US1632009A - Automatic telephone system - Google Patents

Description

June 14, 1927.

C. E. LOMAX AUTOMATIC TELEPHONE SYSTEM Ilm-"- Ilarance E Lamax June 14; 1927.

C. E. LOMAX AUTOMATIC TELEPHONE SYSTEM 2 Sheets-Sheet 2 \!Aug. 5, 1925 lirez-LEE E Lnmax' Patented June 14, 1927. i i

UNITED STATES 1,632,009 PATENT oFFlci-z.l y

CLARENCE E. LOMAX, OF CHICAGO, ILLINOIS, ASSIGNOR, BY MESNE ASSIGNMENTS,`

TO AUTOMATIC ELECTRIC INC., OF CHICAGO, ILLINOIS, A CORPORATION F DELA- WARE.

AUTOMATIC TELEIHONE SYSTEM.

r circuit details and other features not specitically mentioned will be described fully in the following specifica-tion, reference being had to the accompanying drawings .com-

prising Figs. 1 and 2. Fig. 1 is a schematic trunkiug diagram illustrating the particular method ot' trunking between the primary line switches and the secondary" finder switches disclosed in this invention. Fig. 2- shows by means ot the usual circuit dia grams a sufficient amount of the apparatus 'indicated `in Fig. 1 tor a clear understanding ci the operation ot the system.

Y The number of finder switches in each group The trunking diagram shown in' Figyl` will be first briefly explained. In this diagram is shown, in simple forni, thetrunlr ing arrangement between the local subscribers lines, the individual primary line switches, the secondary finder switches, and

the first selector switches. In the paiticular l ay-out shown, the subscribers lines such as A, are divided into groups of 100 lines, each line being connected to an individual primary line switch, such as C. The line switches of each group have their banks multiplied togethenand have access to ten outgoing trunks. The selection ot an :idle trunk is governedby a master switch, suoli as'MS,

one master switch being individual to each group of primary line switches.

The secondary finder switches are also di` vided into groups, the switches ol each group having their banks multiplied together.

is variable depending on traffic conditions. Permanently connected to eachfinder switch is a first selector, such as E. In the lay-out illustrated. it is. assumed that. ten trunks` from each of groups of primary line switches, only the first three groups of which -firstthree of the latter being shown. V.trunk in a particular grou switches terminates in' thelb are shown, terminate in` the bankspof ten groups of secondary iinder switches, onl the *ach of primary line anks ot a different group of secondary finder switches. It is further assumed that Vthe vbanks of the nder'switches contain 2vsets of contacts.

With this arrangement, then, there will be one trunk from each primary'line switch group terminating in each ot' the ten secondary iinder switch groups. In a practical case this particular arrangement as regards the number of trunks per group or the number of groups cannot always be carried out. `In any event, each `primary line switch group vwill have one or more trunks terminating in each secondary finder switch group. Such deviations from the'special case as illustrated fare not departures, however, from the general trunking scheme as outlined.

A distributor switch, sueltas-D, is individualY to each group of secondary iinder switches. By means of a sepa-rate start wire connected to cachot thev Q5 trunks terminating in the banks of the associated group of secondary finder switches, the distributor, having preselected an idle finder switch, starts the selected idle lindei switch to find lthe calling trunk.

The apparatus shown by means of the usual circuit diagrams in Fig. 2, may now befbriefly described. Substation A is anordinary automatic telephone subst-ation having ythe usual talking instrumentalities and ia calling device for controlling the operation of the automatic switches.` The line conductors terminate at the exchange in the individual primary line switch C, which is of the .well known plunger type, modified in circuit arrangements, however, as will be described in detail in connection with its operation. Y y i The master switch MS,:indicated merely by a dottedrectangle, may be of the type 'shown' and described in the U. S. .patent to -Newforth No. 13,901, reissued April 13. 1915, modified to include ansupervisory relay such as relay 204 for an alarm circuit'.

The distributor D 1s a nonaiumerical switch mechanically7 of the well known ro` tar t e whosewi ers have no normal o- `sition and movegin a forward direction only.

lin-e switch, the distributor switch associated with the group ot tinder switches in Whose banks the seized trunk terminates, starts a preselected idle finder switch by means of a start Wire connected to the seit/.ed trunk to hunt for the seized trunk, aliter which it rotates its Wipers to preselcet the next idle tindcr switch in preparation 'tor another call.

