US1720930A - Automatic telephone system - Google Patents

Description

July416, 1929. RF. STEHLIK AUTOMATIC TELEPHONE SYSTEII Filed Sept. 8, 1925 2 Sheets-Sheet l mm A i E M m rl ll L n SQ 2 Sheets-Sheet 2 R. F. STEHLIK AUTOMATIC TELEPHONE SYSTEM Filed Sept. 8, 1925 July 16, 1929.

ITLTEEIET' Ffud l h F. ETEhhk Patented July 16, 1929.

NKTED STATES PATENT OFFICE.

RUDOLPH F. STEI-ILIK, OF CHICAGO, ILLINOIS, ASSIG'NOB, BY MESNE ASSIGNMENTS,

TO AUTOMATIC ELECTRIC INC, OLE CHICAGO, ILLINOIS, A CORPORATION OF DELA- HARE.

AUTOMATIC TELEPHONE SYSTEM.

Application filed September 8, 1925. Serial No. 54,949.

This invention relates to telephone systems in general, but is more particularly concerned with automatic telephone systems in which a connection is established from one subscriber to another through the medium of automatic switches and inter-connecting trunk lines. The principal object of this invention, broadly stated, is the provision of new and improved automatic switches to be employed in establishing a connection bet-ween two subscribers.

Among the switches commonly used in automatic telephone systems are those which are responsive to one or more digits dialled by a subscriber to extend the connection by way of a trunk line to a succeeding switch. A switch of this type is ordinarily provided with a line relay for controlling the operation of the switch and a slow acting release relay which controls the release of the switch. The latter relay is energized by the line relay and due to the slow action characteristic holds up during momentary deenergization of the line relay while the switch is being operated, holds the release magnet circuit open, and maintains ground on the release trunk to provide a holding circuit for the preceding switch or switches previously operated. As soon as the switch has been operated and has extended the connection by way of a trunk to a succeeding switch, the circuit for the line relay is opened which in turn opens the circuit of the release relay. The release relay is sufficiently slow acting to maintain ground on the release trunk to hold up the connection until the release relay of the succeeding switch has been operated toin turn place ground on the release conductor.

Should a defective condition be present in the succeeding switch that would prevent its proper operation, or should any of the conductors of the trunk line extending to that switch be open, which would also prevent the switch from operating properly, no ground potential would be supplied to the release conductor when the release relay of the preceding switch deenergized. This would result in the release of all the preceding switches, and the connection would therefore be destroyed. The subscriber would be given no indication of what had taken place and would continue dialling. Obviously, he would not obtain connection with the desired party. After failing the first time, he might try again. There is a possibility that the same faulty switch or trunk would be selected a second time in the process of setting up the second connection, although he might obtain connection with the desired party by way of some other switches or trunks. However, each time the defective trunk or switch were selected by a preceding switch, failure in the establishment of the desired connection would result.

Obviously, such a condition is undesirable and should be called to the attention of the attendant in charge as quickly as possible so that he may correct the trouble. One of the methods proposed for doing this is to so arrange the switch circuits that the last switch that has been operated and has extended the connection to a trunk connected to a succeeding switch locks itself, and all the preceding switches, in an operated position, and is only unlocked when ground potential is extended back on the release conductor of the last selected trunk. If ground potential is not fed back on the release conductor, indicating failure of operation of the succeeding switch or a defective trunk line, an alarm is sounded. In response thereto, the attendant will immediately correct the trouble or busy the defective trunk or switch and also request the calling subscriber to try again.

The broad idea outlined above is not new, but, heretofore, the methods employed in carrying out this principle, when applied to selector switches, have had the disadvantage in that there existed a short interval during the normal operation of the switch during which a selector switch might become locked to a non-defective trunk extending to a nonfaulty succeeding selector switch. This interval existed during the time immediately after a certain selector had extended a connection to a succeeding selector and before the line relay and the release relay of a succeeding selector had had time to operate. Should the calling subscriber abandon the attempt to establish a connection and replace his receiver'during this interval, the selector last operated and all the preceding switches would become locked in an operated position.

A principal object of this invention, there fore, is the production of a new and improved selector switch which looks itself in an operated position on a defective trunk or a trunk to a defective switch and which eliminates the possibility of its becoming locked on a non-defective trunk extending to a non-defective switch regardless of when a calling subscriber replaces his receiver.

The application of the principle outlined above becomes especially advantageous when applied to primary line switches when sccondary line switches are employed. Ordinarily slow acting relay provided in the secondary line switch which supplies ground potential to the release conductor to hold the primary line switch in operation until the ground is supplied from the succeeding selector switch. lVith he application of this principle, however, the provision of such a relay in the secondary line switch becomes unnecessary.

iticcordingly, another object of this invention is the provision of new primary line switches of the plunger type which utilize the above described principle and which also have improved operating characteristics as will be described hereinafter.

A furth r object of this invention is the production of a new and improved secondary line switch of the rotary type, especially adapted for use in conjunction with the new primarv line switches and selector switch herein described. A special feature of this new secondary line switch is that only one relay is employed to control the operations of the switch.

These objects and novel features and other features not specifically mentioned will l more fully described in the following specification, reference being had to the acconr panying drawings comprising- Figs. 1-5, inelusive.

