US2170669A - Automatic printing telegraph exchange system - Google Patents

Description

Aug. 22, 1939. G. s. VERNAM 2,170,669

AUTOMATIC PRINTING TELEGRAPH EXCHANGE SYSTEM Filed Feb. 20, 1937 4 Sheets-Sheet 1 LLLL TEN LINE RELAYS MULTIPLE T0 In N F IG. 1.

AC DC SUPPLY FIG. 3.

INVENTOR GILBERT 6'. VEfi/VAM ATTO R N EY Aug. 22, 1939.

OPERATORS pm/vrm FIG2.

G. S. VERNAM AUTOMATIC PRINTING TELEGRAPH EXCHANGE SYSTEM Filed Feb. 20, 1937 FINDER com/zero? 33 snare/1 4 Shets-Sheet 2 INVENTOR I G/LBERTS. VER/VAM ATTO R N EY ACORDC J'UPPLY FIG. 4.

s. VERNAM 2,170,669

AUTOMATIC PRINTING TELEGRAPH EXCHANGE SY$TEM Filed Feb. 20, 1937 4 Sheets-Shee t :5

F/NDEMS OTHER U/VE RELAYS LLLLL MULTIPLE TO OTHER GILBERT 5. VE/PNAM ATTORNEY G. S. VERNAM AUTOMATIC PRINTING TELEGRAPH EXCHANGE SYSTEM 4 Sheets-Sheet 4 Aug. 22, 1939. 2,170,669

Filed Feb. 20, 1937 ATTORNEY patented Aug. 22, 1939 AUTOMATIG'PRINTING TELEGRAPH EX CHANGE SYSTEM Gilbert 5. Vernam, Hackensack, N. J assignor to I Postal Telegraph-Cable Company (New York),

New York, N. Y., a corporation of New York Application February 20, 1937, Serial No. 126,769

16 Claims.

My invention relates in general to printing telegraph systems and more particularly to automatic printing telegraph exchanges and concentrator circuits for interconnecting subscribers stations to the central office operators stations.

In automatic printing telegraph exchange systems .heretofore in use of the type employing single wire subscribers lines, special apparatus has been necessary at the subscribers stations, due to the fact that, in general, the automatic system works with a normal low line current whereas in the manual concentration systems the line current is normally much higher. One of the objects of my invention is to provide a line relay circuit at the central office which permits operation with either normal high line current or normal low line current, thus permitting the same type of subscribers' station apparatus to be used interchangeably with either manual or automatic concentration systems, and also permitting the same type of central ofiice equipment to be used interchangeably with either the .standard high line'current subscribers apparatus or with the special low line current subscrib-ers apparatus.

It is a further object of my invention to reduce the number of automatic switches otherwise required, by establishing direct semi-permanent connections between certain subscribers lines and central oifice printers during periods of heavy traffic. It is a further object of my invention to apply the relay circuit to automatic concentrators or printer exchanges of various sizes and also for operating calling signals and controlling the motors of direct connected printers.

Accordingly one of the main ,features of my invention is the provision of a line relay and a cut-off relay in each subscribers line at the central-office, so arranged that an inward call can be initiated either by increasing a normally low line current or by first decreasing and then increasing a normally high line current. I

Another feature of my invention is the provision of separate printer sets at the central oflice which may be plugged directly into a subscribers line, during periods of heavy traffic, without extending the connection through the automatic switches usually employed in establishing connections with subscribers stations. 7

Still another feature is the provision of relays associated with the separate central ofiice printer sets for the purpose of operating a calling signal and automatically starting the printer motors of both the central ofiice and subscribers station printers when a call is made in either direction.

Other features of my invention will be apparent from the following description, as well as from the appended claims.