The secondary linder switch F Vis also a non-numerical ,svitch inechanically'f ot the well known rotary type whose wipers have no normal position and move in a forward direction only. lts function is to hunt tor' the trunk seized by a primary line switch, and when found, to connect the seized trunk to selector E and disconnect the limiting mechanism therefrom.

The first selector E, et which only the sential parts having to do with the operation of finder svvitch F are y"bonn, is -o-tthe well knoivn Stronger vertical and rotary type.

The trunk designated X in Fig. 1, accessible to the linesu'itches ot linesvvitch group 1, such as linesivitch C, and terminatingin the banks of finder-switch group v1 containingl linder switch F, is the trunk shoivn in Fig. Q', extending 'from bank contacts 41- 14C, inclusive, to bank contacts` l5-418, inclusive. The trunk designated Y in Figi 1, accessible to the linesivitch ot linesvvitch group 2, such as linesivitch C', and terminating in the banks of iinder-switch group 1, is the trunk shown in Fig. 2 extending trom bankcontacts sT-4ck, inclusive, to bank contacts iK-41S', inclusive;

Having briefly explained the object ot the invention and the apparatus involved in carrying it outJ the operation ot the equipment: will now be explained more in detail. For this purpose it will be assumed that the subscriber at substation A desires to obtain connection with some other subscriber in the exchange. Then the subscriber at substation A removes his receiver to initiate a call, a direct current bridge is placed across the line conductors 11 and 1Q. A circuit `is thereby closed for the line relay 14e which may be traced 'lrom ground at springs 23, line conductor 11, the bridge at the substar tion A, line conductor 12, springs 2d, through the Winding ofthe line relay 1%, conductor 201, through the Winding oit the supervisory relay 20st, included with the master switch equipment, to battery. Line relay 1st operates and closes a locking circuit tor itself at armature 21; prepares a starting circuit by placingl ground ou the spring 33 at armature 22; prepares an unlockingcircuit at armature 20; and closes a circuit t'or the pull down Windingl 15, vvhichmay be traced from ground by Way ot the Working contact and armature 19, through the pull doivn winding 15, conductor 202 to the open-main. battery supplied from the master switch. The supervisory relay E204 operates and closes a circuit for an alarm circuit (not shown) at armature Q05. This alarm circuit may be any ol the uell known types Which give an alarm after the circuit is maintainedclosed for a period longer than the normal period that relay 204 should be in operation'. rlhe pull doivn winding 15 energizes and operates both the plunger arm 17 and the cutoll armature 1G; the former, through the medium of its plunger (not shown), Aforces the bank springs 81-3t, inclu e, into en gageinent with the contacts -t1-4=l, inclusive, respectively, ity being assumed that the plungers ot this group ot lineswitches inw cluding lineswitch C are standing opposite the trunk accessible at bank contacts 1 -14, inclusive; and the latter disconnects the line conductors 11 and l2 troni the normal ground connection and from the upper terminal ot' the line relay 11i, respectively. The private normal conductor 13 which is connected to the test contacts in the connector banks is placed sutliciently below battery potential from ground by way of Working Contact and armature 21 and through the Winding ot line relay 14 to guard the line to substation A troni being seized by a connector at this time. A circuit is also closed for the cut-ott' Winding 16 'trom ground by Way of the Working Contact and arniature 21, through the winding ol the line relay 14, Working contact and armature 20, through the cut-oli' Winding 16 to battery.

