping the brushes into engagement with the second set of terminals in the selected level, the trunk T-| connected to these terminals being' .the rst trunk in the group leading 'to the called oice as indicated by the first digit dialed. The operation of magnet 28 again causestlie release of relay 25 and the release of relay 2lil causes the release of magnet 28. If the trunk T--'-| isbusy, ythe terminal engaged by test brush 33is connected to ground potential and relay 25 again reoperates 'to cause Vcalling line in usual manner.

nently connected to the terminal engaged by brush 33 causes the operation of relay 25 and magnet 28 to again advance the brushes one step. IIhis cycle of operations is repeated until the brushes engage a set of terminals connected to an idle trunk in the group identified by the nrst digit dialed, whereupon since the terminal engaged by brush 33 `is not connected to ground, relay 24 is no longer short-circuited'and isoperatively energized in series with relay 25. Relay 25 is marginal anddoes not reoperate` in series with relay 24.

A If all of the trunks in the group identied by the rst digit dialed are busy, the advance of the brushes continues until all of the trunks -have been tested and the brushes reach the eleventh rotary position in which case there is no circuit for reoperating relay 25;-and the actuation of the 11th rotary step springs 31 prevents the operation of relay 24 and closes a connection Yfrom the left winding of the busy tone source 38, throughthe upper front contact of springs 31, to the upper winding of relay 2l thereby effecting the transmission of a busy tone to the calling subscriber in usual manner.

YThe operation of relay 24, upon engagement of brushes 3l to 34 with'the terminals of an idle trunk in the group including trunks T-l and T-2, disconnects the calling line from the windingsof relay2l and extends the connection from the calling line through conductors Il and I2, front contacts of relay 24, and brushes 3| and 32 tothe selected outgoing trunk, which is thereupon effective to connect ground potential to the terminal engaged by brush 33 to hold relay 24 r operated and also to prevent selection of this trunk by any other hunting selector having access'thereto. Relay 2| releases, opening the operating. circuit of relay 22; and, after the holding ground is connected by the outgoing trunk to the terminal engaged by brush 33, the release vof relay 22 disconnects ground from conductor :i3 so that the line finder LF-I and cut-off relay of .the subscribers line circuit are thereafter held by` the holding ground supplied by the outgoing trunk circuit. The impulses created by the dialing of the remainingdigits of the calledk subscribers number, are repeated in the-outgoing trunk circuit and the connection is thereby completed in the called oice in usual manner.

When the calling subscriber restores the telephone to normal, the holding ground in the outgoing trunk circuit is disconnected from the terminal engaged by brush 33 thereby causing the 'release of relay 24, the release of the line finder LFI and the release of the cut-off relay of the The release of relay 24 closes a circuit including back contacts of relays 2l, 22 and 24 andthe lower contact of vertical off-normal springs 35, for operating the release magnet 29, thereby eiectingreturn of the shaft and brushes to'normal. Release magnet 29 locks under the sole control of the vertical olfnormal springs 35 so that seizure of selector S-I on another call before its brushes reach normal is ineffective to cause the release 'of magnet 29. When the shaft reaches normal, the return of springs 35 to normal position opens the operating circuit of release magnet 29.

Assume now that the dialing of the first digit after connection of the calling line LI to the selector S-l raises thebrushes to the level with Awhich springs 36 are operatively associated. In

this case, the springs 36 are actuated and, when relay 23 releases after response of vrelay 2l tof.

