US2327877A - Telephone call diverting system - Google Patents

Description

Aug. 24, 1943.

TELEPHONE CALL DIVERTING SYSTEM Filed March 31, 1942 CON/V.

E. L. ERWIIN 2,327,877

2 Sheets-Sheet 1 INVENTOR ELERWl/V 8V v ATTOR/VEV Aug.24,1 943. 'E. L. ERWIN 2,327,877

TELEPHONE CALL DIVERTING SYSTEM Filed March 31, 1942 2 Sheets-Sheet 2 x lNl EN TOR I By ELERWl/V ATTORNEY Patented Aug. 24, 1943 TELEPHONE CALL DIVERTING SYSTEM Edson L. Erwin, Towaco, N. J., assignor to Bell Telephone Laboratories, Incorporated, New

York, N Y., a. corporation of New York Application March 31, 1942, Serial No. 437,007

9 Claims.

This invention relates to dial private branch exchange telephone systems in which automatic switches are employed for establishing connections and more particularly wherein automatic switches at an automatic central ofiice are con-' trolled over a trunk circuit by a subscriber of th private branch exchange. An object-oi the inventionis" to prevent subscribers of the private branch exchange from establishing, by dialing, connections over the trunk to certain points in the central office, as for example a toll operator and'to automatically divert such calls to' an attendant at the private branch" exchange-and'more specifically, the ob-- ject is to prevent the calling subscriber from circumventing such a diversion by irregular manipulation of his dial or switchhook to obtain an unauthorized connection.

Systems have heretofore been employed in which an attempt by subscribers in a dial private branch exchange to obtain a connection to a toll operator results in' the transfer of the calling subscribers line fromthetrunk to an attendant at the private branch exchange which is effected by an arrangement employing an auxiliary autor'natic "switch associated with the trunk at the private branch exchange which switch is actuated. by the dialing pulses transmitted to the trunk, in such a manner that when a certain code is'transmitted, zero for example, thecalling subscribers line is disconnected from the trunk and connected to an attendants position at the private branch exchange. Such a system isdisclosed and claimed in United States Patent switches may take such a position as to permit the undesired connection, While the auxiliary switch having a different setting does not recognize this fact and hence does not cause the call to be transferred to the attendant. I

Further it has been known, as disclosed, for example, in United States Patent 2,040,291, issued .May 12, 1936, to F. A. Bonomi, to prevent a private branch exchange subscriber from obtaining unauthorized toll service which is accomplished by a current reversal back over the trunl; to the private branch exchange which reversal is initiated by the response of the toll operator, which reversalis efiective to cause the calling line to be "transferred to the attendant.

A feature of the present invention resides in means whereby if the pulses transmitted to the trunk occur at a rate per second above a predetermined maximum (12 for example) every other pulse willbe masked or absorbed and only alter: nate pulses will be appliedto the auxiliary switch and transmitted to the centra11 office switches, which alternate pulses will occur at such' a slow rate that both switches should operate in sub-, stantial synchronism and although such pulses Will result in a wrong number? setting of the central ofiice switches, if suchsettingeffects a connection to one of the points to which the calling subscriber is not entitled, the auxiliary switch will be standing in a' corresponding'position and will effect the desired transfer ofthe calling line to the attendant;

Another feature'resides in locating the auxiliary switch and its associated equipmentand circuits 'at the central ofiice end of the trunk instead of at the private branch exchange, whereby the possibility of non-synchronous. operation of the auxiliary and central ofiice switches is re-' duced.

Another feature, whereby the transfer of the calling line to the attendant is effected, resides in means controlled by the auxiliary switchto reverse the polarity of the central ofiice battery supplied to the private branch exchange over the trunkr 1 Another feature resides in means for eiiecting a so-called' interdigit time control whereby if the time interval between successive digits dialed is less than a predetermined minimum (.6 second, for example) a current reversal over the trunk will be effected and the calling line transferredto the attendant.