In Fig. 1, are shown by means of the usual circuit diagrams the switches with which this invention is concerned, employed in extend ing a connection from a calling subscriber to some other subscriber. The substation ll. is an ordinary automatic telephone substation having the usual talking instrumentalities and the calling device for controlling the operation of the automatic switches in the exchange. The line conductors from this suhstation terminate in the individual primary line SVlLCll PLS which is of the well known plunger type, modified in circuit arrangements. however, as will be described in do all in connection with its operation.

The master switch MS, indicated merely by a d tted rectangle, may be of the type own and described in the U. Patent to viorth. llo. 13.901, reissuet April 123, i015, modified to include the usual supervisory arrangement including a relay such as relay 18, which controls an alarm circuit.

The total number of primary line switches are divided into groups of switches, the line switches of each group having access at the prinriry line switch banlts to ten trunks. l lach trunk is connected to a rotary secondary l as SLS connected to the trunk at the bank contacts 2%26, inclusive. The rotary secondary line switches, in turn, have access in their banks to trunks connected to selector switches, such as S connected to the trunk accessible at the bank contacts ll-l6, inclusive. The rotary secondary line switches are of the well known type having no normal position and rotate in a forward direction only. The circuit arrsingen'ients of the rotary line switch shown are a particular feature of this invention and will be described subsequently.

T selector switch S is of the well known Strowger vertical and rotary type. The circuit arrangements are also modified in order to make the selector perform special functions as will be described hereinafter. A lamp, such as L, is provided. for each group of selectors. The alarm relay 91 may be common to all the selectors of the exchange. The utility of the lamp L and alarm relay will become evident from subsequent description.

The selectors, such as S, have access in their banks to other groups of selectors which in turn may have access to still other selectors, the final selectors having access to the connectors as is well known in the art. A sey the calling subscriber to select the proper group of succeeding selectors and also selects a trunk to an idle switch in that group. The connector switch selected by a final selector operates in response to the last two digits dialled by the calling subscriber and extends the connection to the called line, signals the called subscriber, and when the called sub scriber removes his receiver, functions to finally complete the talking connection.

In Fl 24), inclusive, are shown four modifications of the same general type of primary line switch shown and illustrated by the line witch PLS in Fig. 1.

Having described the objects of this invention and briefly described the apparatus involved in carrying them out, the operation of the equipment may now be explained more in detail. For this purpose it will be assumed that the subscriber at substation A desires to obtain a connection with some other subscriber in the exchange. lVhen the subscriber at substation it removes his receiver to initiate a call, a direct current bridge is placed across the line conductors 2 and 3. A circult is thereby closed for the lower winding of the line relay 5 which may be traced from ground by way of closed springs 9, line conductor 3, the bridge at substation A, line conductor 2, closed springs 8, through the lower winding of the line relay 5 to battery. Tl'ie line relay 5 operates and closes a circuit for its own upper winding in series with the pull down winding 6, and for its own middle winding and the bridge cut-off (abbreviated ECO) winding 7. These circuits may be CII ector operates in response to a digit dialled traced from ground through the winding of relay 18 of the master switch MS, upper winding of relay 5, working contact 12 and armature 10, through the pull-down winding 6 to the open main battery supplied at the master switch equipment; and also from the same ground, upper winding of relay 5, working contact 12 and armature 10, working contact 11, through the middle winding of the line relay 5, through the ECG winding 7 to battery. Both the pull-down winding 6 and the B winding 7 are energized and operate both the plunger arm 14 and the cutoff armature 13. The plunger arm 14, through the medium of the plunger (not shown), forces the bank springs 21, 22, and 23 into engagement with the bank contacts 24,25, and 26, inclusive, respectively, it being assumed that tl e plunger-s of this group of line switches, including line switch PLS, are standing opposite the trunk accessible at the bank contacts 24, 25, and 26, inclusive. The armature 13 disconnects the line conductors 2 and 3 from the normal ground connection and from the terminal of the lower winding of the line relay 5, respectively. The line relay 5 is held up, however, by its upper and middle windings, which are cumulatively wound, and the current flow through the upper winding is so heavy that the relay is positively held in spite of a possible momentary reversal of current in the middle winding due to potential being induced in the high resistance cut-oii' winding 7 when current builds up in the relatively low wound pulldown winding 6. The private normal conductor 4 which is connected to the test contacts in the connector banks, is placed sui'liciently below battery potential from ground by way of the working contact 12, arn'iature 10, working contact 11 through the middle winding of the line relay 5, to guard the line to substation A from being seized by a connector at this time. A secondary bridge is also placed across the line conductors 2 and 3 at armature 15 and the working contact. Supervisory relay 18, being in series with windings 6 and 7, operates at this time and closes an alarm circuit (not shown) at armature 19. Any common alarm arrangement which gives an alarm after its circut has been maintained closed for a predetermined period may be used.

It is now evident that the line relay 5 is locked in an operated position and cannot be unlocked until ground potential is extended back from the preceding switches to the bank contact 22. Should the sul scriber at substation A replace his receiver during the time after line relay 5 becomes locked in an operated position and before succeeding switches have been given time to operate to extend the ground potential back to unlock the line relay 5, this relay and the lineswitch PLS would be held locked to the trunk in an operated position were it not for the bridge placed across the conductors 2 and 3 at armature 15 and the working contact. This bridge eit'ects the continuation of the operations of the line switch PLS and succeeding switches to cause the line relay 5 to be unlocked even though the subscriber at substation A should replace his receiver at the critical time, as will become evident in succeeding descriptions.

