US2225478A - Telephone system - Google Patents

Description

Dec. 17, 1940# H. HOVLAND TELEPHQNE SYSTEM Filed Feb. l, 1938 2 Sheets-Sheet l ATTORNEY Dec. 17, 1940. H. HovLAND TELEPHONE SYSTEM Filed Feb. 1, 1938 j sheets-sheet 2 N w aww. m. w

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Patented Dec. 17, 1940 UNITED y STATES TELEPHONE SYSTEM Henry Hovland, Williston Park, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 1, 1938, Serial No. 188,053

11 Claims.

-This invention relates to telephone systems and particularly to systems in which automatic Aswitches are employed in establishing telephone connections.

It is an object of the invention to enable a Vmore eicient use of interoiice trunks in automatic and semi-automatic telephone systems. Other objects are the prevention of double connections and the prevention of false charging on calls from 'message rate .and prepayment coin lines in systems comprising two-way interofce trunks.

The invention is an improved two-way trunk circuit'which is arranged to prevent the establishment of a talking connection if the trunk is seized at both ends and to allow completion of a call over the trunk in case the trunk is seized at one end on a second call before the connection has been released at the other end on a preceding call. According to a feature of the invention, a connection over a two-way trunk arranged for composite signaling thereover is controlled by the calling party and the trunk isheld -busy at the outgoing end, after release'by a calling party, for a longer interval of time than it is 'held busy at the incoming end so as to prevent seizure of the trunk on another callv before the switches at the incoming end of the trunk on the iirst call have returned to normal. Means are provided for transmitting a tone signal to the calling subscriber when the subscribers line is connected to a two-Way trunk which has also been seized at the distant end. A further feature is a two-way trunk circuit arranged to momentarily disconnect the guarding potential from the test conductor at the outgoing end after release of the trunk on a firstv call to enable release of the train of switches at that end in case the trunk is again seized at the distant end von a second call before this train of switches has been released.

To facilitate a clear and complete description of the invention, .an automatic telephone system arrangedV in accordance with the invention is represented schematically in the drawings which form a part of this specification. The invention is, however, not limited to the speciiic arrangements shown in the drawings and is generally applicable to .all automatic telephone systems comprising two-way trunks.

Referring to the drawings, which consist of two gures:

Fig. 1 shows subscribers lines LI and L3, aV

line-finder switch LFI and an associated selector switch SI, a two-way trunk circuit TI and an associated incoming selector switch ISI, and

a connector switch CI, Iall of which are in one cnice of a telephone system; and

Fig. 2 shows subscribers lines L2 and L4, a `line-finder switch LF2 and an associated selector switch S2, a two-way trunk circuit T2 and `s an associated incoming selector switch IS2, and a connector switch C2, all of which are in another oice of the system.

The telephone stations A, B, C and D are of the usual type employed in common battery 10 telephone systems and each includes a dial or equivalent impulse sender for use in controlling the operation of the selector and connector switches by which desired connections are established. Although lines LI and L2 are shown 15 connected only to terminals in the banks `oi `line-finder switches'and lines L3 and L4 `are shown connected only 'to terminals in the banks of connector switches, all of these lines are connected to terminals in the banks of both line- 20 nder and connector switches so that calls can be originated and received at each of the subscribers stations. i

The line-finder, selector and connector switches are ofthe well-known two-motion stepby-step type. For a description of the structure of such switches and their operation when used as selectors and connectors, reference may be had to pages 53 to 67 of the second edition of Automatic Telephony by Smith and Campbell.

For a description of the operation of such linender switches, reference may be had to Patent 1,799,654, granted to R. L. Stokely, April 7, 41931. Each of the switches LFI, LF2, SI, S2, ISI, IS2,

CI and C2 is represented by a set of brushes and 35 a single set of terminal, all magnets, relays and other associated apparatus being omitted with the exception of the line and release relays of the incoming selectors ISI and IS2. Although not shown, additional selector switches may be `included in the train required for completion of a desired connection.