Thepreferred embodiment of my invention is illustrated in the accompanying drawings, in which 1 Fig. 1 illustrates a high current subscribers station of the type usually used with manual concentrators connected to line and cut-off relays at' a central office adapted for use as part of a hundred line automatic concentrator,

I Fig. 2 diagrammatically indicates the remainder "of the apparatus constituting a hundred line automatic concentrator, including the central o'fii'ce operators printer,

- Fig 3 illustrates a subscribers station of the usual automatic or normal low current type adapted to be connected with the same type of line and cut-off relays at the central oifice,

Fig. 4 illustrates a circuit similar to that shown inFig. 1 but adapted to a thousand line automatic concentrator or printer exchange system,

Fig. 5 illustrates diagrammatically further apparatus at the central office for cooperation with the circuit of Fig. 4,

6 illustrates the normal low current subscribers station for connection with the thousand line concentrator or printer exchange system, and

' "Fig. 7 illustrates a circuit arrangement of the printer set for tabling busy lines.

In the drawings the same reference characters are generally used to show similar apparatus in the various figures.

Referring now'to Figs. 1 and 2, the numeral indicates generally a'subscribers station connected through a line 3 to the central olficeline relay 'circuit'design'ated generally as 5. This circuit is associated with the central office apparatus illustrated generally in Fig. 2. The par- 40 ticular apparatus used in thecentral station may be of any desired type and as it does not form a part of this invention it accordingly is shown in' Fig. 2 merely in block diagrams. A particular example of such apparatus is shown in my Patent 1,910,867 issued May 23, 1933.

A more particular description of the circuit shown in Figs. 1 and 2 follows.

Normally a relatively high current, for example, about milliampers flows from negative 50 battery 10 at the central oflice through the upper winding" of cut-off relay II, jack 200, resistance R, back'contact B oi line relay 23, front contact A o-f-cut-off relay ll, line 3 through the winding of polar relay ll, winding of motor control relay 39, armature and front contact A of polar relay l1, through calling key 2! to ground at the subscribers station. This holds cut-oil relay II and polar relay ll energized. Equalizing resistors R are provided in each subscribers line to enable adjustment to correct for differences in line resistance. To call from a subscribers station, the calling key is depressed and released, thus opening and closing the line. When the line is opened cut-ofi relay l I releases, and when the line closes line relay 23 operate through back contact B of cut-off relay H. Polar-relay ll and control relay 33 at the subscribers station release when the line is opened and are again reenergized upon closing of the line but this has no effect except to start and momentarily run the motor 4| during the interval while the line is open at calling key 2 I. The printer cannot be operated, however, as its circuit is open while key 2| is depressed and it is short-circuited immediately through contact A of polar relay i! when key 2i is released. This short -circuit prevents the subscriber from sending during the intervalwhile motor l l is running, but slowing down immediately after key2l is released. Without this short-circuit, it would be possiblefor the subscriber to send signals from his printer keyboard which would vibrate relay 39 and cause motor 4| to continue to run. Such signals would not be properly received at the central offioe as the central ofice printer is not yet connected to the line and might result in lost messages. With the cut-off relay il released and line relay 23 operated, relay contacts 22 and 2 4 are closed and ground is applied to the test bank terminals T of the finder connectors, and slow operating group relay 25 operates toclose switch 21 grounding start lead 29 of the idle position locator or allotter 3i, causing the first idle finder connector 33 to find the calling line and to connect it to operators set 35. When the finder connector cuts in on the calling line, ground is applied to the private bank terminal P energizing the lower windings of relays II and 23 inseries by battery 31. This operates to lock line relay 23 in its operated position and operates cut-01f relay H which removes the ground previously applied to the testbank terminal'l and relay 25 and extends the callingline through front contacts A of relays H were, and the line bank terminal L, which is now'conneoted to the central office printer. Current from a positivebattery associated with the central ofiice printer set 35 causes the subscribers station polar relay IT to release, shorting out motorcontrol relay 39 through back contact B, causing it to release, and starting printer motor 4!. This also permits the line current to traverse printer keyboard and magnet 43 by removing the short circuit. Messages may now be transmitted between the subscribers station and the central ohice. Polar relay I! remains released, with its armature against back contact B during the transmission ofprinter signals, as the armature is held against 3 by a spring (not shown) when the circuit is open and is held against the same contact by current from the positive battery when the circuit is closed.