As a result of the engagement of contacts 21, 22, and 23 with contacts 24, 25, and 26, respectively, a circuit is closed for the switching relay of the rotary secondary line switch SLS which may be traced from the grounded conductor 4, bank spring 22, bank contact 25, through the upper and lower windings of the switchin relay 30 in series to battery. A potential sutliciently below battery potential is placed on the master switch bank contact, individual to the trunk seized, by way of conductor 27 connected to this bank contact to operate the master switch MS in the well known manner. The plungers of all the remaining idle line switches controlled by the master switch MS are then advanced into position opposite the terminals of the next idle trunk. It may be mentioned that during the operation of-the master switch, the open-main battery is disconnected trom the conductor 28, thereby opening the circuit of the pull-down winding 6. The ECO winding is energized sufiiciently over the previously traced circuit to hold both the plunger arm 14 and cut-off armature 13 in an operated position.

Relay 3O energizes and begins to operate. Armature 33 of relay 30 is adjusted to make contact with its working contact before the armatures 32, 34, and 35 are operated. It may be assumed that the rotary line switch wipers 41, 42, and 43 are in engagement with the contacts of a busy trunk line. Ground potential is therefore on the private bank contact 47. soon as armature 33 makes contact with its working Contact, ground is extended from the bank contact 47, wiper 42, working contact and armature 33, armature 34 and the resting contact, to the junction of the upper and lower windings of the switching relay 30. The upper winding of the switching relay 30 is thereby short circuited and deenergized. The magnetic pull of the switching relay 3() is thereby reduced suiticiently to prevent the operation of the other armatures. The magnetic pull caused by the energization of the lower winding of the switching relay 30 only, is sutlicient, however, to keep the armature 33 in engagement with its working contact. A circuit is also closed at the same time from ground on the bank contact 47, wiper 42, working contact and armature 33, armature 34, and the resting contact, interrupter springs 36, through the winding of the motor magnet 31 to battery. Motor magnet 31 energizes, opens its own circuit at interrupter springs 36, and on oeenergizing rotates the switch wipers 41, 12, and 13 in engagement with the next set of bank contacts. Should the trunk terminating in tnis next set of bank contacts also be busy, the circuit for the motor magnet 31 will again be closed from ground on the private bank contact of that trunk. The motor magnet operates as before to rotate the switch wiper in engagement with the next set of bank contacts. In this manner, the switch wipers 41, 42, and 4L3, will be rotated step by step until they come into engagement with a set of bank contacts leading to an idle trunk line.

It will he assumed that the trunk line terminating in the bank contacts 4%, 45, and 46 is the first idle trunk encountered. An idle trunk is indicated by the absence of ground on its private conductor. Therefore, when the wiper 42 comes into engagement with the bank contact 15 no circuit will be closed for the motor magnet 31, and the wipers will be arrested in this position. The short circuit around the upper winding of the switching relay will also be removed, so that this relay will again be energized through its upper and lower windings in series over its 'nitial circuit. The armatures 32, 3 1, and will therefore be operated fully, extending the talking connection by way of the working contacts and armatures 32 and 35, wipers 41 and 4-3, bank contacts 14 and 16, armatures T1 and T l and their resting contacts, through the upper and lower windings of the line relay 51 of the selector S to battery and through the normally closed springs controlled by the springs to ground, respectively. Line relay 51 operates and closes a circuit which may be traced from ground through the slow-aeting relay 53 (the slow acting characteristic being indicated eonvcntionally by the two vertical lines drawn within the rectangle representing the core of the relay) through the release relay 52, working contact and armature 61, winding of the vertical magnet 59 to battery. Neither relay 53 nor the vertical magnet 59 are suf- I'iciently energized to operate at this time, due to the high resistance of the release relay 52. Release relay 52 operates and places ground potential on the release conductor connected to the bank contact 45 at armature 62 and the working contact by way of the normally closed spr ngs controlled by armature 72, thereby grounding conductor at and reversing the direction of current flow in the middle winding of the line relay 5 of the line switch PLS. The middle and upper windings of relay 5 now oppose each other, with the result that the relay deenergizes and removes the auxiliary bridge across the line conductors 2 and 3 at armature 15, and opens the circuit for its own upper and lower windings and for the pull-down winding 6, ineluding the circuit of the supervisory relay 18, at armature 10. By the removal of the auxiliary bridge across the line conductors, the connection is again under the control of the calling subscriber. The ground fed back on the release conductor from the selector S also furnishes the holding ground for the holding winding 7 of the line switch PLS and for the switching relay 30 of the line switch SLS to maintain these switches in an operated condition. The operation of armature 62 also prepares a circuit for the guard relay 55. This relay, however, does not operate at this time since its winding is short circuited by the springs controlled by armature 7 2. A further result of the operation of the release relay is the opening of a point in the circuit of the release magnet 58 at the resting contact of armature 64c and the preparation for the operating circuit of the vertical magnet 59 at the working contact of this armature.