The like trunk circuits TI and T2 are shown in detail and include repeating coils RCI 'and RC2, and composite sets terminating the ends of the interoflice trunk T. The corresponding relays, condensers, resistors, retard coils and conductors of trunk circuits TI and T2 are given reference characters having like tens and units digits. The composite set at one end of trunk T comprises condensers |53, |54, |59 and |60, retard coils |55, |56, |51 and |58 and the composite signaling relay |50. The composite set at the other end of the trunk comprises like elements with corresponding reference characters. A

ground potential compensating circuit is traced from ground through resistor |5I and the upper winding of relay |50, lower windings of retard coils |55 .and |56, lower conductor of trunk T,

- relay 250 being normally connected through back contacts of relays 2I|, 208 and 2I0 to ground. The lower winding of each of relays |50 and 250 is connected in series with a biasing battery to hold the armatures of these relays in normal position.

Assume iirst that trunk T is used on a call from station A to station D. Upon removal of the receiver from the receiver hook at station A, a line nder LFI operates in well-known manner to eX- -tend the line LI to a first selector Sl. In response to the dialing of the rst digit of the called subscribers number, the selector SI selects the desired group and an idle set of terminals therein, the line LI being thereby connected to an idle two-way trunk circuit T| Relay ||0 is operated by the current in a circuit from battery through the upper winding of relay I I0, inner upper back contact of relay |02, left upper winding of repeating coil RCI, outer upper back contact of relay |02, upper back contact of relay IOI, upper brushes of selector SI and line finder LFI, through the line conductors and telephone instrument at station A, middle brushes of line finder LFI and selector SI, lower back contact of relay |0I, outer lower back contact of relay |02, lower left winding of repeating coil RCI, inner lower back contact of relay |02, and lower winding of relay I|0 to ground. Relay IIO disconnects the ground normally connected to the junction between the lowermost and middle windings of signaling relay |50 and closes circuits from signaling battery through resistance lamp |09, upper frontcontact of relay I I 0, back contacts of relays |08 and II I, thence through resistor |52 to ground, through the lowermost winding of relay |50 to biasing battery, and through the middle winding of relay |50, upper windings of retard coils |55 and |56, upper conductor of trunk T, upper windings of retard'coils 256 and 55, ymiddle winding of relay 250, back contacts of relays 2| I, 208 and '2|0 to ground. Relay |50 does not operate since the current through the lowermost winding of relay |50 is electromagnetically opposed to and neutralizes the current through the middle winding; but signaling relay 250 at the other end of trunk T is operated, the energization of ,the middle winding of relay 250 being strong enough to overcome the energization of the biasing winding. The operation of relay I I0 also closes a circuit frombattery through the winding of slow-to-release relay |28, lower front contact of' relay ||0, to ground at the uppermost back contact of relay |20. At its inner upper front contact, relay |28 connects ground through sleeve conductor |00 to the sleeve terminals of trunk TI in the banks of selector switches having access thereto to guard against seizure of trunk circuit TI by another selector. At its inner lower front contact, relay |28 lcloses a` circuit which includes a back Contact of `relay I||`for operating the slow-to-release relayIBI.. Relay |.3I disconnects ground from conductor |35 leading to the group traffic register circuit, closes a circuit for operating the slow-to-release relay |29 and bridges the condenser I I8 across the inner ends of the left windings of repeating coil RCI. Relay |29 connects ground through its outer upper front Contact and a back Contact of relay III to the sleeve conductor |00 and closes a circuit through its outer lower front contact for operating the slow-to-operate relay |30.

The aforementioned operation of relay 250 of trunk circuit T2 closes a circuit for operating relay 232. Relay 232 closes a circuit for operating relay 22|, closes a circuit for energizing the lower, polarizing winding of relay 208, closes a circuit for energizing the upper winding of relay 223, and closes a circuit for charging cond-enser 233 in series with resistor 234 and the winding of relay 220. The energization of the upper winding of relay 223 is eiective to close and hold closed its left contact. Relay 220 operates while condenser 233 is charging and then releases. Relay 221 locks through the outer lower back Contact of relay 206, conductor 236 and both upper front contacts of relay 221; disconnects ground from conductor 235 leading to the group traffic register circuit; closes a circuit foroperating relay 202; closes a circuit through both of its `upper front contacts and conductor 236 for energizing the lower polarizing winding of relay 208, closes a circuit through both of its upper front contacts, conductor 236 and the left contact Yof relay 223 for operating relay 206; and connects ground through its inner upper front contact, conductor 231, the outer upper back contact of relay 2II, and through conductor 200 to sleeve terminals in the banks of selector S2 and other selectors to guard against seizure of trunk circuit TI by any of these selectors. The operation of relay 206 closes a circuit for energizing the lower winding of polarized relay 223, and closes a circuit for operating the line relay 213| of incoming selector IS2. The i energization of the lower winding of relay 223 opposes'and overcomes the energization of its upper Winding to actuate its armature to close the right contact. Relay 223 opens the operating circuit of .relay 206 but relay 206 remains operated in a locking circuit under the control of relays 220 and 227. The aforementioned circuit for operating relay 24| is traced from ground through its right winding, conductor 239, lowermost front contact of relay 202, lower right winding of repeating coil RC2, inner lower front Contact of relay 202, upper winding of relay 208, upper front contact of relay 205, resistor 203, inner upper front contact of relay 202, upper right winding of repeating coil RC2, outer upper front contact of relay 202, conductor 238, and through the left winding of relay 24| to battery. Relay 24| closes a circuit for operating relay 242.

