US2406053A - Telephone system - Google Patents

Description

4. Sheets-Sheet 1 INVENTOR By I L. WRIHT 0H9 Gm ATTORNEY P.- L. WRIGHT Aug. 2%, 1946.

Aug. 2%, 343. wRlGHT ZAQ, TELEPHONE SYSTEM Filed Sept. 2, 1944 4 Sheets-Sheet 2 INVENTOR P. L. WRIGHT I By $42M A T TORNE V MAI/V PRIVATE BRANCH EXCHANGE Aug. 20, 1946. v v p, wm 2,4@6,053

TELEPHONE SYSTEM Filed-se ta, 1944 4 Sheets-Sheetfi INVENTOR RAM RIGHT- A TTORNEY- Aug. 20, 1946. 'P. L. WRIGHT TELEPHONE SYSTEM Filed Sept; 2,1944 4 Sheets-Sheet 4 mkommmfih w #3 mekwmumo a INVENTOR .T m Ma Ma w. M m T A Patented Aug. 20, 1946 TELEPHONE SYSTEM Paul L. Wright, Newark, N. J., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 2, 1944, Serial No. 552,403

Claims.

This invention relates to telephone systems and particularly to systems comprising automatic switching apparatus selectively controlled by subscribers and operators to establish desired connections.

Objects of the invention are the provision and improvement of transmission control in semiautomatic telephone systems and particularly the provision of a trunk circuit arranged for decreasing the attenuation of speech currents transmitted therethrough on connections which include high loss lines or trunks.

According to this invention, a trunk circuit,

employed in establishing connections between an operators position and an automatic branch exchange, includes a pad control relay connected to a talking conductor for operation by ringing current when the ringing key of a connected cord is operated, the impedance of the connection from the talking conductor through the pad control relay being sufiicient to prevent interference with voice current transmission through the trunk circuit and to prevent interference with the transmission of dial impulses. According to a feature of the invention, a pad control relay is connected through a high impedance device to one of the talking conductors of a trunk circuit, the operation of the relay responsive to the connection of a ringing current source thereto being effective to decrease the attenuation of the voice currents transmitted through the trunk circuit, to disconnect the relay irom the talking conductor, and to close a holding circuit independent of the talking conductor.

The invention and its features will be better understood by considering. a system in Which the invention is embodied, one such system being shown in the drawings which form a part of this specification. The invention is, however, not limited in its application to the specific system shown but is generally applicable to systems comprising means including resistance pads for controlling the attenuation of a connection.

Referring to the drawings, Fig. 1 represents an automatic branch exchange comprising a subscribers station A, line LI and line circuit LCI, a line-finder LFI, a selector-connector SCI, an incoming connector ICI and a trunk circuit TCI; and 7 Figs. 2 and 3 and 4 show in a second ofiice a trunk circuit TCZ, jack ended at an operators position, a telephone repeater RI individual to the trunk circuit, and a cord circuit CDI at the operator position.

The subscribers station A is of the usual common battery type and includes a dial or other impulse sender for. controlling the selective operation of the automatic switches through which desired connections are established. The line-finder LFI and selector-connector SCI and incoming connector ICI, may be of the usual two-motion step-by-step type. Reference may be had to Automatic Telephony, 2nd Edition by Smith and Campbell, pages 53 to for description of the structure of such switches andtheir operation when used as selectors and connectors. Reference may be had to the patent to Stokely No. 1,799,654, granted April 7, 1931, for a description of a line-finder switch of the stepby-step two-motion type. Reference may be had to the patent to A. E. Hague No. 1,675,886, July 3,1928, for a description ofa selector-connector of the step-by-step two-motion type. The linefinder LFI and selector-connector SCI and incoming connector ICI are each represented in the drawings by a set of brushes and either one or two sets of terminals, all other apparatus associated therewith being omitted. The line LI is connected to terminals IS in the bank of finder LFI, to terminals I8 in the bank of selector-connector SCI and to terminals 20 in the bankof the incoming connector ICI. The line circuit LC'I, while not shown in detail, comprises the usual line and cut-off relays. The trunk circuit TCI connected to terminals. I9 in the bank of selector-connector SCI in the automatic branch ofiice is associated with a two-conductor trunk line TI extending to the second ofiioe. In the second ofiice, trunk TI is connected to a trunk circuit T02 having a jack at an operators position. The trunk circuits TCI and T02 are shown in detail in the drawings and their operations in establishing connections in both directions over the trunk TI are hereinafter described. The telephone repeater is not shown in detail since such a showing is not necessary; and only so much of the operators cord CDI is shown as is necessary for a clear and complete explanation of applicants invention.

