US2214245A - Telephone system - Google Patents

Description

Sept. 10, 1940. H. G. EvERs TELEPHONE SYSTEM FiledApril 28, 1939 ATTORN EY 5.

Patented Sept. 10, 1940 UNITED STATES -ATENT OFFICE rnmirnormY SYSTEM Harry G. Evers, Chicago, Ill., assignor to Associated Electric Laboratories, Inc., Chicago, Ill., a, corporation of Delaware Application April 28, 1939, Serial No. 270,540

7 Claims.

made for preventing desired connections, result- ,s vised for this purpose. the above-mentioned Jacobsen patent, the `cir- 55A The invention is illustrated in its embodiment ing from calls originating at the paystations, from being established until the correct coin deposits have been made at the calling stations Several different types of circuits have been de- In the arrangement of cuits are so arranged that a desired connection originating at a paystation may be routed to a desired called substation in response to operation of the calling device provided at the calling paystation, without the deposit of a coin in the coinreceiving means provided at the calling station. When, however, the call is answered at the call-ed substation, the connection is broken to prevent communication thereover and is not recompleted until the calling party makes the appropriate coin deposit. This mode of operation is achieved through the provision of paystation adapter circuits, one of which is interposed in the incoming trunk line of each of the first numerical switches, i. e., the first selector switches to which the paystation lines have access. More particularly, each of these circuits is provided with an electropolarized relay which is operative only in response to the reversal of battery over an established calling loop circuit routed by Way of the circuit. This polarized relay is arranged to open a point in the associated trunk line, thus interrupting the connection routed by way of the trunk line and to hold the connection open until a coin is deposited in the coin-receiving means of the call originating paystation.

It is an object of the present invention to provide an improved paystation line control circuit of the general character just described which is arranged for use in conjunction with the individual paystation lines of a telephone system.

It is a further and more specic object of the invention to provide an improved paystation control circuitof the character indicated wherein a unidirectional conductive device is utilized in conjunction with the connection interruptingrelay for the purpose of poling this relay to respond only to current of a predetermined polarity traversing the associated line.

(Cl. 1TB-6.3)

in an automatic telephone system which comprises a plurality of lines, one of which extends to a substation of the paystation type; together with automatic switching apparatus for establishing connections between the lines and including a group oi numerical switches having access to the paystation line. The automatic switching apparatus is operative, on a call originating at the paystation, to route a connection from the paystation line to a second or called one of the 1 lines, the connection as ultimately established comprising a iirsl; loop circuit which includes the rst or calling line and a second loop circuit which includes the second or called. line. In accordance with one feature of the pr'esent invention there is provided in association with the paystation line, a paystation control circuit which comprises a connection interrupting relay including an operating winding and having an operating circuit which parallels a portion of the rst loop circuit. This operating circuit has included therewith a unidirectional conductive de vice in the form of a copper oxide rectifier which functions to prevent the connection interrupting relay from operating when the rst loop circuit is initially completed. IThe indicated relay is -controllable over the called line to prevent the established connection from being used for communicating purposes until the proper coin deposit is made at the calling paystation. Circuit means comprising a plurality of additional relays controllable from the paystation are provided forv rendering the established connection operative for communicating purposes in response to the deposit of a coin in the coin-receiving means of thecalling paystation. One of the last-mentioned relays functions as a line switching relay and its operation is required in routing connections both to and from the paystation terminating the associated line. The arrangement is such that on calls incoming to the paystation line, this. line switching relay is caused to operate in response toseizure of the line by one of the iinal selector or -last numerical switches having access thereto.

Further features of the invention pertain to 4 the particular arrangement of the circuit elements whereby the above and additional operating features are obtained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to theV specification taken in connection with the accompanying drawing, the single ligure of which illustrates an automatic telephone system having embodied therein an improved paystation control circuit of the character briefly described above.

Referring to the drawing, the system there illustrated comprises a plurality of subscribers lines including the lines I and II extending, respectively, to the substations A and B, together with automatic switching apparatus including a pair of line switches I2 and i5, individual to the lines I0 and iI, a selector switch I3 and a connector switch i4 for setting up connections between the various lines. Preferably, each of the line switches is of the well-known rotary type and the selector and connector switches are of the conventional Strowger type employing vertical and rotary motion switching mechanisms for performing the required line selecting operations. The line switches I2, I5, etc., individual to the subscribers lines, are conventionally known as non-numerical switches for the reason that they are ineffective to perform any line selecting functions other than those of selecting idle ones of the selector switches. The selector and connector switches, on the other hand, are commonly known as numerical or directively controlled switches in that they are operative to perform particular line selecting functions in response to impulses transmitted thereto.