When the subscriber at station A dials a digit of the number of the called station, relay ||0 releases and reoperates in response to each dial impulse, thereby momentarily transferring the junction between the lower and middle windings of relay |50 from battery to ground to cause the release and reoperation of relay 250 of trunk circuit T2. The release of relay I0, in response to the iirst impulse resulting from the dialing of a digit, closes a circuit which includes the outer upper front contact of relay ISI, for operating the slow-to-release relay |I|. Relay sh0rtcircuits the left windings of repeating coil RCI ,until all of the impulses inthe train have been of relay 221.

received by relay I l0; and also short-circuits the upper winding of relay |08 but without useful function on a call outgoing from trunk cir- 'cuit TI. Being slow-to-release, relay |28 remains operated while relay I I0 is responding to dial impulses. The release and reoperation of Vrelay 250 of trunk circuit T2`in response to each dial impulse received by relay ||0 of trunk circuit TI, causes the release and reoperation of `relay 232. Each release of relay 232 causes disof relay 223. The lower winding of relay 223 continues to be energizedwhile condenser 224 is charging; butwhen the charging current has decreased sufficiently, the upper Winding of relay 223 actuates the armature to open its right contact and close its left contact. Relay 220 having released after its momentary operation, the closing of the'left contact of relay 223 causes the reoperation of relay 206 and relay 206 again `closes the impulse circuit and the circuit for energizing the lower winding of relay 223 so that relay 233 again closes its right contact. The reoperation of relay 206 also causes the discharge of condenser 224. The capacity of condenser 224 and the resistances of resistors 222 and 225 are of such values as to delay the release of relay 223 a definite and predetermined interval of time, so that relay 206 is released and holds open the circuit of relay 24| for an interval sufficient to insure release of relay 24| and actuation of the vertical magnet of selector ISZ. Upon release of relay 232 responsive to the rst release of relay 250, relay 226 is operated by its upper winding in series with the winding of relay 221. Relay 226 opens its operating circuit but locks through its lower winding under control of relay 223 and closes a circuit for operating relay 2|1; this circuit includes the upper front contacts of relay 221, upper front contact of` relay 226, and the inner lower front contact As soon as relay 223 opens its right contact, relay 226 releases and opens the circuit of relay 2I1; but relay 2|1 is slow in releasing and remains operated until relay 226 releases after the last impulse in the train has been received. Each impulse of the train causes the above cycle of operations of relays 232, 220, 206, 223 and 226 to be repeated. Relay 2|1 short-circuits the right windings o-f repeating coil RC2 and the upper winding of relay 208to reduce the impedance in the impulse circuit while each train of impulses incoming toV relay 250 is being repeated by the opening and closing of the upper contacts of relay 206. Relay 221 is slow to release and since'its locking `circuit is closed each time relay 206 releases and since its winding is momentarily held energized in vseries with the upper winding of` relay 226 each time relay 206 operates, relay 221 does not release while dial impulses are being received by relay 250 and repeated by relay 206.