The above operations all take place automatically responsive to the removal of the receiver at substation A, providing both the trunk extending to the secondary line switch SLS and the trunk to the selector S are not defective and providing the selector relays mentioned are in proper adjustment. Should either the trunk to the secondary line switch SLS or the trunk to the selector S be open, obviously, the operation of the line and release relays in the selector S would be prevented. Dirty contacts or other similar defects might also prevent the normal operation of these relays. In either case, no ground would be fed back on the release conductor and the line relay 5 of the line switch PLS would be maintained locked in an operated position and the circuit of the supervisory relay 18 would be maintained closed, the result being that the alarm circuit controlled by relay 18 would not open at the expiration of a predetermined interval, causing the sounding of an alarm. The attendant would respond and correct the trouble and also request the calling subscriber to redial the number. If the trouble were of a more serious nature, requiring considerable time to correct, the attendant would busy the trunk line and disconnect the line switch PLS from the trunk by forcibly separating the springs 11 and 12 from the armature 10 which would cause its release. The subscriber at substation A could then try again to obtain connection.

It may be assumed, however, that the connection has been extended to the selector S, that line relay 51 and release relay 52 are in an operated position, that the line relay 5 has been de-actuated by the ground on the release trunk which also provides the holding ground for the RC0 winding 7 and switching relay 30. The circuits are now in condition to receive the first digit in the desired number to be dialled by the subscriber. Accordingly, when the subscriber at substation A manipulates his calling device to dial the first digit, the circuit of the line relay 51 is opened a corresponding number of times. Each time line relay 51 deenergizes, it releases its armature 61 and allows the springs controlled by this armature to close. This operation short circuits the high resistance winding of the release relay 52. Both relays 53 and vertical magnet 59 are then energized over a circuit extending from ground through the winding of the relay 53, closed contacts controlled by the armature 61, resting contacts and armature 7 3, armature 64 and the working contact, through the winding of the vertical magnet 59 to battery. Each time the vertical magnet 59 energizes in response to a dccnergization of the line relay 51, the switch wipers 81, 82, and 83 are raised one step. The shortcircuiting of the release relay 52 each time the line relay 51 dcenergizes, gives this relay a slow acting characteristic so that it remains in an operated position throughout the vertical movement of the switch wipers. Relay 53 is also slow acting and also maintains itself in an operated position throughout the vertical movement. In operating, relay 53 closes a circuit for stepping relay 54: at armature 65, which may be traced from ground by way of the working contact and armature 65, interrupter springs 60, vertical off normal springs 77 which are closed upon J the first vertical step of the switch wipers,

through the winding of the relay 5a to battery. Stepping relay 54- operates and prepares the circuit for the rotary magnet 57 at armature 67, and closes a temporary locking circuit for itself at armature 66, at the same time placing a short circuit around the switching relay 56 to keep this-relay from operating at this time.

At the termination of the impulses produced by dialling the first digit of the desired number, the line relay 51 is again maintained in an operated position. Relay. 58 receives insuliicient current to maintain itself oper ated and allows its armature to fall back.-

In so doing, the springs controlled by armature 65 are closed, thereby completing the circuit for the rotary magnet 57 from ground by way of these springs, armature 67 and the workingcontacts, through the winding of the rotary magnet 57 to battery. Rotary magnet 57 operates to rotate the switch wipers into engagement with the first set of bank contacts in the selected level and also operates to open the interrupter springs 60, thereby opening the temporary locking circuit of the stepping relay 54. In deenergizing, relay 54 removes the short circuit around the winding of the switching relay 56 and armature 66, and opens the circuit of the rotary magnet 57 at armature 67 Rotary magnet 57 deenergizes and again closes the interrupter springs 60. If the trunk terminating in the first set of bank contacts is busy, ground will be present on the private bank contact, thereby closing another circuit for the stepping relay 5% from ground on this contact, wiper 82, armature 72 and the resting contact, resting contact and armature 65, interrupter springs 60, oil normal springs 77 through the winding of relay 54 to battery, at the same time maintaining the winding of relay 56'short circuited. Stepping relay 54 again operates to close the circuit of the rotary magnet 57 which operates to rotate the switch wipers into engagement with the next set of bank contacts. It the trunk terminating in the next set of bank contacts is also busy, the above described operation concernig the stepping relay and the rotary magnet again takes place. In this manner, the switch wipers 81, 82, and 83 are rotated step by step until they are brought into engagement with a set of blank contacts presenting an idle trunk, which is indicated by the absence of ground potential on the private bank contact. 5

