US1513777A

US1513777A - Automatic telephone system

Info

Publication number: US1513777A
Application number: US412070A
Authority: US
Inventors: Willis Bernard Darwin
Original assignee: Automatic Electric Co
Current assignee: Automatic Electric Co
Priority date: 1920-09-22
Filing date: 1920-09-22
Publication date: 1924-11-04
Anticipated expiration: 1941-11-04
Also published as: GB169443A

Description

Nov. 4 1924: 1,513,777

B. D. WILLIS AUTOMATIC TELEPHONE SYSTEM Original Filed Sept. 22. 1920 3 Sheets-Sheet 1 7 HM- 1 \1 i b I:

. nuenltu" BET cf UMLUIE. @fif.

Nov. 4 1924. 1,513.777

B. D. WILLIS AUTOMATIC TELEPHONE SYSTEM Original Filed Sept. 22. 1920 I5 Sheets-Sheet 5 Patented Nov. 4, 1924.

UNITED STATES PATENT OFFICE.

BERNARD DARWIN WILLIS. OF OAK PARK, ILLINOIS, ASSIGNOR TO AUTOMATIC ELECTRIC COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

AUTOMATIC TELEPHONE SYSTEM.

Application filed September 22, 1820, Serial No. 412,070. Renewed May 16, 1924.

To all whom it may concern:

Be it known that I, BERNARD D. WiLLIs, a citizen of the United States of America, and a resident of Oak Park, Cook County, and State of Illinois, have invented certain new and useful Improvements in Automatic Telephone Systems, of which the following is a specification.

My invention relates in general to automatic telephone systems, but more especially to private automatic systems of the general type which is in common use for furnishing telephone service to large factories. business houses, etc., and the object of the invention may be broadly stated to be the provision of improved trunk circuits for connecting a private automatic exchange of this character with a main exchange.

A special feature in this connection, which is not limited, however, to use with a private automatic exchange, is the provision of novel and effective means for making an interoflice trunk line busy to switches in the private automatic exchange when the trunk line is taken for use at the main exchange.

There are other features which, together with the above, will be fully described hereinafter reference being had to the accompanying drawings.

I have chosen to illustrate my invention in connection with a private automatic exchange located in a territory served by a manual public exchange and connected with such public exchange by trunk lines. In the drawings, comprising Figs. 1, 2, and 3, I have shown by means of the usual circuit diagrams sufficient of the apparatus in the manual exchange and in a private automatic exchange embodying the principles of my invention to enable the same to be explained and understood.

Fig. 1 shows the manual equipment, which is all old and is included because of its cooperation with the rest of the equipment. Substation A is an ordinary manual telephone substation having the usual line equipment at the exchange. The reference character 0 indicates an operators cord circuit of a type which is in common use. Conductors 36 and 37 are conductors of one of the trunk lines extending to the private automatic exchange under consideration. This trunk line has line equipment at the manual switchboard similar to the line equipment provided for ordinary subscribers lines.

The trunk lines connecting the manual exchange with the private automatic exchange pass through an attendant operators position, commonly referred to as an attendants cabinet, before they terminate in the incoming connector switches which have access to the local lines. The apparatus at the attendants cabinet is associated with the trunk line comprising conductors 36 and 37 and is shown in Fig. 2, being indicated therein by the reference character M. The operation of this apparatus will be fully explained hereinafter.

The incoming connector in which the trunk lines illustrated terminates is shown in Fig. 3 and is indicated by the reference character H. This connector switch is of the well known Strowger vertical and rotary type, and its mechanical construction therefore will need no explanation. The operation of the circuits will be described. The connector H in common with other similar connectors has access to the local lines in the private automatic exchange, one of which is the line extending to the substation A. This substation is an automatic telephone substation similar to the substation A, except that it is provided with a calling device S for controlling the automatic switches which constitute the private automatic exchange switchboard.

The local subscribers lines are provided with individual line switches, the line switch associated with the line of substation A being indicated by reference character C. Theseline switches are of the Well known rotary type, in which the wipers have no normal position and move in a forward direction only, and have access to a group of local connectors. The latter switches are of the type commonly known as selector connectors, such as is shown in the U. S. patent to Dcakin, No. 1,164,479, granted December 1 1, 1915, and have access not only to local lines but also to the trunk lines connecting with the main exchange.