When the idle finder connector switch 33 and its associated printer set 35 were first selected by the allotter 3!, the dial lead of set 35 was momentarily grounded by the allotter, thus operating relay 55, which locks itself'through release key 56. Ground from release key 56 over dial lead 58 serves to hold the finder connector switch to prevent it from releasing. After the message has been transmitted by the subscriber and acknowledged by the central office operator, the connection may be released from the central oifice in the usual way, the operator depressing his release key 56 which releases relay 55 and removes the holding ground from the dial lead allowing finder connector switch 33 to release. The release of the finder connector removes ground from the private bank terminal P. This releases line relay 23 immediately but cut-off relay H is made to be slow releasing and remains operated until the line relay 23 has released and closed the line circuit through the upper winding of cut-off relay ll, so that the control relay remains. operated and the circuit is restored to its normal idle condition. Current from negative battery l operates polar relay I I, removing the short circuit from control relay 39 to stop the printer motor at the subscribers station.

Turning now to Fig. 3 in connection with Figs. l and 2, theoperation of the system with subscribers stations arranged for normal low current lines will be described.

With this system the line 3 norm-ally carries a very low current, for example, about six milliamperes which is notstrong enough to operate cut-off relay H and therefore it is normally released. Likewise the low current is insufficient to operateline relay 23. The polar relay 51 at the subscribers station (Fig. 3) is provided with two windings, winding 53 of high resistance, and winding 55 oflow resistance. Operation of calling key 51 functions to short circuit high resistance winding 53 of polar relay and the line current from battery at the central station is increased to a value sufiicient to operate line relay 23. This grounds the test bank terminals and operates group relay 25 causing the line to be switched through to the central office operators printer set in the same manner as described in connection with Fig. 1. Current from the positive battery at thecentral oificecauses the station polar relay .5! to release thus operating control relay 39 and starting printer motor 4|. The calling key should now be restored to its normal position since the control relay contacts shown at 59 now shunt the high resistance winding of polar relay 5|. Connection is ;now established between the sub scribers station and the operators station. The release circuit for the system shown in Fig, 3 is in general substantially the same as that in Fig. 1, however this difference occurs. After line relay 23 has released, and the polar relay 5| at the subscribers station has released, theline current reduces tothenormal low current value since the shunt is removed from winding 53 and thus allows cut-oi? relay i l H to release restoring the circuit to its normalunoperated position. 7,

Qutwardcalls from the central oflice to the subscribers are handled substantially in the same manner as in the previously constructed circuits, i. e. the eentral ofiice operator operates a dial to causethe finder connector associated with. her setto step its wipers to the terminals of the desired customers line, to test the line to determine whether it is busy, and if not busy to connect with the bank terminal of the line. When the finder connector 33 grounds the private bank terminal P of the called line, the lower windings of line relay 23 and cut-ofi relay II are energized. .In the circuit as shown in Fig. 1 thecut-ofi. relay remains operated and the line thelow current line, both line relays 23 and cut-01f relay II operate upon grounding of the private bank terminal, resulting in a through line connection and the starting of the printer motor. In the low current station system, the line relay 23 may operate slightly faster than the cut-01f relay II in which case a short impulse of current will be sent through group relay 25. In order to prevent such a short impulse applied to relay 25 from falsely starting the position selector on outward calls, relay 25 is made slow operating, so that it will not respond to such short pulses.

The system shown in Figs. 4, 5 and 6, is applicable to a thousand line automatic concentrator or printer exchange system. This may be any type of concentration system including systems in which separate groups of operators printers are used at the central station for inward and outward calls such as illustrated in my prior Patent No. 1,895,113 issued on January 24, 1933, and further may be connected to an exchange system which permits each subscriber to communicate through a tollboard with other systems or through repeater control with other subscribers in a manner, for example, such as described in the patent to F. G. Hallden, No. 1,895,097, issued January 24, 1933. The system generally, as shown in these figures, is broadly the same as that shown in Figs. 1 and 2 so a detailed description only will now be given.