A further feature'resides in means for correcting the pulses transmitted over the trunk and for repeating them to control both the auxiliary automatic switch and the central ofiice switches. The operation of the circuits and arrangement of the invention embodying the foregoing features, will be understood from the following description and accompanying drawings in which:

that portion of Fig. 1 to the left of the brokenline X represents a dial private branch exchange including a subscribers stationA, automatic switching equipment including i a selector corinect'orSC, an attendants position 0', a cenconnector SC and means including a polarized relay P in series with the ring conductor of the trunk, efiective when operated in one direction, 1. e., by a current reversal, to transfer the-line of the subscriber A from the trunk RT to the attendants position, and that portion of Fig. l to the right of the broken line XX, and Fig. 2, represents the equipment associated with trunk RT at the central ofiice end thereof.

Referring to the drawings it will be assumed that it is not desired to permit the subscribers of the private branch exchange to dial certain joints through the central ofiice, a toll operator.

for example, over the dial trunks connecting the private branch exchange with the central office,

while at the same time permitting the subscribers to dial local stations of the central office without hindrance and therefore the trunks to the central ofiice are arranged in accordance with the presentinvention, i. e., are arranged to. permit calls tolocal stations ofthe central ofiice b'ut automatically cause a current reversal back over the trunk to cause the. calling station line to be disconnected from the trunk and transferred to the attendant s switchboard when thelstation dials certain codes; such forex'ample as 0, 110, or 211, which codes are frequently used for calling; a toll operator. It will further'be assumed that the subscriber at station Adialslthe designated code to'select a central office trunk andthen dials the designated. code foraitoll operato'r, which may'beifiz, 110, or 211.

When the subscriber removeshis receiver and actuateshis dial lin accordance with the predetermined trunk-code, the automatic equipment at the privatebranch exchange .(not shown) causes the brushes of the selector connector SC to engage thexbank' contacts to. which central office trunk RT is connected, whereupon ground is temporarily connected to. the ring conductor I of the trunk in a manner described, in line 35 et seq. on page 2 of United States Patent 1,868,326 to G. V. King,'issued- July- '1'9, 1932; whereupon relay 2 at the distant end .of the: trunk operates in the followingcircuit, ring-fconductor of the trunk; lower left winding ofrepeatingcoil 3, contacts4of relay 5', contacts 6' of relay 1-, andwinding of relay 2 to battery. Relay 2 operates and completes a circuit including contacts 8 of relay 1: cooperate. relay 9:which relay, at its contacts l0, connects ground: to the sleeve conductor of the central onic'e connector CON; locks, at its contacts I2 under control of relay l3,-and closes at its contacts [4 a circuit to' operate polarized relay-l5 over its; upper winding, which circuit includes resistance. is, of the; order .015 10,00, ohms.

Relay l5 operatesv and; closes its contacts to operate relay ll. which relay in operating closes ate'sin series with the high resistance?! thereby closing its contacts 28 to operaterelay 1, opens the locking circuit of relay 3,-and by closing its contacts 29'locks relay 2; operated. Relay 9, however, remains operated over its-initial circuit.

Operation. of" relayl first. closes, t contacts 30 r tral office trunk RT accessible to the selector and 3| to connect tip and ring conductors of the trunk from the private branch exchange, to

ground and battery respectively in series with the lower and upper windings of relay l5. Relay 1 continues to attract its outer armature thus opening its contacts 6 to interrupt the original energizing circuit for relay 2, closes its contacts 32 to operate relay 33,

and opens itsv contacts 8 to release relay 9. a 7

loop from the private branch exchange and responds to the dial pulses.

When relay ll operated in response to the initial operation of relay l5, it closed, the trunk loop to the central ofllce over contacts 2|. and 23 of relays 22 and 2 4,r'e-

spectively, and when the line finder found the line thus-causing. the operation of relay 1, this operationof relayl'5, conductor 38,'contacts=39. oiv a relay 35', contacts of relay M, and winding of relay 3.5,to battery. Relay35 in operating closes its contacts 42 thereby completing a locking cir-. cuit for itself independent of contacts 3110i relay llzwhich :circuit can be traced from battery,

windingof relay 35,-contacts' 40 of relay M, contacts 42 of relay 315;, and contacts 34 of relay 1 to ground.