Having described the apparatus briefly I will now proceed to point out more in detail the novel features of my invention and explain the operation thereof. For

this purpose it will be assumed that the subscriber at substation A, in the manual public exchange, desires to secure a connection with the subscriber at substation A, who is connected to the private automatic exchange. When the receiver is removed at substation A a circuit is completed over the line conductors 11 and 12 for the l ne relay 2, which operates to light the line lamp L. When the operator at whose position the lamp appears perceives the signal.

she will answer the call by inserting the answerin plug of an idle cord circuit, cord circuit 6 for example, in the answering jack J. As a result the cut oil relay 3 is energized, the line relay 2 is deener ized, and the line lamp L is put out I ey K being thrown to listening posit on, the operator may now talk to the calling subscriber, and on finding that he desires to be connected with a subscriber in the private automatic exchange, she will insert the calling plug of the cord circuit in use in the jack of an idle trunk line extending to such private automatic exchange. Assuming that the trunk line shown in the drawing is the one taken for use, when the'plug is inserted in jack J a circuit is completed for the cut off relay 30 in series w th sleeve relay 17 and the former relay is energized to cut off the line relay 31.

Before considering the operations which take place at the private automatic exchange responsive to the insertion of the plug n jack J it will be necessary toexplain briefly the normal circuit conditions in the trunk line. A repeating coil is inserted in the trunk line at the attendants cabinet and it will be observed that a polar relay 52 is connected around the terminals of the condenser which joins the two left hand windings of this repeating coil. Thus relay 52 may be said to be bridged across the trunk conductors 36 and 37 At the main exchange the line relay 31 is n orma ll bridged across the trunk line in series wit the battery and it follows therefore that the polar relay 52 will be normally energized, the current flow in its winding being in such a direction that its armature is held in position as shown in the drawing. Due to the fact that the high resistance 40 (about 12,000 ohms) is included in the circuit of the polar relay, the line relay 31 at the main exchange is not operated under normal conditions.

It will be seen that the upper trunk conduct-or 36 connects to battery at the line relay while the lower trunk conductor 37 connects to ground. When the plug is inserted in jack J as described hereinbefore, conditions are reversed and the upper trunk conductor 36 will extend to ground through the upper winding of the supervisory rela 16 while the lower trunk conductor 37 wi go to battery by way of the lower winding of the same relay. It follows, therefore,

that when the plug is inserted in the jack the direction of current flow over the trunk line will be reversed, and as a result the polar relay 52 will operate its armature 61' so as to close its working contact. .This closes a circuit for the slow acting relay 54 which upon energizing prepares a circuit for relay 53 at its armature 65, connects ground to the release trunk conductor 78 at its armature 64 (in order to make the trunk line busy to the local selector connectors), and at its armature 66 completes a control circuit extending over conductor 7 5 to the line relay 100 of the connector H, Fig. 3. It is noted at this point that although a circuit is completed for relay 53 at armature 65 of rela 54, the said relay 53 is not operated at t is time, because its winding is short circuited over a path which extends by way of armature 63 to the grounded armature 61 of the polar relay 52.

At the operators cord circuit the supervisory relay 16 is not energized due to the inclusion of the high resistance 40 in the trunk circuit, and the supervisory lamp L will be lighted. The other supervisory lamp L is of course extinguished due to the fact that the double wound relay 15 is energized over the calling subscribers line circuit.

The manual operator will now throw her key K to ringing position in order to signal the attendant at the private automatic exchange. By this operation ringing current is transmitted over the trunk line, causing the polar relay 52 to vibrate its armature, whereupon the short circuit is intermittently removed from around the winding of relay 53 and the said relay 53 is energized. Up'on energizing, relay 53 breaks the connection to the contact of the polar relay 52 at armature 63 thereby insuring that the short circuit will not be reestablished. Relay 53 also closes a circuit for the answering lamp L at its armature 62. When the attendant sees the lamp L lighted, she will answer the call by throwing her listening key K thereby connecting her head set across the trunk line. The operation of key K also closes a circuit for the relay 51 which upon energizing locks itself at its armature 59, the locking circuit including the winding of relay 53. Relay 51 also prepares a circuit for the supervisory lamp L at its armature 58, and at its armature 60 short circuits the resistance 40. When the resistance 40 is short circuited relay 16 in the operators cord circuit at the main exchange is energized and the supervisory lamp L is extinguished, thus notifying the operator that the attendant ill ber of the desired station, and the latter will;

complete the connection as follows:

The calling device key K is first operated in order to cut the calling device in on the control circuit extending to the line relay 100, after which the dial may be operated in accordance with the first digit in the local number of the substation A. The slfow acting release relay 101 being now in energized position, the resulting deenergizations of the line relay serve to transmit a series of im ulses to the vertical.,magnet 107 over the fol owing path: ground at G armature 112 and its resting contact, armature 113 and its working contact, oif

normal springs

132 and 134, wiudin of the slow acting series relay 102, and tie winding of the vertical magnet 107 to battery. By the operation of the vertical magnet the wipers 140-142, inclusive, are raised step by step and are brought to rest opposite the level of bank contacts in which the line of substation A is terminated. The slow acting relay 102 is energized in series with the vertical magnet, and retains its armature 114 attracted throughout the vertical movement of the switch, thereby maintaining the circuit of the vertical magnet intact after the off normal springs have been shifted, which occurs at the first vertical step. At the, end of the vertical movement relay 102 falls back and transfers the operating circuit to the rotary magnet 108.

The attendant will now operate her calling device in accordance with the linal digit in the desired number, and the line relay 100 is deengerized a plurality of times as before, this time, ho\vever,'sending impulses to the rotary magnet 108 as follows: ground (i armature 112 and its resting contact, armature 113 and its working contact, oti'

normal springs

132 and 133, armature 114 and its resting contact, winding of the slow acting series relay 106, armature 125 and its resting contact, resting contact of armature 120 and the said armature, and the winding of the rotary magnet 108 to battery. By the operation of the rotary magnet the wipers 140142, inclusive, are rotated over the bank level opposite which they have been raised and are finally brought to rest in engagement with the particular set of contacts in which the line of substation A is terminated, these contacts being indicated in the drawings by reference characters 143-145,

- inclusive. The slow acting relay 106 is energized in series with the rotary magnet 108 and malntalns its armatures attracted throughout the rotary movement of the switch. At its armature 127 relay 106 connects the test Wiper 141 with the test relay 1'04 and at its armature 128 relay 106 closes a shunt around the break contacts at armature 120 of the said test relay in order to guard against the possibility of having the rotary magnet circuit opened. while the test wiper is passing over busy test contacts during the rotation of the switch.

Assuming now that the line of substation A is busy when called, when the test wiper 141 comes to rest in engagement with test contact 144 it will find a ground potential signal to the calling subsz-riber and to either or both of the operators provided they are across the trunk line. This is the usual signal which is transmitted to the calling party to notify him that the called line is busy.

Having explained briefly the operation which takes place when the called line is busy it will be assumed now that the called line is idle. Under these circumstances, and assuming that the line is connected with as before described, when the test wiper 141 comes into engagement with test contact 144 it will find no ground potential and the test relay 104 will not be energized. Then when the slow acting relay 106 talls back a circuit will be completed for the switching relay 105 as follows: from ground at G by way of the working contact of armature 115 and the said armature, armature 119 and its resting contact, lower winding of the switching relay 105, resting contact of armature 127 and the said armature, test wiper 141, test contact 144, winding of the switching relay 148 of the line switch C, and the winding of the stepping magnet 150 to battery. Upon the closure of the above circuit relays 105 and 148 are energized in series and the latter relay clears the called line of its normal battery and ground connections in the line switch (1. Due to a mechanically interlocking device controlled by the armature of the line relay 141-9, relay 148 operates its armature, or armatures, only about half way and the wipers of the line switch are not connected.

