US2405194A - Telephone system - Google Patents

Description

405,1 sheets-sheet 1 J. w. DEHN ETAL.

TELEPHONE SYSTEM Filed July 2o, 1945 TILI UI|I NWS Aug., 6 E 4 /fv VEA/Tops M v4.w

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TELEPHQNE SYSTEM1 Filed July p, 194sv 4 sheets-sheet 2 AT'ToR/vy Aug. 6, 1946.

J. W. DEHN EVAL TELEPHONE SYSTEM Filed July 20, 1945 4 Sheets-Sheet 1'?,

ATTORALSV @me www /A/VE/VTORS Patented Aug. 6, 1946 TELEPHONE SYSTEM Joseph W. Dehn, Great Neck, and Myron C; Goddard, Garden City, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation'of New York Application July 2o, 1943, s eiai No. 495,489

7 Claims.

This invention relates to telephone systems and particularly to those in which connections are established by automatic switches.

In automatic switching systems using line finders or trunk finders it is usual to have a plurality of such :nders in groups serving groups of lines improve the flexibility of group allotting arrangeor trunks. Associated with. each finder group is a start circuit actuated by a line or trunk over which a call has been initiated and this start circuit energizes a particular iinder except when this finder is in use. When the rst finder is in use it transfers the start lead to the nextfinder of the group. The start lead is thus transferred to the next in order finder in a group or subgroup in a sequential manner by the preceding finder as the finders are taken for use.

In the system disclosed herein both line finders and trunk finders are used, each having other mechanisms permanently connected thereto, the line finder having a trunkand a dual selector connected thereto and the trunk finder having a sender connected thereto. This arrangement is used as a matter of economy to obviate the use of links or connectors between the finders and the senders and for the sake of speed in operating these mechanisms.. Both a line finder and a trunk nder are automatically energized by an incoming call, the line finder hunting for the calling line and the trunk finder hunting for the trunk permanently connected to the hunting line finder. Thus a sender is automatically connected with the calling line in a short space of time by the action of the line and trunk finders. The circuits from the sender dial tone mechanism and dial pulse responsive mechanism extend through the trunk finder, the trunk circuit, the line under and over the line to the station mechanism of the calling subscriber.

One group of lines is served by a particular group of line finders and a plurality of groups of trunks connected to these line finders is served by'a particular group of trunk finders. A start circuit for energizing the trunk nders is common to the trunks of a group and also common to the finders of this group which serve these trunks. The finders are operated in a set sequence during busy periods as the finders are taken for use. However, during non-busy periods if calls come into the ofce from a given group of lines over spaced periods and each call is disconnected before another call comes into the office the same finder would be used for all of these successive calls. In the case of the trunk finders which each have a sender permanently connected thereto this would mean that the same sender would be'repeatedly connected to the calling lines of a group if the calls in the group were spaced apart so that each switching connection is completed and disconnected before another call from this group of lines enters the oiiice.

nl.The object -of thisrinvention is, therefore to ments in automatic switching; systems in which there is a tendency to limit the operation of mechanism to a single channel during light load periods.

A feature of the present invention consists in automatically operable preference control mechanisms for shifting the Vpreference for selecting finders arbitrarily during light load calling periods and in a sequential order during busy load Y calling periods.

Another and related feature of the invention consists in means associated with the preference control mechanism automatically operated by periodic switching operations in the office for determining kwhen the order of preference for Fig. -2 illustrates the mechanism trunk finders and senders shall be normally advanced and when it shall be arbitrarily advanced.

These and other features will be discussed more fully in the following description as illustrated in the accompanying drawings in which:

Fig. 1 .illustrates a line finder, a line-finder selector trunk, a Vdual selector, and associated selectors and villustrates in detail the mechanism of'a-trunk-finder circuit; Y Y

associated with a number o fsubgroups of trunks Aand trunk-finder circuits for shifting the preference of trunk` findercircuits in a sequential order as the finders are taken for use;

.'Fig. 3 illustrates anY automatically operable arrangement for arbitrar-ily shifting the preference of trunk iinders during -light load calling periods and for placing the :nder preference mechanism in the normal sequential order during busy load calling periods; and v Y Fig,` 4 illustrates the grouping arrangement of line finders, trunks, selectors and the trunk finders and the senders.