Assume now that a call is initiated at the subscribers station A, that the line circuit LCI and a line-finder LFI are operated to extend the line LI to a selector-connector SCI,- and that the selector SCI is operated under the control of the dial at the calling station to select the terminals It! to which the trunk circuit TCI is connected. Upon seizure of the terminals I9, the selectonconnector SCI temporarily connects ground to brush I1 and therethrough to sleeveconductor O 23 of trunk circuit 'ICl, thereby effecting the operation of relay 24. Relay 24 locks through its inner lower front contact to conductor 23 independent of relay 31, and connects conductor 23 to the winding of relay 26. With relays 24 and 25 operated, a circuit is closed for operating relay 3!, this circuit being traced from battery through the upper winding of'relay 3!, upper continuity back contact and outer lower back contact of relay 32, uppermost front contact of relay 26, lower right winding of repeating coil 25, inner upper front contact of relay 26, conductor 22, brush N3 of switch SCI, middle brush of finder switch LFl, line LI and station loop,

upperbrush of finder switch LFl, brush l of switch SCI, conductor 2|, outer upperfront contact of relay 26, upper right winding of repeating coil 25, inner upper front contact of relay 2'4, outer upper back contact and lower continuity back contact of relay 32, and lower winding of relay St to ground. The-operation of relay 3| closes a circuit for operating relay 38 and closes a-circuit foroperating relay 5| of the trunk circuit T62 atthe distant end of trunk TI. The operation of relay 38 oftrunk circuit TCI closes a-circuit for operating relay 31, and relay 31 connects ground to sleeveconductor 23 to hold relays M -and 25 operated and to holdthe selector connector SCi andlline finder LF! operated 'after the ground temporarily connected to brush ll of the selector-connector SCI is disconnected therefrom, The circuit for operating relay-5| is traced from battery through the upper winding-ofrelay-5l, l.ower back contact of relay 50,'1ower conductor of trunk Tl and conductor- 42 of trunk circuit 'I'Cl, lowermost front contact-of relay 26, middle lower front contact of relay fil, outer lower continuity backcontact of relay-28, innerlower front contact of relay 26, lower left winding of repeating coil 25, middle of relay and through the lower winding of relayr5l to ground. Relay 36 is polarized and the currentthrough itslower winding is in the non-operating direction so that relay 36 is not operatedat this time. The operation of relay 5| oft-trunk circuit TC2 closes a circuit for operating' relay 84. Theoperation of relay 84 connects ground to the sleeve of multiple jacks 95 as albusy indication, closes a circuit for lighting the busy lamps 91, and closes a circuit including the inner lower front contact of relay 5! foroperatively energizing the winding of relay 6 2. The operation of relay 62 closes a circuit including the lower backcontact of relay 82 "for lighting the-trunk lamp 96; and disconnects ground from the lower outer transfer spring of relay 53 to prevent the operation of eitherof relays 54 and 58. 'At its upper front contact, relay 52 closes a circuit including the outer lower front contact of relay '5! for operating-relay 50. The operation of relay 50 closes a locking circuit through its outer lower front contact to ground at the inner-lower front contact of relay 84, relay 62 beinglocked through its upper front'contact to-the same ground. Relay 50 disconnectsthe conductors of trunk Ti 4 from the windings of relay 5| and'connects these conductors, through front contacts of relay 62 and back contacts of relay 63, to the windings of line relay 5T. Relay 5| is thereby released; and relay 5! is operated in the circuit through the conductors of trunk Ti and the bridge across these conductors comprising the lower winding of relay 35 of trunk circuit TCi. Relay 84 is held operated, after relay 5! releases, under the control of relay 51. With relays 50, 52 and 84 operated, a connection is closed from the ringing current source 85, through condenser 81', lowermost front contact of relay 85, inner upper back contact of relay ,82, inner lower front contact of relay 52' and inner'lower front contact of relay 55, to the lower conductor of trunk Ti, thereby to transmit an audible ringing tone to the calling subscriber at station A.