Relay 35 also closes its contacts thereby shortcircuiting, contacts, |;8 .,'.of relay I! to maintain closure of the loop. over contacts 2| and 23 of relays Z-Zjand 24, respectively. On the -first in'-. terruptionof the trunk loop at the dial contacts of the. calling;station, relay-l5 releases therebyreleasing relay 1'! which; closes its contacts, 44 to operate relay 45, the circuit for. which canbe ace ro ro nd cont s 3 f rela c nuc con acts 4. rot-r y -,-c nta s of relay 41 contacts 4,80? relay 49,- and'winding-of relay 4.5 to b tt ry; --Rc ay in o eratin locks at its contacts 50 under control ofrelay 4 9. Clor f nta s :Qt e1a 5 p r t u l rl n c o ur f nta sjfi "of this. same re (45 closes, a circuittofcperate relay 54 which can be traced from ground,-outer left contacts of relay 2, contacts 55-.of relay 1, contacts 560! relay 4|, conductor 51, contacts 53 ofirelay 45, and winding of relay 541to battery;- -'Closure of contacts 58 of relay 24; and contacts 5,9 or relay; 5.4 close parallel operating paths for relay. 60.

Closure ofcontacts 6,! ofrelay 54 completes-"a;

' circuit to operate relay 4'!- which can be traced from ground, contacts 61 of relay54; conductor 62 contacts 63- of.'. relay 4|, conductor '64, and.

winding of relay 41' to batterywhi'ch latter relay closes its contacts 65 to lock itselfunder control x i Further, I closure of contacts'iit'ofrelay 54 -cz aguses-opera of the back contacts- 44' of v relay 'll.

tion of steppinglmag net E1 of auxiliary switch" which circuit 'can be tracedfrom ground on conductor 57', over contacts 6,6 cf relayi 54, :to'batte'ry in. series 'withmagnet G ll" Switch lit-is of the typewhich stepson-the release of its rotary 'magmal position at this time;

Relay E l in operating opens net. and therefore .it, doesnot moye'irom its norclosure'efiected whenrelay lloperated, but, operits". contacts 23 thereby introducing a, break in the original loop relay 22 and therefore the operation of'relay 24 is ineffective to-open the loop.

When relay 49 operates, however, it opens its contacts 48 to deenergize the slow-to-release relay 45. When relay 45 finally releases it opens its contacts 69 thereby opening the loop to the central oflice thereby causing a stepping movement of the switches thereat. Prior to the release of relay 45, relay 49 is held operated over contacts of relay 45 which relay (49) closes its left-hand contact thereby short-circuiting relay 22 and insuring its released condition butwhen relay 45 operates this short circuit is removed and relay 22 operates in series with relay 49 in a circuit from ground on conductor 35, contacts 19 of relay I1, winding of relay. 22, lefthand contacts and windingof relay 49 to battery. Relay 49 remains operated in this circuit.

The release of relay 45 opens the circuits of relays 54 and 24 which latter relay, however, is slow to release. Relay 54 in releasingopens its contacts E5 to deenergize the rotary magnet 51 thereby causing the switch 68 to move into its first position. Relay 24 in releasing recloses its contacts 23 but due to its slow-to-release characteristic this does not occur until relay 22 has operated whereby the pulsing loo-p to the central ofnce remains open at contacts 2! of relay 22.