General description The grouping arrangement of the mechanism of an automatic switching system in a small tele-A phone vareafo'r which this invention is used is shown in Fig. 4. Reference may be had to my copending application, Serial No. 495,487, filed July 20, 1943, for a complete descriptionof different units of mechanism shown herein in diagrammatic form and also for a completedescription of the organization and operation of these units of mechanism.v Two groups of lines 400 and 4H are shown served by two subgroups of line finders each of which may comprise ten line finders. The ten line finders dill-402 have ten trunks M13-404 and Aten dual selectors 405-406 permanently connected thereto. The same arrangement is used for each subgroup'as shown for' the :ten line finders 4i8-4I9 which served the group of lines 4H and -haveten trunks 420- 42| and ten .dual selectors 422-423permanently connected thereto.Y The sequence of operation of a line finders in a subgroup of li-ne'iinders is well known inthe art and therefore only 'a general description of this operation need be given. The energization of line equipment responsive to an incoming call actuates what is known as a group start relay which energizes a line iinder and marks a segment on the line-finder commutator to guide the line finder to a particular level of* switch tterminals where the calling line may be found. The line iinders are allotted for use in `a 4sequential order depending upon the busy `condition of the line iinders in the subgroup. This allotting is controlled by the transfer of the line-finder start circuit sequentially as the finders become busy which circuit` is carried through contacts of a relay in each line iinder.

In the presentlsyste'm the operation of a line finder such as 40| .fromy its start circuit automatically energizes mechanism in its associated trunk 403 which establishes a start circuit for a trunk -iinder so that both the-line finder and the trunk Afinder are hunting simultaneously. The line iinder 60| hunting for the calling line and the trunk finder, Figs. 1 and 2,-hunt for the trunk 403 connected to the hunting line nder 40|. The trunk-iinder start circuitis common to the line nders and associate-d 'trunks of the line iinder subgroup as shownand thereforefany trunks in this-subgroup will `establish a start circuit for subgrouprstart relay` 4|4. Likewise any trunk in a second group 420-42Iwi1l establish a circuit for group start relay 4I3. Each start relay controls a subgroup of trunk finders.

Trunk iinders 4I I-4I2 are diagrammatically shown for serving theiirst subgroup of trunks and trunk iinders- 4 I 5 and-4 I 6 for serving the second subgroup of trunks, each of these trunk linders being the same as shown in'Fig. l of the drawings associated With the sequence circuit and subgroup start circuits of Fig. 2. A 'shown diagrammatically in Fig. 4, -theloperation `of `'subgroup start relay 4I4w-i11 actuate trunk finder 4|2 if it is idle and if Ait is busy will actuate trunk nder 4I I or another trunk finder in the subgroup and at the same time will mark a commutator segment for guiding the trunk nder. As previously stated eachtrunk -nder `has a sender permanently connected thereto and therefore the sender 426 is connected through the trunk to the calling line when the trunk finder-412 is actuated. Sender 421 may be connected through the trunk to the same calling lline when trunk nder 4II is actuated because trunk cfin1deri4l2 is in use. This may be any trunk of the group which actuates the trunk finder for associating a sender through the trunk to the line. The second and other groups of line nders, trunks, trunk finders and senders are Aoperated and associated together as outlined for the rst group.

From the foregoing it may be seen-that the normal sequence of operating finders is economical 'for'all automatic switching oiices and' would not require any auxiliary transfer devices in a large oce in which more than one nder is normally in use. However, in smaller telephone areas there are periods when incoming calls over lines in a group are spaced apart and the rst nder in a subgroup is taken for use on each incoming call over this period of light load. In the case of the present automatic switching system such as is fully described in copending patent application," Serial No. 495,487, iled July 20, 1943, and particularly to this system when used as an unattended oice this normal procedure would overwerk particular trunknders 'and senders in the subgroup alt any one time.

. gized repeatedly by successive calls because these calls .are spaced so that there is only one call Since a sender 426 is permanently connected to this trunk nder this sendervwill be continuously used to the exclusion of the other senders and if this sender becomes faulty in its operation it may prevent completing a number of telephone connections.