In the connector H when the switching relay 105 energizes it establishes a locking circuit for itself at its armature 124, opens the rotary magnet circuit at armature 125, and at

armatures

122 and 126 connects up the line wipers 140 and 142. Ringing current from the generator Gen is now periodically projected out over the called line to operate the bridged ringer at substation A, in order to notify the subscriber thereat that he is being called. It is noted that the return path for the ringing current includes the upper winding of the ring cut off relay 103, and when the called subscriber responds ofi relay will be operated. Upon energizing, relay 103 looks itself at armature 117, breaks the rin ing circuit at armatures 116 and 118, an at the working contacts of these same armatures finally completes the talkin circuit.

aving described the operation of the connector H in completing the connection it will be convenient now to return to the equipment at the attendants cabinet and consider the operations whlch take place there. It is understood of course that the calling device ke K is left in operated position only while the attendant is using the calling device to operate the connector H. When the last digit is called therefore, key K is restored and the line relay 100 is maintained energized over the original circuit which was established at armature 66 of relay 54. When key K is restored a circuit is completed for the supervisory lamp L as follows: ground at G, off

normal springs

131 and 130, conductor 76, up-

; per contacts of key K working contact of armature 58 and the said armature, normally closed contacts of key K and through the lamp L to battery. The supervisory lamp L remains lighted until the called subscriber answers. When this occurs the battery feed relay 50 is energized and breaks the locking circuit of relay 51 at armature 56. Relay 51 accordingly deenergizes (listening key K having been restored by this time) and breaks the circuit of the supervisory lamp L, thus puttlng out the light and notifying the attendant that the local party has answered. Relay 51 also removes the short circuit around resistance 40, but this operation is of no effect because the said resistance is short circuited at armature 57 of relay 50.

The circuit over which conversation is held is of the usual character, as will be observed, and will need no explanation. When the subscribers are through talking they will hang up their receivers. When the subscriber at substation A replaces his receiver the supervisory relay 15 in the cord circuit '0 will fall back and light the supervisory lamp L. When the subscriber at substation A hangs up his receiver the battery feed relay 50 will deenergize and remove the short from around the high resistance 40, which is included in the circuit of the supervisory relay- 16. Relay 16 is accordingly deenergized and lights the supervisory lamp L Responsive to the disconnect signals the manualyoperator Wlll pull down the connection. When the plug is withdrawn from jack J the cut ofl' relay '30 is deenergized and the-line relay is again connected in bridge of the trunk conductors 36 and 37. By this operation the direction of current flow over the trunk and of the polar relay 52 is again reversed, t at is, the current flow in the trunk circuit is now in normal condition, and the polar relay 52 is accordingly operated to move its armature 61 to the position shown in the drawing, so that its make contact is broken. Relays 54 and 53 are now deenergized, and the former relay breaks the control circuit extendin to the line relay 100 of the connector The line and release relays 100 and 101 are now dcenergized, the latter relay breaks the looking circuit of relays 103 and 105, and relays 100 and 101 jointly close a circuit for the release magnet 109. By the operation. of therelease magnet the connector H is restored to normal position in the usual manner and is thus made ready for use in setting up another connection.

Having described the process of setting up a main to branch exchange connection, it will be convenient now to explain briefly the process of setting up a connection in the opposite direction. It will be assumed that the subscriber at substation A desires to call the subscriber at substation A. Since there is nothing novel in the operation of the local switches in the private and automatic exchange the operation ofthese switches in connecting with an interoifice trunk line need not e explained in detail, and it has not been considered necessary to show the circuit of the local selector connector, which as before stated may be similar to the selec tor connector shown in the patent to Deakin. When the receiver is removed at substation A the line switch C operates automatically to extend the associated line into connection with a trunk line leading to an idle selector connector, and the line and release relays in the selector connector which is taken for use are then energized in the usual manner in order to prepare the switch for 0 eration under control of the calling device' S at the calling station. Assuming that the main oflice trunk lines are terminated in the tenth level of the selector connectors, as is the usual practice, the calling device will now have to be operated in accordance with the digit 0. By this means the wipers of the selector connector are raised step by step to the tenth level, after which the automatic trunk selecting movement is initiated and the Wipers are rotated into engagement with the contact set in which the first idle trunk line is terminated. It will be assumed for convenience that the trunk line shown in the drawing is the first idle trunk line and the wipers of the selector connector will therefore establish connection with bank contacts -82, inclusive, Fig. 2. When the connection is established a ground potential is placed upon the test wiper of the selector connector in the usual manner and a circuit is completed byway of the said test wiper,