Any desired number of the subscribers lines extending to the automatic switching apparatus of the system may be arranged to serve substations of the paystation type. The lines extending to these substations are individually provided with paystation control circuits identical in arrangement with the circuit I6 shown in association with the line I0 extending to the paystation A. Briey described, this paystation control circuit comprises a line relay RIID, a slow-to-release line switching relay RI 50, a combined cutoff and line switching relay RIGU, and a'bridging relay RI I0 which includes an operating winding arranged to be bridged across the conductors of the line I0 in series with a unidirectional conductive device in the form of a copper oxide rectifier I33. The relay RIIO is arranged to control a line switching relay RI00 to interrupt an established connection routed from the substation A over the line I0 to a desired called substation when the call is answered at the called substation. For the purpose of recompleting the connection, in order that communication may be held thereover, there are provided three control relays RI20, RI30 and RI40, the second of which is of the marginal type and is controllable from the substation A to cause the restoration of the relays RI I0 and RI50 and the operation of the relay RI60, thereby to recomplete the established connection.

The apparatus provided at the substation A conventionally comprises coin-receiving means which includes a coin deposit chute, a fragment of which is indicated at I'I. Coins deposited in this chute are arranged to control the operation of a switching device I8 which is provided for the purpose of controlling the relay RI30 embodied in the paystation control circuit I6. In addition to the switching apparatus embodied in the line terminating exchange, there is provided the usual ringing current apparatus together with a paystation tone current source and an answer tone current source. Since the illustration of these elements is not essential to an understanding of the invention, they have been omitted from the drawing. In order more clearly to describe the operation of the paystation control circuit I6, a portion of the relay equipment embodied in the connector switch I4 has been illustrated in detail. More particularly, this 'equipment comprises a line relay R40 and a back bridge or battery reversing relay R30 which is arranged to operate in the usual manner when a call routed by way of the connector switch I4 is answered at the called substation.

Referring to the operation of the apparatus and considering first a call initiated at the substation A, when the calling party removes the receiver provided at this substation from its supporting hook or cradle, a loop circuit is completed over which the line relay RI'IQ is energized. This circuit extends from ground by way of the lower winding of RI'IU, RAUM, RAI58, RAIUS, the conductor I0b, the switch springs I9 and 20, the

' bridge across the conductors of the line I0 at the substation A, the conductor Illa, RAIOI, RAI52, RAISI and the upper winding of R110 to battery. When this circuit is completed, the relay Rill) operates to open, at RAIZ, a point in the circuit traced hereinafter for energizing the relay RI60, and to prepare at RAII, a circuit for energizing the slow-to-release relay RI50. At RAI'II the relay R110 also completes a path for impressing a paystation tone voltage upon the control conductor Ic, this path extending from one terminal of the tone current source by way of CMB, the condenser |45, RAI'II and RAIBZ to the conductor i0c. Completion of this path is without effect on local calls of the character under consideration, this path forming a link in a circuit for transmitting to an operators position the usual paystation identifying signal on toll calls originating at the station A. At RAI'I3, the relay RIlO completes an auxiliary loop circuit including the resistor H4 and extending to the line switch I2. The line switch I2 responds to the completion of the auxiliary loop circuit by operating to select an idle one of the selector switches to which it has access. Assuming, for example, that the selector switch I3 is the first available idle selector switch, when the wipers of the line Switch l2 encounter the trunk line extending to this selector switch, the auxiliary loop circuit is switched through thereto. As a result, the selector switch I3 is conditioned to respond to the rst series of impulses of the directory number designating the substation desired by the calling party. In addition, the relay equipment of the selector switch I3 operates in the usual manner to impress ground potential upon the control conductor of the trunk line incoming thereto, this ground potential being extended through the operated line switch I2 to the private conductor I0c of the line I0, thereby to mark the line I0 as busy in the bank contacts of the connector switches having access thereto. When ground potential is impressed upon the conductor I0c, the slow-to-release relay RI50 is energized over a circuit including this conductor, RAIGZ, RAIlI and the winding of RI 50. Upon operating, the relay RI50 completes, at RAISE, a circuit for energizing the lower winding of the relay RI30 in parallel with the lower winding of the relay RI I0, one branch of this circuit extending by way of RAMI andthe lower winding of RI30 to battery and the other branch extending by way of the resistor I I and the lower winding of Ri I0 to battery. At RAi55, the relay RI50 opens a further point in the above-mentioned circuit for energizing the relay RISE). At RAI54, the relay RI50 completes an obvious holding circuit for itself.

