USRE18917E - Telephone system - Google Patents

Description

J. WICKS TELEPHONE SYSTEM Aug. 15, 1933. I Re. 18,917 Orlglnal Flled Aug. '7, 1925 15 .'Sheets-Sheea` l Hambr- Jn-m Malfa J. wIcKs v Aug. 15, 1933. TELEPHONE SYSTEM Re. 18,917

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TELEPHONE SYSTEM John Wicks, Oak Park, Ill., assignor, by mesne assignments, to Associated Electric Laboratories, Inc., Chicago, Ill., a'Corporaton of Delaware Original N0. 1,723,224,

dated August 6, 1929,

Serial No. 48,693, August 7, `'1925. Application for reissue August 5, 1931.

Serial N0. 555,333

64 Claims.l (Cl. 179-27) the so-called inverted type in which talking b'afttery and-holding potential are suppliedfroma first selector rather than a connector. This case may be considered as an improvement on my Pate' ents No. 1,667,405 and No; 1,681,913 patented April 24, 1928 and August 21, 1928 respec-;

tively.

One object of the invention is the provision of arrangements whereby the switch-overoperation v (which takes place automatically when the -final digit in a given number is dialled) is'used to bring about the switch-through operation in the directors (which are employed to receive the number dialled by the calling subscriberand to route the call tothe desired oice and nally to the desired line). The utility of this arrangement lies in the fact that, Withthe switch-through operation or the director under the control of the switch-over operation in the switch train, the directors may be used in handling calls in which the telephone numbers d0 not contain a uniform number of digits without adding special apparatus to the director in order to cause it to be switched through and freed from the connection at one point or another in accordance with the number of digits in the telephone numbers.

Another object of the invention is the production of a new and improved toll service trunking system to operate in conjunction with the present inverted system to enable toll operators to complete the connections to the subscribers lines through switches operating along the same line as the other switches in thesystem and to be able to retain the necessary supervision over the connection and to control. the starting of the application of ringing current to the called line.

Another object more or less closely associated with the preceding one is the production of an arrangement whereby the intermediate selectors and the connectors in a given oflice may form a part of the toll servicev trunking system as well as a part of the regular subscriber-to-subscriber tunking` system, and to this end certain novel circuit arrangements have been devised to permit the intermediate selectors and the connectors to be used in both systems, without unnecessarily 'complicating these switches.

According to another feature, a new and improved rotary connector has been provided for use in the inverted system to take care of the situato the vfirst series of impulses received from the non-inverted system, to repeat the remaining imder to be understood best.

tion in which a certain subscriber-.desires several lines having a common call number.

Another object is theproduction of an arrangement whereby two oiiices, onecontaining switches operated according to the inverted system, and the other containing switches 'operatedaccordingtothe non-inverted system, may vboth form part ofthe same multi-oilice. trunking system, and

vto this end an incomingselectorhas been pro-f videdin the invertedofiice' to operate responsive pulsesto thevother selectors and `to the connectors,

to supplytalking battery to the calledline, and to reverse battery` to the non-inverted oflice when the calledfsubscriber responds. i j

;.v-Another object of the invention is the pro-y visionof a new and improved impulse repeating arrangement-'to enable impulses 'to be repeated satisfactorily over vlong trunk lines. To this end a repeater has been provided for insertion into an Y outgoing trunk line when the trunlclineI is of great length, together with certain modifications in the distant incoming selector to adapt. it to'respond to the reversals of current in the trunk line brought about by the repeater to operate the switches in the distant oice. c

Another object of the invention is the production of an arrangement for completing calls to an operators position and for automatically controlling the rst selector repeater from the trunk line leading to vthe operators position to switchthrough and cut-off all the bridges form the'talking circuit and to` hold up dependent upon ground hsupplied to the release trunk conductor at the opbodying the principles o f the invention to enable the invention to be understood, and Fig. 17 is a Y diagram showing how the sheets' ofv drawings on which Figs. 1-16 appear should be arrangedin orf Fig. 1' vshows a three conductor toll service trunk extending from the jack J at the toll board to the selector S located in the same oice as the toll board. This drawing shows also the cord circuit CC which is one of the cord circuits used by thetoll operator to complete connections to the Referring no w to the accompanying drawingsl l hereinafter.

toll service trunks through the jacks such as J.

Fig. 2 shows the transmission selector repeater on the incoming end of the inter-oilice trunk line comprising conductors 9 and 10 and accessible to the selector S, Fig. 1.` This selector repeater is of a special design so as to give the operator the correct supervision and so as to enable the operator to control the starting of the application oi' ring-l ing current to the called line.