test contact 81, release trunk conductor 78, armature 64 and its resting contact, and the winding of relay 55 to battery. Upon energizing, relay 55 establishes a locking circuit or itself at armature 68, disconnects the section of the trunk line extending to the connector H by means of armatures 67, and 71, and at armature 69 prepares a circuit for relay 54. The selector connector having finished its operation the ailing line is extended through to the

trun conductors

77 and 79, and the battery feed relay 50 is accordingly energized over the line circuit. Relay 50 now completes a circuit for relay 54 at its armature 56, and the latter relay, upon energizing, connects ground to the release trunk conductor 7 8. If the selector connectors have the precise circuit arrangement shown in the Deakin atent this latter operation is of no particu ar utility, for the conductor 78 will already have been grounded at the test wiper of the selector connector in use. Selector connectors of a later type, however, or some of them at least, place only a temporary ground on the test wiper, and require a holding circuit to be established in order to maintain them in operated position. The circuit arrangement shown provides for the use of a switch of this character if desired.

The connection with the trunk line having been established, it remains to be seen how the operator at the distant exchange is signalled. This is accomplished by the short circuiting of the resistance 40 at armature 57 of the battery feed relay 50. When resistance 40 is short oircuited, the line relay 31 which is bridged across the trunk line at the main exchange is energized and lights the line lamp L Upon perceiving the signal the operator will answer the call by plugging into the jack J and will then extend the connection to any desired party, the subscriber at substation A, for example, in the usual and well understood manner.

When the plug is inserted in jack J B the direction of current flow over the trunk circuit and through the winding of the polar relay 52 is of course reversed, as explained in the previous case, and. the. polar relay is operated to close its make contact. This, however, is of no eflect at the present time, since relay 54 has already been energized. The circuits which control the supervision are the same as have been already described. When the subscriber at substation A hangs up his receiver the supervisory lamp L is lighted by the deenergization of relay 15. When the subscriber at substation A hangs up his receiver relay 50 deenergizes and connects resistance 40 in series with relay 16, which thereupon deenergizes and lights the supervisory lamp L The operator will then pull down the connection. In case the selector connector in use is provided with a series holding relay as is the Deakin selector connector, it will release as soon as the calling subscriber hangs up his receiver, but in case the selector connector is of the type which is held up over the release trunk ahead, it will not be released until the operator at the main exchange pulls down the connection. When this occurs the polar relay 52 of course returns to normal position, relay 54 deenergizes, and ground is removed from the release trunk conductor 78.

The foregoing describes the operation of the system in the daytime. After business hours the attendant will not be present at her position and naturally cannot be relied upon to complete incoming calls. It is necessary therefore to provide other means for completing such connections, and in order to take care of; this situation each trunk line is connected through to some particular local station. Each trunk line thus becomes in effect an individual line connecting the associated local station with the main exchange. The settin up of these night connections will now fie explained. Assuming that the trunk line shown in the drawings is to be connected through to the line of substation A for night service, the attendant will actuate the night key K' and will leave the same in operated position. When the key is operated lamps L and L are disconnected, ground is connected to the make contact at armature 56 of relay 50, thus preparing a circuit for relay 54, and ground is connected through a resistance to conductor 75, thereby completing the control circuit of line relay 100 of the connector H. Since this control circuit remains closed all night the resistance is included in the circuit in order to reduce the flow of current and prevent the relay from overheating. The attendant will now throw her calling device key K to connect the calling device S into the control circuit of the line relay 100, after which the calling device may be operated to control the connector H for the purpose of connecting the trunk line with the line of substation A. The connector operates in the usual manner and when the subscriber answers he is informed that his telephone is being connected for night service. The operator will now restore the calling device key and her listening key. Having extended the other trunk lines through to their respective night service stations in the same manner the attendant may now leave her position.

The operation of putting through calls when the night service connections are up will doubtless be obvious but it may be well to describe the same briefly. Considering first, incoming calls from the main exchange, itwill be clear that when the manual operator plugs into a trunk line and a circuit for relay 54, which pulls up and places groundon the test contact 81 and the multiplesthereof, in order to make the trunk line busy to the selector connectors in case one of these should be operated. In

' case a party whose telephone is connected for night service desires to call a main exchange subscriber he may do so simply by lifting his receiver from the hook. This operation, "supposing the subscriber at substation A' to be calling, energizes relay and short circuits the resistance 40, thereby operating the line relay 31 to light the line lamp L Relay 50 of course operates relay 54 now as in the previous case in order to ground the test contacts in the selector connector banks and make the trunk line busy. In case anyone should wish to use one of the telephones which is not connected for night service, he may do so of course, and can call the main exchange as described hereinbefore, just the same as though the night service connections were not up.