At RAI 53, the relay RI50 completes the operating circuit for the relay RI I0, this operating circuit paralleling a portion of the loop circuit extending to the selector switch I3. More particularly, the operating circuit for the relay RI I extends from ground through one of the windings of the line relay conventionally embodied in the selector switch i3 over one side of the established loop circuit, through the upper winding of RI I0 and by way of RAI I I, the rectier I33,'RAI53, the other side of the established loop circuit extending to the selector switch I3 and the other winding of the line relay embodied in this selector switch to battery. The rectier |33 included in this circuit is so poled that current is prevented from traversing the operating winding of the relay RI I0 at this time.

At its armatures RAIBI, RAI52, RAI`I and RAI58, the relay RI50 completes a loop circuit extending from the calling substation through to the selector switch I3 in an obvious manner. At RAI52 and RAI53 and their respective associa-ted resting contacts, the relay RI50 opens two points in the above-traced circuit for energizing the line relay RI '10, causing this line relay to restore. Upon restoring, the relay RI 'l0 opens, at RAI T3, a point in the above-described auxiliary loop circuit including the resistor Ilfi. At RAIII, the relay RI'ID opens a point in the operating circuit for the relay RI50. At RAI'i2, the relay RI'I0 prepares a point in the previously mentioned circuit for energizing the relay R I E0.

When the relay RI50 operates to complete the above-traced circuits for energizing the respective lower windings of the relays RI I0 and Rill, the relay RI30 operates to complete, at RAIBI, an obvious circuit for energizing the slow-acting relay RI20. At RAI32, the relay RI30 opens a point in another circuit, traced hereinafter, for energizing the relay RI. The relay RI 20, upon operating, opens, at RAI2I, a second point in the last-mentioned circuit. Due to the resistance of the resistor I I5 the ow of current over the abovetraced circuit for energizing the lower winding of the relay RI i0 is insuicient to cause this relay to operate in response to operation of the relay RI 50.

Following the operation of the relay Ri20, the switching apparatus is conditioned to respond to the impulses making up the digits of the directory number designating the desired called substation. Assuming, for example, that the substation B is the called substation and, further, that the selector switch I3 and the connector switch I4 are utilized in extending the connection to the called line I I, the selector switch I3 responds to the impulses of the rst digit dialed at the substation A by operating to select an idle one of the connector switches having access to the called line, which connector switch, in the present case, is assumed to be the switch I4. When this connector switch is selected by the switch i3, the calling loop circuit is extended thereto so that the line relay R40 is energized thereover. This line relay responds to the impulses of the second and third digits by causing the Strowger switching mechanism embodied in the connector switch i4 to operate to the position corresponding to the called line II. At the end of the third and nal digit, the called line is tested in the usual inanner to determine the idle or busy condition thereof and if found busy the usual busy tone signal is returned to the calling party. If, on the other hand, the line I I is idle at the time it is selected, the control apparatus of the connector switch Ill functions to impress ground potential upon the control conductor of this line for the purpose of marking the line as busy and to project ringing current over the line in order to energize the signal device provided at the called substation B.