the last impulsel ofthe train, relay 23 closes a circuit for .operating release magnet 29.' This circuitincludes the lower contact of springs 35, lowermost -back contaotof relay 26, normally opencontact of springs 36, front contact of relay 25, back contact' ofrelay 23, to ground at relay 22.- The operation of magnet 29 causes the return o f the shaft and vbrushesto normal. Although levelsprings 36 are restored to normal as soon as the brushes leave the level, release magnet 29is` held through the lower contact of the vertical oli-normal springs V35 Yto ground at the upper front-contact of magnetV 29. ,When the shaft and brushes reach normal, this circuit is'bpened bythe return ofV springs 35 to normal; whereupon the winding of release magnet '29 is energized in series with the lower winding of .relay-26; back contact of relay 26, and front contact of relay 22, to'ground at the upper front contact of'inagnet 29.V Release .magnet 29 is thus held operated and the energization *ofL the lower winding' of relay 26 is effective to close, at the inner upperfront contact of this relay, a circuit for energizing 'the upper-winding to complete the actuation of relay 26. Relay 26 is lockedthrough its'upper-windingto ground at the lower front contact f release magnet`29 until thismagnet releases. AWhen' magnet 29 releases, due to the opening-of the'` circuitf through the lower winding of relay '26 va'tthe innerV lower back contact of relayY 26, the upper winding` of relay 26 'remains energized in 'a circuit which includes the lower contact of the eleventh rotary step springs 3'! and the inner lower" front contact of relay 22. l

"Thus the brushes are restored to` normal to absorb' the rst digit and to render thetest brush 34 operatively effective instead of brush 33. Wheny thecallingsubscriber dials the second digit of the called subscribers number, the brushes are raised step by step to the level corresponding toY this digit. Relay 23 is reoperated in series With the vertical-stepping magnet upon release of relay 2l lin response tothe first impulse of the train.` Whenthe 'shaitmoves out of normal posiltion the actuation of 'springs 35 closes the circuit for operating relay" 25 and relay 25'locks under controlof the rotary-stepping magnet 28 as'hereinbefore described.' When relay 23 releases, after theresponseof relay'Zl "to' the last impulse of Athe train, the'rotary-stepping magnet 28 operates to advance the brushesinto engagement vwith the iirst setiof terminals'in the selected level, which level Vwe will again' assume to be the level 'shown -inthedrawingV` vWith' relay 26 locked operated,

the dial `tonelsource .30 is. disconnected and ground is connected directly to the upper winding ofrelay 2l. And with relay 26 locked operated, the testbrush 34 is connected through a front Contact of relay 26, a back contact of relay 24, back contact of rotary-stepping magnet 23 and upper contact of the VON spring 35 to' the winding of relay 25. If the connector C-I, which is connected to the first'set o-flterminals, is busy, the ground potential encountered by brush 34 causes the reoperation of relay 25. Relay 25 locks and closes the circuit for operating the rotary-stepping magnet 28 to advance the is not reoperated, since the-terminal engaged by testbrush 34 is not connected toground,-but the windingof relay 24 is operatively energized in series with the winding of relay 25. Relay v24l disconnects test ,brush 34 from the winding of relay 25 and connects it to ground at the inner lower front contact of relay 22 to immediately guard against seizure of the selected connector switch yby another selector having access thereto. The operation oi relay 24 also causes the release of relay 2| and connects the calling line through conductors, II and I2 and brushes ,3I and 32 to the seized connector. The operation of the line and release relays (not shown) of the seized connector are effective in usual and well-known manner to connect a holding, ground potential to the terminal engaged by brush34. The release of` relay 2| opens the operating circuit of relay 22. Being slow in releasing relay `22 does not'disconnect ground from conductor I3 until ground in the .connector has been connected to the terminal engaged by brush 34 so as to hold relays 24 ,and 26 and to hold the line nder LF-I and cut-off relay of line LI in usual manner.

When the calling subscriber dials the last two digits of the called subscribers number, theconnector switch is selectively operated to connect with the .terminals of the called subscribersline. The testing of the called line, ringing of the called station, and establishment of the talking connection when the call is answered all Aoccur in well-known manner, the connection being held under control of the calling station. When the ca llingsubscriber releases the connection by restoring the telephone to normal, the holding ground in the connector circuit is disconnected from the terminal engaged by brush .V34 to cause the release of relay 24 and the return of switches yS-I and LF-I to normal. i V

Sincethe selector S-I maybe used'as a second or Vother intermediate selector, the release magnet 29 when operated connects ground through its lower front contact and the lower back contact oi springs 31 to conductor I3 thereby preventing reseizure of selector S--I s by'a preceding `selector having access thereto until lthe switch is completely restored. Otherwise reseizure of vthe selector after release by the calling subscriber on a first call might cause the immediate false operation vand locking of relay 25.