The lighting of the trunk lamp 98 associated with jack 55 attracts an operators attention and indicates that there is a call waiting to be answered. Whenan operator answers-the call by inserting the plug ill of a cord CD! in jack 95, relay [5% is operated by the closure of a circuit from ground at the inner upper front contact of relay 8%, sleeve conductors of jack 95 and plug iili, inner lower back contact of relay N3 of cord CDi and the winding of relay I06, to battery. The operation of relay I55 closes a circuit from battery through the lower winding of retard coil I39, lower front contacts of relay I06, lower winding of relay I55, lower back contact of key H33, ring conductors of plug l8! and jack 95, conductor 92, windings of retard coil and relay 8!, a back contact of relay 53, conductor 9!, tip conductors of jack and plug llll, upper back contact of key 593, upper winding of relay 55, upper front contacts of relay IE5, through the middle winding of retard coil ms to ground. Relayfi i is thereby operated to close a circuit which includes the upper front contact of jack 95 for operating relay 53; Relay 53 looks to the ground at the contact of jack 95 independent of relay 8|, closes a circuit for holding relay 84, closes a circuit for holding relay 59, closes a circuit for operatively energizing the lower winding of relay 82, closes a circuitincluding the outer lower front contact of relay 5'! for operating relay 64, and interchanges the connections between the windings of relay E'I'and the conductors of trunk Tl so as to reverse the direction of current through the lower winding of polarized relay 36 of trunk circuit TC! and thereby cause'the operation of relay 35. The operationof relay 82 closes a looking circuit through the upper winding of this relay to battery at the lowermost front contact of relay 62, opens the circuit path for lighting the trunk lamps 96, and opens the connection for transmitting an audible ringing signal to the calling station. The operation of relay 64 opens the short-circuit normally connected across condoctors 65 and 65, which connect with the input side of one of the amplifiers of the telephone repeater, and closes a short-circuit across resistor 68 and condenser 69. The bridge, consisting of the winding of relay 8! and the windingsof retard coil 86, across tip and ring conductors of jack 95 is opened by the operation of relay 63 and again closed by the operation of relay 54. The continued closure of this bridge effects the operation of supervisory relay I05 of the cord thereby extinguishing the rear cord supervisory lamp I02. The aforementioned operation of relay 82 connects the winding of relay 52 to conductor 9!, whereby relay '52 is operatively energized in series with the upper Winding'of relay I05 and middlewinding of retard coil I09.

The operation of relay. 52 closes a circuit for operating relay 53; and the operation of relay 53 completes the talking'connection between the,

relay I05 of cord CDI, condensers, I01 and N33,.

back contacts of relay I I3, front contacts of talking key I I4, to the operators telephone set. The aforementioned operation of relay 36 oftrunk circuit TCI closes a circuit for operating relay 32 and-a circuit for operating relay 39. The operation of relay 32 interchanges the connection between the windings of relay 3| and the conductors of line LI thereby to reverse the current through the conductors of line LI. Relay 39 looks under control of relays 31 and 38 and connects the upper high resistance winding of-relay 36 in parallel with its low resistance winding.-

The answering operator may further extend the connection by plugging the other end of the cord into a jack of a called, extension, into the jack of another tie line or into the jack of a trunk to the central ofiice.

If the receiver hook at the calling station is operated to recall the operator, relay 3| of trunk circuit TCI is alternately released and reoperated, thereby alternately opening and closing the connection through the low resistance winding of relay 36. The operation of relay 36 is maintained by its upper, high resistance winding, but relay 5'! of trunk circuit TC2 releases and reoperates each time the low resistance bridge across the conductors of trunk TI is opened and closed by relay 3 I. The release and reoperation of relay 5? cause the release and reoperation of relay 64, whereby the bridge across conductors 9| and 92 is alternately opened and-closed to re-- lease and reoperate supervisory relay I05 of cord CDI and thereby flash the answering supervisory lamp I92. If the telephone instrument is replaced at the calling station, to release the connection, before the cord CDI is disconnected from jack 95, relay 3| of trunk circuit TCI, relays 51 and 64 of trunk circuit T02, and relay I05 of cord CDI are released in succession, thereby causing the answering cord supervisory lamp I92 to be lighted as-a disconnect signal. The continued release of relay 3| causes the release of relay 38;