Fig. 3 shows an intermediate selector which is the term applied to the simple selectors coming in the switch train following a battery feed selector i'n the same oiiice. This intermediate selector is accessible to the transmissionselector repeater Fig. 2 and to the incoming selector repeaters shown in Figs. 11 and l2. This selector has the usual line and release trunk conductors and it also has an extra conductor 160 coming into it from the transmission selector repeater and passby relays 303 and 307 of Fig. 4 is performed by relay 406, Fig. 5, with the aid of the rotary mal spring 414 and its associated contact. i

Fig. 6 is a rotary connector switch which is otherwise similar to the connector shown in Fig. 4, and it is used for calling ordinary lines and also for lin'es associated in groups with the number of the first line assigned as the call number of the entire group. n

Fig. 7 shows a toll recording trunk circuit which is accessible to the first selector repeater'shown in Eig. 8 through the conductors 797-798. 'I'his trunk circuit is used for extending connections to the toll recording operator and it brings about the operation of the switch-through relay. 721 of the first selector repeater, Fig. 8, vso as to remove all bridges at the iirst selector repeater to give a clear talking circuit through to the manual switchboard.

Fig. 8 vshows the iirst selector repeater which is accessible tothe line switch LS associated with the line of the substation A. This selector reoil-norpeater is provided with the necessary arrange-h ments for feeding talking battery to the calling subscriber in all cases and to the called subscriber on local calls. Itis also provided with Vthe neces-v sary arrangements for cooperating with the director shown in Figs. 13-16.

Fig. 9 shows a reverse battery repeater which responds to the impulses received by the line relay 803 to reverse the direction of the current flow over the conductors '818 and 819 to operate the electro-polarized relay 1001, Fig. '11, to bring about the operation of the automatic switches in the distance omce.

' Fig. ioshows a reverse battery repeater somewhat similar to the one shown in Fig. 9, but difi'ering somewhat in detail as will be pointed out Fig. 11 shows an incomingselector repeater ac;

peater is designed to operate responsive to the reverse battery impulses transmitted by the reverse battery repeaters shown in Figs. 9 and l0. It also supplies talking battery to the called line and it controls the switch-over and metering operations of the ilrst selector repeater shown in I Fig. 8. i

Fig. 12 shows an incomingselector repeater which performs functions similar to those of the incoming selectorgrepeater shown in Fig. 11 as regard the supplying of talking battery to the called line, but instead of being accessible to a rst selector repeater operating in the inverted system, it is accessible to an ordinary two wire selector in a distant office through the medium oi' the usual outgoing repeater.

Figs. 13-16 taken together show a director selector individual to the first selector repeater, Fig. 8, and one oi' the directors accessible to the director selector.

The portion of the director shown'in Fig. 13 comprises the sequence switch S which is used to distribute the various series of impulses received by the director to the various operating magnets ofthe registers concerned. This drawing shows also the sequence switch S' which is used Ato control the various steps in the sending operation as will hereinafter appear.

FigL 14 shows the director 'selector which re-y sponds to the first oilice digit to select a director in the proper group. This drawing shows also the sending portion of the director, which comprises the pick-up and stop relays 1246 and'1247, together with the sending switch SS which counts the impulses as they are being transmitted and consequently terminates the transmission when the required number has been transmitted in a given case.

'I'he portion of the director shown in Fig. 15 comprises the oilice register OR which responds to the second and third cnice digits, together with the intermediate distributing frame IDF through the medium of which the code digits transmitted Von calls to any ofilce may be altered at will. J

The portion of the director shown in Fig. 16 comprises the four digit registers DR-l-DR--4 which are used to register the four subscriber digits in a number after the otlice digits have been taken care of by the director selector, Fig. 14, and by the oiiice register 0R Fig. 16. Regarding the mechanical construction oi' the apparatus shown, the various selectors, selector repeaters and connectors Vare mechanically of the well-known vertical and rotary type of switch in which the bank contacts are arranged in horizontal rows or levels'. The oilice register OR. Fig. 15, is also of this type.

'I'he various line switches such as LS are mechanically ofthe well known rotary -type of switch in which the wipers are advanced in one direction only and are advanced upon the back stroke of the associated stepping magnet. The sameis true o! the sequence switch S', Fig. 13, and the sending switch SS, Fig. 14.