The private bank terminals of line finders in these thousand line concentrators are open when the corresponding line is idle, grounded when busy and connected to battery when the line is calling. In answering an incoming call the line finders hunt for battery on the private bank terminals instead of hunting for a ground on separate test bank terminals, as in the case of the hundred line combination finder connector, described above. For this reason the line relay 23 is wired so that, when operated, it connects the battery through its own winding and that of the cut-off relay II to the private bank terminals P. Except for this change in wiring the relay circuit shown in Fig. 4 is substantially the same as that shown in Fig. 1.

Since this concentration and exchange system may provide for selective service as set forth in the above patents referred to, dial IOI has been shown in connection with the subscriber's station, in Fig. 5. However, the system could be used without the dial if this selective service is not required, in which case the subscribers station circuit of Fig. 1 would be used in connection with the line and cut-0.1T relays of Fig. 4.

At the subscribers station, as shown in Fig.

4, a calling dial IOI is shown and a milliammeter I03 is provided in the line. Printer magnet coils 43 are normally short circuited by the make or front contact A of polar relay I1 and conductor I9 to permit operation over longer lines, without a detrimental decrease in the normal current due to resistance in the printer contact.-

This short circuit also serves to prevent false starting of the printer motor before the call is completed. To make a call, calling key 2| is depressed and released thus releasing cut-off relay II and operating line relay 23 and causing the line finder I05 to connect the line through to a selecting circuit. As the line relay of the selecting circuit and its battery tap resistor are of lower resistance than the winding of the cutoiT relay II and its battery tap resistor, the line current now increases. This increase may be observed in meter I03 indicating that the calling station may dial. The off normal contacts I02 of the dial I are arranged to connect a local negative potential from battery I01 to the line and prevent the printer motor M from starting in response to: the dial pulses. The dial of the subscribers station may now be operated to actuate the switch of the selecting circuit to select the class of service required. Assuming that the subscriber called in order to send a telegram, the digit dialed will be such that a group of secondary line finders associated with central ofiice operators printers will be selected. The first idle secondary finder I09 of this group will operate to extend the line through to operator's printer set III. When the line is switched through to the central oflice printer, the line current reverses, polar relay II releases, thus removing the short circuit from the printer magnet 43, short circuiting control relay 39 through back contacts of the polar relay, thus starting printer motor 4|. The circuit is now established for the subscriber to transmit his message. After the desired communication has been concluded the circuit is released in the same manner as that described in connection with Fig. 1.

In the low current system as shown in Fig. 6, the line circuit normally extends through both the high and low resistance windings 53 and 55 respectively, of polar relay 5|, through a calling key designated generally as H3 and through a back contact II5 of control relay 39 to ground. The high resistance winding 53 limits the line current to a low value, for example about six milliamperes, such that cut-ofi relay II (Fig. 4) releases and line relay 23 does not operate. Polar relay 5I will operate on this low current however. When the calling key is operated it connects dial lamp II! and its shunt resistance across the high resistance winding 53 of polar relay 5| and removes the short circuit from the dial pulsing contacts by opening contacts H8. The line current now increases from the normal value up to a higher value, for example about forty milliamperes. The increased line current operates line relay 23 (Fig. 4) and causes the circuit to be switched through the line finder I05 to the selecting circuit I 01 (Fig. 5). The line current now increases to a higher value, for example about sixty milliamperes, and the dial lamp III lights up indicating that the circuit is ready for dialing. The oif normal contacts II9 of the dial IOI open the windings of control relay 39 to prevent starting of the motor even though polar relay 5I responds to the pulses.

The dial at the subscribers station may now be operated to connect the subscriber through the primary line finders and secondary line finders I05 and I09 to the operators printer set III. The subscriber may likewise be connected through a tollboard jack or a repeater control unit for local exchange service, as described in my Patent 1,895,113 or the Hallden Patent 1,895,- 097. As this particular connection is not needed for an understanding of my invention as disclosed herein, illustration thereof has not been made.