When the call is answered at the called substation, a second loop circuit, including the called line I I, is completed between the substation B and the connector switch I4. The back bridge relay R30 is energized over this second loop circuit and operates its armatures RA3I to RA34, inclusive, to reverse the direction of current ilow over the calling loop circuit in the usual manner. When the direction of current flow over the calling loop circuit is reversed, the rectifier I 33 no longer blocks the passage of current through the upper winding, of the relay RI I0 with the result that this Winding is energized and the relay RI I0 operates. Upon operating, the relay RI I0 first completes at the X contacts associated with RAII2 an obvious path for short-circuiting the resistor I I5. Following the completion of this short-circuiting path, the current flow through the lower winding of the relay Rl I0 is sumcient to maintain this relay operated independently of its upper winding. At RAIII, the relay RII 0 opens a point in its operating circuit, thus clearing the established connection of the shunt impedance represented by the upper winding of the relay RI I0. At RAI I4, the relay RI I0 prepares a circuit, traced hereinafter, for impressing across the conductors of the line I0 the voltage of the answer tone source, not shown. At RAI I3, the relay RI I0 completes a circuit for energizing the relay RI 00, this circuit extending from ground at RAISG by way of RAI I3 and the winding of R900 to battery. Upon operating, the relay RI00 completes, at RAI02, an obvious holding circuit for itself. At RAIIJI and RAI03, the relay Ri00 opens two points in the established connection extending between the calling and called substations A and B, respectively, and completes the above-mentioned circuit for conducting answer tone current through the receiver provided at the calling substation A. More particularly, the path traversed by this current may be traced as extending from one terminal source of the answer tone current by way of CI05, the condenser |04, RAI I0, RAI03, the conductor I0b, the springs l0 and 20, the receiver at the calling substation, the conductor I 0a, RAI 0I, the resistor I0'I and the exchange battery to the opposite terminal of the answer tone current source. Reproduction of this current by the receiver provided at the calling substation serves to signal the calling party that the desired connection has been obtained. At RAIOI and RAI03, the relay Ri also completes a circuit for energizing the upper winding of the relay RI 30 in series with the winding of the relay RI40. This circuit extends from ground by Way of the adjustable resistor I 44, the winding of RI40, the upper winding of RI30, RAI03, the conductor Illb, the switch springs I9 and 20, the bridge across the line conductors at the substation A, the conductor Illa, RAI 0I and the resistor I0'I to battery. When energized over this circuit, the relay RI40 operates to open, at RAMI, the operating circuit for the relay RI30. Due to the energized condition of the upper winding` of RI 30, this relay remains in its operated position. At RAIIIZ, the relay RIM) prepares the previously mentioned second circuit for energizing the line switching relay RI 60. At RAI43 the relay RI40 recompletes the previously described auxiliary loop circuit, which circuit now extends through the switches I2 and I3 to the operated connector switch M. The purpose of recompleting this auxiliary loop circuit is to maintain the switches I2, I3 and M operated, it being pointed out that the calling loop circuit is now interrupted at the armatures RAIOI and RAIO3 of the operated relay RIOS. It is pointed out that by making the resistor M4 adjustable, the resistance of the circuit serially including the winding of RMO and the upper winding of RISO may be adjusted for positive response of the marginal relay RISO regardless of the resistance of the line lO or of another line with which the control circuit IS may be associated.

The calling party, upon hearing the answer tone signal, is required to deposit a coin in the coin deposit chute I1 of the coin-receiving means in order to cause the connection to be recompleted. Passage of this coin through the chute I1 serves to actuate the switching device I8 so that the springs I9 and 2I are closed and the springs I9 and 2O are opened. As a result, the resistor 22 is serially included in the above-traced circuit for energizing the upper winding of the relay RI3O in series with the winding of the relay RMO. Inclusion of the resistor 22 in this circuit serves to decrease the current flow thereover to a point such that the relay RISO is insufficiently energized to remain operated. Accordingly, the relay RISO restores to open, at RAI3I, the operating circuit for the relay RI2O and to prepare, at RAI32, a further point in the circuit for energizing the relay RISO. Thereafter, and when the relay RI 2O restores, the last-mentioned circuit is completed, this circuit extending from ground by way of RAISS, RAI2I, RAI32, RAM2 and the lower winding of RISO to battery. When energized over this circuit, the relay RISO operates to complete, at the X contacts associated with RAlSS, an obvious holding circuit for itself, this holding circuit including the grounded conductor IO@ of the line IO. At RAISI and RAISA, the relay RISO opens two further points in the previously traced loop circuit for energizing the line relay RI1O and prepares two points in an alternative loop circuit extending from the substation A to the connector switch M. At RAISZ, the relay RISO opens the holding circuit for the slow-to-release relay RISO, causing the last-mentioned relay to restore. Upon restoring, the relay RISO completes, at RAISS, a multiple holding circuit for the relay RISO and prepares, at RAI 52 and RAI S8, two additional points in the above-mentioned alternative loop circuit. At RAISI and RAIS1, the relay RISO opens two points in the initially completed loop circuit extending from the substation A to the switches I3 and M. At RAIS3, the relay RISO opens a further point in the previously traced operating circuit for the relay RIIO. At RAI54, the relay RISO opens an additional point in its own holding circuit. Finally, at RAISS, the relay RISO opens a point in the common portion of the operating and holding circuits for the relays RIOO, RIIO, RlZO and RISO. As a result, the relays RIOS and RI lO are caused to restore, the first-v mentioned thereof functioning to open, at RAIOI and RAIOS, two points in the above-traced circuit traversed by the answer tone current and two points in the above-traced circuit for ener'- gizing the upper winding of the relay RISO in series with the winding of the relay RMO. At these same armatures and their respective associated resting contacts, the relay RIOO completes the above-mentioned alternative loop circuit extending from the substation A to the connector switch M, thereby to complete the desired communicating or talking circuit between the calling and called substations. When the relay RMO restores in response to the release of the relay RI OO, it opens, at RAM3, the above-traced auxiliary loop circuit including the resistor |14, thereby to clear the established talking circuit of the impedance represented by this resistor. Following the release of the relay RMO, no further operation of the apparatus occurs until the connection is cleared out.