It may be assumed that the first idle trunk line encountered is the one terminating in the bank contacts 845, 85, and 86. When the wiper 82 comes in engagement with the ungrounded bank contact 85, no circuit will be closed for the steppin relay 54, or, in turn, for the rotary magnet 57. The switch wipers will, therefore, be arrested in this position. The absence of ground on the private wioer 82 also removes the short circuit around the winding of the switching relay 56. Consequently the switching relay 56 is energized over a circuit which may be traced from ground by way of the working contact and armature 62, springs controlled by armature 72, normally closed rotary oft-normal springs 7 8, through the winding of the switching relay 56, interrupter springs 60, vertical ofl'normal springs 77, through the winding of relay 5% to battery. Relay 5% is not operated over this circuit because of the high resistance of the switching relay 56. Relay 56, however, operates and performs the following function: By the operation of armature 72, the normally closed springs controlled by this armature are opened, thereby removing the short circuit around the winding oi the guard relay 55 and, by so doing, inserting this winding in the previously traced circuit of the switching relay 56. The guard relay 55 energizes but is slow to pull up (indicated by the solid black section on the upper end of the rectangle representing the core of this armature) and does not operate immediately. The operating circuit of the vertical magnet 59 is opened at armature 73. And by the operation of armatures 71 and 74, the circuit of the line relay 51 is opened at the resting contacts of these armatures and the talking connection is ere tended by way of the working contacts of these armatures, wipers 81 and 83, bank contacts 84: and 86, to the windings of the line relay of the succeeding switch similar to line relay 51. Line relay 51 immediately deenergizes and opens the circuit of the release relay 52. Relay 52 being short circuited again, assumes a slow acting characteristic and maintains its armatures in an operated position for a short interval after its circuit is opened. Under normal operating conditions, the release relay 52 remains in an operated position, after the circuit of the line relay 51 is opened and the talking connection is extended to connect the succeeding switch, longer than is necessary for the operation of both the line relay and the release relay of the succeeding switch similar to relay 51 and relay 52, respectively, to take place. Furthermore, relay 55 is sufficiently slow-to-pullup so that under normal operating conditions sufiicient time elapses for the operation of the release relay of the succeeding switch before relay 55 has had time to operate, but this relay will operate fully before the release relay 52 drops back providing it is not again shortcircuited. By the operation of the succeeding switch however, which may be a selector similar to the selector S, ground is placed on the release conductor which is extended back from the succeeding switch by way of bank contact 85, wiper 82, arn'iature 72 and the working contact and again short circuits the relay 55 and prevents its operation. This ground also furnishes the holding ground for the switching relay 56 and also for the ECG winding 7 of the line switch PLS and the switching relay 30 of the lineswitch SLS. Shortly thereafter release. relay 52 deactuates and allows its armatures to fall back to normal.

The reason for relay 55 having the characteristics as described may now be appreciated. An ordinary quick-acting relay would function equally well except that, since it would operate as soon as relay 56 removed the short-circuit around its winding, the interval required between the dialling of consecutive digits would be increased by the length of time it would take the guard relay 55' to deactuate. By preventing relay 55 from operating under normal conditions, this increase of time is eliminated.

Impulses produced by the dialling of the second digit causes the operation of the succeeding switch in a manner identical to the manner in which the selector S was operated. This succeeding switch also operates in re sponse to impulses produced by the dial to raise its wipers opposite the level corresponding to the second digit and then rotate its wipers to select an idle trunk to another switch. Each succeeding switch has access in its banks to trunks to other switches, the number of switches to be operated to complete the connection depending on the size of the exchange. The final switch, in whose banks the line of the called substation termi nates, is a connector switch which operates in response to the last two digits dialled to select the called line, signal the called substation and when the subscriber at the called substation removes the receiver, operates to finally complete the talking connection.

The subscribers may now hold conversation as they desire. At the termination of the conversation both subscribers will replace their receivers on their respective switchhooks. Replacing the receiver at substation A causes the deenergization of certain relays in the connector switch thereby causing the removal of ground potential from the hold ing conductor extending through the switches associated with the connection. He-

moving ground from the holding conductor opens the circuit for the ECG winding 7, for the switching relay 30, and for the switching relay 56, as well as for the switching relays of the other selectors associated with the connection. The ECO winding 7 deenergizes, allowing the cut-off armature 13 and the plunger armature 14 to restore to normal. Opening the circuit of the switching relay 30 causes this relay to dcenergize and restore its armatures back to normal. Likewise, relay 56 deenergizes, restoring its armatures to normal thereby closing a circuit for the release magnet 58 at armature 73, which may be traced from ground through the winding of relay 53, springs controlled by armature 61, resting contact and armature '78, armature (A and the resting contact, Verticaloffnormal springs 76 which were closed upon the first vertical step of the switch wipers, through the winding of the release magnet 58 to battery. Release magnet 58 operates to displace the double-dog that holds the switch wipers in position, thereby allowing the switch wipers to restore to normal.

In the foregoing description normal operating condition has been assumed. In order to describe special features and functions of the selector switch S, it will be assumed that either the trunk line terminating in the bank contacts 84;, 85, and 86 is defective or that the switch terminating in this trunk line contains some defect which prevents its operation. In either case no ground potential will be extended back to the bank cont-act after relay 56 has operated to extend the connection to the switch to which this trunk is con nected. As briefly descibed in preceding paragraphs it is very desirable that the attention of the attendant be called to any such condition as soon as aossible and this is ac complished in the following manner: As stated above, the winding of the guard relay 55 is included in the circuit of the switching relay 56, as soon as relay 56 operates to extend the connection to the line relay of the succeeding switch, but is sufliciently slowto-pull-up to give the line and release relays of the succeeding switch time to operate before it itself can fully operate so that normally ground potential is extended back to again short-circuit relay 55. However, since ground potential is not extended back to the bank contact 85 because of the faulty condition assumed to be present either in the trunk line or the switch terminating the trunk line, the guard relay 55 will operate over its previously traced circuit, remembering that release relay 52 is still in an operated position. In operating relay 55 closes a looking circuit for itself at armature 68 in series with the switching relay 56, thereby locking this selector in an operated position. At armature 70, an alarm circuit is closed which may be traced from ground by way of armature and the working contact, through the signal lamp L, through the winding of the supervisory relay 91 to battery. Relay 91' operates and closes an alarm circuit at armature 92. The attention of the attendant is attracted by the alarm, and with the aid of the lampL, he may quickly locate the switch that is locked in an operated position, correct whatever trouble he may find, and also advise the calling subscriber to try again.