It willnow be noted that auxiliary switch 68 and the centralofiice selector switch have both moved in substantial synchronism into their first positions. When relay I1 reoperates at the end of the first pulse, relays 22, 49 and 41 release and prepare the circuit for the next pulse, 'i. e., the release of relay H which release reoperates relay 45. As before mentioned relays 45 and 24 are slow in releasing and under normal pulsing, i. e., below l2 pulses per second, relay 24 is not fully released to close its contacts 23 until relays 22 and 49 are released by the operation of relay ll. Therefore relay 24 controls the length. of the pulse intervalat normal speeds. When relay ll releases for the next pulse relay 45 reoperates as before provided relay 4'! has released'thereby again closing the loop at its contacts 69. If, however, relay 41 has not released when the contacts 44 of relay I! close relay 41 remains locked and the circuit of relay 45 is open at con tacts 45 of relay 4'! and therefore relay-45 does not operate to close the loop and therefore acts to insure a minimum interval during which the loop is open, at dial speed which is slightly higher than normal.

The arrangement just described is substantially similar to the impulse correcting arrangement disclosed in United States Patent 2,249,854 issued July 22, 1941, to T. C. Rehm, in which relays I3, [4 and i5 of both figures, and relay H of Fig. 2, respectively correspond to relays 45, 24, 49 and 22 of the present application.

Cancellation of alternate pulses If the pulses occur at a considerably faster than normal rate, i. e., l2 or more pulses per second, relay 24 which is slow to release will not release until after relay 45 is released and therefore relay 45 controls the length of the closed loop interval.

At this speed relay 4'! will not release between the successive openings of contacts 44 of relay I! and hence only alternate pulses will be transmitted to the central office and to the switch 68 which will occur at such a slow rate that there will be no question of these switches operating Interdz'gz't time control When relay fifl operated as before described it closed its contacts H to operate relay 12 which in turn operates relay .13. An alternate energizing circuit for this latter relay was also closed when relay 22 operated. Relay 5G is slow to release and therefore holds operated between pulses.

At the end of the first digit if the interval between the last pulse of the first digit and the first pulse of the second digit is greater than a predetermined interval (six-tenths of a second, for example) relay 5!] will release due to the release of relays 24 and 54 and opens the locking circuit of relay 12 which is independent of relay l3. Relay 12 will therefore release and in turn release relay 13 provided relay 22 has released. Relay 13, however, is also slow to release and if the nextor second digit is'dialed too soon and relay 60 reoperates before relay l3 had fully released, a circuit is completed to operate reversing re1ay 5 in a circuit as follows: ground, contacts ll of relay '52, contacts '35 of relay l3, conductor l6 and right-hand winding of relay 5 to battery. Relay 5 in operating opens its contacts 4 and 26 and closes contacts Ti and 18 thereby reversing the polarity of the battery supplied'back over the trunk RT to the private branch exchange where it causes operation of polarized relay P in the ring conductor of the trunk. Operation of relay P operates relay 19 which disconnects the line of the calling subscriber from the trunk RT and connects it 'to the attendants position 0 in the manner shown in United States Patent 2,040,291 to F. A; Bonomi, issued May 12, 1936. The attendant thereupon talks with the subscriber to ascertain what number he hasdialed. It will therefore be noted that relays 54, 5! l2 and 13 are effective to divert the calling line to the private branch exchange attendant if the interval between successive digits is less than a predetermined minimum. 7

First digit 1 When the selector 5%! has stepped to the first terminal as the result of a single pulse and relay 60 releases, ground is connected at contacts 89 of relay 65 to operate the release relay 8! of the switch to restore it to normal. This circuit can be traced from ground, contacts 82 of relay 35, conductor 33, contacts of relay 55, brush and the first terminal of switch 58, contacts 84 of relay 85, contacts 86 of the off-normal switch ON and winding of the release magnet 8! to battery thereby releasing the switch 68 and restoring it to normal.

The disclosure of the present invention shows an arrangement for preventing the subscriber from Obtaining a connection to an operator at the central oflice whose code may be 110, 211, or 0, depending on the type of central ofice in which the trunk terminates and we will now assume that the subscriber has dialed the code which, if not interfered with, will call in a toll operator, access to which operator the subscriber is not entitled.