An auxiliary preference shifting mechanism is shown in- Fig. 3 which is automatically responsive to light and heavy load conditions. During normal heavy load conditions this mechanism causes the regular sequence starting mechanism to function as described above but during light load periods when calls are' spaced apart it alters fthe. preference for thev trunk finders after each call and thus alters the preference for senders connected to these trunk finders. This not only has the advantage of using different sender mechanism for successive calls but has the advantage of providing telephone service at all times even though a sender mechanism is faulty in an unattended oiiice.

Detailedv description One of a group of lines is shown in Fig, 1 connected to a subscribers stationA which causes the energization of a line nder and line iinder selector trunk circuit such as |00 and IlI respectively when the subscriber A removes the receiver from the switchhook. The line nder and line nder selector trunk circuit operate in a manner fully .described in copending application Serial No. 495,487, filed July 20, 1943, and therefore need not be described herein. VThe energization of the line finder |00 by a subgroup start relay also causes the energization of the line finder selector trunk IUI which connects ground to the start lead |45 for energizing trunk nder subgroup start relay 203. Four such relays are shown 200, 20|, 202 and 203 for four subgroups of trunks and therefore for four subgroups of trunk iinders. As shown in Fig. 4 the start lead |45 and therefore the start ground relay 203 are common to all trunks in a subgroup. The operation of relay 203 is shown for connecting ground to segment I of commutator |22 and for energizing relay I32 of the trunk nder which latter circuit may be traced from ground on'contact I of the start ground relay 203, contactl of relay 205, contact 2 of jack 233, lead 25|, contact 3 of relay |35 winding of relay I32 to battery. If this trunk iinder is busy and relay |35 of this trunk nder is operated the ground from contact I of start relay 203 extends to the next sequential trunk finder 245 for operating a relay the same as relay I32 in the latter trunk nder. This circuit would extend from contact I of relay 203, Contact I of relay 205, contact 2 of jack 233, contact 3 of relay |35 operated, contact 2 of relay |26 thence over lead 253, contact 3 of jack 233, contact 2 of jack 234, contact of relay 26, to a relay such as relay |32 in trunk finder 245. Relay 206 in this second trunk finder is the same as relay |35 in the first trunk finder of the subgroup.

'Thesequence circuit or preference circuit for otherv trunk nder subgroups is shown in Fig. 2. Relays 201 and 208 are in an intermediate subgroup of trunk finders and are the same as relay |35 shown in Fig. 1. lRelays 209 and 2|!)A are in another intermediate'subgroup and relays 2| I and 2 I2 lare in the end subgroup. VStart ground relay finders represented by relays 2|| and 2|2. Relay 20| energized by the start circuit of another subgroup of trunks extends its start circuit through the group of trunk finders represented by relays 209 and 2|0. Relay 202 extends its start circuit to the subgroup of trunk nders represented by relays 267 and 208 as shown. Relay 203 extends its start circuit to the subgroup represented by the trunk nder in Fig. 1 and the trunk finder represented by relay 206. These relays 200 to 203 also connect ground to segments of commutators in the respective trunk nders depending upon which trunk nder is energized, By reference to diagrammatically grouped units shown in Fig. 4 it may be seen that the operation of start ground relay 4|4 representing a relay such as 203 mayr associate ground from its contact I with any cornmutator segment of the trunk nders in the group. When trunk finder 4| 2 is energized by relay 4|4, the trunk nder is arrested in its motion when it reaches the rst level of terminals. These terminals are indicated as terminals 409. When trunk iinder 4| I is energized by relay 4|4, it is arrested on the second row of terminals indicated at 4|0 since the ground from contact I of start relay 4 I 4 is connected to the second segment of the commutator of trunk finder 4| I.