A bridge is also placed across the talking conductors at armature 69 and the working contact, the purpose of which may now be explained. It is evident that any condi tion that prevents ground from being extended back over the release conductor from the succeeding switch, after the operation of re lay 56, will cause the selector S to be locked on the corresponding trunk line. A certain interval of time exists after the switching relay 56 has operated and before both the line relay and the release relay of the succeeding switch operate to extend the ground back on the release conductor. hould the calling subscriber replace his receiver any time during this interval, the direct current bridge across the talking conductor would ordinarily be opened and therefore immediately prevent the line relay of the release relay from performing their normal operations,

and thereby prevent ground from being extended back over the release conductor. Again the relay 55 would function to lock the selector S in an operated position to a trunk to a succeeding switch, and the alarm would also be sounded. As soon as the attendant made an investigation he would find that no trouble existed and would release the selector S mechanically. Although not serious, this is an undesirable feature and the possibility of the selector locking itselfin an operating position on a non-defective trunk leading to an unfaulty switch is pre vented by the placing of the bridge across the talking conductors at armature 69 and its working contact. In this .case even though the subscriber replaced his receiver at the critical time, this bridge would cause the normal operation of the succeeding switch. Then as soon as ground was extended back over the release conductor relay 55 would be short circuited and would deenergize, releasing its armatures. The length of time that the alarm circuit would be closed at armature 70 would be too short to cause the alarm signal to be operated or the lamp L to be lighted so that this incident would pass unnoticed. As soon as the bridge is again opened by the release of armature 69, the succeeding switch and also the preceding switches, in this case being line switch PLS, the line switch SLS, and the selector S, would be released in the manner described above.

To complete the description of the operation of the selector S, the functions performed when all the trunks in a certain se lected level are busy, will now be described. For this purpose it will be assumed that the selector has been operated to raise its wipers opposite a certain level and has commenced its rotary movement in search for a trunk to an idlesucceeding switch. It will be further assumed that all the trunks on this level are busy. The selector wipers will therefore be rotated step-by-step testing each of the bank contacts for an idle trunk including thelast set in the switch banks and then be rotated past the last set of bank contacts. The regular Strowger step-by-step automatic switch assumed in this case has ten sets of bank contacts in each level. The switch wipers will therefore be rotated off the last set of bank contacts to the eleventh position. On the switch shaft a set ofcams is provided, namely cam- 79 and cam 80. When the switch shaft rotates to its eleventh position, the cams function to separate springs 78 and the springs controlled by spring 75. At spring 7 5, the normal ground connection to the lower winding of the switching relay 51 is disconnected and a ground upon which there is superimposed a busy tone from the busy machine B is sub-- stituted therefor. The busy tone which is then audible to. the calling subscriber indicates to him that all trunk lines to the particular group to which heis calling are busy.. r he opening of springs 7 8 prevents the switching relay 56 from operating when the switch wipers are rotated to the eleventh position, which otherwise would result, by reason of the absence of ground on the wiper 82.

Certain points in connection with special features of the separate switches described may now be briefly summarized. According to this invention, a plunger type line switch has been provided which locks itself in position to the selected trunk andis not unlocked until ground is extended back on the release conductor from the succeeding switch. This is accomplished by providing the line relay of the line switch with a holding winding. This winding energizes in series with the ECG winding and may therefore be of fairly high resistance. For this reason the unlocking operation is quite independent of the ordinary re. stance that might exist in the release conductor. A. rotary secondary lineswitch has also been provided being novel in that only one relay, a combined line and switchingrelay is required for controlling its operations. A selector switch has been provided which locks itself in an operated position upon detective trunks or trunks to detective switches, and eliminates the possi' bility of its being locked in an operated position upon a nondei'ective trunk leadingto a non-detective switch regardless of when a all in; subscriber might replace his receiver.

Fig. shows a niodilied form of primary line switch in which somewhat simpler circuit arrangement is used. hen the receiver is removed at the calling station a bridge is closed across conductors 202 and Q03, thereby energizing the line relay 205. Upon energizing, relay 205 closes a circuit tor the pull-down winding 206 at arn'iature Q and also, at this same armature, connects its own lower winding in series with the cutott winding The line switch now operates substantially as described in the case of the line switch shown in F 1, the connection being extended to a secondary line switch such as SL8, Fin. 1 and thence to a selector. The primary line switch is locked up due to the energization of its low r inding in series with the winding 107. When the line and release relays of the selector pull up, n'roun d is extended back on the release trunk conductor. with the result that the lower winding of the line relay is short circuited, and the relay is dcenergized.

The tendency for a reversal. of current to take place in the ECO winding 7 due to the build ing' up of current in the pull-down winding has been reduced to such an extentthat it does not seriously in tort-ere with the operation of the line relay This is accomplished by modifying the core structure and by suitably proportioning the windings 0 and 7. By increasing the number of turns in the winding 6 and by reducing the number of turns in the winding 7 the inductive effect may be considerably reduced without interierino; with the proper operating characteristics of these windings.