Dialing of the first digit 1 as before described causes the-switch 68 to advance one step whereupon the release ma'gnet operates to restore the switch and thuscancelling the digit. The second digit 1 again advances the switch 58 to the first .terminal whereupon the switch is. released ant returned .to. normal as before; The third dig-it consisting oiiten'pulsescauses: the switch 68 to advance, step. by step,- to the tenth terminal whereupon when relay 60 releases, ground on conductor 83 is connected over contactstll of relay Eli, the brushes 68, at its tenth terminal, contacts 81 of relay 85,-and conductor 16 to battery in series with the reversing relay 5 -to cause the calling line to be disconnected from the trunk at the private branch exchange and transferred to the attendant as before described. Operation of relay 19"at the private branch exchange causes the trunk loop to be opened whereupon av disconnect is given to the central ofiice equipment.

When the central ofiice switches restore the div verting equipment restores to normal.

First digit 2 If we assume that the central ofiice equipment is arranged to call in a toll operator in response to the receipt of the code 211 and that the subscriber dials this code, the first digit-,2 will cause the switchesto advance to the second terminal and the release of relay 6!) will supply ground over thes'econd terminal of the switch and contacts 88 of relay 8'5 to operaterelay Bit-which relay in operating closes its contacts 90 to operate the release magnet 8| in a circuit which can be traced from ground, contacts 98 of relay 89, contacts 9| of relay'85 and contacts 36 of the offnormal switch ON to the release magnet. Operation of'therelease magnet restores the switch 58 to normal. Operation of relay 89 also closes its contacts 92 to lock itself to ground under control of relay 35. Relay 8!! also closes its contacts 93 to operate relay 85 which is thereafter also held under control of relay '35. When the second digit '1 is dialed the selector 68 advances as before to the firstterminaland when relay Bll'releases a circuit is completed to operate the reverse relay 5 over contacts 94 of relay 85, now operating. Operation of relay 5 causes disconnection of the calling line from the trunk and its transfer to the attendant as before.

I First digit 0 The third code which the present invention is arranged to intercept is zeroor ten pulses and if this digit is first dialed, the result will be the same .as in the case when code 110 wasdialed, f

First digit 2 and second digit other than 1 In case a subscriber dials a code in which the firstdigit is 2 and the second digit some other number than 1, relays 89 and 85 will be operated at the completion of the first digit and the release of relay 6!! will connect a ground over the brush of switch 68 and whichever terminal it is standing on, other than 1, to operate relay 4| when relay, I3 is released which occurs a short interval after relay releases. opens at its contacts 40 the energizing circuit for relay 35 which releases and closes its contacts 95 to connect ground over conductor 96 to operate the release magnet and restore the selector switch Relay 4| holds over its right winding and contacts 91. to ground at contacts 34 of relay 1. Relay 4l operated also opensits contacts 56 to prevent operation of relay 54 and any further move- Operation of relay All ment of the selector switch'68 by subsequent'pulsing which, however, will be repeated, in the man'- 'n'er previously described, in response to the subscribers dial to control the central officeswitches.

First digit other than 1,2 or 0 If the subscriber dials for. the first digit of a code any number from 3 to 9, which will advance switch 68 to a corresponding terminal, which ter- 1 minals it will be noted are connected together, the release of relay Ell will cause relay 4| to operate and relay 35 to release thus releasing switch 68 and disabling it from further movement in re sponse to further dialing.

Disconnect The release of relay 1 releasesrelay 5, if operated, and reconnects' relay 2' to the ring conductor of the trunk. Relay 1 in releasing reconnects ground at its contacts 98 to the tip of the trunk over the outer right contacts of relay 33 which relay operated when relay 1 operated.

Release ofrelay I also opens its contacts 32- thereby deenergizing relay 33'but this relay is slow to release and holds ground on the'tip of the trunk for a short interval after contacts 3I of relay 1 open.

When relay 33 finally opens its outer right contacts ground is removed from the tip of the trunk causing the equipment at the private branch exchange to return to normal in a wellknown manner.