It will be seen that I have devised a novel and highly eflicient trunk circuit for connecting a private automatic exchange with a main exchange, including features which will doubtless be found to be of utility in systems difi'ering widely from the particular system shown and described herein. A feature of considerable importance consists in the provision of a polar relay which is normally energized in order to maintain the trunk line in idle condition and accessible to the selector connectors. Since this ,relay must remain energized in order to same as though it had been taken for use at the main exchange. This feature is very advantageous therefore and is obviously applicable not only to trunk circuits connecting a private automatic exchange with a manual exchange but also to trunk circuits connecting a private automatic exchange with another automatic exchange.

For these reasons, and because of other features which also have a wide range of utility, I do not wish to ,be restricted to the precise form of my invention which is shown and described herein, but desire rather to include and have protected by Letters Patent all forms of my invention which may hereinafter be devised, limited onl by the scope of the appended claims. a at I claim 'as myinvention is: 1. In a telephone system, a trunk line,

means for associating a test potential with said trunk to make the same busy, and a relay permanently bridged across said trunk and normally energized over said ti'unk'to prevent the application of said test potential.

2. In a telephone system, a trunk line and test terminal therefor, means for placing a potential on said terminal to make the trunk line busy, a polar relay energized over the trunk to prevent the application of said potential to said terminal, and means for reversing the direction of current flow over said trunk to deenergize said relay.

3. In a telephone system, a trunk line and test terminal therefor, means for placing a potential on said terminal to make the trunk line busy, a polar relay energized over the two sides for the trunk in series to prevent the application of said potential to said terminal, and operator controlled means for reversing the direction of current fiow over said trunk to deenergize "said relay.

4. In a telephone system, a twp way trunk line, a relay at one end of said trunk and normally energized thereover to maintain the trunk in idle condition, means efi'ective when the trunk is taken for use at the other end for deenergizing said relay, and a test circuit completed by said relay to make said trunk busy.

5. In a telephone system, a two way trunk line connecting two exchanges, a relay in one exchange normally energized over said trunk line to maintain the same in idle condition, means effective when the trunk line is taken for use at the other exchange for deenergizing said relay, and a circuit closed by said relay to make said trunk line busy in the first exchange.

6. In a telephone system, a two way trunk line, means at one end of said trunk line responsive to normal current flow thereover for maintaining said trunk line in idle condition, and means for reversing the current flow in said trunk line when the same is taken for use at the other end for making the trunk line busy.

7. In a telephone system, a two way trunk line connecting two exchanges, a polar relay in one exchange normally energized over said trunk line to make said trunk line idle.

and means for reversing the direction of current flow in said trunk line when the same is taken for use at the other exchange, the said relay being responsive to such current reversal to make the said trunk line busy. I

8. In a telephone system, a two way trunk line connecting two exchanges, means in one exchange for maintaining a current flow over said trunk line in one direction when the trunk line is idle and in the opposite direction when the trunk line is in use, and

sive to current flow over said trunk line for maintaining the same idle or busy under the conditions named. respectively. y

9. In a telephone system, a two way trunk line connecting a manual exchange with an automatic exchange, electromagnetic means energized responsive to normal current How in said trunk line for rendering the same accessible to switches in the automatic, exchange, and means for altering the current flow when said trunk line is taken for use at the manual exchange whereby said first means is operated to make said trunk line busy to said switches.

10. In a telephone system, a two way trunk line connecting a manual exchange with an automatic exchange, a polar relav responsive to normal current flow in said trunk line for rendering the same accessible to switches in the automatic exchange, and means for reversing the current flow when the trunk line is taken for use at the manual exchange, the said polar relay being responsive to such current reversal to render the said trunk line busy to said switches.

