US2133682A - Telephone system - Google Patents

Description

Oct, 18, 1938. P. BAKKER TELEPHONE SYSTEM Filed March 3, 1937 10 Sheets-Sheet l wwzwflwxi 5 Q H 22223 E53 d? T: 25 56523 9 $356 25252 I .i J $2 I: NBQELQF 56 $5 1 Bi SE20: 5: E23 V55 :5 $9 0.. v $7233 $5232 I p WE INVENTOR. Pl ER BAKKER Z 62? A TTORNEY.

P. BAKKER Oct. 18, 1938.

TELEPHONE SYSTEM 10 Sheets-Sheet 2 Filed March 3, 1937 1N VEN TOR.

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FROM F169 FROM SEL. BANKS & F!G.7

TO DIAL BACK SEL.

ATTORNEY.

Oct. 18,1938.

FIG. 5

TO F182 WARNIN6 TONE P. BAKKER TELEPHONE SYSTEM Filed March 3, 1957 10 Sheets-Sheet 5 NTERRUPTER START INVENTOR. PiER BARKER ATTORNEY.

Oct. 18, 1938 P. BAKKER TELEPHONE SYSTEM PIER BAKKER ATTORNEY.

Oct. 18, 1938. P. BAKKER 2,133,682

TELEPHONE SYSTEM Filed March 3, 1937 10 Sheets-Sheet 7 To FiG 2 TO EXCHANGE'B FIG. 7

TC-3- CONTINUATION POLARIZED T0 INVENTOR,

PEER BAKKER ATTORNEY.

FIG.

Oct. 18, 1938. P. BAKKER TELEPHONE SYSTEM Filed March 3, 1937 10 Sheets-Sheet 8 +0 FIG.9

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PIER BAKKER ATTORNEY.

Oct. 18, 1938. P. BAKKER TELEPHONE SYSTEM Filed March 3, 1937 10 Sheets-Sheet 9 Patented Oct. 18, 1938 UNITED STATES TELEPHONE SYSTEM Pier Bakker, Chicago, 111., assignor to Associated Electric Laboratories, Inc., Chicago, 111., a corporation of Delaware Application March 3, 1937, Serial No. 128,736

13 Claims.

The invention relates in general to telephone systems but more particularly to inter-exchange calls in which an operator is required to complete connections between automatic subscribers lines overtrunk lines connecting two automatic exchanges.

The main objectof the invention consists in the provision of improved circuits whereby an automatic subscriber in one exchange in order to call an automatic subscriber in another exchange must first call a control operator over a trunk extending from the automatic exchange to the manual control exchange, after which the control operator dials back over this same trunk to operatethe-automatic switches to complete the desired connection. 'Whensuch connection is complete the control'operator may disconnect and free her trunk leaving the connection between the two automatic subscribers established over one of the trunklines connecting the two automatic exchanges. The switching system herein disclosed is an improvement over those previously developed, an exampleof which is shown in'the Saunders application, Serial No. 45,845, filed October 21, 1935.

An additional feature of the invention relates to the provision of automatic timing "equipment which afiords a means of releasing an interoflice connection after apredetermined'timeinterval.

Other features relate to the circuit refinements in the dial back repeaterwhich enables this type of service to be given.

The drawings, Figs. "1 to 10, inclusive, show by meansof the usual circuit diagrams, a'sufficient amount of apparatus to enable the invention to be described and understood.

Fig. 1 shows a trunking diagram ofatelephone network employing the principles of this invention. The network consists of the automatic exchange A, a second automatic exchange B, a manual exchange Cwhichh'ousest-he control operator, and a second manual exchange v.D which may be considered as a tollcenter and in which, incidentally, the operators positions are not .provided with dials. Automatic exchange Ais connected with each of the other exchanges by a plurality of two way trunks.

The two way trunk TWT--I terminates in exchange A upon thelin'e'circuit LCl,-and-anrincoming first selector; andis accessible fromibanks of the outgoing secondary line eswitch OGS. In exchange iC this :trunk terminates -in line equipment usually .-found in :a manual switchboard,

and iii-addition means are provided whereby a dial may be associated therewith.

The two way trunk TWT2 terminates in exchange A upon the trunk circuit TC-2 and the incoming secondary line switch ICSZ; and at exchange D upon line equipmentusually found in a manual switchboard.

"The twoway trunkTWT-3 terminates in exchange A upon the trunk circuit TC-3 and the incoming secondary line switch -ICS--3; and at exchange B .may be considered as terminating upon a simplex to loop repeater of the general type shown in the 'Wicks Patent No. 1,682,100, issued August 28, 1928, but with a circuit arrangement such that the local seizure of the repeater causes a splash of generator to be impressed upon the simplex of the trunk.

Each station in exchange A terminates in a line switch (such as LS-l), which has access to trunks leading to local first selectors. The tenth level in the banks'of the local, dial back, and incoming first selectors are multipled and trunked to dial back repeaters. These same trunks are also multipled to'the banks-of incoming secondary line switches ICS2 and ICS3.

Each dial back repeater is connected to an outgoing secondary line switch (such as OGS), which has access to trunks leading to line circuits (such as LCI), which are the terminus of two way trunks (such as TWT-l), connecting exchange A with exchange C. Each'dial back repeater'has in association-a dial back first selector. which is operable only by the control operator at exchange C.

The ninth level in the banks of the local, dial back, and incoming first selectors is multipled and carried to trunk circuits (such as TC2), which are the terminus of two way trunks (such as TWT2), connecting exchange A with exchange D. Each trunk circuit is connected to an incoming secondary line switch (suchas ICSZ) which has access to trunks leading to the dial back repeaters.

The eighth level in the dial back and incoming first selectors is multipled and carried to trunk circuits (such as TC3-) ,which are the terminus of two way trunks (such as TWT3) connecting exchange A with exchange B. Each trunk circuit is connected to an incoming secondary line switch (such as ICS3) which has access totrunks leading to the dial back repeater. Attention is directed to the fact that there are no trunks connected to the eighth level in the banks -of the local first selectors.

Trunk circuit TC3, when seized from exchange A, functions as an automatic to automatic impulse repeater. The circuit to this repeater employs loop pulsing, and the repeater repeats pulses over the simplex of the trunk to exchange B. The supervision from exchange B is the well known high resistance supervision used on simplex dialling circuits, and is repeated as standard reverse battery supervision to the switches of exchange A.

Trunk circuit TC-3 functions as a line circuit when a subscriber in exchange D dials the digit representative of exchange A. A splash of ringing current on the simplex of the trunk causes the circuit to be extended by way of the incoming secondary line switch ICS3 to an idle dial back repeater. The dial back repeater then extends the call by way of the outgoing secondary line switch OGS to an idle line circuit (such as LC-I) and thence to the control operator at exchange C.

Trunk circuit TC2, when seized from exchange A, functions as an automatic to manual ring down repeater to signal the operator at exchange D.