The trunk nder shown in Fig. 1 operates in the following manner for connecting a sender to the calling line. For this connection it may be assumed that the trunk ilnder shown Ain Fig. 1

is used and that therefore the start circuit is extended by relay 203 through contact 3 of relay |35 and the winding of relay |32 operating the latter relay, and that segment I of the commutator |32 is connected with ground by the operation of relay 203. Relay |32 associates ground with the sleeve lead extending to the sender and operates the stepping relay |36. The circuit for operating relay |36 may be traced from ground through contact 2 of relay |32, contact of Vertical magnet |3|, Contact 2 of rotary magnet |30, upped winding of relay |36 to battery connected to contact 5 of relay |35. The stepping relay |36 establishes a circuit for the vertical magnet which steps the shaft up one step placing the 'commutator brush |23 on the first segment. l'I'hecircuit for the vertical magnet may be traced from battery through contact 5 of relay |35, vertical magnet |3I, contact 4 of relay |26, contact of relay |36 to ground on contact 2 of relay |32. The vertical off-normal springs |25 are closed and the circuit for relay |36 is opened by the contact of the vertical magnet |3I. Relay |36 releases which causes the release of the vertical magnet which now recloses the circuit to relay 36. If brush |23 is not associated with-ground on the rst terminal, relay C and the vertical magnet continue to step the brushes ofthe trunk nder in 4an upward movement until the commutator brush reaches the segment which is grounded by the operation of the group start relay which in this case is relay 203. Since, for this example, segment of commutator |22 is connected |54 to |6| of the trunk first level of terminals. established through its with ground, the brushes finder are arrested on the A circuit fory relay |26 is upper winding in series of relay |36. This circuit may be traced from ground on contact 2 of relay 203, segmentl of commutator |22, brush |23, upper windingof relay 26, contact 2 of the rotarymagnet-|36.;

with the upper winding upper winding 5 of relay |35.

ated position to prevent further stepping. Relay |26 is made slow in operating to give a short time interval between the last ,vertical step and the first rotary step which is brought about by the operation of relay |26. Relay |26 locks through its lower winding in series with the vertical magnet to battery on contact 5 of relay 35. This places relay |26 under the control of relay |35 and causes its release when relay |35 operates. The operation of relay |26 transfers the stepping circuit from the vertical to the rotary magnet. 'I'his circuit may be traced from battery through rotary magnet |30, contact 4 of relay |26, contact of relay |36 to ground on contact 2 of relay 32. The rotary magnet thus lsteps the shaft around until the sleeve wiper |51 reaches a terminal whichis connected to battery through-the winding of a cut-off relay in the trunk circuit connected to sleeve lead |50. This circuit extends from ground on contact 3 of relay |32, lower winding ofrelay |36, contact 2 of relay |35, resistance |2|, upper winding of relay |20, sleeve brush |51 to battery through the cut-01T relay winding in the trunk circuit. The trunk circuit cut-01T relay operates in this circuit and in so doing connects ground with the sleeve lead |50 in place of battery. Relay |36 is thus held operated to prevent further stepping of the brushes in a rotary movement. Relay |20 is also sufficiently operated to close its contact 9. 'I'his energizes the lower winding of relay |20 over a circuit extending from battery through this winding and contact 9, contact of the rotary magnet, contact of relay |36 to ground'on Contact 2 of relay |32. The operation of relay |20 connects leads |46 to |49 and |5| to |53 directly to the mechanism of sender 320 and establishes an operating circuit for relay |35. This operating circuit may be traced from battery through the upper winding of relay |35, contact I0 of relay |20 to ground on contact 2 of the olf-normal contacts |25. The operation of relay |35 closes the sleeve lead ground from the trunk to the sender which may be traced from ground on sleeve lead |50, brush |51., contact 4 of relay I 20, contact 2 of relay |35 to sleeve lead extending through the sender. Relay |35 also closes a locking circuit for relay |20 traced from battery through its lower winding and contact 9 of relay |20, contact 4 of relay |35 to sleeve lead Relay |35, as has been previously described, transfers the start lead 25| to the next in order trunk nder. The start lead extends from lead 25| through the associated armature, contact 2 of relay |26, lead 253, contact 3 of jack 233, contact 2 of jack 234, through the contact of relay 206 to a relay the same as relay |32 of the preceding trunk nder. 'I'he operation of relay 35 removes battery through its contact 5 from the upper winding of relay |36 and from the vertical magnet circuit and relay |26 and also releases relay |36 by short-circuiting its lower winding through its contact 2. This releases relay |26 so that the start circuit may extend through its contact 2. The circuit for relay |32 is opened through the continuity contact 3 of relay |35 and relay |32 is made slow to release so that ground is'held on the sleeve lead extending to the dual selector and the sender until a short period of time has elapsed so that the mechanism of the associated circuits has had time to function. l