but relay 31 is held operated until the answering operator disconnects, the circuit for holdin relay 3'! being traced through the back contact of relay 38, a front contact of relay 39 and the front contact of relay 35. When the operator disconnects plug IIlI from jack 95, relays 52 and 63 release; the release of relay 52 causes the release of relay 53; the release of relay 53 causes the release of relays 84, 62 and 59; and the release of relay 62 causes the release of relay 82. The release of relay 84 removes the busy indication from the sleeves of the trunk jacks95 and extinguishes the busy lamps 91. The aforementioned release of relay 53 also interchanges the connections between the windings of relay 5! and the conductors of trunk TI, thereby reversing the current through these conductors to effect the release of relay 36 of trunk circuit TCI. The release of relay 36 causes the release of relays 32 and 31; and the release of relay 31 causes the release of relays the busy lamps '91.

24, 26 and'39. whereby'the trunkcircuit TCI is again normal. 1

Assume next that an operator has answered a call with plug IIII of cord CDI, that the call is fora station in the dial branch exchange and that the operator inserts the plug I19 in jack to extend the connection over trunk TI and that the talking and dialing key I Id of cord CDI is in operated position. Relay 8| of trunk circuit T02 is thereby operated by the current in a circuit from battery through the lower winding of relay I I I, outer lower back contact of relay I I3, a front contact of talking key-I I4, conductor I54, a back contact of relay I5! of the operators position,

conductor I52, a front contact of key H4, lower back contact of ringing key II5, ring conductors of plug H9 and jack 95, conductor 92, windings of retard coil 89 and relay 8|, back contact of back contact of relay I51 of the operators position, conductor I53, a front contact of key IM', outer upper'back'contact of relay II3, upper winding'of relay I I I, and inner upper back contact of relay II3, to ground. The operation of relayBI closes the circuit, including the upper front contact of jack. 95, for operating relay 63. Relay 63 looks to ground at the upper front contact of jack 95, opens the circuit for operating relay 8|, and closes circuits for operating relays 8d, 59 and 82. The operation of relay 84 connects ground to th sleeve of jacks 95 and lights I conductor 9|, tip conductors of jack 95 and plug II 9, upper back contact of ringing key II5, a front contact of talking and dialing key II 4, conductor I5I, upper back contact of relay I51 of the operators position, conductor I53, another front contact of key II4, outer upper back contact of relay I I3, upper winding of relay I II and inner upper back contact of relay H3 to ground. The resistance of relay 52 is high so that the current in this circuit effects the operation of relay 52 without effecting the operation of relay III. The operation of relay 52 closes a circuit for operating relay 53. The operation of relay 53closes a circuit for operating relay 54, connects the conductors of trunk TI to conductors 55 and59 leading to the telephone repeater, and also bridges the winding of retard coil 60 and winding of relay 6| in series across these conductors to cause the operation of relay 3| of trunk circuit TCI in the dial private branch exchange. The circuit for operating relay 3| is traced from battery through its upper winding, upper continuity back contact of relay 32, outer'lower back contact of relay 32, middle lower back contact of relay 26, lower left winding of repeating coil 25, inner lower back contact of relay 2-6, conductor 42, lower conductor of trunk TI, inner lower front contact of relay 50, inner upper back contact of relay 62, lower continuity back contact of relay ,58, winding of retard coil 69, winding of relay 6|, upper continuity back contact of relay 58, upper front contact of relay 53, upper front The operation of relay 50 upperiback' contact of relay, outerrupp'er back." contact of relay 32, lower COIltiIIUitYZbaCkT con= tact ofzrelay 32; and lowers-winding of relay3I,

to'ground. The oper'ationofirelay 3.1 causes'the" successive operations .i'of relays .238" 3:1, the operation of-relay 3'! connecting. ground to conductor 23 to mark the trunk circuit TCI busy in the banks of selector-connectors havingaccess' thereto. The operation of relay 3| also closes a circuit for operating the line relay 45* ,of the incoming connector ICI; Thiscircuit is traced from battery through the right winding of relay 65, right back contact; of'relay II, conductor '43,

inner lower back contact of relayz i, inner front contact of relay. 3!, innerlower back contact of relayZB, outer upper back contact of relay 24,

upperright winding of repeating coil 25, middle upper back contact of relay 24, upper back contact ofrelay 28, lower winding of relay 36, outer. upper back contact of relay 25, lower right winding of repeating coil 25, inner upper back contact of relay 26, outer lower back contact of relay28,

middle lower front contact of relay 3|, lowermost back contact of relay 25, conductor, left back contact of relay 41, and left winding of relay,

to ground. The incoming connector ICI is thereby prepared for response to dial impulses.