Trunk circuit TC2 functions as a line circuit when the operator at exchange D plugs into the jack associated with trunk TWT-2. A ring on the trunk causes the circuit to be extended by way of the incoming secondary line switch ICS-2 to an idle dial back repeater. The dial back repeater then extends the call by way of the outgoing secondary line switch OGS to an idle line circuit (such as LC-I) and thence to the control operator at exchange C.

The dial back repeater, when seized from either first selectors or incoming secondary line switches, functions to extend the call by way of the outgoing secondary line switch OGS to an idle trunk to exchange C. The control operator may extend the call manually, or may complete the call by dialling back over the same trunk. The associated dial back first selector is seized by momentarily placing booster battery on the simplex of. the trunk.

When a call has been reverted by the control operator, the operator on receiving answering supervision may disconnect from the control trunk or supervise the call as the conditions may require. In the event that the operator disconnects, the dial back repeater will free the two way trunk thus making it available for other calls. The release of the dial back repeater is now under control of the calling party, or of the automatic timing equipment when it is used.

The automatic timing equipment provides a means of releasing an established call after a predetermined time interval, and is brought into operation when the called party answers. The automatic timing equipment is not effective so long as the control operator supervises the connection, nor is it effective when the call is originated at a toll board.

Line circuit LC-I when seized by an outgoing secondary line switch, functions as a ring down repeater to signal the operator at exchange C.

Line circuit LC-I functions, as the name implies, when the control operator at exchange C plugs into the jack associated with trunk TWTI, and extends the connection to the associated incoming first selector.

It is evident that calls involving the control operator at exchange C would fail in the event of either of two busy conditions. Provision has been made such that the outgoing secondaries will cease to hunt, and will give the calling party the busy tone, should all the trunks to exchange be in use. Likewise the incoming secondaries will cease to hunt, and will give the calling party the busy tone, should all the dial back repeaters be in use.

Fig. 2 shows a portion of the circuit of the dial back repeater DBR. Conductors H, I2 and I3 represent the three conductor trunk leading to the individual dial back first selector associated with this dial back repeater. Conductors I4, I and I6 represent the three conductor trunk from the banks of the first selectors and from the bank of the incoming secondary line switches associated with the trunks to exchange B. Conductors I'I, I8, I9 and 20 represent the four conductor trunk from the banks of the incoming secondary line switches associated with the trunks to exchange D. Conductors 59I, 592, 593 and 594 lead to the individual timing unit associated with this dial back repeater.

Fig. 3 shows a continuation of the circuit of the dial back repeater DBR, and the outgoing secondary line switch OGS associated therewith. Conductors 460, 46I, 462 and 463 represent the four conductor trunk leading to line circuit LCI. Conductors I013, I014 and I016 lead to the all trunks busy equipment.

Fig. 4 shows the line circuit LCI located in exchange A, conductors 460, 46!, 462 and 463 represent the four conductor trunk from the banks of the outgoing secondaryline switches,such as OGS of Fig. 3. Conductors 464, 465 and 466 represent the three conductor trunk leading to the individual incoming first selector associated with this line circuit. Line circuit LC2 represents the equipment upon which trunk TWT-I terminates in exchange C.

Fig. 5 shows the individual timing unit associated with the dial back repeater of Fig. 2 and the master timing unit common to exchange A.

Fig. 6 shows a portion of the trunk circuit TC3. Conductors 25, 26 and 21 represent the three conductor trunk from the bank of the first selectors. Conductors I0II and I012 lead to the all trunks busy equipment.

Fig. '7 shows a continuation of trunk circuit TC--3, and the incoming secondary line switch ICS3 associated therewith. Conductors I4, I5 and I 6 represent the three conductor trunk leading to the dial back repeater of Fig. 2. Conductors 33 and 34 represent the two conductor trunk to exchange B.

Fig. 8 shows a portion of the trunk circuit TC2. Conductors 28, 29 and 30 represent the three conductor trunk from the banks of the first selectors. Conductor I012 leads to the all trunks busy equipment.

Fig. 9 shows a continuation of trunk circuit TCZ, and the incoming secondary line switch ICS2 associated therewith. Conductors I1, I8, I9 and 20 represent the four conductor trunk leading to the dial back repeater of Fig. 2. Conductors 3| and 32 represent the two conductor trunk to exchange D. Conductor I0II leads to the all trunks busy equipment.

Fig. 10 shows the all trunks busy equipment. This equipment includes a relay chain (I040, I050 and I060), comprising one relay for each trunk between the outgoing secondary line switches and the line circuits to exchange C, and a normally energized relay (IOI0) whose circuit is multipled to all dial back repeaters.

The various relays diagrammatically shown throughout the drawings are of the general type ordinarily employed in :automatic telephone systems. The secondary line switches are :of the well known single motion pawl and ratchet type arranged to advance their wipers upon eachdeenergization of the motor magnet.

It is believed that a most thorough understand-- ing of the invention can be obtained by describing the various circuit operations of the dial back repeater in the following order:

1. Call from exchange A to exchange C.

1. Seizure of the .dial back repeater. 2. Selection of an idle trunk to exchangeC. 3. Seizure of first idle trunk to exchange C. 4. Control operator answers. 5. All trunks busy.

2. Reverted call from-exchange C to exchange A.

1. Seizure of dial back selector. 2. Repeating impulses to the dial back selector. 3. Control operator receives answering supervision. 4. Control operator disconnects. 5. Timing equipment releases connection. 3. Call from exchangeC to exchange A. 4. Call from exchange A to exchange D.

1. Seizure of TC-2 by automatic subscriber. 2. Signalling operatorat exchange D. 5. Call from exchange D'to exchange A.

1. Seizure of TC2 by operator at exchange D. 2. ICS-Z selects trunk'to idle DBR. 3. Operator at exchange C answers. 4. All trunks to exchange C busy. 5. All dial back repeaters busy. 6. Call from exchange C .to exchange B.

1. Seizure of TC3 by first selector. .2. Repeating impulses .to exchange B. '3. Called party answers. '7. Call from exchange 3 to exchange A.

1. Seizure of TC3 byautomatic subscriber. 2. ICS3 selects trunk to idle DBR. 3. Release of the connection. 4. All trunks to exchange C busy, or all dial back repeaters busy.

CALL FROM A TO C Seizure of dial back repeater Assume that a subscriber-in-exchange A desires to call exchange C; he willfirst remove his receiver, whereupon his line switch will extend the connection to an idle local first selector. The calling subscriber then dials the digit 0, with the result that the first selector willproceed to select a trunk leading to an idle dial back repeater. Referring now to Fig. 2, it will be assumed that the trunk so selected is that represented byconductors I4, I5 and I6.

Relay 20!] operates; over conductor I4 and I5 and the calling subscribers loop; closes relay 2I'0 at 202; closes relay 330 by way of 202, 236 and conductor I46; and closes relay 35D by way of 204, conductor I49 and 314.

Relay 330 operates; and removes a multiple ground, by way of 224, 244, I50, 332 and I013, from the all trunks busy relay 10H].