The invention is, of course, applicable to other selector circuit arrangements. It may, for instance, be applied tothe selector disclosed -in the .application of H. Hovland, Serial No. 319,993,

iiled February 21, 1940, in ,which `selector the brush switching relay has a single Winding and its operating circuit is arranged so as to :prevent false operation in case the selector is used as an incoming selector and is seized ona second call after the release magnet is energized at the end of a rst call.

What is claimed is:

1. kIn a telephone system, a selector switch vcomprising a bank of terminals divided into levels and a set ofv brushes including two test brushes 'for connecting with any set of terminals in any level, means .responsive to ya ltrain of impulses incoming to said switch for stepping the brushes into.alignment with the terminals of .the level corresponding'to thenumber of impulses Vin the train, means for stepping the brushes into engagement with any set of terminals in any selected level, groups of trunks connected to said bankptwo of said groups Vbeing connected to ter- 'minals in one level, a test relay, a-control relay having a normally closed contact for connecting said test relay to one of said test brushes and a normally open contact for connecting said test relay to the other of said test brushes, and means comprising Va set of contact springs actuated when said brushes are stepped into alignment witha particular level for returning the brushes yto normal to condition the switch for selective response to a second train of wimpulses and for operating said control relay. 'v

Y 2. In a4 telephone system, a selector switch comprising a bank of terminals divided into levels and a set of brushes including two test brushes for connecting withany set of terminals in any level, means responsive to a train'of impulses incomingtosaid switch for stepping the brushes into alignment with thek terminals of the level corresponding to the number of impulses in the train, means for stepping the brushes into engagement with any set of terminals in any selected level, groups of trunks, a different two of said `groups being connected to each of a pluralityoi levels of said bank, a test relay, a control relay for connecting the one or the other of said test brushes to said test relay, and means comprising a set of Acontact springs actuated when saidbrushes are stepped into alignment with a particular level for returning the brushes to normal to condition the switchvfor selective response to a second ,train of impulses and for operating said control relay to switch from one test brush to the other.

3. In a telephone system, a selector switch comprising a bank of terminals divided into levels and a set of brushes including two itest brushes for connecting with any set of terminals in any level, means responsive to a train of impulses incoming ,to said switch for stepping the brushesA into alignment with the terminals of the level corresponding to the number of impulsesA in the train, means for stepping the brushes into engagement with any set of terminals in any Vselected level, groups of interoice trunks, a different group for each of a plurality of levels, groups of local trunks, a different group for each level, a test relay for testing the trunks of any level, a control relay having a normally closed contact for connecting one of said test brushes to said test relay and a normally open contact .for connecting the other of said test brushes to said test relayand means compris- -ing a set of contact springs actuated when said -brushes .are stepped into alignment with a particular one of said vlevels for returning the brushes to normal to condition the switch for selective responseto a second train of impulses `and for operating said control relay to switch frornone test brush to the other.

4. ,In a telephone system, a selector switch comprising a bank of terminals divided into levels and a; set of brushes including two test brushes :for connecting with any set of terminals in any level, means responsive to a train of impulses incoming 4to said switch for stepping the brushes into alignment with the terminals of the level vcorrespondingito the number of impulses in the train, 'means for stepping the brushes into engagement with any set of terminals in any selected level, groups of trunks connected to said bank, two of said groups being connected to terminals vin one level, the test conductors of the trunks in one group being connected to test terminals engageable by one of said test brushes and the test conductors of the trunks in the other to test terminals engage.-

able by the other of said test brushes, the other terminals in said level engageable by said test brushes being permanently marked with busy potential, a test relay, a control relay having a normally closed contact for connecting said test relay to one of said test brushes and a normally open contact for connecting said test relay to the other of said test brushes, and means comprising a set of contact springs actuated when said brushes are stepped into alignment with a particular level for returning the brushes to normal to condition the switch for selective response to a second train of impulses and for operating said control relay.