The sequence switch S, Fig. 13, is a switch of a simple typein which the wipers are advanced in one plane only by the operating magnet 1205 and' are' restored to their normal position by the releasemagnet 1206 under the control of the oil-normali contacts 1207. 'I'he digit registers V,DR+1-Dlit4, Fig. 16, are of the same type oi' cessible to the rst selector repeater showniinf' Fig. 8 through the trunk line comprising con-'Q ductors 8l8'and 819. This incoming selector re-' v switch mechanically.

The invention having been described generally, a detailed description of the operation oi' the apparatus shown will now be given. For this purpose it will beassumed that the subscriber at substation A, desiring to converse with the subscriber at substation A', removes his receiver and dials the digits in the number of the subscriber at substation A.

Call from substation A to substation A' When the receiver is removed at the substation A, a circuit is closed over line conductors'701 and '702 for line relay 704 of the line switch LS. Line relay 704 energizes over the calling line and places ground on the associated private normal conductor '718 leading to the banks of the connector switches having access to the line of substation A, thereby making this line busy to those connectors, and also closing a circuit through the combined switching and cut-oi relay '703 and the motor magnet '705. Line relay '704 also at armature '720 connects test wiper 711 through armature '707 and its resting contact to the junction of relay '703 and magnet' '705. Accordingly, if the trunk upon which thewipers of the switch are standing is busy the ground potential encountered by test wiper '711 shunts relay '703 and prevents it from energizing and ailords a direct circuit for the self interrupting stepping magnet '705. Accordingly stepping magnet '705 operates in a buzzer-like manner to advance the wipers '710-'713 in search of an idle trunk line.

When an idle trunk line is reached, the stepping magnet '705 ceases to operate, and relay 703, being no longer short circuited, energizes in series with stepping magnet '705, which however does-not operate at this time on account of the relatively high resistance of relay '703. Upon operating, relay '703 disconnects test wiper '711 from the junction of its own winding and that of magnet '705 and connects it to the grounded private normal conductor '718, thereby extending ground to the release trunk conductor '715 of the seized trunk; connects up the meter operating wiper '712 at armature '708; and at armatures '706 and '709 disconnects conductors '701 and '702 from line relay '704 and ground and extends them by way of Wipers '710 and '713, conductors '714 and '71'7, and the normally closed contacts controlled by armatures '751 and 754 to the windings of line relay '722 of the first selector repeater. Line relay '722 now energizes over the callingline and,

closes at armature '756 a circuit for the release relay 723. Relay '723 energizes and places ground on the local-ground supply conductor '748 at armature '758, prepares the vertical magnet circuit at armature '759; and at armature '75'7 places ground through the low resistance lower winding of relay '721 on the release trunk conductor '715 to maintain the 'switching relay '703 of the line switch LS operated and to maintain the calling line guarded after the falling back of the slow acting line relay 704. Although the current for holding switching relay '703 operated is obtained through the lower winding of relay '721, the proportion between the windings of the two relays is such that relay '721 is notoperatively energized at this time, nor is it operatively energized in case several other holding relays obtain their current through its lower winding. The function of this winding will be brought out hereinafter.

The calling subscriber now manipulates his calling device in accordance with the first digit in the desired number, being informed of the condition of readiness of the exchange apparatus by the dial tone current transmitted to the calling line from the dial tone lead, Fig. 14, through the resting contact and armature 1325, conductor 1301,working contact and armature '7'71 of relay '728, which is energized from release trunk conductor '715, and the resting contact and armature '782.

When the calling subscriber turns his calling device, the circuit oi lline relay '722 of the ilrst selector repeater is interrupted a corresponding number of times. 'Each time it falls back, line relay '722 closes at armature '756 a. circuit for the vertical magnet 1319 of the director selector, Fig. 14 as follows: from ground by way of armature '753 and its resting contact of relay '721 of the rst selector repeater, armature '7560! line relay 722 and its resting contact, armature 759 and its working contact, armature '788 and its resting contact, conductor 1302, armature 1324, relay 1312, and vertical magnet 1319 to battery. By the operation of vertical magnet 1319 the wipers 1329-1331 are raised step by step and come to rest opposite the desired level of bank contacts. Relay 1312 is energized in series with vertical magnet 1319 upon the rst impulse of current being delivered to the latter, and is maintained energized throughout the series of impulses to the Vertical magnet on account of the fact that it is slow acting. At `armature 1323 relay 1312 prepares a circuit for the stepping relay 1311 and opens a point in the circuit of rotary magnet 1314. As soon as the off-normal contacts 1315 operate, as they do upon the completion of the rst vertical step, the circuit of relay 1311 is completed as follows: from ground by way of the grounded conductor '748, Fig. 8, resting contact and armature r780, conductor 1304, working contact and armature 1323, off-normal contacts 1315, and stepping relay 1311 to battery. Relay 1311 energizes over the above circuit and prepares at armature circuit at the normally closed contacts controlled by armature 1323, whereupon rotary magnet 1314 energizes and advances the wipers 1329-1331 into engagement with the first set of bank contacts in the selected level, and near the end of its stroke, it opens at interrupter contacts 1306 the locking circuit of relay 1311, whereupon relay 1311 falls back and opens the circuit of rotary magnet 1314 at armature 1322, and opens a further point in its own locking circuit at armature 1321. Rotary magnet 1314 now falls back and closes its interrupter contacts 1306 again.