Relay 24| of incoming selector' ISZ releases and reoperates in response to each impulse of the first train repeated by the contacts of relay 206 thereby causing the brushes of selector IS2 to be selectively advanced to the desired group of terminals in usual andwell-known manner. Relay 242 is slow to release and remains operated idle set of terminals in the selected group and extend the impulse repeating circuit f- rom conductors 238 and 239 tothe line relay (not shown) of the connector circuit C2. Relays 24| and 242 are thereupon released and sleeve conductor 231 is connected to holding ground potential in the connector circuit in usual manner. The last two trains of impulses repeated by relay 206 are eiective to advance the brushes of connector switch C2 to establish connection with the called line L4; inusual and well-known manner. The line being idle, the usual ringing circuit is closed and when the receiver is removed at station D to answer the call, the connector C2 reverses the current through conductors 238 and 239 and the upper winding of polarized relay 208 to cause the operation of relay 206. Relay 208 disconnects ground from the lower windings of relay 250 and connects battery thereto to cause the operation of relay |50 of trunk circuit TI as an answering supervisory signal. Relay |50 closes a circuit, which includes the front contact of relay |30, for operating relay |0I; and relay |0| reverses the current through the calling line LI.

If the called subscriber returns the receiver to normal before the calling subscriber releases the connection, the resulting reversal of current by the connector circuit causes the -release of relays 208, |50 and |0I. When the calling subscriber returns the receiver to normal, relay ||0 releases thereby reconnecting the junction between the lower windings of relay |50 to ground 'to cause the lrelease of relays 250 and 232 of trunk circuit T2. The release of relay 232 causes the discharge of condenser 233. Since relay 232 does not reoperate, relays 206 and 221 release, causing the release of relay 202, and disconnecting ground from conductor 231. Relay 2|1 operates when relay 206 releases and releases when relay 221 releases. The release of relay 206 opens the outgoing supervisory circuit over conductors 238 and 239 to cause the release of the line relay of the connector circuit and the release of relay 208 if not already released. Whenthe connector C2 also disconnects holding ground from sleeve conductor 231, selector IS2 is returned to normal, and trunk circuit T2 is again ready for seizure by a selector S2. The connector is returned to normal in usual manner as soon as the receiver is returned toinormal at the called station. The aforementioned release of relay |I0 also causes the operation of relay ||1, and the successive release of slow-to-release relays |28, I3| and |29 and the release of slow-to-operate relay |30. The release of relay |3| 'causes the release of relay ||1. Relays |3| and |29 maintain the connection of ground to the sleeve conductor |00 for an interval of time after relay I0 releases so that trunk circuit T| cannot be again seized on another call until the selector IS2 has been restored to normal. When ground is disconnected from conductor |00, the selector SI and .line finder LFI are restored to normal in usual and well-known manner.

On 'a call from station B to station C, line L2 is extended through line nder switch LF'2 and by the selective operation of selector S2 under control of the dial at station B to an idle trunk circuit T2. Thereupon trunk circuit T2 operates as did trunk circuit TI, and "trunk circuit T| operates as did trunk circuit T2, on the call above describedfrom station A to station D.

. 'In case thel trunkcircui-t Tl is seized byl a selector Si at about the same time that trunk circuit T2 is seized by a selector- S2, relays 110,

128, llfand 129 of trunk circuit TI operate as hereinfbefore described in. about the same interval of 'time that relays 210, |50-, 132, 121 and |02 operate due to seizure of trunk circuit T2; and relays 210, 228, 231 and 229 of trunk. circuit T2 operate' in about the same interval of time that relays 11d, 2513, 232, 221 and 202 operate due to seizure of trunk circuit T1. The operation of relay 129 closes a circuit for operating relay 111; andthe operation of relay 229 closes a circuit for operating relay 21 1. Relays ISU-and 230 are slow in operating so that relays 1 1 I and 211 will operate and prevent the operation of relays 130 and 230. Relay 111 locks under control of relay 128, connects relay 110 directly to the calling line, connects ground to the junction between the-lower windings of relay 150 to cause the release of relay 250, disconnects sleeve conductor 100 from the sleeve conductor 13T of incoming selector ISI, connects ground to conductor 114 to start the intcrrupter mctor 11.5 if `not already operating, closes a circuit path from tone source 1 13 through Y condenser 112 to one of the conductors of the calling line, closes a circuit for operatingrelay 102 ifl not already operated, and causes the release of relays 13E), 131 and129. A similar series of operations results from the operation of relay 211 of trunk circuit T2. Each of the calling stations is thus disconnected from the trunk so that a wrong connection does not result. When the calling subscriber at the station connected to trunk TI returns the receiver to normal, relays 111), 128, 111 and |02 release; and when the calling subscriber at the station connected to trunk T2 returns the receiver to normal, relays 210, 228, 211 and 2112 release.