5. In a telephone system, a selector switch comprising a bank of terminals divided into levels and a set of brushes including two test brushes for connecting with any set of terminals in any level, means responsive to a train of impulses incoming to said switch for stepping the brushes into alignment with the terminals of the level corresponding to the number of impulses in the train, means for stepping the brushes into'engagement with any set of terminals in any selected level, groups of trunks, a different two of said groups being connected to each of a plurality of levels of said bank, the test conductors of the trunks in one group of each of said -plurality of levels being connected to test terminals engageable by one of said test brushes and the test conductor of the trunks in the other group of each of said plurality of levels being connected to test terminals engageable by the other of said test brushes, the other test terminals in each of said plurality of levels being permanently marked with busy potential, a test relay, a control relay for connecting the one or the other of said test brushes to said test relay, and means comprising a set of contact springs actuated when said brushes are stepped into alignment with a particular level for returning the brushes to normal to condition the switch for selective response to a second train of impulses and for operating said control relay to-switch from one test brush to the other.

6. In a telephone system, a selector switch comprising a bank of terminals divided into levels and a set of brushes including two test brushes for connecting with any set of terminals in any level, means responsive to a train of impulses incoming to said switch for stepping the brushes into alignment with the terminals of the level corresponding to the number of impulses in the train, means for stepping the brushes into engagement with any set of terminals in any selected level, groups of trunks connected to said bank, two of said groups being connected to terminals in one level, a test relay, a control relay having a normally closed contact for connecting said test relay to one of said test brushes and a normally open contact for connecting said test relay to the other of said test brushes, means comprising a release magnet for restoring the brushes to normal, means comprising a set of contact springs actuated when said brushes are stepped into alignment with a particular level for operating said release magnet, a shaft-controlled contact for opening the operating circuit of said magnet when the brushes reach normal, a holding circuit for said magnet including a front contact of said magnet in series with an operating winding and a back contact of said control relay, said circuit effective to hold said magnet operated and operatively energize said winding of the control relay upon the opening of said shaft-controlled contact, and means comprising another winding of said control relay and a contact of said control relay closed by the energization of said operating Winding for maintaining said control relay operated, the energization of said other windingv being eiective to actuate said back contact of said control relay to open said holding circuit and thereby release said magnet.

7. In a telephone system, subscribers lines, a selector switch comprising a bank of terminals divided into levels and a set of brushes including two test brushes, groups of trunks connected to said bank, two different groups to each of a plurality of levels, means for connecting a calling one of said lines to said switch, an impulse sender at the calling station, means responsive to a train of impulses from said sender for moving the brushes from normal to select a desired level, a set of contact springs actuated only upon selection of a particular level, means comprising a release magnet for restoring the shaft and brushes from any oit-normal position to normal, means including a normally open contact of said springs for operating said release magnet upon selection of said particular level in response to the first train of impulses incoming to said switch to restore said brushes to normal, a test relay, a brush-switching relay having a contact normally connecting said test relay to one of said test brushes and a normally open contact for connecting said test relay to the other of said test brushes, means for operating said switching relay upon restoration of said brushes from said particular level when selected in response to the first train of impulses and for locking said relay until the connection is released by the calling subscriber, means for advancing the brushes step by step in a level other than said particular level selected responsive to a rst train of impulses and in any level selected responsive to the next train of impulses after the restoration of the brushes from said particular level to hunt for a set of terminals connected to an idle trunk in the selected level,means including said test relay and said one of the test brushes for testing the terminals in said level when the level is selected responsive to the iirst train of impulses, means including said test relay and said other test brush for testing the terminals in said level when the level is selected responsive to a train of irnpulses following restoration from said particular level when selected in response to a rst train of impulses, means associated with trunks in the group tested by said one test brush connecting a busy potential to the test terminals engageable by said othertest brush, means associated with Y trunks in the group tested by said other test brush connecting busy potential to the test terminals engageable by said one test brush, means including said test relay for seizing the iirst idle trunk in the group being tested, means includ-Y ing said seized trunk for completing a talking connection with a called line, means for maintaining the connection between said calling line and said seized trunk until the connection is reother calling line to saidV switch until the brushes have been restored to normal.

PAUL W. WADSWORTH.l

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