Relay 2 I0 operates; supplies holding ground to the preceding switches by way of 534, conductor 59I, 2I3, impedance I25 and conductor I6; prepares the operating circuit of relay 368 at 2I5; and at 2I1 closes the circuit from test wiper-383 to the motor magnet 380.

Relay 356 operates; prepares a circuit to signal the distant operator at '35I; and closes the circuit of relay 34.0 .at352.

Relay340 operates; prepares the circuit to relay 310 at 342; and at 34I connects battery through a resistance to the-dial lead.

CALL FRoM A TO 0 OGS selects trunk to idle LC'1 If the wipers of the outgoing secondary line switch are resting on a busy trunk, relay 366 is shunted by a circuit extending from ground on the bank contact occupied by test wiper 383, by way of 315, 36I, conductor I35, 2I1, conductor I36, 362, conductor I41, 238, conductor I48, 38I to the motor magnet 380.

Motor magnet 380 operates, and at 38I cuts the high resistance upper winding of relay 310 in series with its own circuit.

Motor magnet380 restores as a result of the reduced current flow; advances the associated wipers one step, and again shunts relay 316 at 38 I. Motor magnet 380 will continue to act in this fashion until an ungrounded bank contact is reached by wiper 383. Relay 310 is slow to operate because of its normally short circuited lower winding, and will not operate when its circuit is only momentarily closed in series with the motor magnet 386.

CALL FROM A To C OGS seizes the first idle trunk When the rotary switch wipers engage the first idle trunk, assumed to be that represented by conductors 460, 46I, 462 and 463, the absence of ground at wiper 383 will render effective a circuit from ground at 2I5, 253, conductor I31, relay 360, 362, conductor I41, 238, conductor I48, 38I, through motor magnet 380. The motor magnet will not energize at this time because of the high resistance of relay 368.

Relay 360 operates; locks itself to battery at 362, thus opening the circuit of motor magnet 386; extends busying ground forward by way of 2E5, 253, conductor I31, 36I, 315 to wiper 383 and the contact upon which it is standing, and thence to relay 420 by way of conductor 462 and MM; closes the circuit of relay 310 in series with motor magnet 380 at 364; and impresses ring back tone on the calling line by way of wiper 386, condenser 33M, 363, conductor I32, 249, 241 and conductor I4.

Relay 429 operates; extends talking conductors 460 and I to the left side of the repeating coil R4-I; and connects conductor 463 to the simplex of the line at 422.

The motor magnet 38!) will not energize in series with the upper winding of relay 316, but the latter relay Will operate and lock itself upat 3:1I; :remove the short circuit from its lower windingat 312; open the circuit of relay 350 at 1314; completes the signalling circuit at 313; and replaces the previous ground on wiper 383 by that-at 315. Following the operation of relay 319, and prior to the restoring of relay 356, a splash of generator signals exchange C by way of 35I, 31-3, wiper 38-2, conductor 463, 422, simplex of the trunk TWT-I, relay 456, 442 and condenser 461.

Relay 450 operates; locks its second winding at 452; and brings in the line lamp L-I by way of 45I and 443.

Relay 350 restores and opens the circuit of relay 340. Following the restoration of relay 350, and :prior to the falling away of relay 346, it will be noted that the simplex of the trunk is drained of any possible condenser charge by being connected to battery at 34I. Relay 340 restores and extends the simplex of the trunk by way of 34 I, 33 I, through the lower windings of relays 320 and 3I0, and thence through the upper windings of relays 320 and 3I0 to battery.

CALL Fnon A TO 0 Control operator answers The control operator at exchange C responds to the lighted line lamp by inserting the plug of an idle cord circuit into the jack J, whereupon relay 440 energizes and completes a circuit from ground at key K2, through key K--I, 442, relay 450, simplex of trunk TWTI, 422, conductor 463, wiper 382, 313, 35I, 34I, 33I through the lower windings of relays 320 and 3I0, thence through the upper windings of relays 320 and 3I0 to battery. Relay 320 does not operate on normal exchange voltage, but relay 3I0 operates and closes the circuit of relay 300 at 3I2.

Relay 300 operates; supplies a multiple ground to conductor I6 at 30I, thus placing the release of the connection under control of the operator; closes a multiple circuit to relay 360 at 303; and by way of 302 and conductor I4I closes a circuit to the lower winding of relay 240.

Relay 240 operates; removes the ring back tone from the calling line at 240, removes the normal short circuit from the right hand winding of impedance I25 at 245; reverses battery on the calling line by way of 243 and 241; closes the circuit of relay 290 at 242; and at 24I closes the circuit to the left hand winding of relay 260, but without effect at this time.

Relay 290 operates and closes an intermittent circuit to slow to operate relay 295 through the weighted spring contact 292.

If the calling line is a pay station line, a tone, superimposed on conductor I6, flows through the left hand winding of impedance I25 and induces a tone in the second winding thereof which is audible to the control operator.

Relay 295 does not operate when first energized by relay 290, but as the vibration of the weighted spring 292 decreases in amplitude, the circuit to relay 295 is closed for longer and longer intervals. Finally relay 295 operates; closes its locking circuit at 296 and short circuits the right hand winding of impedance I25 at 291, thus removing the pay station tone from the trunk.

Transmission battery is supplied to the calling subscriber through the windings of relay 200, and voice currents pass to the trunk by way of condensers I26 and I21.

CALL FROM A To C All trunks busy With reference to Fig. 10, it will be noted that the second chain relay I050 has been connected via conductor I016 to conductor 462 of the trunk extending to LC-I, Fig. 4. Other chain relays are connected in a similar manner, it follows then that a circuit will be completed for relay I030 each time all trunks in the group become busy.

Relay I030 operates; and by way of I033 and conductor I014 removes ground from the last bank contact accessible to wiper 383. Other springs on relay I030 likewise remove ground from similar last bank contacts in the remaining outgoing secondary switches.

We will now assume that all the trunks accessible to OGS are busy when the dial back repeater DBR is seized from the local first selector. In this event wiper 383 will engage grounded contacts, and the switch will continue to search, until wiper 383 engages the contact to which conductor I 014 is connected, whereupon relay 350 energizes.

Relay 360 operates; closes relay 310 in series with motor magnet 330 at 364; transfers its own circuit at 362; and extends busy tone to the calling subscriber by way of the last contact in the bank and wiper 385, 353, conductor I32, 249, 241 and conductor I4.

Relay 310 operates; closes its locking circuit at 31I and opens the circuit to relay 350 at 314.

Relay 350 restores; and opens relay 340 at 352.

Relay 340 restores; but without effect.

When the calling party restores his receiver, the release of the connection will take place as previously described.

REVERIED CALL C To A Seizure of a dial back selector The control operator may extend the call through exchange C, in which case no further operation occurs in the circuits under discussion until the calling subscriber disconnects.