The manner in which the 'connection is released depends upon the circuit arrangement of the control apparatus embodied in the connector switch M. For example, this apparatus may be so arranged that the release of the connection is under the control of the last party to hang up. Alternatively, this apparatus may be so arranged that the release of the established connection is entirely under the control of the calling party. Assuming the latter type of arrangement, when the calling party replaces the receiver provided at the substation A upon its supporting hook or cradle, the calling loop circuit is interrupted with the result that the switches M, I3 and I2 are released in the order named and in a manner Well understood in the art. During the release of the connector switch M, the control apparatus of this switch functions to remove ground potential from the private conductor of the line II and to substitute battery potential thereon, thus identifying the line I I as idle in the bank contacts in the connector switch having access thereto. Similarly, when the switch train is released, ground potential is removed from the private conductor IOC and battery potential is impressed upon this conductor to mark the line IO as idle. When the conductor IOC is disconnected from ground, the relay RISO is deenergized and restores. Thus, the paystation control circuit IS is fully released and is conditioned for further use.

On a call incoming to the substation A, the line switching relay RISO is caused to operate to extend the desired connection to the called substation immediately the line iO is seized by the final selector' or connector switch through which the call is routed. For example, if `a cal] is routed from the substation B over the line II and through the line switch I5, the selector switch I3 and connector switch I4 to the line lO at a time when the last-mentioned line is idle, the control apparatus of the connector switch I4 functions to impress ground potential upon the private conductor IOC of the line IO shortly following the selection of this line. In response to this operation, the line switching relay Rl!) is energized over a circuit extending by way of the grounded conductor IOc, RAI'IZ, RAISS Ia-nd the upper winding of RISO to battery. Upon operating, the relay RISO first completes at the Y contacts associated with RAIS3, the previously traced holding circuit for itself. At RAIS2, the relay RISO opens a point in the previously traced operating and holding circuits for the relay RISO. At RAISI and RAiSlI, the relay RiSO opens two points in the previously traced operating circuit for the rela-y RI1O and extends a connection to the called substation A so that ringing current may be projected over the line EO for the purpose of energizing the signal device provided at the substation A. During the release of the connection, the relay RISO is deenergized and restores when the control apparatus of the connector switch M operates to re- 'IISu move ground potential from the control conductor Ille. i On calls' of the character just described, i. e., calls incoming to -the substation A, the relay RIBU is the'onlyvrelay which is affected, the remaining relays of the control circuit I6 serving no purpose in the completion of calls of this character. l v

While there has been described what isat present considered to beth'e preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is: contemplated to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is: A

1. In a telephone system, a subs-tation, a iirst line extending to said substation, a second line, automatic switching means for establishing a connection between said lines on a call originating at said substation, said connection comprising a rst loop circuit including said first line and a second loop circuit including said second line, a relay, an operating circuit for said relay, said operating circuit paralleling a portion of said rst loop circuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in response to completion of said first loop circuit, means operative to cause current to traverse said operating circuit in the proper direction to cause operation of said relay when said second loop circuit is completed, means responsive to operation of said relay for preventing communication over said connection, and means controllable from said substation for subsequently conditioning said connection for communication thereover.