As a result of the engagement ot contacts 31-34, inclusive, With contacts 45t-44, inclusive. resliectively, the talking loop is e1;-` tended to tl e l[inner switch F; ground potential is placed on the tinder switch bank con! tact 47, and on the start conductor (o, there-- by closing a circuit through the winding olE the start relay 77 to battery; and by reason of the ground by Way ot' the Working contact and armature 21, through the u'iruling ot line relay 11i, working contact and arinature Q0, bank contact 232, bank contact /l-Q, conductor 203, a potential sutliciently lirlou' battery potential placed ou the n'iastcr switch bank contact to which conductor 202i is connected to operate the mater switch MS in the n'cll known manner. The plungers oit all the remaining idle liuc switches controlled by thc master switch MS are then advanced into position before the terminals ol the next idle trunk. .lt may be mentioned that duringl the operation ot' the master switch, the vopcnuuain batteri is disconnected from the conductor i202 thereby opening` the circuit 'tor the pull down winding 15. The cut-oli' winding' lo is euer, sed sutliciently over the previously traced c`.cuit to hold both the plunger arm t7 and thc cutotf armature 18 in an operative position. Start relay 77 operates and placesi ground on the start conductor 80 at armature S1 and. the Working contact. It is assnn'ied that naar Switch r is an@ a. this um@ mi that the wipers 7 3 and 74 of the dstributer switch D are in engagement with bank contacts 71 and 72, respectively.. It the linder switch wipers 51H54, inclusive, happen to be standing on the bank contactsl 45-48, inclusive, when the trunk terminating in such bank contacts is seized, the switching relay 6l. will energizeimmediately. i

It is assumed, however, that the wipers 51-54, inclusive, are standing on a set of bank contacts other than bank contacts 454-48, inclusive. A circuit may `then be traced from ground on the conductor 80, the resting contact and armature 79, wiper 74, bank contact 72, the resting contact and armature 68, interrupter springs 63, through the winding of the stepping magnet 62 to battery. Stepping magnet 62 energizes and lil .opens its own circuit at the interrupter springs 63. Stepping magnet 62 deenergizes` and upon releasing its armature moves the' wipers 51-54, inclusive, one step, and again vcloses its own circuit at the interrupter springs 63. ,Stepping magnet 62 operates a second time, opens its own circuit at interrupter springs 63, and in deenergizing moves its switch wipers 51-54, inclusive, another j step. This action continues until the wipers 51454, inclusive, are moved so that wiper 53 comes into engagement with a grounded contact indicating that the wipers 51-54,` inclusive, have `found a calling trunk. If this happens tev be some `trunk terminating in bank contacts accessible to the wipers 5154,

inclusive, before the trunk terminating in the bank contacts --48, inclusive, for example, the trunk iterminating in bank contacts 45v48, inclusive, the wipers 51-54, inclulsive will be arrested on bank contacts '45@482 inclusive, and proceed to extend descriptions.

that connection. The calling trunk terminating in bank `contacts `45--48, inclusive, Awill ythen be 'found by the next idle finder switch as will be understood from subsequent terminating in the bank contacts 45-48, inclusive, is the first calling trunk appearing in the finder switch banks. A circuit may now be tracedvfor the switching relay 61 of the nderswitch lt from ground by way of' the. bank contact47 engaged by wiper 53, throughthe upper winding of the switching relay 61 to battery. Switching relay 61 energizes and closes a point in the chain ycir- Y cuit to -be described later, at armature 64; ex-

tends the talking loop by way of armatures 65 and 69 and their workingcontacts, armatures 106 and 109 and their resting contacts,

to the upper and lower windings of the `line relay 101 of theselector D to battery and ground; opens .the circuit of the stepping magnet 62 at armature 68; prepares a circuit for the stepping magnet 'at armature 67 ,l

and places direct ground potential Von the private normal conductor 13 by way of the working contact and armature 66, wiper 52, bank contact 46, bank contact 42, and spring 32, thereby also fully `energizing the cut-oli winding 16 and short circuiting the line relay 14. Line relay 14 deenergizes and opens f the circuit for the pull down winding 15 at armature 19; removes ground potential from bank contact 47 and the start conductor 76 at armature 22, thereby openingthe initial energizing circuit for the'upper winding of the switching relay 61 and the circuit for the start relay 77; opens tlieinitial circuit for energizing the cut-off winding 16 and for placing the initial guarding potential on the private normalconductor 13 at armature 20; y and opens its own locking circuit which in" cludes the winding of the supervisory relay v'204, at armature 21.

l It' the line relay failed to be unlocked for any reason, such as the failure of the finder f' initial energizing'circuit of switching relay or the Y 61 until sufficienttime'has elapsed holding circuits for the lower winding of switching relay 61 to beclosed, which will be described subsequent-ly.