We will now assume that the calling subscriber desires a number which necessitates that the control operator revert the call back over trunk TWTI. The operator momentarily depresses key K--2, thereby inserting the booster battery BB in the simplex circuit of the trunk.

Relay 320 operates because of the increased current flow and closes a circuit by way of 3I2, 322, conductor I44, 232, through the upper winding of relay 220.

Relay 220 operates; prepares a holding circuit for itself through the upper winding of relay 230 at 22I; prepares its own releasing circuit at 222; and effects the seizure of the dial back selector by completing a circuit from conductor I I, 221, resistance I22 (now shunted by 285), right hand winding of relay 260 (now shunted by 255 and resistance I2I), conductor I30, 3I I, conductor I3I, 225 and conductor I2.

When booster battery is removed from the simplex of the trunk, relay 320 will restore and at 322 will remove the short circuit from the upper winding of relay 230, thus allowing the latter relay to operate in series with relay 22 0.

Relay 230 operates; opens the operating circuit of relay 360 at 238, and of the upper winding of relay 220 at 232; opens the circuit of relay 330 at 236; prepares a holding circuit to relay 250 at v 234; prepares a holding circuit for itself and a releasing circuit for relay 220 at 232; and extends holding ground to the timer equipment by way of 23I and conductor 592.

Relay 330 restores; removes the lower windings of relays 3I0 and 320 fromv the simplex of the trunk and extends that circuit through the upper windings of relays 320 and 3I0 to battery. Relay 320 will not operate on normal exchange battery. The simplex of the trunk is now in readiness for the operator to dial back over the same.

REVERTED CALL C TO A Repeating pulses to the dial back selector The control operator at exchange C now depresses key KI and dials the number of the called station, the impulse springs interrupting the simplex of the trunk in accordance therewith.

Relay 3I0 follows the impulses as received; and at each deenergization opens the circuit of slow to release relay 300; closes a circuit by way of 3I2, 304, conductor I42, 254, through relay 210 in parallel with the lower winding of relay 280; and opens the loop to the dial back selector at 3I I.

Relay 210 operates at the beginning of each series of impulses and remains energized throughout the series; short circuits. the upper winding of relay 320,.by way of 273 and conductors. I39

and I40, toprevent the possibility of its operatingduring impulsing; and at 2'II closes a circuit to.

the upper winding of relay 289.

Relay 290 operates; shunts the righthand winding of relay 2'60 and resistance I2I' at 285 to improve dialling; and opens the transmission. circuit at 28I and 283. to prevent dial clicks fronr being heard by the calling subscriber.

Relays 210 and 289 deenergize after each series of impulses and restore the previously discussed circuit to the dial back selector. The non inductive resistance I22 prevents completely openingthe loop circuit to the dial back selector during.

the transfer from the direct loop to the loopin series with the right hand winding of. relay 260:

The non inductive resistance I2I reduces the impedance of the loop to the dial back selector;

Rnvnnrno CALL C To A Operator receives answering supervision We will now assume that the control operator has dialled the number of the called subscriber and, when he answers, the connector in use will reverse battery in conductors II and I2. The

resulting reversal of current flow through the right hand winding of relay 260 will cause the twowindings of that relay to oppose each other and bring about the operation of the relay.

Relay 266 operates; closes the upper windings of relays 249 and 259 at 262; closes the circuit oi relay 330 by way of 262, 236, 264, and conductor I46; and short circuits the resistance I22 at 25L Relay 256 operates; opens the operating circuit to relay 360 at 253; removes the resistance I 2| at 255 to improve transmission; opens the circuit of relays 216 and 280 at 254; connects the right hand winding of relay 259 directly across conductors II and I2 at 255 to prevent an interruption of the loop to the dial back selector when relay 3I9 restores; prepares a holding circuit for its lower winding at 252; and closes timer relay 50!! by way of 25I and conductor. 593.

Relay 339.0perates and at 33I inserts both windings. of relays 3H! and relay 329 in the simplex of the trunk. The resulting reduction. in current. flow causes relay 456. at exchange C to restore. and bring. in disconnect lamp L-2. by way of 45.I

and'445.

The control operator may supervise the call. by retaining the connections to the trunk. The operator can be recalled by either the calling or called parties since relay 33B is under control. of both relay 299 and relay 260. The calling party cannot release at this time because ground is maintained on conductor I6 by way of I25, con- .ductor I38 and 3M.

REVERTED CALL G TO A Control operator disconnects Upon receiving answering supervision the control operator may disconnect by withdrawing theplug from jack J, thereby opening the simplex oi the trunk.

Relay 35E restores; opens the circuit of relay 3% at BIZ; and closes the locking circuit to relay 259 by way of 312, conductor I43, 234, and 252.

Relay 390 restores; removes ground at I from conductor I6, thereby placing the release of the connection under control of the timing equipment; opens the circuit of relay 240 at 392; and

opens the circuit: of relay 360 at 303.

Relay: 360; restores, and. shunts out the upper winding of relay 31.6at: 364.. The motor magnet 380 operates and opens the circuit to relay 310 at 38I.

Relay 319 restores and opens: the circuit to motor magnet 38.0 at; 3.11:. The motor magnet 38!) restores and advances the associated wipers one step;

Relays 350 and 340'- operate at this time but without effect.

Voice currents complete the circuit for trans,- rnission from the calling and called lines by way of; condensers I23 and I524;

REVERTED CALL 0 TO A.

Timing equipment releases connection With reference to Fig. 5, it may be said that the individual timing unit ITU is in association with the dial back repeater of Fig. 2, and that the master timing" unit MTU is common to exchangeA.

The timing equipment is placed in operationwhen the called party answers on a connection established over the dial back switch train. ITU searches-for and finds the present location of the wipers of MTU, thereby establishing the start of a predetermined time interval during which the connection will be maintained. A warning tone LWill be placed on the transmission circuit onehalf minute before the expiration of the time interval, thus giving the subscriber an indication that the connection isabout to be released. The connection will be released at the end of the time interval even though the calling subscriber is still on the line. A detailed circuit explanationof the timer equipment will now be given.

Relay 239 operates when the control operator prepares the circuit of Fig. Zfor dial back operation, and at 23I places holding ground on conductor 592.

Relay 259 operates when the called party answers-on a dial back connection, and at 25I closes a circuit by way of conductor 593 to relay 569.

Relay 500' operates; prepares the stopping circuit at 5M; and closes a circuit to motor magnet 540 byway of 592-, 5I2; and 525.

Motor magnet 540 operates and at 541- closes the-circuit of relay 5).

Relay 5H1 energizes; and at 5I2 opens the'circuit of motor magnet 540.

Motor magnet 546 restores; advances the associated wipers one step; and at 544; opens the circuit of relay 5I0.

Relay 5"! restores; and at 5I'2' again closes the circuit of motor magnet 540.

Motor magnet 546 operates; and again at 54I. closesthe circuit of relay 5H]. This cycle of operation continues untilwiper 543 finds a grounded bank contact, thereby aligning ITU with the position occupied by MTU at the moment. When alignment results, a circuit is closed by way of wiper 553, wiper 543, 52I, through the upper windings of relays 520 and BM to battery at 59L Because of this circuit relay 5I0 will not restore and again complete the circuit of motor magnet 540.