The aforementioned operation of relay 5c closes circuits for maintaining the operation of relays 84 and 59, and closes a circuit from ground at the inner upper back contact of relay. 64, through a front contact of relay 5B, resistor fil, conductor 92, ring conductors of jack 95 and plug H8, lower back contact of ringing key H5, a front contact of key IM, conductor I52, lower back contact of relay 551 of the operators position, conductor I55, another front contact of key H4, lowermost back contact of relay H3 and through the lower winding of relay II I to battery. Resistor 61 limits the current to prevent operation offrelayfI I I. When the operator dials each of the digits of the number of the called station in the dial branch exchange, the ofi-normal contacts of dial I58 are actuated to eifect the opera-,

tion of relay I55 and to eiiect the operation of relay I51, the circuit for operating 'relay I57 not being shown. With relays I56 and I5! operated, the windings of relay I I I are disconnected from the'tip and ring conductors of plug II9so as to place relay :32 under the control of the impulse contacts of the dial E]; The circuit for operating relay 52 is now traced from battery through the winding of relay 52; outer upper front contact of relay 82, conductor BI, tip conductors'of jack 95 andplug I19, upper back contact of ringing key H5, a front contact of talking and dialing key H4, conductor I5I, impulse contacts of dial I56, front contact of relay I56, conductor I52, a front contact of key II I, aback contact of key H5, ring conductors of plug IIS and jack 95, conductor 92, resistor 81, a front contact of relay 5 and a back contact of .relay 64, to ground. Relay 52 is alternately released and reoperated responsive to each impulse cre- .ated by the operation of dial I50, the alternate release and reoperation of relay52 causing. a like release and reoperation of relay 53. The release of relay 53 in response to the first dial impulse of each digit closes a circuit for operating relay 50; 53 -andziit; a'ndrthe circuitfor energizing'the lower winding of relay 3| includes'the upper conductor of trunk TI and upper front contacts of relays 50; 53 and'58; Relay58 is slow-to-release and remainsoperateduntilrelay'53 has reoperatedafter the lastiirnpulseof a train; Relay SI of trunk circuit TCI 'is thereby released'and reoperated in response to 'the dial impulses created by' the dialing ofv the digits of the called'number in the dia1 private branch exchange with which connection isxdesired', The release of relay 3I, in response to the first dial impulse of each digit, opensthe bridge acrossv conductors Hand to release the "line relayL IE of connector 'ICI and closes a circuit from" ground at its lower back contact, through aback contact of relay-39 and a front contact ofrelay-38, for operating relay 28. Relay'28 is a slow-to-release relay which remains operated during the receiptof each train of impulses by relay 3i. Whenrelay 3| reoperates at'the end of the firstdial impulse, the windings'of line relay 45 ofthe connector are energized'in separate groundedcircuits, The circuit for-energizing the right Winding of'relay 45 includes conductor 43, inner'front contact of relay 3i and inner lower front contact of relay 28; and the circuit for energizing the left winding of relay 45' includes conductor 44, the middle lower front contact of relay SI and the outer lower front contact of relay 28. The release and reoperation of relay 3I responsive to the succeeding dial impulses ineach train causes the alternate release and' reoperation of relay 45. At the end 'of each of these trains of dial impulses relays 31 and 45' remain operated and relay 28 releases to connect the lower winding of relay 36in series with the windings of "relay 45; The response" of relay 45 to the dialing of the digits of the number of the calledstation operates connector ICI to select and test the terminals connected'to thecalled line; and," if the line is idle, ringing current is transmitted to the called station. When the called subscriber. answers, relay 3'! is operated, therebyefiectinga reversal of the current through the lower winding of' relay 36 to operatively-energize this winding. The operation of relay 36 closes a circuit for operating relay 3'2 and a, circuit for operating relay 39.