The lineswitch shown in Fig. 3. employs another method for preventing the line relay from being deenergized due to the tendency of the current being reversed in its holding winding. Briefly its operation as concerns this feature is as follows: The removal of the receiver at the calling substation places abridge across the conductors 802 and 303 thereby closing a circuit tor the upper winding of the line relay 305. Line relay 805 0pcrates and at armature 316 closes a circuit for the lower locking winding and the ECU winding; 307 in series. lhe current immediately begins to build up and attains practically its full value before the magnetic pull produced by the current 110W in the holding winding is sufiicient to operate the cut-oil armature 313. When the current has attained this value, armature 313 is operated and the circuit for the upper winding of the line relay 305 is opened at spring 309 and its restingcontact and at the working contact thereof a circuit for the pull-clown winding 306 is closed which may be traced by way of the spring 309 and its working contact, working contact and armature 310 through the pull-down winding 306 to the open main battery at the master switch equipment. Qurrcnt then begins to build up in the pulldown winding 306 and the same transformer action results between this winding and the ECO winding 307. However, since this does not take place until after the current in the RC0 winding 307 has practically attained its tull value, the tendency to actually reverse the current in this winding; and the lower winding of the line relay 305 with which it is in series is reduced to the extent that the opera tion of the line relay 305 is not etl'ected.

The lineswitch shown in Fig. 4 is another modification of this general type of lineswitch. In this case the removal of the receiver at the calling station closes circuit from ground through the lower winding of the line relay 405. closed springs 409. conductor 403, through the bridge at the calling station to conductor 402, closed springs 408, through the upper winding of the line relay 405, through the ECO winding 407 to the open main l attery at the master switch. The upper winding of the line relay 405 is 01" suiiiciently higl. resistance to prevent the opera tion oi? the arn'iature 413 due to the current flow in the RC0 winding 407 at this time. Line relay 40?) operates and closes another circuit for the ECU winding 407, including the lower winding of the line relay 405 and the working contact and armature 416 and at the same time closes a circuit for the pull down winding 400 at annature 410 and the working contact. Again the building up of current in the pull down winding 400 causes a transformer action which tends to produce a reversal of current in the ECU winding 40? and the lower winding: of relay 405 with which it is in series; but because of the initial current flow in the coil 40? and in the windings of the line relay 405 before the circuit of the pull-down winding 400 is closed, this effect is not sutlicient to produce a deactuation of the line relay 405.

In Fig. 5, is shown a lineswitch of this same general type which employs still another method for assuring the proper operation of its line relay. In this lineswitch, removal 0T:

the receiver at the calling substation closes a circuit. which may be traced from ground through the upper winding of line relay 505, springs 508, line conductor 503, bridge at the calling substation, line conductor 502, normally closed springs controlled by the spring 509, lower winding of the line relay 505, through the pull down winding 506 to the pen main battery at the master switch. Line relay 505 and the pull-down winding 506 energize; the line relay closes a circuit for the RC0 winding 507 at armature 516 and a holding circuit 'lorthe pull-down winding at armature 510 and the pull-down winding operates the plunger arm 51 1 and the cut-off armature 513, opens the initial circuit of the upper winding of the line relay 505 at springs 508, and short-circuits the lower winding of the line relay 505 at springs 509 and the working contact. In this arrangement no attempt is made to prevent the momentary reversal of current in the B00 winding 50? due to the building up 01 current to pulldown winding 500 but any such tendency is made ineffective due to the slow acting characteristic which is given to the line relay 505 by short-circuiting its lower winding.

Having described this invention. its operation and utilities and how the different objects are accomplished, what is considered new and is desired to have protected by Letters Patent is set forth in the appended claims.

IVhat is claimed is:

1. In a telephone system, an automatic switch, a trunk line, means for operating said switch to select and connect with said trunk line, a relay in said switch, means including said relay for testing'said trunk line for a defective or non-defective condition, means tor operating said relayafter said trunk has been tested providing said trunk is found de iect-ive, and means for preventing the operation of said relay it said trunk is found nondefective.

2. In a telephone system, an automatic switch, a trunk line. means for operating said switch to select and connect with said trunk line, a relay in said switch, means including said relay for testing said trunk line for a defective or a non-detective condition, a signal, means for operating said relay after said trunk has been tested providing said trunk is found defective to operate said signal, and means for preventing the operation of said relay if said trunk is found non-defective.

3. In a telephone system, a trunk line we tending to an automatic switch, a second switch and means for operating it to extend a calling line to said trunk line, a release trunk conductor included in said trunk line, means in said first switch controlled over the calling line for grounding said conductor, means for locking said second switch on the selected trunk only in case said first switch fails to ground said conductor, and means for automatically unlocking said second switch in case the calling party abandons the call.

4. In a telephone system, a switching device for selecting an idle trunk line of a group of trunk lines accessible thereto, a second trunk line terminating in said switching device, a single relay in said switching device having sole control of all the operations of said switching device, and means for controlling said relay over a non-talking conductor of said second trunk line to control said operations as set forth.