Relay 520 operates; locks its lower winding at 522 to conductor 592; opens its own operating circuit at 52 I; opens at 526 the incomplete circuit to motor magnet 540"; at 524 prepares the circuit of relay 530; and closes the circuit of 560 by way of 528 and 536.

Relay 560 operates; starts the interrupter I at 5.64; prepares the warning tone circuit at 56I; and connects motor magnet 556 to the interrupter circuit by way of 562 and 512, For the purpose of explanation it may be assumed that the interrupter I delivers ground pulses at one half minute intervals and that each pulse has a duration of one fourth second.

Motor magnet 55!! operates at the next pulse and closes the circuit of relay 510 at 55l.

Relay 51!] operates; locks up to the pulse at 512 and opens the circuit of motor magnet 550.

Motor magnet 550 restores and advances the associated wipers one step.

'Relay 510 restores at the end of the pulse and again extends the interrupter circuit to motor magnet 550. Relay 510 and the motor magnet 550 will operate as described so long as relay 560 remains operated.

It will be noted that the banks associated with wipers 543 and 553 have been connected tail to tail and contact for contact; the banks associated with wiper 542 and 552 have been connected with a slip of six; and the banks asociated with wipers 544 and 554 have been connected with a slip of seven. We will assume that the position of the switches and wipers was as shown in Fig. at the instant that ITU aligned itself with M'I'U. It follows then that on the fifth interrupter pulse thereafter (representing approximately a three minute time interval) a warning tone will be placed on the dial back transmission circuit by way of 51I, 56I, wiper 552, wiper 542, 53I, 523, conductor 594, conductor I45, 319, I26, conductor I33, 291, and 283. This tone will advise the subscribers that the connection is about to be released, and at the next step of wiper 554 a circuit is closed by way of wiper 544, 524, and 532 to relay 530.

Relay 530 operates; locks itself to conductor 592 at 532; opens the circuit of relay 560 at 536; and removes ground from conductor 59I at 534.

The release of the dial back repeater and all associated switches takes place when ground is removed from conductor 59I; it being remembered that relay 300 restored when the control operator disconnected from the trunk.

The control of the connection, following the disconnect of the control operator, is in the hands of the calling subscriber; for he at any time may restore his receiver and bring about the restor ation of relay 200, followed by that of relay 2"], thus disconnecting the dial back repeater from grounded hold conductor 59I at- 2.

CALL FROM C To A When the operator at exchange C wishes to establish a call through exchange A she will insert the plug of an idle cord circuit into the jack J, Fig. 4, thereby energizing relay 440.

Relay 440 operates; and closes relay M0 by way of key K2, key K-I, 442, relay 45D, simplex of trunk TWT-I, 442 and 432.

Relay 4H! operates; connects the upper left hand winding of repeating coil R4-I to the incoming first selector trunk at M3; and closes the circuit of relay 400 at 4| I.

Relay 400 operates; ground conductor 462 at 40I, thus making the trunk busy in the banks of the outgoing secondary line switches; closes the circuit of the left hand winding of relay 430 at 403 but without eifect at this time; and completes the loop to the incoming first selector associated with LCI, by way of conductor 465, 465, 423, lower left hand winding of repeating coil R4-I, right hand winding of relay 430, 425, upper left hand winding of repeating coil R4I, 421, M3 and conductor 466. Relay 430 will not energize at this time because its two windings now aid each other.

The operator will now dial the number of the desired station. Relay 4| I follows the impulses received over the simplex of the trunk, and when at normal opens the circuit of relay 400 at 4, and the loop to the incoming first selector at 3. The switches in exchange A follow the repeated impulses and complete the desired connection. Relay 400 remains operated during each series of impulses.

When the called party answers, battery will be reversed over conductors 465 and 466, thus causing the operation of relay 430. The latter relay connects the bottom (high resistance) winding of relay 4H] in series with the simplex of the trunk at 432. The reduced current flow therein results in the required supervision at exchange C.

When the called party disconnects, the current flow through relay 430 is again reversed and the latter relay functions to remove the high resistance winding of relay 4I8 from the simplex of the trunk, disconnect supervision is thus given the operator at exchange C.

When the operator withdraws the plug from Jack J the simplex circuit is opened to direct current flow, with the result that relay 4l0 restores; opens the loop at M3 to allow the release of the incoming switches; and opens the circuit of relay 400 at 4| I, thus removing busying potential in the banks of the outgoing secondary line switches.

CALL FROM A TO D Seizure of TC-2 by subscriber Assume that a subscriber in exchange A desires to call exchange D; he will first remove his receiver, whereupon his line switch will extend the connection to an idle local first selector. The calling subscriber then dials the digit 9 with the result that the first selector will proceed to select an idle trunk circuit leading to exchange D. Referring now to Fig. 8, it will be assumed that the trunk so selected is that represented by conductors 28, 29, and 30.

Relay 830 operates; by way of 881, 853, and conductor 28 from ground supplied by the selector seizing TC2; and bridges relay 800 across the calling loop by way of conductor 29, conductor H, 942, windings I and 3 of R9-I, 944, conductor I86, and conductor 30.

Relay 800 operates; closes relay 8! at 802; and closes relay 868 by way of 804, 854, conductor I86, 914, and conductor I19.

Relay 8l 0 operates; closes relay 820 at M2; and disconnects relay 890 from the all trunks busy equipment 8I6.

Relay 820 operates; extends ground by way of 823, impedance I88, 881, 853 to conductor 28 to hold the preceding switches; prepares its own holding circuit at 822; and opens the incomplete circuit to relay 888 at 82L Relay 860 operates; closes its locking circuit at 864; and connects generator to the upper winding of relay 810 by way of 86I and condenser CALL FROM A To D Signalling operator at D Generator by way of NH, 81l, upper winding of relay 810, conductor I15 and M2 causes an energization of said relay sufiicient to energize make first contacts 812.

Relay 810 operates; looks through its lower winding at .872; closes relay m :by way :of .814 and conductor :I .16; and closes relay 1848 :at 81.6.

Relay 840 operates, but without effect :at this time.

Relay 92'!) operates; opens relay .860 at 914; closes relay 859 by way .of 842, conductor I78, 912, and conductor I11; and :at 916 and .918 impresses generator upon trunk conductors '3I and 32.

Relay 859 operates; locks :itself at 852; and transfers the operating circuitof relay .860 byway of 854 to break springs of relay 800.

Relay 869 restores; opens its incomplete looking circuit .at 864; .and at .862 opens the circuit of relays 819,840 and 919.

Relay 848 restores, but without efiect at this time.

Relay 919 restores; prepares the operating circuit to relay 869 at .914; and removes generator from trunk conductors 3.I and '32 at 918 and 918. The slow release characteristics of relays 86D and 918 determine the length of the ring sent out on conductors v3! and 32.