Relay 39 looks under'control of relays 31 and 38 and connects the upper winding of relay 36 in parallelwith its lower winding. The operation of relay 32 interchanges the connections between the windings of relay 3i and the conductors of trunk TI thereby to reverse the currentrthrougrh the winding of relay EI of trunk circuit TCZ to cause the operation of relay 6i, and relay GI.

closes. a circuit for. operating relay 64. The, operation of relay short-circuits resistor 68 and condenserfiil; opens the short circuit across conductors 65: and 65,- closes the talking circuit, opens the connection of conductor 92 to ground through resistor El 'toimprove the transmission balance, and connects thewinding of relay SI and retard coil 881' across the tip and ring conductors of the jack 95 and plug H9." When the talking and dialing key'of cord CDI is restoredto normal, relay 52 is held operated in series with the upper wind ing of relayI I I andrelay SI is operated in series to extinguish the" supervisory lamp IIB'. Relay" 8| is operated but performs no useful function atthistime.

If the subscriber at the called station alternately restores and removes the telephone, relay 4'! is alternately released and reoperated, thereby causing the alternate release and reoperation of relay 35. Relay 99 remains operated, but relay is alternately released and reoperated to reverse the current over the conductors of trunk TI, thereby effecting the alternate release and reoperation of relay 5| of trunk circuit T02. The alternate release and reoperation of relay 6| causes the alternate release and reoperation of relay 64, thereby to open and close the bridge across conductors 9I and 92, relay 52 being held operated during flashing in series with the upper winding of relay I II. The energization of the upper winding of relay I I I in series with relay 52 and the energization of the lower winding of relay I I I in series with resistor 6'! are ineffective to hold relay III operated and relay III is alternately released and reoperated under the control of relay 64, to flash supervisory lamp H8 in the cord circuit. When the telephone is restored to normal at the called station, relay 4'! releases and the reversal of the current through conductors 43 and 44 causes the release of relays 36 and 32 of trunk circuit TCI. The release of relay 32 reverses the current in the conductors of trunk TI to cause the successive release of relays BI and B4 of trunk circuit T02. The release of relay 64 opens the bridge across the'conductors 9i and 92 to cause the release of relay III to light the supervisory lamp H8. Relay 52 remains operated to ground in the cord circuit until the cord is disconnected from jack 95. When the operator disconnects plug I I9 from jack 95, relays 63, 52 and 8I release. The release of relay 63 causes the release of relay 82; and the release of relay 52 causes the release of relay 53. The release of relay 53 causes the release of relay 54 and opens the bridge across conductors of trunk TI thereby to release relay 3i of trunk circuit TCI. The release of relay 54 also causes the release of relay 84, whereby the busy indication is disconnected from the sleeve conductor of jacks 95 and the busy lamps 9'! are deenergized; and causes the release of relay 59 whereby the trunk circuit T02 is restored to normal. The release of relay 3| of trunk circuit TCI causes the successive release of relays 38, 31 and 39 and causes the release of relay 4'! of connector ICI. The release of relay 3? disconnects ground from conductor 23 to terminate the busy indication in the banks of the selectors having access to the trunk circuit TCI. The release of relay 41 causes the release of relay 46 and the connector is restored to normal in usual manner.

If on a call initiated by the operator to a called subscriber in the dial branch exchange, a busy condition is encountered, busy tone from the connector is transmittedthrough repeating coil 25, trunk TI and trunk circuit T02 to the calling operator. Since the called station has .not answered, relays GI and 64 are not operated so that the talking connection is open at the lower front contact of relay 64; but the condenser 69 and resistor 68 constitute a path suitable for the transmission of the busy tone from the conducthe dial private branch exchange is a toll call re- 10 quiringhigher grade transmission, or if *the' above described call to astation in the dialp'ri vate branch exchange is of such a character, the Dad control III relay of'trunk' circuit T02 is operated by application of ringing voltage toconductors BI and 92. If plug I M is-connected' toc jack ringing key I03 is operated and if plug H9 is connected to jack 95 ringing'key H5 is is bridged across conductors 9| and 92 and'th'e voltage applied to conductor 92, through the elements of gas-filled tube 83 and the winding of relay I0 causes the break-down of the tube and u the operative energiz'ation of the relay.- Relay 1 19 looks through conductor 94 to ground'at the upper contactof jack 95 as long asthe' plug of the cord connected to jack 95 at the same-time opening the connection through tube'83 to conductor 92. The operation of relay III short; i cuits the series resistors of pad PI and opens the bridge across these resistors to decrease the attenuation of the talking currents. While the specific arrangement showns'a gas-filled tube as being the high impedance element normally-con necting the winding of relay III to the talking conductor 92, a thermistor may be used-in place of the tube. What is claimed is:

1. In combination in a telephone system, a' trunk circuit including an attenuation pad comprising resistors normally connected inseries. with the talking conductors of said'trunk cir-H cuit, an operators cord connected, to said trunk circuit, a'pad control relay for short circuiting' -said resistors, means including a normally high impedance element connecting said relay to one of said talking conductors, a'direct current source connected to the talking conductors of said cord;

a ringing .current source, and a ringing key in said cord for connecting said ringing current source to the talking conductors of said cord in place of said direct currentsourcefthe voltage" of said direct current source being insumci'ent and the voltage of said ringing current source being suflicient to effect the operative energiz'ation of the winding of said'relay.

of .said direct current source, "the voltage of said direct current source being insuflicient and the voltage of said ringing current source'being sufficient to effect the operative energization of the winding of said relay. contacts actuated bythe operative energization ofthe Winding of saidre-1 lay for disconnecting the winding from said'one talking conductor, and means for locking said relay operated as long as said cord is connected to said trunk circuit.

3. In combination in a telephone system, a trunk circuit including an attenuation'pad com-' prising resistors normally connected in series With the talking conductors. of'sai'd trunk circuit, an operator's cord connected to said-trunk 'CllClll'G,

a pad'control relay for short circuiting said-ire: 1;

sisters, means including-a normally high impedance element connecting-saidqrelay toone of said talkingz condu'ctors, a direct current source connected 'tothe talking conductors of said cord, a ringing current source, a ringingkey in said cord forconnecting said ringing current source tomthe talkingconductors of. said cord in place of said directcurrent source,the voltage ofisaid directcurrentsource being insufficient and the voltage of said ringing current source being, sufficientto effect the operative energization of the winding: ofsaid relayrand means. rendered effectiveby the operation of said relay for maintaining 'thefshort-zcircuit across the resistors of said padswhileasaid cordis; connected to said trunk-:circuit;

475 11131 21 telephone system comprising subscribers lines; trunk-lines, andanoperators link fonuse in-establishing-rconnections between said lines, said trunkin'cluding a source of direct current, a: source of ringingcurrent and means for; connecting thesource' ofringing current to a connectedvline-in: place. of the source of direct current; a-trunki circuit individually terminating one of :said, trunk. lines, an: amplifying repeater insthe' talkingconnectionthrough said: trunk circuit, an attenuation pad connected in series with the talking conductors of said -trunk circuit, aerelay in said trunk circuit, a variable impedance element-connecting the winding of said relay to: a: talking conductor of said trunk circuit, saidelement" normally having a high impedance, theconn'ection'ofthez'direct current source in said link to the talking conductors of said trunk circuit being ineffective'to decrease the impedance of saidele'ment and the connection of the source of ringing current in said link to the talkingcon'ductors ofsaid trunk circuit being effective to decrease the impedance of said element 'to' effect the 'operationof said relay, and means includingcontacts' of said relay forrendering said pad'inefiective to. attenuate current transmitted throughithe talking conductors of' said trunk circuit;

In a telephone system comprising subscribers lines, trunk lines, and an operators link for usein establishing connections between said lines,'.said link including a source of direct cur rent, a source of'ringing current and means for connecting: thesource of ringing current to a connected line inplace of thesource of direct current;,a trunk circuit individually terminating one =ofsaid trunk lines, an amplifying repeater in the :talking :connection through said'trunk circuit, an attenuation pad. connected in series with the ttalk'ingconductors of said trunk circuit, a relay in said trunk circuit, a variable impedance element connecting the Winding of said relay to a talking conductor of said trunk circuit, said element normally having a highimpedance, the connection of the direct current source in said link to the talking conductors of said trunk circuit being ineffective to decrease'the impedance of said element and the connection of the source of ringingcurrent in said link to the talking con- ,ductors of' said trunk circuit'being effective to decrease theimpedance of said element to effect the. operation of said'relay, and means including contacts of said relay for rendering said pad ineffective to attenuate current transmitted through the-talking conductors of said trunk circuit and for disconnecting said relay from the talking conductor of said trunk circuit.