The sleeve circuit in the trunk |0| has ground extended thereto, rst, by the line lnder,B .ndsec-y nd,.' by arelay in they dual. selec-.tor- |02. `When this 'ground is initially connected irorrr'the line finder to the trunk" circ 't, a' 'trunk start relay-is operated which establishes thestart circuit? for the trunk liinder andat the same timeconnects battery through the winding of the trunkY cutoff Vrelay Wi'ththe sleeve lead |50 extending to the sleeve terminal of the Vtrunk` finder terminal-bank. As previously described fwhen "theV 'trunk finder ndsthis terminal, relay '|20 is operated'sulciently to close 'its contact 9. 'This operation takes placethrough the 'upper winding of relay |20, lower winding of `relay |36 to' ground on contact 3 of relay |32 and when this circuit is established the cut-off relay in the trunk is operated in such a manner as to connect ground instead of-battery with the sleeve lead |50 and relay |20 -is locked through its lower winding as described. The ground extends over lead |50 through contact 4 of relay and contact 2 of relay |35 after the 'i latter relay is operated, thence over sleeve llead tothe sender for holding the off-normal ground sender relay 32| 'and' as noted on the drawings the operationl of relay 32| operates relay 324. The energization of relay |32 in the trunk finder causes the operation of off-normalrelays 32| and 324 so that the sender is prepared to function 'as soon as it is connected Vto the trunk circuit and linefinder. This automatically establishes the dial tone circuit and Vcauses dial tone to be connected with conductors extendingrto "the subscribers line and thereafter establishes a connection between vthe calling subscribers line and pulse responsive equipment in the 'sender for setting the sender according to dial pulses transmitted by the calling subscriber. l Y Y The nder preference control circuit j 300 is shown connected to one sender rcircuit 320 and it is also 'shown that this control circuit is cornmon to all senders of the telephone office. Since each trunk nder has lasender permanently connected. thereto the device 300 is a sender preferencecontrol circuit as well as a trunk-finder control circuit but'will be known hereinafter as the finder preference control circuit. Reference to the copending application, Serial No. 495,487, led'July 20, 1943,'may be had for the detail operation of the senders shown diagrammatically in Figs. 3 and 4. For convenience in referring to the larger disclosure, relay numbers-'are shown in the sender 320 in brackets which are the 'same as the numbers given to these relays in the aforementioned copending patent application, the numbers which are not in brackets will be used in this description. The control features ofV the mechanism of the nder preference control' circuit 300 depend upon the frequency with which senders of the oilice are taken `for use by incoming calling-lines. Theenergiaaticn of relay 32| when thev sender is taken for use, establishes .a circuit for relay from ground on `the contact of relay 32|, contact of relay 303, contact 3 and winding of relay 30|, resistance 3|0 to battery. This ground places a shunt on relay 302 through its contact 3 so that relay 302 does not operate atthis'time. The operation `of relay 324 opens a circuit at one point which is later established for the rotary magnet 304 of switch 301. Switch 301 is of the type which is moved `one step when the rotary magnet 304 is energized `and deenergized and requires. the lenergization. of release magnet 305 to return it to its normal position.

During normal busy load periods more than-one sender is in constant useY and -thereorej-the ous' senders maintains -relay 30|constantly operated` and .the normal preference circuit shown in liigfZ1 for the trunknders is in use so that when one iinder is busy the next in order finder in the sequence is energized in the Vmanner previously explained.