11. In a telephone system, a two way trunk line connecting two exchanges, means in one exchange for maintaining a current flow over said trunk line in one direction when the trunk line is idle and in the opposite direction when the trunk line is in use, automatic switches in the other exchange for connecting with said trunk line, and a polar relay responsive to current flow over said trunk line for rendering said trunk line accessible to said switches when idle and busy to said switches when in use.

12. In a telephone system, a two way trunk line connecting two exchanges, means in one exchange for maintaining a current flow over said trunk line in one direction when the trunk line idle and in the opposite direction when the trunk line is in use, automatic switches in the other exchange for connecting with said trunk line and provided with testing means for determining whether the trunk line is busy or idle, and a polar relay responsive to current flow over said trunk line in one direction for causing the trunk line to test idle, and responsive to current flow in the opposite direction for causing the trunk line to test busy.

19 In a telephone system. a trunk line, a signal for indicating when a call is received over said trunk line, a relay controlling said signal, a circuit for said relay, a short circuit around said relay to. prevent the same from energizing, and means responsive to ringing current received over said trunk line for opening said short circuit to permit said relay to operate.

14. In a telephone system, a trunk line, a signal at one end of said trunk line for indicating when a call is received thereover, a relay controlling said signal, a circuit for said relay, a short circuit around said relay to prevent the same from energizing,

means for closing said circuits when the trunk line is taken for use at the other end, and means responsive to ringing current received over said trunk line for opening said short circuit to permit said relay to operate.

15. In a telephone system, a trunk line connecting a manual exchange with an automatic exchange, a high resistance bridged across said trunk line at the automatic exchange, means for setting up connections over said trunk line in both directions, means for short circuiting said resistance whenever a connection is established, a line relay operated responsive to the short circuiting of said resistance when the call originates in the automatic exchange, and a supervisory relay responsive to the short circuiting of said resistance when the call originates in the manual exchange.

16. In a telephone system, a trunk line connecting twoexchanges, a line relay and battery bridged across said trunk line at the first exchange, a high resistance normally bridged across said trunk line at the second exchange, means for short circuiting said resistance when the trunk line is taken for use at the second exchange to operate said line relay to signal an operator at the first exchange, means for extending a connection over said trunk line from a calling subscriber in the first exchange to a called subscriber in the second exchange, and means for short circuiting said resistance when the called subscriber answers in order to give the operator in the first exchange a supervisory signal. 7 17. In a telephone system, a trunk line connecting a main exchange with a private automatic exchange and terminating at the latter point in an automatic switch, said switch having automatic off normal release mechanism, means for establishing a control circuit for said switch when the trunk line is taken for use at the main exchange, means for operating said switch to connect with a subscribers line, and means for then establishing the control circuit over a new path to retain the switch in position independent of the main exchange connection or the connection to said subscribers line.

18. In a telephone system, a trunk line connecting a main exchange with a private automatic exchange and terminating at the latter po nt in an automatic switch, means for establishing a control circuit for said switch when the trunk line is taken for use at the main exchange, means for operating said switch to connect with a subscriber-s line, means for then establishing the control circuit over a new path to retain the switch in position independent of the main exchange connection or the connection to said subscribers line, and a resistance included in the new control circuit to reduce the flow of current therein.

19. In a telephone system, a trunk line connecting a main exchange with a branch exchange and terminating at the latter point in an automatic switch, other switches in the branch exchange having access to said trunk line for establishing branch to main exchange connections, means for operating said first switch to connect said 7 in an automatic switch, other switches in the branch exchange having access to said trunk line for establishing branch to main exchange connections, means for operating said first switch to connect said trunk line with a subscribers line in the branch exchange, means for retaining said switch in position independent of subscriber or main exchange control, and means responsive to the removal of the receiver on said subscribers line for making said trunk line busy to the said other switches.

21. In a telephone system, a two-way trunk circuit, means including automatic switching mechanism for completing a connection to a called subscribers line over said trunk circuit in one direction, multiple terminals for said trunk circuit accessible to automatic switches through the medium -of which a calling subscriber can set up a connection over said trunk circuit in the opposite direction, and a relay. bridged across said trunk circuit and arranged to feed talking current to the called subscriber in the first connection and to the calling subscriber in the second connection.