6. InaItelephone' system comprising subscribers 'li nes, trunk lines, and an operators link for 1'2 use in establishing connections between said lines, said link including a source of direct current, a source of ringing current and means for connectingithe source of ringing current to a connected line in place ofthe source of direct current, a trunk circuit'individually terminating one of said trunk lines, an amplifying repeater in the talking'connectionthrough said trunk circuit, an attenuation pad connected in series with the talkingconductors of said trunk circuit, a relay in said trunk circuit, a variable impedance element connecting the Winding of said relay to a talking conductor of said trunk circuit, said element normally having a high impedance, the connection of the direct current source in said link tov the talkingconductors of said trunk circuit being in.- eifective to decrease the impedance of said element and-the connection of the source of ringing current in said link to the talking conductors ofsaid trunk circuit being effective to decrease the impedance'of said element to effect the operation of said relay, means including contacts of said relay for rendering said. pad ineffective to attenuate-current transmitted through'the talkingconductor of said trunk circuit, and means for locking said relay operated as long as said link is connected to' said trunk circuit.

7. In a telephone system, a trunk circuit, an amplifying. repeater inv the talking connection through said trunk circuit, an attenuation pad includedin said talking connection, a relay in said trunk circuit, a gas-filled tube connecting the winding. of said-relay to a talking conductor of saidtrunk circuit, a source of direct current connected to said talking conductor during the existence ofa talking connection through said trunk circuit, the connection of said direct current source to said talking conductor being inefiective to energize said tube; a ringing current source and means for connectingsaid ringing source to said conductor in place of-said direct current source to effect the energization of said tube and operation of said relay, and meansincluding contacts of saidrelay for rendering said pad ineffective to. attenuate; current transmitted through said talking: connection.

8-. In a telephone system,. a trunk circuit. an amplifying repeater: in the talking connection through said trunk circuit, an attenuation pad included in said talking connection, a relay in said. trunk circuit, a. gas-filledtube connecting the winding of said relay to a talking conductor of said trunk circuit, the impedance of said tube being high enough, to prevent interference with thetransmission oftalking' current through the trunkcircuit', to prevent interference with the transmission of dial impulses and to prevent the operation of said relay While talking current is being transmitted through the trunk circuit, vmeans for applying a ringing voltage to said talking conductor to effect the breakdown of the tube and operation of said relay, and means including contacts'of said relay for disconnecting the winding of said relay from said tube and talking. conductor. and for rendering said pad ineffective to attenuate current transmitted through said talking connection.

9. In a telephone system, a trunk circuit, an amplifying repeater in the talking connection .through said trunk circuit, an attenuation pad included in said talking connection, a relay in said trunk circuit, a gas-filled tube connecting the winding of said relay to a talking conductor of said trunk circuit, the impedance of said tube being high enough to prevent interference with the transmission of talking current through the trunk circuit, to prevent interference with the transmission of dial impulses and to prevent the operation of said relay while talking current is being transmitted through the trunk circuit, means for applying a ringing voltage t said t 1 ing conductor to effect the breakdown of the tube and operation of said relay, means including contacts of said relay for rendering said pad inefiective to attenuate current transmitted through said talking connection, and means for locking said relay while said talking connection is maintained.

10. In a telephone system, a trunk circuit, an amplifying repeater in the talking connection through said trunk circuit, an attenuation pad included in said talking connection, a relay in said trunk circuit, a gas-filled tube connecting the winding of said relay to a talking conductor of said trunk circuit, the impedance of said tube being high enough to prevent interference with the transmission of talking current through the trunk circuit, to prevent interference with the transmission of dial impulses and to prevent the operation of said relay while talking current is being transmitted through the trunk circuit, means for applying a ringing voltage to said talking conductor to effect the breakdown of the tube and the operation of said relay, means including contacts of said relay for rendering said pad ineffective to attenuate current transmitted through said talking connection and for disconnecting the Winding of said relay from said tube and talking conductor, and means for locking said relay while said talking connection is maintained.

PAUL L. WRIGHT.

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