5. In a hunting switch, a stepping magnet, a double wound relay, a test wiper, an initial energizing circuit for said relay including both windings in series, contacts actuated when the relay is partially energized for connecting said test wiper to said magnet to operate the same to select an idle trunk, a branch of said magnet circuit for shunting one winding of said relay to prevent complete energization thereof until an idle trunk is found, and contacts on said relay actuated when the relay is fully energized for disconnecting said test wiper from said magnet.

6. In a telephone system, a switching de vice having wipers, a trunk line terminating in said device and normally disconnected from the wipers of said device, a relay and a magnet in said device, means for partially energizing said relay over a circuit including. a conductor of said trunk line to close a circuit for said magnet, and means for fully op' erating said relay to open the circuit of said magnet and connect said trunk line to said wipers.

7. In a telephone system, a switch having a line relay, means for operating said switch responsive to the energization of said line relay, a self-locking circuit for said line relay, and means responsive to the operation of said switch for reversing the polarity of said relay to cause the same to deenergize and break said locking circuit.

8. In a telephone system, a switch having a line relay, means for operating said switch responsive to the energization of said line relay, a. self-locking circuitfor said line relay, said circuit including two windings of said line relay in series, and means responsive to the operation of said switch for shortcircuiting one of said windings and. reversing the direction of current flow in the other of said windings to deenergize said line relay and open said locking circuit.

9. In a telephone system, an automatic switch, a trunk line, means for operating said switch to select and connect with said trunk line, a relay in said switch normally shortcircuited, means for removing said short-circuit operable when said switch connects with said trunk line to cause said relay to test said trunk for a defective condition, and

means for making said relay slow-acting so as to allow sulficient time for the operations in the trunk line responsive to the connection to take place before the test is completed.

10. In a telephone system, an automatic switch, a trunk line, means for operating said switch to select and connect with said trunk line, a relay in said switch, means for starting a. current fiow in the winding oi said relay when said switch connects with said trunk line, means for stopping the current flow in said winding providing said trunk line is non-detective, and means for causing the current tlow to continue pro i'ding said trunk line is defective.

11. In a telephone system, a first switch, a first trunk line terminating in a second switch, a second trunk line terminating in a third switch, means for operatin said first switch to connect with said first trunk line, means in said first switch for starting the operation of said second switch to connect ith said second trunk line and for maintaining said secondswitch connected thereto providing said second trunk is defective, and means in said third switch for disabling said last means and for maintaining said second switch connected to said second trunk independent or said first switch.

12. In a telephone system, a first switch, a first trunk line terminating in a second switch, a second trunk line terminating in a third switch, means for operating said first switch to connectwith said first trunk line and for operating said second switch to connect with said second trunk line, means in said first switch for locking itself and said second switch operated providing said second trunk line is detective, means in said third switch for disabling said locking means in said first switch providing said second trunk is non-detective, a release con ductor extending back from said third switch through said second switch to said first switch and relay windings in said first and said second switch energized in multiple over said release trunk to maintain said first and second switches operated.

13. In a telephone system, a first switch, a first trunk line terminating in a second switch, a second trunk line terminating in a third switch, means for operating said first switch to connect with said. first trunk line and for operating said second switch to connect with said second trunk line, means in said first switch for locking itseltand said second switch operated providing said second trunk line is defective, means in said third switch for disabling said locking 1 means in said first switch proyiding said second trunk is non-detective, a release conductor extending back from said third switch through said second switch to said first switch, relay windings in said first and said second switch energized in multiple over said release trunk to maintain said first and said second switch operated, a third trunk line, means for operating said third switch to connect with said third trunk line, and means in said third switch for locking itself operated and for maintaining said relay windings energized providing said third trunk line is detective.

lat. In a telephone system, a calling line terminating in a first switch, a first trunk line erminating in a second switch, a second trunk line terminating in a third switch, means responsive to the initiation of a call for operating said first switch to connect with said first trunk and for operating said second switch to connectwith said sechnd trunk, means in said first switch for locking itself and said second switch operated providing said second trunk is defective, means in said third switch for disabling said lOCk ing means in said first switch providing said second trunk is non-detective, a third trunk line, means responsive to a digit dialed at the calling station for operating said third switch to connect with said third triml; line, means in said third switch for locking all three switches operated providing said third trunk line is defective, means in said first switchator preventing said first and said second switch from being locked in case the call is abandoned before a digit is dialed, and means in said third switch for preventing all three switches from being locked in case the call is abandoned after a digit is dialed.

15. In a telephone system, serially related primary and secondary line switches, a line relay in said primary line switch, and a con trolling relay ror said secondary line s itch energized in series with said line relay upon the operation of said primary line switch.

16. In a telephone system, a line switch, a selector switch, means for operating said line switch to connect withsaid selector switch, a holding circuit closed at said selector switch responsive to the establishment of said connection and including a relay in said line switch, a sell locking relay in said selector switch, and means for introducing said relay into saidholoing circuit upon the operation of said selector switch.

17. In a telephone system, a primary line switch, a secondary line switch, and a selector, all in serial relation, a controlling re lay in said secondary line switch, means for initiallyenergizing said relay over a circuit extending forward from said primary line switch, and meansftor holding said relay. over a circuit extending backward fromsaid selector.

In witness whereof, I hereunto subscribe my name this 5t-l1,clay ot September, A. D.'

RUDOLPH F. ST'EI-ILIK.

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