The line relay 1010i the selector D energizes over the previously traced circuit including the talking loop and operatesto close afcircuit `for the releaserelay 102 at its armature 104.7 Release relay 102Y0perates and places ground potential on the holding conductoim a t arn'iature 105'and the working contact. Ground on the holding conductor 100 Vcloses'the holding circuit .tor the lower winding oli the switching.relay6l and also Vcloses the circuit for the stepping magnet 75 l It will be assumed,v however; by way of the working contact and armature that the trunkv seized by the line switch C 67, bank contactf71, wiper' 7 springs 86,' through the winding of the stepping magnet 7 5, armature 84 and the resting contact, tobattery. The slow acting'relay 70' is also operated in multiple with the stepping magnet, 75 over an obvious circuit.

p vMotor magnet 75 operates and opens its own circuit at interrupter springs 86. In releasinghmotor magnet 75 movesthe distributor switch" wipers 73 and 74 into engagement witlrbank contacts 7l and 72', associated with the next finder switch. If this finder switch is busy, ground will be present onbank contact 71, being pted back Vover the holding conductor of the ycorresponding selector'. A circuit Vwill therefore again be closed for motor magnet 75. Motor magnet 75 will energize, open its own `.circuit at interruptersprings .86, and deenergizing\vill move the distributor switch wipers 73 and7l1 another step. Thisaction will continue until the wipers 78-and` 74: are resting on the bank contacts to which the controlling conductors ol" an idle linder switch are connected. rThe distributor switch LD is then .in readiness to start this idle linder switch when another call is initiated over a trunk extending to this particular tinder switch group. Ground is maintained on contact 71 to prevent the distributor wipers from becoming engaged with contacts 71 and 72, during the time that the finder switchF is associa 'ed with the present connection. 1

The slow-acting relay is energized in multiple with the stepping magnet and is maintained energized during the entire hunting operation of the .fiitepping magnet. Being slow-acting, relay 70 remains operated Jfor an interval after the wiper 73 stops on a contact representingl an idle Vlinder switch and maintains the starting conductor S0 disconnected `from the wiper 7d at its armature 79 during this time. Start relay 77 deenergizes when `ground is removed from `start conductor 76 and removes ground from the start conductor 80 at armature 81 betere the slow-acting relay 70 has had time to deenergize and again connect start conductor` 80 to the distributor wiper 74. The vfunction of the slow acting relay 70 is to keep the start conductor 80 disconnected trom the wiper 74: tor an interval. long* enough to allow the line relay 14 to deenergize, and the start relay 77 to deenergize and remove ground from the start conductor 80. It this circuit were not held open during this time and the distributor D immediately 'found an idle iinder switch, this idle finder switch might be uselessly oper- :iter betere the start relay 77 deenergized and removed ground 'from the conductor 8O at armature S1.

The operation of the selector E is well known to those skilled in the art. It will therefore sutiice to mention that the selector E operates in response to the first digit dialle'l by the subscriber at substation A to raise its wipers opposite thelevel of bank contacts corresikmnding to the digit diallcd and then automatically rotates its wipers in search for an idle trunk to a succeeding switch. llhen an idle trunk is found, the

switching relay oi the .selector E, indicated.

in the drawings by the rectangle 103. operates and disconnects the line relay 101 and extends the talking` loop by 4way of armatures 100 and 10!) andthe working contacts, wipers 111 and 113, bank contacts 121 and 123 to the line relay o'f the succeeding switch, similar to line relay 101. The holding conductor is also extended to the succeeding switchat armature 107, and a release circuit (not shown) connected to the resting contact of armature 10ft `is held open at armature 108. The line relay of the succeeding switch operates and closes a circuit Ylor the release relay of that switch similar to release relay 102. The release relay operates and places ground on its release conductor which is ted back by way of bank contact 22, wiper 112, armature 107 and the working contact before the release relay 102, whose circuit was opened when line relay 101 deencrgized, has had time to fall back. Consequently there Vwill beno interru ption oi" ground on the l'iolding conductor 100. The switching relay 103 is. also maintained operated 'from ground on conductor 100.

The connection is extended in this manner through one or more other selectors, depending on the size ot the telephone system, each being responsive to a digit dialled by a subscriber at the calling substation A, to a connector in whose banks the line of the calling substation terminates. The connector operates in response to the last two digl .i dialled by the subscriber at substation A, raises its wipers and rotates them into engageinent with the set of bank contacts at which the line of the called substation terminates, causes the substationto be signalled, and finally completes the talking connection between the calling subscriber at substation A andthe called substation when the subscriber at the called substation removes his receiver.