After connection has been established at the central oifice, positive potential from the central ofiice is supplied to the line thus reversing the line current causing polar relay 5| to operate to close contact I20 energizing control relay 39 and starting the station printer motor 4|. The control relay also removes the short circuit from the keyboard contacts and printing magnet 43 and establishes a short circuit around the high resistance winding 53 of polar relay Hand the dial lamp IIT. Calling key II3 should now be restored to the normal central position. Release of this circuit is controlled from the central office in the same manner as that described in connection with Fig. 3.

In the event the central station desires to be connected with a particular subscribers line such connection is established by dialing from outward o'perators printer such as I2I through selector I23 and connector circuit I25 to private and line bank contacts P1, L1, and through the line to the particular printer station, in the manner disclosed in my Patent No. 1,895,113.

If a local station-to-station call between two subscribers is desired after operating calling key II 3 and waiting for dial lamp II! to light, as described, one digit is dialed to select this class of service. This increases the line resistance at the selecting circuit to extinguish the dial lamp until the line is connected through the secondary finder to the repeater control unit at which time the dial lamp is again brightly lighted. Further operation of the dial controls selectors and connectors to extend the circuit to the called station line after which the calling key is restored to its normal center position. This reconnects the high resistance winding of the polar relay 5| into the line, reduces the line current to normal value and causes the repeater control unit to connect the calling and called station lines through a single line reepater connecting a positive potential to both lines.

This system is not illustrated herein but is fully set forth in the Hallden Patent No. 1,895,097.

The positive potential thus applied causes polar relays 5! at both the called and the calling station to operate their respective control relays 39 starting the motors 4| and increasing the line current to its normal working value, in the same manner as set forth above in the description of the circuit of Fig. 6.

To release these connections the calling key at the subscribers station may be operated to its clear out position, to the left as shown in Fig. 6. This opens the contact 121 removing the short from clear out relay I29 and allowing 'it to operate in series with control relay 39. The clear out relay locks itself in operated position through contacts NH and establishes a short circuit around control relay 39 which then releases stopping motor 4| and reducing the line current again to the normal non-operating value. This low current permits a slow acting relay, not shown, associated with the single line repeater, not shown, to release the connection. Current from a negative battery on the line then operates through polar relay 5| releasing the clear out relay and restoring the circuit to normal.

The high current circuit, such as shown in Fig. 4, may be used in substantially the same manner for local exchange service. For adapting this system to such service a resistance 20, of substantially the same resistance value as high resistance 53 of polar relay M, is shunted across calling key 2|. After operating calling key 2| and receiving the usual indication in meter I03, one digit is dialed on dial IOI to select the local exchange service. Calling key 2| is then operated to reduce the line current by reinserting resistance 2| in circuit, causing the repeater control unit to connect the calling and called station lines through a single line repeater, connecting a positive potential to both lines. When the positive potential is applied to line 3 current reverses in meter I93.- The subscriber then releases calling key 2'I and circuit is established in the manner described above.

Release of this connection may be accomplished by manipulation of key 2|, to reduce the line current to a low value as described above. This drop in current permits the slow acting relay associated with the single line repeater to release, opening the line connection. The circuit at subscribers station then returns to its normal unoperated condition in a manner similar to that described in connection with Fig. 1.

In an automatic concentration system of either of the types described, during periods of heavy traffic all of the printer sets at the central oilice for receiving or sending telegrams and their associated automatic switches may be busy, with the result that incoming calls or outgoing messages may be delayed. Additional printer sets can be provided, but if these are connected in the usual way additional automatic switching equipment must also be provided. This involves a considerable expense for equipment that is only used for a comparatively short time each day. During such periods, certain subscribers stations will have enough traffic to keep a central office printer continually busy, or nearly so, and it is, therefore, desirable to table them or, in other words, to connect them directly with certain central ofiice printer sets. need for rapid switching on such connections, as they are set up and left connected for fairly long periods of time. Arrangements are provided, therefore, for making these connections with cords and jacks to avoid the expense of providing automatic switches. As there may be some idle periods, even during busy hours, on such circuits it is desirable to equip them with calling and motor control arrangements.