The splash of generator impressed on trunk conductors 3| and 32 will actuate the usual ring down equipment at exchange D and, when the operator answers, transmission will occur. Transmission battery is supplied to the calling party through relay 800.

If the calling line is a paysta'tion line, atone, superimposed on conductor 28, flows through the left hand winding thereof which .is audible to the operator at exchange D by way of conductors I82 and I81, and contacts 934 and 936.. The operator cuts off the tone by ringing on the trunk, thus actuating relay 968.

Relay 969 operates; and closes relay .930 by way of 983, conductor I"II, '83'l.,-and conductor .l'l2.

Relay .93!) operates; locks itself at 93].; .andremoves the second winding of impedance I 8.8 from the talking conductors at .934 and 936.

The .callin party may re-signal the operator by momentarily operating the hookswitchor dialling another digit. In either event relay 890 will momentarily restore and close relay 860.

Relay 959 operates and locks itself .at 864 whereupon relays 81B, 848, 910 and 860 function as before explained to impress a splash of generator on conductors 3! and 32.

The operator, uponanswering, must again ring to remove the paystation tone, for relay 899 opened the locking circuit of relay 930 at8IlI.

The weighted spring 842 of relay 840 will vibrate following the restoration of that relay, thus maintaining an intermittent circuit to relays 820 and 858 so that the latter will not restore as the result of the deenergization of relay BIO, should the subscriber delay unduly Iin releasing the hookswitch when resignalling the operator.

The release of the connection is under control of the calling party, and the restoration of the receiver to the hookswitch'will open the dirsuit of relay 899.

Relay 999 restores; opens relay 8Il at 802 and relay 939 at 8ill; and closes relay 859.

Relay 868 operates; locks itself at 864 and causes relays 819, 840 and 910 to operate and-send ring-01f generator over trunk conductors SI and 32.

Relay 8I9 restores; and opens relay 820 at 8:2 and relay 838 at .8I4.

Relay 889 restores; and opens relays B49, B10, and 919.

Relay E49 restores; and its weighted spring 842 vibrates, thus opening and closing .the circuit to relays 820 .and .850. The latter relays are slow :to release and do not restore immediately, but as the vibrations of 842 decrease in implitude relays.8 2 0 and 1850 are opened for longer and longer periods until restoration finally occurs.

Relay 829 restores; and opens relay 830 at 825 and impedance I88 at 823, thus causing the release of the preceding switches.

Relay 858 restores; and again connects relay 830 .to conductor .28 inorder that TC-2 may be available from the selector banks.

CALL FROM D To A Seizure of TC2 by operator The operator at exchange D may ring on trunk conductors 31 and 32, thereby bringing about the energization of relay i960.

Relay960 operates; and closes relay 880 by way of I822, I882, conductor ID'II, 96l, 98I, conductor I13 and 8.21. The short circuited lower winding of relay 880 makes said relay slow to energize, thus it will not operate should 'relay 960 momentarily respond to a surge or condenser discharge on the trunk.

Relay 880 operates; prepares its locking circuit by way of 881, conductor I14, relay 900 and 982; places ground on conductor '28 by Way of 881 and 853 to make TC2 busy in the banks of the selectors; closes motor magnet 950 by way of 885, conductor I66 and 901; and connects wiper 953 of ICS2 by way of 945, conductor I68, 883, conductor I61 to relay 940.

Relay 960 restores when the operator removes generator from the trunk, and opens the operating circuit of relay 889 at 96-I, whereupon relay 990 energizes in series with relay 880.

Relay 900 operates; opens the operating circuit of relay-880 at -90I to prevent the release of relay 999 should the operator again ring in the trunk; opens motor magnet 959 at 901; prepares the circuit of relay 940 at 905; and closes the left hand winding of relay SID at 902, but without effect at this time.

Motor magnet 95!! restores and advances the associated wipers'one step. If ICS-2 is now resting on a trunk to a busy dial back repeater, wiper 953 will be grounded, thusshunting'out relay 940 and closing motor magnet 950 by way of 945, conductor I68, 883, conductor I61 and 95I.

Motor magnet 950 operates; opens its own circuit at 95I, and advances the associated wipers one step upon deenergizing. This cycle of operations will continue until an idle dial back repeater isfound.

CALL FROM D TO A I C'S2 selects trunk toz'dle DBR When the rotary switch wipers engage the first idle trunk, assumed to be that represented by conductors l1, I8, I9 and 20, the absence of ground at wiper 953 will render efiective a circuit from ground'at 905, 933, relay 940, 95I and motor magnet 950. The motor magnet will not energize .at this time because of thehigh resistance of relay 940.

Relay 940 operates; closes the loop to DBP by way of conductor I'I, 955,942, winding I of R9-I, right hand winding of relay 9"], 994, conductor I85, 834, conductor I84, winding 3 of R9I, 944, 5954 and conductor I8; and places ground on conductor 20by way-of 941 and 952; thus rendering inoperative the time release equipment of The dial back repeater will extend the conncction over an idle trunk to exchange C, in the manner previously explained. In the event that the call is not answered by the operator at exchange C, the operator at exchange D may release the connection by again ringing on the trunk, thereby energizing relay 960.

Relay 960 operates; and closes relay 920 by way of 963 and 903.

Relay 920 operates; provides a holding circuit for relays 900 and 880 at 926; and closes relay 930 at 922.

When the ringing current is removed from the trunk, relay 960 will restore and open the circuit of relay 920.

Relay 920 restores; opens the circuit to relays 900 and 890 in series; and to relay 930. Due to its slow to release characteristics, relay 930 remains operated long enough to allow the release of relay 800.

Relay 900 restores; opens the loop to DBP at 904; and causes the release of ICS2 by opening the circuit of relay 940 at 905.

CALL FROM D To A Operator at exchange C answers When the control operator answers, the associated dial back repeater reverses battery over conductors I1 and I8, thus causing the energization of relay 9I0.

Relay 9I0 operates; opens the incomplete circuit of relay 810 at 9I2; and closes relay 920 at 9I I.

Relay 920 operates; prepares its holding circuit by way of 042, I, conductor I10 and 92I; closes relay 860 by Way of 9I I, 924, 914, and conductor I19; closes relay 930 at 922; and provides holding ground for relays 900 and 880 at 926.

Relay 860 operates; but without eiiect at this time.

Relay 930 operates; removes ground from relay 940 at 933, but without effect as relay 940 is held from ground on conductor I9; closes a holding circuit for itself at 93I and opens the original holding circuit of relays 900 and 880 at 932, thus placing the release of this switch under control of relay 920.

The control operator at exchange C will now extend the call over the dial back switch train in the manner previously explained. When the called party answers, the control operator can disconnect from the control trunk, thus placing the release of the entire switch train under control of the called subscriber.

The called subscriber can recall the operator at exchange D by momentarily operating his hookswitch. Such action, as heretofore explained, will cause the dial back repeater to momentarily reverse battery on conductors I1 and I8, thereby causing the deenergization of relay 9I0.