The two subscribers may now hold oonversation. After the conversation is terminated, both ,subscribers will replace their receivers on their respective switch hooks. These operations bythe calling and called subscribers cause the release of the connector switch in the well known manner. The release oi the connector removes ground from the holding conductors extending through the selectors, including the holding conductor 100. Ren'ioving ground from the holding conductors o'lI the selector switches associated with the connection causes the deenergization ot the switching relays ot the selectors. Release circuits for the various selectors are closed by armatures, suchas armature 108, and theira'espective resting contacts. to restore the selectors to normal. Removal ol ground from the holding conductor 100 opens the circuit for the lower winding of the switching rela-y 61. Switehing relay 61 thereupon deenergizcs and restores its armatures back-to normal. The circuit ot the cut-ofi winding 10 is opened at armature 6G which thereupon .also deenergizes. The plunger of arm 17 and the cut-ott1 armature 18 areithen released. It may be statedl atthis time that it is assumed that the` line switches, such as C, are equipped with self restoring plungers; that is, as soon as a plunger arm is released, it

isforced back into engagement with the shaft by 'the action of a spring and its position with respect to idle ltrunks is again at substation vA. By the removal of ground:

from holding conductor 100, ground is also removed from the distributor bank cont-actv 71. lilith the release of the switchingxrelay` 61,'the hnder switch F is again in position y ready to be selected to extend another call.

The finder switch F, however, will not beA selected until the wipers ofthefdistribiitor D are again rotated into engagement with the bank contacts 71 and 72.

The itunction of the chain relay 7 8 andth'e circuits associated therewith will now be described. VIt is evident to those skilled in the art that traliic conditions never demand that there be as many Vlinderswitches in a group as there are incoming trunks terminating in the banks of that group.` Theretore, if no means were taken .to preventit, a call might be extended over a trunk terminating in agrou of finder switches when all the finder switches ot that grouppwere`- busy which ordinarily would result"` in the A chain relayV failure `ot vthel connection.` 78 whose circuit includes an armature and a working contact on the switching relay ,oi'4 alltlie inder switches of agroup is provided 'for each group. i The rectangle labeled 61 represents the switching yrelay vsimilar to switching relay 61 of the irst finder switch in the same group containing'finder switch F. The working 4contact 'of the armature 64; included in thechain circuit is grounded. Vhen all the finder switches of a group are busy, `all the switching relays'y similar to and including switching relay 61 will be energized. The chain circuit will be completed, extending from ground on the working contact and armature 64 4through the corresponding working contacts and ary matures oi the other busytinder switches, ai'- niature G4 and the working contact, through thewinding of the chain re1ay`78 to battery. The armature 83 ot' the chain relay 78 is a, specially constructed armature having 25 working contacts.` The release conductor of cach trunk terminating in the banks ofthe group ot' finder switches containing finder switch .F is connected to one of these working` coiitacts. The `chain relay 78 operates over the previously traced circuit, and places ground on the release conductor of each of the 25 trunks terminating in the banks of this vgroup ot iinder switches, thereby marking all these trunks engaged, even though some ci them are not being used "inan actual connection. The release conductors of the i case, each ot the 25'release` conductors are 'i connected-to a cont-act ina differentniasterl switch bank, there being 25 'master switches,

oneftor each group of' line switches.' 'It' the plu'ngers ot the line switches otany oflthe groups of line switches are positioned before a trunk leadingto'this particulargroup of. 4finder switches, the master switch control-` ling thisgroup of lineswitches will immediatelybe operated to movethe plungers of the line switches opposite a trunk extend-r ing to some other group `of i'inder switches. As a result, the plungers of no line switches of any group will be standing opposite-a trunk leading `toV this group* `of finder switches, 'and consequently no morefcall's will' be trunked tothis particular group of ind'er f switches.