To take care of such cases, a group of central oflice printer sets is provided, each set being connected as shown in Fig. 7. Each line circuit, as shown in Figs. 1 and 4, has a line jack 290 for use with cords such as 20! of the tabling printers. When it is desired to table a particular line, a cord MI is plugged into the line jack 200. The extra contacts on the jack ground the P bank contacts, making this line test busy to all outgoing calls, and operating both the line relay and the cut-off relay. The subscribers line 3 is now disconnected from its regular line and cutoff relay and connected to relays II and 23 associated with the tabling printer set shown in Fig. 7.

If the subscribers station is of the normal low current type, as shown in Fig. 3 or Fig. 6, relays H and 23 will remain released when the cord is plugged in. To call, the subscriber operates his calling key in the usual Way, thus increasing the line current and operating relay 23. Relay 28 looks itself through release key 203 and operates relay I I. The line is now connected through the B contacts of relays II and 23 to the central office printer. Relay 23 also operates control relay 39 and lights the guard lamp 299 to call the central ofiice operators attention to the call. The control relay connects positive battery to the central oifice printer motor, starting it, and to the line, starting the subscribers printer motor in the usual way. The subscriber may now send his message.

To release the connection, the operator depresses release key 203, releasing relay 23 which connects negative battery through its A contact and the winding of relay I I to the line. A guard There is no lamp is extinguished and the control relay'is released at this time. The negative battery on the line stops the subscribers printer motor in the usual way and cuts in the high resistance'winding of relay 5|, which releases relay 1 l, restoring the circuit to normal.

To call the station, the central oflice operator depresses call key 205 to operate relays l I and 23, after which the circuit functions the same as during an inward call. In the case of a high current line. relay ll remains operated after the call is released, and the relays function in the same manner as the usual line and cut-off relays for calling.

The remaining features of the system are substantially similar to those illustrated in Figs. 1 and 4 and need not be described in detail. The circuit connections of this printer may be readily followed by a reference to the particular description made in connection with the other figures.

While the description given above refers generally to a'singe subscribers'station and its connection with the central ofiice, it is, of course, understood that in a system of automatic'printing telegraphy, such as described, any desired number of subscribers may be connected to the central operators station in the usual manner.

While my invention has been described with particular reference to the attached drawings, such description is not intended as a limitation thereon. .My invention and the particular features for which I desire protection are defined in the appended claims.

What I claim is:

1. In a printing telegraph system, a subsciibers station, a central ofiice, a line circuit from the subscribers station interconnecting said station and oflice, means at the central office for establishing connection between the subscribers line circuit and an idle printer at said central ifice, and means operative alternatively in response to an increase, or a decrease and a sub sequent increase in normal line current for operating said first named means.

2. In a printing telegraph system, a plurality of subscribers stations, a line from each subscribers station to a central office, means at the central office for connecting said subscribers stations to an idle printer, and means operative alternatively in response to a breaking of said subscribers line circuit and subsequent closing of said line, or to increased current on said line circuit for operating said connecting means.

3. A printing telegraph system as claimed in claim 1, further comprising a release key and means operative upon manipulation of said release key to restore said system to its original state.

4. In a printing telegraph system, a subscribers station, a subscribers printer, a central oflice, a printer and a finder switch at said central office, a line connecting said subscribers station to said central office, means operative alternatively in response to an increase, or a decrease and subsequent increase of current in said line current to operate a selecting means, means responsive to operation of said selecting means to connect said line to said central printer, and means responsive to said last named connection, to operate said subscribers printer.

5. In a telegraph concentrator circuit, a cutoff relay, a line relay, line windings on said relays connected with a source of current, other windings on said relays connected with a source of current and normally in an open circuit, means responsive to a change in line current to change the current through said line windings, and means responsive to said change in current to close the circuit through said other windings on both said relays.