'Ihe further operation depends upon whether the first set of contacts in the selected level is busy or idle. If this set of contacts is idle, switching relay 1313 energizes right away, seizing the associated director. Assuming, however, that the director associated with this set of contacts is busy, there is a grounded potential on the test contact encountered by test wiper 1330, with the result that switching relay 1313 is shunted and does not operate and that a circuit is closed for stepping relay 1311 which energizes and closes a circuit for rotary magnet 1314 with the result that the wipers 1329-1331 are advanced another step.

This alternate operation of relay 1311 and rotary magnet 1314 continues until an idle set of lil) ' bank contacts is reached, which set it will be assumed is the one comprising conductors 1332- 1334. When this idle set of bank contacts is reached, stepping relay 1311 is unable to operate over test wiper 1330, and switching relay 1313 (being no longer short circuited) energizes in series with relay 1311, which latter relay is unable to energize on account of the relativelyhigh resistance of relay 1313. Upon energizing, relay 1313 opens a point in the test circuit and grounds the wiper 1330 at amature 1326; opens a point in the circuit of release magnet 1318 at armature 1327; and at armature 13,25 disconnects the dial tone lead from conductor 1301; at armature 1324 disconnects the impulse conductor 1302 from the local vertical magnet circuit and extends it by way of the working contact of armature 1324 to wiper 1329; and at amature 1328 connects up wiper 1331.

In the seized director, the switch-throughcontrol relay 1345, which is associated with the outgoing control conductor 1337, now energizes in series with the lower winding of relay '729 of the first selector repeater, Fig. 8 over the following circuit: from ground by way of the lower winding of the impulse relay 729, Fig. 8, normally closed contacts controlled by armature 784, conductor 1305, armature 1328 and its working contact, wiper 1331, bank contact 1334, conductor 1337, resting contact and armature 1351 in multiple with the sending contacts 1361, and the switch-through-control relay 1345 to battery.

- The two relays 729 and 1345 energize in series over this circuit and relay 1345 opens at armature 1350 a point in the switch-through circuit so as to prevent a premature operation of the switchthrough relay 730 of the lrst selector repeater, Fig. 8, when the stop relay 1347 of the sender operates.

In the rst selector repeater, relay 729, upon energizing, prepares a locking circuit for its upper winding at armature 772, and at amature 773 opens a point in the circuit of vertical magnet 736 and closes a circuit for relay 726 as follows: from ground by way of the grounded conductor 748, resting contact and armature 780, armature 787 and its resting contact, armature 773 and its working contact, normally closed Oifnormal springs controlled by spring 735, and relay 726 to battery. Relay 726 now energizes and prepares a circuit for stepping relay 727 at armature 764, and at armature 766 prepares a circuit for vertical magnet 736.

In the seized director, release relay 1344 energizes through the grounded test wiper 1330 of the director selector, bank contact 1333, and the release trunk conductor 1336 and removes ground from the release conductors 1342 and 1358.

The calling subscriber now manipulates his calling device in accordance with the remaining digits in the desired number, and when the next digit is dialled, the resulting impulses of current transmitted by line relay 722 of the first selector repeater are transmitted over the previously traced circuit through conductor 1302 to amiature 1324 of the director selector and thence by way of the working contact of armature 1324, wiper 1329, bank contact 1332, conductor 1335,

'slow acting seriesrelay 1201, Fig. 13, distributing wiper 1210 of the sequence switch S, vertical magnet operating conductor 1221, and vertical magnet 1402, Fig. 15, of the oflice register OR to battery. Accordingly, the vertical magnet 1403 operates to raise the wipers 1411-1414 of the oilice register 0R to a position opposite the desired level of bank contacts of the omce register. Relay 1201 is energized in series with vertical magnet 1401 on the first impulse of current and, being slow acting, maintains its armature 1203 attracted throughout the vertical operation. Upon energizing, relay 1201 closes a circuit for the asso- -ciated slow acting relay 1202 which operates and prepares at armature 1204 a circuit for magnet 1205 of the sequence switch S.