As soon as one of the finder switchesV of this'ygroup becomes idle, the chain vcircuit will bez openedat 'the armature of the idle finderswitch corresponding to armature '64I' and the chain' relay 7 8 will fbe deenergized.` The guarding ground placed on the release' Vvconductors Vof the trunks will lthen be ref movedat armature' 83. Another `call may'V now be extended over 'any one of the remar-in-VA ing idletrunks terminating in`this group ot finder switches. i e Y i Chain 'relay 78 has an additional function 'in that it removes 'battery from one terminal of the stepping lmagnet 7 5 at armature 84 wlienall the finder switches of the group are busy, thereby preventing useless rotation of thefdistributor, wipers 7 3 and 74 i in search for an idle finder when all the finder switches are busy.

AIn Fig; 2 is shown the test jack 110 hav- 1ng one of the terminals connected to bank' contact 71 and the other o the terminals grounded.` This test jack may be at any conf venient place, its'purpose being to manually busy-guard the finder F and the 'associated selector E against being selected to extend a connection for `any reason thatmight occur.

By connecting the two `terminals of thetest' jack 110, ground is placed lon the corresponding Contactin the distributor switch bank,` thereby preventing the `distributor D from selecting the Afinder switch F.

VShould acall be extended grounded inthe well known manner bythel u to the substation by someother subscriber in theex-` I change, the cut-off vwinding 16 is energized connector which has been selected by the .l

calling subscriber.`

winding 16 is not sufficient to actuate the plunger arm 17,- but operatesthe cut-olf armature-18m disconnect the 'line conductors'll and 12 from theirnormal connec- The magnetism proi. duced byy the energization oi the" 'cut-ott" tions to ground and the upper terminal of line relay 14 by separating springs 23 and 24, respectively. Whenv the subscriber at substation A removes his receiver in response to the ringing of the bells at his substation, the direct current bridge is closed across the line conductors l1 and l2 which operates relays in the connector inthe well known manner to complete the talking connection.

Having fully described the operation and novel features of this'invention, what is considered new and is desired to have protected by Letters Patent will be pointed out in the appended claims.

VWhat is claimed is:

1. In a telephone system, a group ot line switches, a plurality ot groups of finder switches, a group of trunks accessible to said line switches each trunk terminating in a different group of finder switches, means for automatically operating any line switch to seize an idle trunk, and means for starting an idle finder switch of the group in which the seized trunk terminates to find the seized trunk.

f2. In al telephone system, a plurality of groups of line switches, a plurality of groups of finder'switclies, outgoing trunks from each group of line switches, each group o'l line switches having a trunk terminating iii each group of finder switches and each group of finder switches terminating a trunk from each group of line switches, means for operating a line switch to seize an idle trunk accessible thereto, and means for starting an idle finder .switch of the group in which the seized trunk terminates to `tind the seized trunk.

3. In a telephone system, a plurality of groups of line switches, a plurality of groups of finder switches, each group of finder switches containing a less number of finder switches than the number of groups of line switches, outgoing trunks from each group of line switches, each group of line switches having a trunk terminating in each group of finder switches and each group of iinder switches terminating a trunk from eachv Vsaid relay, means or operating said line group of line switches, means for operating a line switch to seize an idle trunk accessible thereto, means for starting an idle finder switch of the group in which the seized trunk terminates to find the seized CII to, andlineans for o erating said start relay to it terminating in said group of finder switches, and means for energizing the relay individual to the trunk seized to start an idle finder switch of the group to hunt for the seized trunk.

6. In a telephone system, a plurality of groups otline switches, a group of finder switches containing a less number of finder switches than the number of groups of line switches, a trunk from each group of line switches terminating in the banks of said .group of finder switches, a relay individual to each of said trunks, means for operating any line switch to seize the trunk accessible toit terminating in` said grou of finder switches, means forenergizing t e relay in- A Y divi-dual to the trunk seized to start an idle linder switch of the group to hunt for the seized trunk, and means for incal'iacitating said iirst means when all the finder switches are in use.

7. In a telephone system, an individual line switch, a line relay in saidswitch having a winding, subscriber controlled means for energizingl said winding to initially operate said relay, means for operating said line switch responsive to the operating of said relay, means for maintaining said winding energized independent of said subscriber, and means for automatically short circuiting said winding after the completion of the operation of said lineswitch.