From the foregoing description it will be observed that battery will be reversed back over the trunk to'the private branch exchange to call in the attendant and disconnect the calling line from the trunk if thefirst cligit dialedis zero, if the code dialed is or 211, or if, due to manipulation of the dial or switchhookjtrains. of pulses are transmitted, which trains are separatedfrom each other by less than a predetermined time interval, six-tenths of asecond in the arrangement disclosed.

Further, it will be noted that if pulses are transmitted which recur at a rate faster than the central ofiice switch can-accurately follow,

i. e., twelve or more per second, alternate pulses are masked or absorbed and only every other pulse is permitted to control the auxiliary trunk.

switch and the central office equipment.

In other words, the auxiliary switch associated with the trunk follows and registers one or more of the first three digits dialed into the central ofiice. When the first digit registered on the auxiliary switch does not form a part of. a recognized toll code, i. e., 3 to 9, inclusive, further dialing is ignored and the call goes' through in a normal manner. If the first digit dialed.is:0, or 0 is preceded by 1, the call is diverted to the attendant. Further, if the first digit is 2 andthe second digit is 1 the call will'also be diverted.

Further, the pulses transmitted to the central oflice are corrected and if the rate of recurrence.

is greater than the ability of thecentral ofilce switches to correctly register them every other 'pulse will be absorbed and therefore the pulses applied'to the auxiliary switch, and transmitted to the central oflice, will be slow enough so that both the auxiliary switch and the central ofiice switches can operate in synchronism. Obviously, such alternate pulsing will result in an improper setting of the central office switches but if a code starts with 0, 10, 21, or 110 thecalling line will be diverted to the attendant and further control the central ofiice equipment prevented, if the dial at the station calling is retarded in such a manner that the digit dialed is split into two trains of impulses which are too closely spaced, i. e., if the digits are dialed so as to succeed each other too rapidly, the'call is likewise diverted. i

It will, of course, be obvious that, by different connections of the terminals of auxiliary switch 68, operation of reversing relay can be eifected in response to other digit pulses, either singly or in combination.

What is claimed is:

1. In a telephone system, an automatic exchange, an operators position thereat, a central office, a trunk line extending from said exchange to said central office and terminating in automatic switches thereat, a subscribers line terminating in said exchange adapted to transmit groups of dial impulses to extend said line to said trunk and to control said automatic switches at the central office, means in said trunk at the exchange responsive to a reversal of current over said trunk to disconnect said line from the trunk and connect it to the operators position, and other means in said trunk efiective in response to a predetermined minimum time interval between successive groups of dial impulses transmitted to cause a current reversal to be transmitted over said trunk to said exchange.

In a telephone exchange system, an automatic exchange, an operators position thereat, a central oiiice, a trunk line extending from said exchange to said central cities and terminating in automatic switches thereat, a subscribers line terminating in said exchange adapted to transmit groups of dial impulses to extend said line to said trunk and thereafter to control said automatic switches at the central office, means in said trunk for repeating said impulses to said central office switch, an auxiliary switch associated with and responsive to said repeating means adapted to advance in step with said central ofiice switches, means in said trunk at said exchange responsive to a reversal of current over the trunk to disconnect said line from the trunk and connect it to the operators position, and means responsive to a predetermined setting of said auxiliary switch for reversing current over said trunk, said repeating means being arranged to suppress alternate group impulses when said impulses recur faster than a predetermined rate whereby simultaneous advance of said central office and auxiliary switches is insured.