Relay 9I0 restores; and completes the circuit of the upper winding of relay 810 at 9I2; whereupon relays 810, 840, 910, 850 and 860 function as before explained to impress generator on trunk conductors 3i and 32, thereby signalling the operator at exchange D. The vibrations of the Weighted spring of relay 840 prevents the release of relays 850 and 920, so that TC2 will not release as a result of the momentary reversal of current on conductor I1 and I8.

When the called party restores his receiver at the end of the conversation a splash of ring on? current is impressed upon conductors 3I and 32, in the manner just described. And, as

the vibrations of relay 840 decrease in amplitude, the circuit to relays 920 and 850 are open for a longer period during each oscillation until finally the aforementioned relays deenergize.

Relay 920 restores; and opens the circuit of relays 880, 900 and 930.

Relay 900 restores; and at 904 opens the loop across conductors I1 and I0, thus causing the release of the dial back repeater and switch train. The release of DBR removes ground from conductor I9, thus causing the deenergizatlon of relays 940 and 930.

Relay 350 restores; and again connects relay 830 to conductor 28, thus making TC2 available to calls from the selector banks.

CALL FROM D TO A All trunks to C busy In the event that all the trunks from exchange A to exchange C are busy it is desirable that the operator at D receive a busy indication without needlessly tying up a dial back repeater. As before explained, this busy condition results in the energization of relay I030, which in turn closes the circuit of relay I000. A circuit is thereby closed by way of I004 and conductor I012 to all trunk circuits incoming to the dial back repeaters of exchange A. In the case of TC2 the circuit extends through M6 to relay 890.

Relay 890 operates; and impresses busy tone on conductors 29 and 30 by way of 892 and 894. The busy tone is induced in the line side of repeating coil R9-I and is audible to the operator at exchange D when she plugs into the jack associated with trunk conductors 3| and 32.

The busy condition just described does not prevent a subscriber in exchange A from making a call to exchange D, for relay 8I0, which operates on a local connection, opens the circuit of relay 890 at 0I6.

The operation of relay 960 is rendered inefiective, should the operator ring on trunk conductors 3I and 32 during an all trunks busy period,

by the removal of ground from conductor I01I at I002.

CALL FROM D To A All DBRS 1711 11 Conductor I013 connects the normally energized relay IOI0 to all dial back repeaters in exchange A, in the case of BBB. this conductor is normally grounded by way of 332, conductor I50, 244, and 222. It follows then that relay IOI0 will deenergize when all dial back repeaters are in use.

Relay IOI0 restores; and closes the circuit of relay I020 at IOI2.

Relay I020 operates; removes ground from conductor I01I at I022; and grounds conductor I012 at I024. 1

As a result of the foregoing, the operator at exchange D will receive a busy indication and the trunk circuit will be protected as previous ly explained.

CALL FROM 0 TO B Seizure of TC3 by first selector As indicated by the trunking diagram Fig. 1, exchange B can only be called by the control operator at exchange 0', through the medium of a dial backfirst selector or an incoming first selector.

We will assume that the control operator reverted a connection to a dial back first selector, and now dials the digit eight, with the result that the dial back selector will proceed to select an idle trunk circuit leading to exchange B. Referring now to Fig. 6, it will be assumed that the trunk so selected is that represented by conductors 25, 26 and 21.

Relay 616 operates; by way of 665, 695 and conductor 25 from ground supplied from the selector seizing TC-3; bridges relay 626 across the calling line by way of 616, conductor 62, winding 3 of R1!, 146, conductor !64, and 664 to conductor 21, and by way of 614, conductor !6!, winding I of R1-!, 144, conductor 166 and 662 to conductor 26; and opens the circuit of relay 686 at 61 2.

Relay 626 operates; closes relay 636 at 622; and completes a dial lead to exchange B by Way of relay 656, 624, conductor -!59, and the midpoint of windings 2 and 4 of repeating coil RL- l.

Relay 656 operates; and opens the incomplete circuit to relay 6!6 at 65!.

Relay 636 operates; and at 631 supplies ground to conductor 25 to hold the preceding switches.

CALL FROM 0 TO B Repeating impulses to exchange B Trunk circuit TC3 is now prepared to repeat impulses to exchange B. Relay 626 follows the impulses of each series and, when at normal, opens the dial lead at 624 and closes the circuit of relay 646 by way of 622 and 632.

Relay 646 operates; opens the incomplete circuit to relay 6!6 at 643; closes relay 656 by way of 63! and '64! to maintain the latter during the impulsing period; and at 642 connects resistance I69 in parallel with relay 656 to improve pulsing.

Relay 656 restores momentarily, following the deenergization of relay 626 and prior to the operation of relay 646, but without effect as relay 6! 6 is slow to operate.

Due to their slow release characteristics relays 636 and 646 remain operated during impulsing.

The opening and closing of the simplex of the trunk causes the switches in exchange B to operate and complete the dialled connection.

CALL FROM C To B Called party answers When the called party answers, the .equipment at exchange B places a high resistance in the simplex of the trunk thereby causing relay 656 to deenergize and close the circuit of relay 6!!) by way of 65!, 643, 633 and 6.

Relay 6!6 operates; locks itself to ground at 63! and opens its originating circuit at 6! and closes relay 666 at 6I3.

Relay 666 operates; and reverses battery to conductor 26 and 21 at 662 and 664.

The circuit is now complete for transmission. Transmission battery is supplied to the calling party through relay 626. The transmission circuit to and from the called party is by way of the repeating coil R1-!.

The called party can recall the operator at exchange D (assuming that the connection originated thereat) by momentarily depressing this hooksw-itch. The equipment at exchange D follows the hookswitch and places the high resistance out and in the simplex of the trunk. Relay 656 will follow the supervision received over the simplex and will cause relay 666 to send reverse battery supervision back over the calling switch train.

The release of the connection is under the control of the calling party and, when the receiver is restored, the circuit of relay 626 will be opened.

Relay 626 restores; opens relay 636 at 622; opens relay 656 at 624; and closes relay 646 at 622.

Relay 636 restores; opens relays 616 and 656 at 63!; opens relay 646 at 632; causes the release of the preceding switches by removing ground from conductor 25 at 631; and closes relay 666 at 635.

Relay 666 operates; prepares its locking circuit at 662; and closes relay 656 at 664.

Relay 6!6 restores; and grounds conductors 25 at 6!5, thus making TC3 busy in the selector banks.

The operation now depends upon whether or not the called subscriber has restored his receiver to the switch hook. In the event that he has restored, relay 656 will remain operated by virtue of its lower winding which is energized over the simplex of the trunk.

Relay 646 restores; and opens relay 666 at 645.

Relay 666 restores; removes ground from conductor 25 at 665 and connects relay 616 thereto; and opens relay 656 at 664.

Relay 656 restores; whereupon TC3 is again at normal.