8. In a telephone system., an individual line switch, a line relay in said switch having a winding, subscriber controlled means for energizing said winding to initially operate switch responsive to the operation of said relay, means for maintaining said winding energized independent of said subscriber, asucceeding switch, means for operating said succeeding switch in response to the operation of said line switch, and means for deenergizing said winding upon the completion of tli-e operation of said succeeding switch.

9. In a telephone system, a subscribers line, a line switch individual to said line, a relay in said switch having a winding, subscriber controlled means for energizing said winding to initially operate said relay, means for placing a guarding potential on a conductor of said line upon the energization of said relay, and means for maintaining said `titi of said relay, means for maintaining saidv winding energized independent ot said sub-- scriber, a succeeding switch, means for operating said succeeding switch in response to the operation of said line switch, and means for deenergizing said winding upon the completion of the operation of said succeeding switch without removing said guarding potential from said conductor.

11. In a telephone system, a group of lineswitches, a plurality of groups ot linder switches, a group of trunks accessible to said lineswitches, each trunk terminating ina ditl'erent group of finder` switches, means for operating any lineswitch to seize an idle trunk, and means coimnon to the finder group in which said seized trunk terminates and individually associated with-an idle `linder switch of that group for starting that linder to find said seized trunks.

12. In a telephone system, a plurality of groups of lineswitclies, a plurality of groups ot linder switches each group of tinder switches containing aless :number ot' l'inder switches than the number' of groups of lineswitches, outgoing trunks from each group of lineswitches, each group of lineswitches having a trunk terminating in each group of .finder switches and each group of linder switches terminating a trunk from each group of lineswitches,a distributor switch for each tinder switch group including means :tor preselecting idle switches of that group, means for operating a lineswitch to seize an idle trunk accessible thereto, means responsive thereto for starting a preselected idle finder ot the group in which theseized trunk terminates to find the seized trunk, and means for disabling the distributor of and for preventing the seizure of an idle trunk which terminates in a group of linder` switches when all the linder switches of that group are busy. a

13. In a telephone system, a subscribers line terminating in an individual lineswitch, a group of finder switches, a 'distributor including means for preselecting anidle one of said linders, a trunk line outgoing from said lineswitch and terminating in the banks oi' said finder switches, means on said subscribers line for operating said lineswitch to seize said trunk, and meansassociated with said trunk andrdistributor for extending a start circ-uit only if said distributor has preselected an idle finder. v

14. In a telephone system, a plurality of groups of lineswitches, a plurality of groups of linder switches, trunks outgoing from said lineswitches and terminating in said iinder switches, a distributor switch for each vtinder group including means for preselecting idle iinders of that group, switching relays in said linders, a chain circuit extending Vthrough contacts on the switching relays of a group, and a relay energized overv said chain responsive to an all finders busy condition in that group to remove battery from said distributor and place an artificial busy on the idle trunks terminating in that group.

15. In a telephone system, a subscribers line terminating in individual lineswitches, a plurality of trun is outgoing from said lineswitches, plurality of groups of finder switches, one of said trunks terminating in each of said finder switch groups, a distributor for each group including means for preselecting ali-idle finder of that group, means it'or operating one ot' said lineswitches to seize an idle trunk, a relay energized when that trunk is seized to extend a start circuit towards a l'inder switch of that group, and means for completing said start circuit only if said distributor has completed the presel lection of an idle tinder.

116. In a telephone system, a plurality ot' groups of lineswitches, a plurality of groups of finder switches, outgoing trunks from each group oit' lineswitches, each group ol lineswitches having a trunk terminating separately in each group of linder switches and each group ot' linder switches terminating a trunk individual to each group ot' lineswitches, means for operating a lineswitch to seize`an idle trunk accessible thereto, and

means for starting an idle finder switch of the group in which the seized trunk terminates to nd t-he seized trunk.

1T. In a telephone system, a plurality of groups of lineswitches, a Aplurality oi groups of inder switches, a plurality of groups of trunks, each group of trunks being associated with a group of lineswitches and having each of its trunks connecting that group of lineswitches to a different group of finder switches and each group of finder switches terminating a trunk from each group of lineswitches, means for operating a lineswitch to seize an idle trunk accessible thereto, and means to start an idle finder switch of thegroup in which the seized trunk terminates to lind the seized trunk.

,In witness whereof, I hereunto subscribe my name this 30th day of July, A. D. 1925.

CLARENCE E. LOMAX.

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