During light load periods when the incoming calls are spaced apart and only one sender is energized at' Va time Y'the following procedure takes place .in the `nder preference control circuit as follows. During vthis period the rst sender energized operates its relay 32| which connects ground through the winding of relay 30| which operates. Relay 322 is operated to release the sender. The latter relay holds'ground through the winding of relay 30| until the sender has released which releases both relays 32| and 324. Relay 322 is slow to release after the other mechanism of the sender has been released and therefore .the release of -relay 324 causes a circuit to be established fromground through contact 2 of relay 322, contact of relay 324, contact of relay 323, contact 2of relay 30 I, rotary :magnet 304 to battery energizing the rotary magnet 304.l The sender having released causes the release of relay 322 thus removing ground from rotary magnet 304; removing ground from the winding of relay 30| and contact 3 of relay 302. When ground is removed from the rotary magnet'304 the brush 308 steps from its normal position to terminal The removal of ground from the winding of relay 33|and from'armature 3 of relay 302 causes a circuit to be established for relay 302 from battery through resistance 3| I; winding of relay 302, contact V3 of relay 30| to ground on contact 2 of relay 309. Thus, both relays 30| and 302 are operated. The operation of relay 302 establishes a circuit to arbitrarily change the preference lof all trunknder circuits by operating relays 204 and 205. It will now be apparent that if start ground relay 203 'is'energized the start circuit extends 'from ground on contact I of relay 203 through the inner contact of relay 205, contact 2 of jack -234 to trunk iinder Y245 which -is the second trunk finder in the sequence of the start circuit. Like- Wise ifany of relays 200, 20| -or 202 are energized the start circuit extends to the second trunk finder of the subgroup instead of the first trunk finder. `By vreference to Fig. 4 it will be seen that the second :trunk iinder 4| l `has sender 421 permanently connected thereto whereas the rst trunk finder 4|2 has sender 426 permanently connected thereto. Therefore, in shifting the preference of the trunk finder arbitrarily as above-outlined the preference is shifted Vto a different sender than would be used if this preference had not been altered. The rotary oft-normal contact 336 is closed when brush308 steps to the iirst terminal and with both relays 30| and 302 operated a circuit is established for release magnet 335 :from ground on contact of relay 30|; This causes the brush 308 to restore to its normal position but ydoes not cause the release of either 'relays 30| or 302.

When the next trunk finder and sender are taken for use relay 132| of this sender is operated which again connects ground to lead 330 which in vthis case short-circuits the winding of relay 30| causing its release. Relay 302 is, however, held operated from ground over lead 330, continuity contact 3 of relay 33| released, winding of Arelay-332, resistance 3|I to battery. This trunk liinder and sender are the second preference finder and sender since relays 204 and.205.;are operated. Uponlthe-operation of? relay322 for `the release of this sender and the release of relays Fig. 2 to normal by releasing relays 204 and 205 and thus the first trunk finder is the preferred finder for the next successive call.v The third spaced call which is the next successive call again operates a sender relay 32| causing the operation of relay 30| and the release of this sender brings about the operation of relay 302 and the operation of the rotary magnet 304 causing the brush 308 to step forward. The operation of relay 302 again shifts the sequence circuit by the operation of relays 304 and 305 and the release of relays 30| and 302 by the subsequent operation and release of a sender again shifts sequence circuit of the trunk finders as explained. When the switch 301 moves away from its normal position, the rotary o-normal contact 306 is closed and therefore, at any time that relays 30| and 302 are both operated or both released, the release magnet 305 is energized through contacts 2 of relays 302 to ground on contact of relay 30| and therefore this switch is released very often during calls which'are spaced apart. In fact, this release occurs after each cycle of operation when relays 30| and 302 are both operated or both released for shifting the preference circuit of Fig. 2. However, when a busier period occurs the switch may advance a number of steps without having the trunk finder preference shifted.

Let it be assumed now that after a number of spaced operations of the switching equipment of the oice including senders, when only one switching equipment is operating at one time, a busy period occurs and that there are a number of over-lapping calls and therefore that more than one sender is in operation at one time for completing switching connections. These overlapping operations hold ground continuously on lead 330 since a relay the same as relay 32| is operated in more than one sender and the operation of relay 32| in one sender overlaps the operation of relay 32| in another sender. If relays 30| and 302 are normal, the rst sender operated during this busy period will operate relay 30| and place the shunt ground on contact 3 of relay 302 to prevent the operation of relay 302. If relays 30| and 302 are operated, the first sender operated during the busy period will release relay 30| by placing a ground shunt through contact 3 of relay 302. This will continue until such time as all of the senders in operation at any one time have released and the preference for the trunk finders and senders will be according to the position of the relays in the finder preference control circuit 300, that is, with relays 204 and 205 operated, the second finder and associated sender are preferred and with relays 204 and 205 released the first trunk finder and associated sender are preferred. This, of course, depends upon whether relay 30| is held operated at the time or whether relay 302 is held operated.