In the event that the called party had not restored, relay 656 will restore when its upper winding is opened at 63! because the current flowing through its lower winding and the simplex of the trunk is limited by the high resistance at exchange B, and is not sufficient to maintain the relay in an energized position. The restoration of relay affords a locking circuit for relay 666 by way of 65! and 662. Thus the trunk to TC3 is made busy in the selector banks until such time as the called party restores.

When the called party restores, the high resistance will be cut out of the simplex of the trunk, whereupon relay 656 will operate and open relay 666 at 65!.

Relay 666 restores; removes ground from conductor 25 at 665; and connects relay 616 thereto; and opens relay 656 at 664.

Relay 656 restores; whereupon TC3 is again at normal.

CALL FROM B To A Seizure of TC3 by automatic subscriber Assuming now that a subscriber in exchange B desires to call a subscriber in exchange A, he will first remove his receiver, whereupon his line switch will extend the connection to an idle local first selector. The calling subscriber than dials the digit required to select an idle trunk to exchange A. Referring now to Fig. 7, it will be assumed that the trunk so selected is that represented by conductors 33 and 34.

The equipment at exchange B will next impress generator upon the simplex of the trunk, thereby energizing relay 686 by Way of 612, condenser 666! conductor I58, 164 and windings 2 and 4 of R1--! in parallel to conductors 33 and 34.

Relay 666 operates; and closes relay 696 at 68!.

Relay 690 operates; grounds conductor 25 at 695, thus making TC3 busy in the selector banks; closes motor magnet I50 by way of 693, conductor I54, and TM; and prepares the operating circuit of relay I00 at 69I. The normally short circuited lower winding of relay 690 causes that relay to be slow to operate, and as a result it will not energize should relay 600 operate momentarily as the result of a line surge.

Relay 680 restores; when generator is removed from the simplex of the trunk; and removes the short circuit from relay I00 at 68I. The energizing circuit of relay I00 extends from ground at 65I, 562, conductor I56, relay I00, conductor I55, 69I and relay 690.

Relay I00 operates; opens relay 600 at I04, connects relay 650 to the simplex of the trunk at I02; closes relay I I 0 by way of 693, conductor I54, and TM; connects wiper I52 to motor magnet 150 by way of MI, I05, I6I, and I5I; prepares the circuit of relay I40 at I03; and opens motor magnet I50 at IOI.

Motor magnet I50 restores and advances the associated wipers one step. If ICS-3 is now resting on a trunk to a busy dial back repeater wiper I52 will be grounded, thus shunting out relay I40 and closing motor magnet I50 by way of I4I,I05,I6I and I5I.

Motor magnet I50 operates; opens its own circuit at I5I, and advances the associated wipers one step upon deenergizing. This cycle of operaticns will continue until an idle dial back repeater is found.

CALL FROM B TO A ICS3 selects trunk to idle DBR When the rotary switch wipers engage the first idle trunk, assumed to be that represented by conductors I4, I5, and I6, the absence of ground at wiper I52 will render effective a circuit from ground at I022, I002, conductor I0II, 66I, conductor I53, I03, I3I, relay I40, I5I and motor magnet I50. Motor magnet I50 will not energize in series with the high resistance relay I40.

Relay I40 operates; closes the loop to the dial back repeater by way of conductor I4, I54, I44, winding I of RL-I, relay H0, conductor I63, 652, 616, conductor I62, winding 3 of Rl-I, I46, I53, conductor I5; prepares its own holding circuit at MI; and connects relay I30 to wiper I52 at I42. The dial back repeater associated with trunk conductors I4, I5, and I6 now returns ground on conductor I6, thus energizing relay I30 by way of I52 and I42.

Relay I30 operates; prepares a holding circuit for relay 690 in series with relay I00 at I32, thus placing the release of TC3 under control of the dial back repeater; and transfers the holding circuit of relay I40 to conductor I6 at I3I.

The dial back repeater and associated outgoing secondary lineswitch function as before described to signal the control operator at exchange C. When the control operator answers, battery is reversed over conductors I4 and I5, thus reversing the current in the operating winding of polarized relay H0.

Relay IIO operates; and closes relay I20 at II I.

Relay I20 operates; opens the simplex of the trunk at I22, thus causing the restoration of a relay at exchange B in order that paystation tone may become audible on conductors 33 and 34; and closes an intermittent circuit to relay 660 by way of I33, weighted spring HI and conductor I5 I. Relay 660 does not energize until the weighted spring I2I comes to rest.

Relay 660 operates; closes its locking circuit at 662 recloses the simplex of the trunk at 664, thus causing the removal of paystation tone from conductors 33 and 34 at exchange B; opens the original holding circuit of relays 690 and I00 at 662; and opens the incomplete circuit of relay I40 at 66I.

Busy tone is placed on conductor I 6 by way of condenser I003, MI and I52, thus affording a discriminating tone which becomes audible to the control operator in exchange C, and advises her that the call is from exchange B.

CALL FROM. B To A Release of the connection The control operator will now revert the call by way of a dial back first selector to the desired subscriber in exchange A, in the manner already described. If the control operator supervises the connection, the release of TC-3 is under the control of the last party to release, otherwise the release is under the control of the calling party, or of the timing equipment.

When the calling party releases, the equipment at exchange B removes the high resistance from the simplex of the trunk, thereby causing the energization of relay 650.

Relay 650 operates; opens relay 660 at 65I; and opens the loop circuit to the dial back repeater at 652, thus giving the control operator disconnect supervision. When the dial back repeater releases, ground is removed from conductor I6, thus opening the circuit of relays I30 and I40.

Relay I30 restores; and opens the circuit of relays 690, 700 and 660. Relay 660 is made slow to release so that relay 690 will restore first and thereby open the operating circuit to relay I00.

Relay 660 restores; opens the simplex of the trunk at 664 thus allowing the release of the equipment in exchange B; and at 665 connects relay 610 to conductor 25, thus making TC3 available from the selector banks.

In the event that the calling party remains on the line, the timing equipment will eventually remove ground from conductor I6, thereby opening the circuits of relays I40 and I30.

Relay I40 restores; opens the loop to the dial back repeater at I44 and I46, thus causing its release, as well as the deenergization of relays H0 and I20.

Relay I30 restores; and opens relays 690 and I00 at I32.

When the called party restores his receiver, the high resistance is removed from the simplex of the trunk, thus causing the energization of relay 650.

Relay 650 operates; and opens relay 660 at 65I.

Relay 660 restores; opens the simplex of. the trunk at 664, thereby releasing the repeater in exchange B; and at 665 connects relay 610 to conductor 25, thus making TC3 available from the selector banks.

CALL FROM B TO A All trunks to C busy or all dial back repeaters busy As has been previously explained, relay I000 operated when all of the trunks from exchange A to exchange C are busy, while relay I020 operates if all of the dial back repeaters are in use. In either event ground is placed on all branches of conductor I0I2, and ground is removed from all branches of conductor I0II.

If, during either busy condition, TC3 is seized

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