3. In a telephone exchange system, an automatic exchange, an automatic central oifice, a trunk line therebetween, a subscribers line terminating at said exchange, a subscribers station dial adapted to transmit groups of impulses over said line to connect the line to the trunk and to thereafter transmit impulses over the trunk to the central ofiice, automatic switches at the central cifice, pulse repeating means including a source of direct current associated with said trunk at the central office and adapted to control said central office switches in accordance with the pulses transmitted over the trunk, and an auxiliary stepv-by-step' switch at the central oflice also controlled by said repeating means and arranged to move in substantial synchronism with said central office switches, and means responsive to a predetermined setting of said auxiliary switch by said repeating means for reversing the direction of said direct'current source over said trunk to the exchange. I

4. In a telephone exchange system, an automatic. private branch exchange, an automatic central office, automatic switches thereat, a trunk line therebetween, a subscribers line terminating at said exchange, means for transmitting impulses over said line to connect the line to the trunk and to thereafter transmitgroups of impulses vover the trunk, pulse repeating means including a source of direct current associated with said trunk at the central office and adapted to control said automatic switches in accordance with the pulses transmitted, an auxiliary step-by-step switch at the central oflflce also controlled by said repeating means and arranged to move in substantial synchronism with said central oflice switches and to return to normal at the end of each group of pulses, and means responsive to predetermined first and second settings of said auxiliary switch in response to the first two groups of impulses transmitted to reverse the connection of said current source to the trunk.

. 5. In a telephone exchange system, an automatic private branch exchange, an automatic central ofiice, automatic switches thereat, a trunk line therebetween, a subscriberfs line terminating at said exchange, means for transmitting impulses over said line to connect the line to the trunk, and to thereafter transmit groups of impulses over the trunk, pulses repeating means including a source of direct current associated with said trunk at the central office adapted to control said automatic switches in accordance with the pulses transmitted, an auxiliary step-by-step switch at the central office also controlled by said repeating means and arranged to move in substantial synchronism with the central office switches and to return to normal at the end of each group of pulses, and means responsive to predetermined first, second and third settings of said auxiliary switch in response to the first three groups of impulses transmitted to reverse the connection of said current source.

6. In a pulse repeater and corrector, an incoming line, an outgoing line, a relay associated with said incoming line responsive to impulse trains transmitted thereover, means responsive to an initial operation of said relay for closing said outgoing line, means for opening said outgoing line eifective a predetermined time interval after the release of said relay to open said outgoing line, other means responsive to a succeeding reoperation of said relay to close said outgoing line and means efiective when the succeeding rate of operation and release of said relay is above a predetermined maximum to prevent every othersuccessive release of said relay from opening said outgoing line.

7. In a'pulse repeater and correotor, an incoming line, an outgoing line, a relay associated with said incoming line operated and released in accordance with current of impulses transmitted thereover, means responsive to operation of said relay to close said outgoing line, means responsive to release of said relay to open said outgoing line, and means responsive to the successive release and operation of said relay at a rate faster than a predetermined maximum to hold said outgoing line closed over every other release interval of said relay.

8. In an impulse repeater, an incoming line, an outgoing line, an impulse receiving'relay associated with said incoming line, means for repeating to said outgoing line trains of impulses transmitted over said incoming line and received by said receiving relay, said repeating means comprising a sloW-to-release relay for determining the minimum duration of said repeated impulses when and if the rate of the received impulses is lower than a first critical rate, and also comprising a fast-operating relay and a. slowcperating relay for determining the minimum duration of said repeated impulses when and if the rateof the received impulsesis equal to or in excess of said critical rate, and other means comprising a second slow-release. relay for preventing the repetition of every other received impulseif and when the received impulses are equal to or in excess of a; second predetermined critical rate higher than-said first critical rate.-

9. In a telephonesystem, an automatic exchange, an operators positionthereat, a. central oflice, a trunk line extending to said exchange from said central ofiice and terminating in auto.- matic switches thereat; a subscriber's line terminating in said exchange adapted to transmit groups of dial impulses to extend said line to said trunk and to control, said automatic switches at the central ofiice, means in saidtrunkvat the exchange responsiveto a reversal of current over lease relay, effective in response to a predetermined minimum time. interval between successive groups of dial impulses transmitted to cause a. current reversal to be transmittedover said trunk to said exchange.

EDSON'L. ERWIN.

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