During this temporary busy4 period each sucperiod, relay 302 is operated f l0 cessive sender operated vand released-on overlapping calls associates `ground with the stepping circuit for energizing rotary magnet 304 and the switch is advanced as each sender is released. The circuit for energizing the rotary magnet 304 is over conductor 33|. Assuming that relay 30| is operated this circuit extends from contact 2 of relay 32-2, contacts of relays 324 and 323, lead .33| to Contact 2 of relay 30|, rotary magnet 304 to battery. Upon the release of each sender this circuit is closed and opened causing the switch to advance one terminal. Under this condition if the switch advances to terminal 6 responsive to the operation and release of six consecutive senders without any spaced periods in the operation of 4these senders, relay 309 is operated from ground connected to brush 30S. Relay 309 in operating establishes a locking circuit to ground on contact of relay 32| of the energized sendersl so that the continuous overlap operation of 'senders maintains relay 309 locked in an operated position. Relay 309 in operating opens the circuit extending over lead 330 to relays 30| and 302 and opens the ground circuit from its contact 2 which is used to operate relay` 302 as previously described. This causes the release of relay 30| establishing a circuit for restoring switch 301 to normal by energizing the release magnet 305 'and prevents any further arbitrary shifting as long as there are overlap operations of the senders. If relays 204 and 205 were operated they are now released and the normal 'sequence operation as shown in Fig. 2 remains in use as long as this busy condition ensues. The next light load period when thelast sender in use has released and no other sender of the office is in use for a switching connection removes ground from lead 330 and the locking circuit for relay 309 is thus open and this relay now releases. The next sender operated will thus operate relay 30| and if a spaced light load period 'follows the busy period, the arbitrary shifting of the preference again takes place under the control of relays 30| and 302 as operated. by the spaced operation of the senders.v

Frequently, during light load periods three .or four calls may enter the office in an overlapping order and thereafter the calls may be again spaced apart. During this period when more than one sender is in operation at the same time, relay 30| or relay 302 would .remain .operated and the other relay of this pair would remain normal. Each of the four senders during.. this overlapping period would, upon releasing, advance switch 301 one step so that at the time the fourth sender released'brush 308 would'have reached the fourth terminal. Assuming that relay 30| had been operated during this' overlapping at 'the time of releasing the fourth sender by removal of ground from lead 330 which removes the shunt from relay 302. This voperates relays 204 and 205130 arbitrarily shift the preference of the trunk finders and Vassociated senders. On the other hand if relay 302 was operated during kthe overlapping period and relay 30| was released, then the release of the fourth sender would cause the release of relay 302. In either case both of relays 30| and 302 wouldv either be operated or released and consequently a circuit is established for the release magnet 305 to cause Athe switch 301g`to restore to normal. Therefore at this time the normal shifting arrangement of preference shown in Fig. 2 is not used sincev the light loadperiod of 'incoming calls continues even'thoughior'a short period a small number of calls enter the ofiice in overlapping order. y

`From thel foregoing it is vapllarent that the auxiliary device 30D is common to and under control of thev senders in the. oflice for arbitrarily switching the order of preference of trunk iinders and senders during light -load calling periods on each occasion after the release of the sender when no sender in the oice is in use for a switching connection so that the senders serving each group of lines are used in successive order during light load periods but automatically control the reversion to the normal preference shifting control shown in Fig. 2 during heavy load calling periods.