vIn case the trunk circuits T1 and T2 are both seized at about the same time, the operating times of the various relays may be such that relay of trunk circuit T1 operates before relay l 1 1 operates or that relay 230 of trunk `circuitfTZ operates before relay 211 operates. In this case if relays |11 and 230 are operated, the calling station connected to trunk circuit T1 is blocked and receives a tone as above described; but the call incoming to trunk circuit'TI from trunk circuit T2 will be completed. Or if relays 21 l and |30 are operated, the calling station connected to'trunk circuit T2 is blocked and receives a tone but the vcall in-y coming to trunk circuit T2 from trunk circuit T1 will be completed. When the calling subscriber whose call is blocked returns the receiver to normal, the releasev of relay 110 or 210 causes the release of relay |28 or 228. 'I'herelease of relay 128 or 228 disconnects ground from the sleeve conductor 100 or 2130 and causes the release of relay 111 or 211. Relays 11| and 211 are slow in releasing lto insure release of the selector S1 and line finder LFI or selector S2 and line finder LFZ before reconnecting conductor 100 or 200 to ground and to the sleeve conductor 13'1 or 231 of the associated incoming selector ISI or IS2.

From the foregoing description, it is apparent that the trunk circuit at the called end of the trunk is restored to normal by the successive release of two slow-to-release relays,for instance, relay 221- of trunk circuit'TZ and the release relay of the associated connector; whereas the trunk circuit atthe calling end is notrestored to normal until completion of the successive release of three slow-to-release relaysfor instance, relays 128, 131 and 129 oftrunk circuit T1. The trunk circuit at the called end may, therefore, be again seized on a second call before the trunk circuit at the calling end ofthe iirst call .has restored to normal. In this case, operation of the composite signaling relay, or 250, before the near trunk circuit has been restored to normal causesthe operation of relays 132 and 1 I 1, or relays 232 and 2H, in multiple. Relay 111, or 211, locks under control of relay 12S, or 229, and disconnects ground from the sleeve conductor 100, or 200, to allow the line iinder and first selector switches used on the first call to restore to normal. When relay- 129, or 229, releases, relay |11, or 211 releases and the sleeve conductor 100, or 200, is then connected to the sleeve conductor of the associated incoming selector to guard against seizure by one of the rst selectors. 'Ihe second call is thereupon completed by operation of the incoming selector anda connector vat the one end of the trunk under the control of the dial at the calling station at the other end of the trunk.

What is claimed is:

l. In a telephone system, subscribers lines, a two-way trunk arranged for composite signaling thereover, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, and means at each end of the trunk and controlled by the calling subscriber for terminating the marking of said trunk as busy, said means being effective to terminate the busy marking of said trunk at `rthe end to which the calling line is connected after the marking of said trunk as busy is terminated at the end to which the called line is connected.

2. In a telephone system, subscribers lines, a two-way trunk arranged for composite signaling th'ereover, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, means responsive to the release of the connection by the calling subscriber for terminating the marking of said trunk as busy and for causing the return of said selector switches to normal, and means for delaying the return of the switch through which the calling line is connected to said trunk until a suiicient interval of time has Yelapsed for said switch through which the trunk is connected to the called line to have been restored to normal.

3. In a telephone system according to claim 2, r

means responsive to Vseizure of said trunk at the end connected to said called line for temporarily interrupting the marking of the trunk as busy at the end connected to said calling line if the last-mentioned seizure occurs prior to the release of the switch through which said calling line is connected tothe trunk.

4. In a telephone system, subscribers lines, a two-way trunk, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, means at each end of said trunk effective upon simultaneous connection of calling lines to both ends of said trunk for preventing the connection of said trunk to a called line at either end, means at one end of the trunk `for transmitting to a calling line a Signal indicating that the call cannot be completed 75 due to seizure of the trunk at the other end, and means at each end of the trunk and controlled by the calling subscriber for terminating the marking of said trunk as busy, said means being effective to terminate the busy marking of said trunk at the end to which the calling line is connected after the marking of said trunk as busy is terminated at the end to which the called line is connected.