6. In a line relay circuit, a line relay, a cut-off relay, windings on said relays, a line circuit'normally closing said cut-off relay winding and normally leaving said line relay winding open circuited, other windings on said cut-off and line relays, said last named relay windings being mor mally open circuited, means responsive to opening and closing of said line circuit to open said normally closed relay winding and close said normally open relay winding, and means responsive to the closing of said normally open winding to close the circuit through said other relay wind-l ings. v 7. In a line relay circuit as claimed in claim 6, means for opening said second named relay. circuit, and means responsive to the opening of said circuit to restore the circuit to normal.

8. In a linerelay circuit, a line relay, a cut-off relay, windings on said relays, a line circuit normally connected through said line relay windings, the current in said line circuit being normally insufiicient to operate said relays, other windings,

normally open circuited, on said cut-off line relays, and means responsive to an increase of current in said line to close the circuit through said last named windings.

9. In a printing telegraph exchange system, a subscribers station, comprising a polar relay, a motor control relay and a calling key, a line connected to said subscribers station, a line relay circuit, comprising a slow release cut-ofi relay, a line relay and. a group relay, means operative upon operation of said calling key to cause said line relay to operate said group relay, selective means operative upon energization of said group relay to connect said relay circuit to a central oflice printer, local windings on said cut-off and line relays, means responsive to said last named connection to energize said series windings, and means responsive to energization of said local windings to connect said line to said central office printer and. to release said motor control relay.

10. In a printing telegraph system, a subscribers station, a central office, a line from said subscribers station to said central oifice station, a printer set at said central office, a line control relay circuit operative alternatively in response to an increase, or a decrease and a subsequent increase of normal current in said line for controlling the establishing of a circuit between said subscribers station and said printer set, and means for connecting said printer set directly into said line.

11. In an exchange system, a station connected over a normally closed line with a central office, a source of line current at said central oiiice, a cut-off relay at said central office normally energized by said line current, a normally deenergized line relay at said central office, means at said station for momentarily decreasing said line current to release said cut-off relay and to energize said line relay to initiate a call at said central oflice, means responsive to said call to energize both said relays and means responsive to the release of said call to release said line relay.

12. In an automatic printing telegraph exchange system, a plurality of stations connected over single wire lines to a central office, relays in each line at the central office for. controlling automatic switches, said relays being provided with means operative, alternatively by increasing the line current above normal, or by first decreasing the line current below normal for operating said automatic switches and then increasing it to normal.

13. In a printing telegraph exchange system, a plurality of subscribers stations, a central office, a line from each subscribers station to said central ofii-ce, printer sets at said central ofiice, means for connecting anyone of said printer sets directly into any one of said lines, and relay means in said printer set, operative alternatively in response to a decrease below normal and subsequent increase to normal or to an increase above normal of the line current to establish communication between said subscribers station and said central ofllce.

14. In an automatic exchange system, a plurality of subscribers stations, a central oiiice, line circuits from said subscribers stations to said central ofiice, means at said central ofiice for communicating with said subscribers, a source of line current at said central oiiice, and means in each subscribers line operative alternatively in response to. increase in line current, or a decrease and subsequent increase to normal of said line current to establish connection between the subscribers station and the central office communicating means.

15. In an automatic telegraph exchange system, a cut-off relay, a line relay, line windings on said relays, a source of current for said line windings, means responsive to a change in current in said line windings to change the current through said windings, normally open circuited local windings on said relays, and means responsive to said change in current in said line relays to close the circuit through said local windings.

16. In an automatic exchange system, a station connected over a normally closed line with a central oflice, a source of current at said central office,a line relay and a cut-ofi relay at said central station normally in circuit with said source, means atsaid station for. increasing said line current above normal, means responsive to said increase in line current to open said cut-off relay circuit to initiate a call, and means operative in response to initiation of said call-to energize other windings on said relays.

GILBERT S. VERNAM.

Images