What is claimed is:

l. In an automatic switching telephone oflice, switches, groups of lines, a plurality of senders, a plurality of iinders in groups each having one of said senders permanently connected thereto operable for connecting said senders to said lines, means responsive to a call overa line for operating a finder, the lines of each group preferring saidiinders in a particular order, mechanism for normally advancingr the order of preference, se.- quentially as` said finders and sender-sare taken for use during busy load calling periods and an auxiliary device for arbitrarily advancing said order of preference during light loadcalling periods. Y f

2. In an. automatic switching telephone, office. switches, groups of lines, a plurality of'Y senders, afplurality of finders in groups each having one of said senders permanently connectedthereto operable for connecting said senders to said lines, means responsive. to acall over a linev foroperating a finder, the lines of each group preferring said iinders in a particular order, mechanismrfor normally advancing the order of preference sequentially as said iinders and senders are taken for use during busy load calling periods, an auxiliary device for arbitrarily advancing saidorder of preference during light load calling periodsand a control mechanism automatically operatedu by periodic switching operations in said cnice for determining when the order of preference for nders and senders shall be normally advanced and when it shallbe arbitrarily advanced.

3. In an automatic switching telephonek office, switches, groups of lines, a plurality of senders, a plurality of iinders in groups each having one of said senders-permanently connected thereto operable for connecting said senders tosaid lines, means responsive to a call over a line for4 operating a finder, the lines of each group preferring said finders in a. particular order, mechanism for normally4 advancing the order of preference sequentially as said finders and senders are taken for use during busy load calling periods, an auxiliary device for arbitrarily advancing said order of preference during light load calling periods on each occasion after a switching connection is completed and no other switching connection is in progress of completion and a control mechanism automatically operated by periodic switching operations in said oiiice for determining when the order ofI preference for finders and senders shall be normally advanced and when it shall abitrarily be advanced.

4'. In an automatic switching telephone office, switches, groups of lines, a plurality of senders, a plurality of finders in groups each having one f said senders permanently connected thereto operable for connecting said senders to said lines, means responsive to a call over a line for operr12 ating a finder, the lines of. each group preferring said finders in a particular order, mechanism for normally advancing the order of preference sequentially as said finders and senders are taken for use during busy load calling periods, an auxiliary device common to and under thel control of the senders in said ofce for arbitrarily switching said order of preference during'light load calling periods on each occasion after the release of a Isender when no other sender of the oliice is in use for a switching connection so that the senders serving each group of lines are used in a successive order during light load periods and a control mechanism automatically operated by' periodic sender operations in said office for determining when the order of preference for finders and senders shall be normally advanced and when it shall be arbitrarily advanced.

5. In an automatic switching telephone oiflce, groups of lines. a group of line iinders serving each group of lines. each line nder having a trunk permanently connected thereto, senders. a group of trunk iinders common to each group of line finders and trunks, each trunk iinder having one of said senders permanently connected thereto, means responsive to a call over a line for operating aline finder and a trunk finder for connecting a sender to thev calling line, said trunk finders and senders being operated in a preferred order, mechanism for normally advancing the order ot preference sequentially as ysaid trunk iinders and senders are taken for use-during busy load calling. periods, an auxiliary device for arbitrarily advancing said order of preference during light load calling periods on each occasion after lswitching connection is completed and' no other switching connection is in progress of completion and a control mechanism automatically operated by switching operations in said oiilce for. determining when the order of preference for trunk iinders and senders shall be normally advanced and when it shall be arbitrarily advanced.

6. In an automatic switching telephone oflice,A groups of lines, a plurality of finders inV groups, means responsive to a call over-a line for operating a iinder, the lines of each nder group preferring said nders in a particular order, mechanism for. normally advancing the order of preference sequentially as the findersl are taken for use during busy lead. calling. periods andan auX- iliary mechanism for arbitrarily advancing said order. of preference after each call when. one finder. completes a telephone connection and is released before another call requires the use of a finder.

7.. In an automaticswitching telephone office, groups of lines, a plurality of iindersin groups, means responsive to a call over a line for operating a finder, the lines of each finder group preferring said finders in a particular orden. mechanism for normally advancing the order of preference sequentially as the iinders are taken for use during busy load calling periods, an auxiliary mechanism for arbitrarily advancing` said order of preference after each call when one finder completes a telephone connection and isreleased before another call requires the use of a finder, and a control mechanism automatically operated by periodic switching operations responsive to each call for determining when the order of preference for nders shall be normally advanced and when it shall be arbitrarily advanced.

JOSEPH W. DEHN'.y MYRON C. GODDARD.

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