22. In a telephone system, a trunk line connecting a manual exchan e with an automatic exchange and terminating at the latter exchange in an automatic progressively movable switch, a repeating coil dividing said trunk line into two inductively connected sections, an operators switchboard in the automatic exchange to which the second section of said trunk line is multiply connected, relay mechanism actuated over the first section when the trunk line is taken for use at the manual exchange to close a control circuit for said switch over a conductor separate from the talking circuit, and calling mechanism at said switchboard for interrupting said control circuit in accordance with digits in a subscribers telephone number to operate said switch.

23. In a telephone system, a trunk line and test terminal therefor, automatic switches having access to said trunk line, means for placing a potential on said terminal to make the trunk line busy to said switches, a polar relay controlling the application of said busy potential, and means for reversing the direction of current flow in said trunk line to control said polar re- 24. In a telephone system, a trunk line, automatic switches having access to said trunk line, multiple test terminals for said trunk line in the banks of said switches, a polar relay having an armature selectively responsive to currents of difi'erent polarities flowing over said trunk line, a source of test potential, and circuit arrangements controlled by said armature for controlling the application of said potential to said test terminals.

25. In a telephone system, a two wa trunk line, means at one end of said trun line responsive to normal current flow thereover for maintaining said trunk line in idle condition, an automatic switch for connecting with said trunk line at the same end, and means for reversing the current flow in said trunk line when the same is taken for use at the other end for making the trunk line busy to said switch.

26. In a telephone system, a two way trunk line connecting two exchanges, a polar relay in one exchange normally energized over said trunk line to make said trunkline idle, an automatic switch in said one exchange for connecting with said trunk line, and means for reversing'the direction of current flow in said trunk line when the same is taken for use at the other exchange, the said relay being responsive to such current reversal to "make the said trunk line busy to said switch.

27. In a telephone system a two way trunk line connecting two exc anges, means in one exchange for maintaining a current flow over said trunk line in one direction when the trunk line is idle and in the opposite direction when the trunk line is in use, said means comprising a line relay and an operators cord circuit together with suitable battery connections, automatic switches in the second exchange, and a polar relay in the second exchange responsive to current flow over said trunk line for maintaining the same idle or busy to said switches under the conditions named, respectively.

28. In a telephone system, a trunk line terminating at one end in an automatic switch, a polar relay normally energized over said trunk line, means for reversing the direction of current flow in said trunk line when the same is taken for use at the other end, and an operating circuit for said switch ing the direction of current flow in said,

trunk line when the same is taken for use at the first exchange, and a control circuit for said switch closed under control of said polar relay responsive to such current reversal.

30. In a telephone system, a trunk line, an automatic switch for extendin a connection over said trunk line in one irection, a signal for calling an operator to extend a connection over said trunk line in the other direction, a polar relay normall energized over said trunk line, means or o erating said polar relay by reversing t c direction of current flow over said trunk line to make the trunk line busy to said switch, and a circuit for said call signal closed under control of said polar relay re-.

sponsive to ringing current received over said trunk line.

31. In a telephone system, a trunk line, an automatic switch for extending said trunk line to a called line, an operators call siglnal, a polar relay bridged across said trunk e and normally energized, a control circuit for said switch closed under control.

of said relay responsive to a reversal of current in said trunk hue, and a circuit for said call signal closed under control of said relay responsive to ringing current received over said trunk line. Y

32. In a telephone system, a two way trunk line, two switches associated with said trunk line at the same end thereof, one switch for extending a connection to said trunk line and the other for extending a connection from said trunk line, a polar relay normally energized over said trunk line, and means for operating said relay by reversing the direction of current flow in said trunk line to make the trunk line busy to one switch and close a control circuit for the other switch.

33. In a telephone system, a trunk line connecting two exchanges, means whereby an operator in the first exchange can signal an operatorin the second exchange, a relay at the second exchange energized by the operator at the second exchange when she answers the call, means including an automatic switch for extending the call to a line in the second exchange, a battery feed relay in the second exchange for supplying talking current to the called station, a supervisory signal at the first exchange, a circuit including a conductor of said trunk line for controlling said signal, said circuit including a resistance at the second exchange, contacts on both said relays for short circuiting said resistance to control said signal, and a locking circuit for the said first mentioned relay controlled by said battery feed relay.

Signed by me at Chicago, Cook County Illinois this 18thday of Se tember 1920.

fiERNARD DARWI WILLIS.