2. In a telephone system, a substation including means for receiving coins or the like, a rst line extending to said substation, a second line, automatic switching means for establishing a connection between said lines on a call originating at said substation, said connection comprising a rst loop circuit including said rst line and a second loop circuit including said second line, a relay, an operating circuit for said relay, said operating circuit paralleling a portion of said first loopcircuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in response to completion of said rst loop circuit, means operative to cause current to traverse said operating circuit in the proper direction to cause operation of said relay when said second loop circuit is completed, means responsive to operation of said relay for preventing communication over said connection, and means operative in response to the deposit of a coin in said coin-receiving means for conditioning said connection for communication thereover.

3. In a telephone system, a pair of lines, means for establishing a connection between said lines, said connection comprising a rst loop circuit including the calling one of said lines and a second loop circuit including the called one of said lines, a relay, an operating circuit for said relay, said operating circuit paralleling a portion of said iii-st loop circuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in response to completion of said rst loop v circuit, means operative to cause current to traveise said operating circuit in the proper direction to cause operation of said relay when said second loop circuit is completed, means responsive toI operation offsaid relay for opening said first loop circuit, and means controllable over said calling line for recompletings'aid first loop cir- Cuit.

4. In atelephonelsystem` a substation including means for receiving Acoins or the like, a rst line extending to said substation, a second line, means for establishing a connection between said lines on a call originating at said substation,said connection comprising arst loop circuit including said first line and 'a' second loop circuit-including said second line, a relay, an operating circuit for `said relay, said operating circuit paralleling a portion of said-'first loop circuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in'response to completion of said'rst loop circuit, means operative to cause current to traverse said operating circuit in the proper direction to cause operation of said relay when Said second loop circuit is completed, means responsive to operation of said relay for opening said rst loop circuit, and means operative in response to the deposit of a coin in said coin-receiving means for recompleting said sec- 3nd loop circuit.

5. In an automatic telephone system, a pair of lines, means including impulse responsive switches for establishing a connection between said lines, said connection comprising a first loop circuit including the calling one of said lines and a second loop circuit including the called one of said lines, a relay, an operating circuit for said relay paralleling a portion of said first loop circuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in response to completion of said rst loop circuit, means operative to cause current to traverse said operating circuit in the proper direction to cause operation of said relay when said second loop circuit is completed, an auxiliary loop circuit paralleling a portion of said first loop circuit, means responsive to operation of said relay for opening said rst loop circuit and for completing said auxiliary loop circuit, and means controllable over said calling line for recompleting said flrst loop circuit and for interrupting said auxiliary loop circuit.

6. In an automatic telephone system, a substation, a first line extending to said substation, a second line, automatic switching apparatus including non-numerical switches and impulse responsive numerical switches, a line relay individual to said rst line, a loop circuit including said first line and the winding of said line relay, said loop circuit being completed in response to the initiation of a call atsaid substation, means responsive to operation of said line relay for extending said loop circuit through one of said non-numerical switches to one of said numerical switches, whereby a connection may be routed through said switches. ,to said second line, a second relay, an operating circuit for said second relay, said operating circuit paralleling said loop circuit as extended to said one of said nonnumerical switches, means comprising a unidirectional conductive device for preventing said second relay from operating when said operating circuit is initially completed, means controllable over said second line for causing current to traverse said operating circuit in the proper direction to cause operation of said second relay, means responsive to operation of said second relay for opening said loop circuit and for completing an auxiliary loop circuit extending through certain of said switches by way of which said connection is routed, and a relay controllable from said substation to open said auxiliary loop circuit and to recomplete the loop circuit extending through said train of switches, said relay being released in response to the release of said connection.

'7. In a telephone system, a pair of lines, means ior establishing a connection between said lines, said connection comprising a first loop circuit including the calling one of said lines and a second loop circuit including the called one of said lines, a relay, an operating circuit for said relay, said operating circuit paralleling a portion of said first loop circuit, means comprising a unidirectional conductive device included in said operating circuit for preventing said relay from operating in response to completion of said first loop circuit, means operative to cause current to traverse said operating circuit in the proper direction to cause operation of said relay when said second loop circuit is completed, means responsive to operation of said relay for opening said rst loop circuit, means including a line switching relay individual to said calling line and controllable thereover for recompleting said rst loop circuit, said line switching relay being released in response to the release of said connection, and a numerical switch having access to said calling line, said line switching relay also being operative in response to seizure of said calling line by said numerical switch.

HARRY G. EVERS.

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