5. In a telephone system, subscribers lines, a two-way trunk, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, means at each end of said trunk eiective upon simultaneous connection of calling lines to both ends of said trunk for preventing the connection of said trunk to a called line at either end, means at one end of the trunk for transmitting to a calling line a signal indicating that the call cannot be completed due to seizure of the trunk at the other end, like signaling means at the other end of the trunk, and means at each end .of the trunk and controlled by the calling subscriber for terminating the marking of said trunk as busy, said means being effective to terminate the busy marking of said trunk at the end to which the calling line is connected after the marking of said trunk as busy is terminated at the end t'o which the called line is connected.

6. In a telephone system, subscribers lines, a two-way trunk arranged for composite signaling thereover, means comprising a selector `switch for connecting a calling line to one end of said trunk, like means for connecting a calling line to the other end of said trunk, means comprising a selector switch for connecting said one end of the trunk to a called line, like means for connecting the other end of the trunk to a called line, means responsive to seizure of said trunk for marking it busy at both ends, means responsive to release of the connection by the calling subscriber for terminating the marking of said trunk as busy and for causing the return to normal of the selector switches through which the trunk is connected to the calling and called lines, `and means for delaying the return of the one of the last-mentioned switches which connects the calling line to the trunk until a suiicient interval of time has elapsed for the return to normal of the other one of the last-mentioned switches.

7. In a telephone system according to claim 6, means responsive to seizure of said trunk at the end connected to said called line for temporarily interrupting the marking of the trunk as busy at the end connected to said calling line if the lastmentioned seizure occurs prior to the release of the switch through which said `calling line is connected to the trunk.

8. In a telephone system, subscribers lines, a two-way trunk, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, and means at each end of the trunk and controlled by the calling subscriber for terminating the marking of said trunk as busy, said means comprising three slow-to-release relays released in succession at the end of the trunk to which the calling line is connected and comprising only two slow-to-release relays released in succession at the end of the trunk to which the called line is connected. 9. In a telephone system, subscribers lines, a two-way trunk arranged for composite signaling thereover, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, means at each end of the trunk and controlled by the calling subscriber for terminating the marking of said trunk as busy, said means being effective to terminate the busy marking of said trunk at the end to which the calling line is connected after the marking of said trunk as busy is terminated at the end to which the called line is connected, and means effective upon simultaneous connection of calling lines to both ends of said trunk for preventing the `connection of said trunk to a called line at either end and for transmitting a tone signal to each of the calling subscribers.

10. In a telephone system, subscribers lines, a two-way trunk, means comprising a selector switch for connecting a calling line to said trunk, means comprising another selector switch for connecting said trunk to a called line, means responsive to seizure of said trunk for marking said trunk busy at both ends, means responsive to the release of the connection by the calling subscriber for terminating the marking of said trunk as busy and for causing the return of said selector switches to normal, means for delaying the return of the switch through which the calling line is connected to said trunk until a suificient interval of time has elapsed for said switch through which the trunk is connected to the called line to have been restored to normal, and means responsive to seizure of said trunk at the end connected to said called line for temporarily interrupting the marking of the trunk as busy at the end connected to said calling line if the last-mentioned seizure occurs prior to the release of the switch through which said calling line is connected to the trunk.

11. In a telephone system, subscribers lines, a two-way trunk, means comprising a selector switch for connecting a calling line to one end of said trunk, like means for connecting a calling line to the other end of said trunk, means comprising a selector switch for connecting said one end of the trunk to a called line, like means for connecting the other end of the trunk to a called line, means responsive to seizure of said trunk for marking it busy at both ends, means responsive to release of the connection by the calling subscriber for terminating the marking of said trunk as busy and for causing the return to normal of the selector switches through which the trunk is connected to the calling and called lines, means for delaying the return of the one of the last-mentioned switches which connects the calling line to the trunk until a suflicient interval of time has elapsed for the return to normal of the other one of the last-mentioned switches, and means responsive to seizure of said trunk at the end connected to said called line for temporarily interrupting the marking of the trunk as busy at the end connected to said calling line if the last-mentioned seizure occurs prior to the release of the switch through which said calling line is connected to the trunk.

HENRY HOVLAND.

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