At the end of the digit, relay 1201 falls back and completes at armature 1203 the circuit through armature 1204 and its working contact for stepping magnet 1205 whereupon the wiper 1208 and 1210 are advanced one step. The slow acting relay 1202 falls back shortly and opens the stepping magnet circuit at armature 1204.

When the next digit is dialled, the resulting series of impulses is transmitted over the circuit previously traced to the wiper 1210 of the sequence switch S, and from thence over the rotary magnet operated conductor 1222 to the rotary magnet 1402 of the office register 0R. Rotary magnet 1402 operates to advance the wipers of the oice register into engagement with the desired set of bank contacts, which set it will be assumed is the one comprising contacts 1421- 1424.

The wipers 1208 and 1210 of the sequence switch S are advanced at the end of the rotary movement in the manner above described, wiper 1210 extending the operating conductor to conductor 1231 and wiper 1208 connecting the nterrupter 1209 to the starter conductor 1341 leading to the sender, Fig. 14. This latter operation results in the starting of the sender to transmit the necessary digit combination to trunk the call to the called oilice, as will be later explained.

When the next digit is dialled, the resulting impulses of current are transmitted over the conductor 1231 to stepping magnet 1501 of the digit register DR-l with the result that'the wiper of this register is set on the corresponding bank contact.

Similarly, the next three series of impulses are transmitted over conductors 1232-1234 to the stepping magnets of the digit registers DRl-DR4 respectively, with the result that their respective wipers are set on the corresponding bank contacts.

The wipers 1208 and 1210 of the sequence switch S of course are advanced in the usual manner at the end of each vseries of impulses, and at the end of the last series, wiper 1210 advances to a dead contact and the Wipers remain in this position until the director is released.

Reverting now to the point at which the interrupter `1209 is connected to conductor 1341 at wiper 1208` of the sequence switch S at the end of the setting of the oiice register OR, upon the first subseduent closure of' the interrupter 1209 arr impulse of current is transmitted over conductor 1341 and through the resting-contact and armature 1356 to the stepping magnet 1360 of the sending switch SS. Magnet 1360 thereupon energizes preparatory to advancing the wipers 1363 and 1364 and it falls back and advances these wipers one step when the impulse is terminated` Wiper 1364 engages a dead contact, but wiper 1363 encounters a grounded contact, closing a circuit for the pick-up relay 1346. Pick-up relay 1346 energizes and places a local ground on the release trunk conductor 1336 so as to insure the holding of the director until the transmission of the digit is completed; closes at armature 1353 a circuit through armature 1357 and its resting contact and over conductor 1343 for the stepping magnet 1213 of the sequence switch S', which magnet energizes preparatory to advancing the wipers 1211 and 1212; and at armature 1351 it removes the shunt from around the sending contacts 1361 of the sending switch SS.

Accordingly, on each subsequent energization of stepping magnet 1360 of the sequence switch SS, the out-going control circuit is opened at contacts 1361. Since the connection at the intermediate distributing frame IDF, Fig. 15, is such as to predetermine that the first code digit to be transmitted is digit 2, the operation of stepping magnet 1360 continues under the control of the interrupter 1209 until two interruptions have been produced in the out-going control circuit at contacts 1361, the i'lrst interruption being produced while wiper 1364 is standing on its rst bank contact, which is dead, and the second interruption being produced while the wiper 1364 is standing on its second bank contact, associated with the rst impulse stop conductor. Accordingly, when magnet 1360 falls back at the end of the second interruption in the out-going control circuit and advances the wipers 1363 and 1364, wiper 1364 encounters the second impulse stop conductor, which is grounded, thereby closing a circuit for stop relay 1347 as follows: from ground by way of the stop-conductor-distributing wiper 1212 of the sequence switch S', first code-digit stop conductor 1241, wiper 1411 of the ofnce register OR, bank contact 1421, IDF jumper 1431, the second impulse stop conductor, the associated contact in the bank of the sending s witch SS, wiper 1364, and stop relay 1347 to battery. Upon energizing, stop relay 1347 shunts the sending contacts 1-361 at armature 1354; closes a point in the switch-through circuit at armature 1355, which circuit, however, is maintained open at armature 1350 of the switchthroughy-control relay 1345; opens the circuit of stepping magnet 1213 of the sequence switch S' and closes a locking circuit for itself at armature 1357, whereupon the wipers 1211 and 1212 of the sequence switch S advance one step; and at armature 1356 it disconnects magnet 1360 of the switch SS from the start conductor 1341 and connects it instead through the local self-interrupting contacts 1362 to wiper 1363. Acordingly, magnet 1363 operates in a buzzer-like manner to advance the wipers 1363 and 1364 until wiper 1363 encounters the ungrounded contact shown in its bank. When this contact is reached magnet 1360 ceases to operate, and the circuit of the slow acting pick-up relay 1346 is opened. This relay, being slow acting, does not fall back for the time being. f

In the rst selector repeater, Fig. 8, the impulse relay 729 falls back each time the outgoing control circuit of the directoris interrupted as hereinbefore pointed out, and upon each deenergization, closes a circuit at armature 773 and its resting contact through armature 766 and its working contact, armature 779 and its resting contact for vertical magnet 736. Accordingly, vertical magnet 736 operates to raise the Wipers 791-793 opposite the second level of bank contacts in response to the two impulses of current which it receives. Upon the closure of oir-normal contacts 734, the circuit of stepping relay 727 is completed from ground by way of the grounded conductor 748, Working contact and armature 764, armature 786 and its resting contact, and the off-normal contacts 734. Relay 727 thereupon energizes and prepares a circuit for rotary magnet 737 at armature 767, and at amature 768 completes a locking circuit for itself from ground by way of armature 783 and its resting contact, armature 768 and its working contact. interrupter contacts 769 of rotary magnet 737, and hence to battery by way of the off-normal,l contacts 734 and stepping relay 727.

It will be noted that, whereas relay 726 initially energized through the working contact and armature 773 of the impulse relay 729 and through the normally closed contacts controlled by olf-normal springs 735, its circuit is shifted so as to include the armature 773\and its resting contact, and the normally open contacts controlled by olf-normal spring 735 when the olf-normal springs operate. Accordingly, relay 726 is maintained energized in multiple with the vertical magnet 736 during the vertical operation, and it falls back when the vertical operation ceases.

When relay 726 falls back it completes, at the normally closed contacts 'controlled by armature 764, a circuit through armature 767 and its working contact for the rotary magnet 737, whereupon rotary magnet 737 pulls up and advances the wipers of the switch into engagement with the first set of bank contacts in the selected level and at the interrupter contacts 769 opens the circuit oi' stepping relay 727, whereupon relay 727 falls back and opens the circuit of rotary magnet 737 at armature 767. Rotary magnet 737 now falls back and closes its interrupter contacts 769 again.

From this point the operation depends upon whether the trunk line terminating in the first setk of bank contacts is busy or idle. Assuming this trunk line to be idle, switching relay 731 energizes right away in series with stepping relay 727. Assuming however on the other hand that this trunk is busy, test wiper 792 encounters ground on the busy test contact with the result that a circuit is completed through armature 785 and its resting contact, interrupter contacts 769, and off normal contacts 734 for stepping relay 727. Accordingly relay 727 energizes again and closes the rotary magnet circuit with the result that another rotary step is taken.

This alternate operation of the stepping relay and the rotary magnet continues until an idle trunk is reached, which trunk it will be assumed is the one comprising conductors 794 and 796. When this idle trunk is reached, stepping relay 727 ceases to operate on account of the fact that test wiper 792 encounters an ungrounded test contact, and switching relay 731, being no longer short circuited, energizes over the following circuit: from ground by way of the grounded release trunk conductor 715, switching relay 731, interrupter contacts 769, off-normal contacts 734, and stepping relay 727 to battery. Relay 727, however, does not pull up over this circuit on account of the relatively high resistance of the switching relay 731. Relay 731, upon energizing, removes ground from armature 768 of stepping relay 727 and closes a locking circuit for relay 729 at armature 733. This locking circuit includes armature 776 and its resting contact and armature 772 and its workng contact. Relay 731 also disconnects conductor 1305 (coming from the director) from the lower Winding of impulse relay 773 at the normally closed contacts controlled by armature 784 and extends it by way of the normally closed contacts controlled by armature 775, and wiper 791 to the seized trunk line; opens the test circuit and connects test wiper 792 to the groundedrelease trunk conductor 715 at arma-

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