US1779492A

US1779492A - Carrier-current telegraph system

Info

Publication number: US1779492A
Application number: US345057A
Authority: US
Inventors: Wilton T Rea
Original assignee: American Telephone and Telegraph Co Inc
Current assignee: AT&T Corp
Priority date: 1929-03-07
Filing date: 1929-03-07
Publication date: 1930-10-28
Anticipated expiration: 1947-10-28
Also published as: FR699469A; US1779493A

Description

Oct. 2 1930. T. REA 1,779,492

CARRIER CURRENT TELEGRAPH SYSTEM v Filed March 7. 1929 3 Sheets-Sheet '1 :INVENTOR ATTORN EY Oct. 28, 1930.

CARRIER CURRENT TELEGRAPH SYSTEM Filed March '7, 1929 3 Sheets-Sheet 2 c I 1/ l 35 l losyv INVENTOR ATTORNEY w. T. REA 1,779,492

-Oct 28,1930. w. 'r. REA

I CARRIER CURRENT TELEGRAPH SYSTEM Filed March 7, 1929 5 Sheets-Sheet 3 iN VENTOR WZ'Rew ATTORNEY Patented @ct. 28,1936

PATENT OFFICE barren srras "WILTON T. REA, OE FLUSHING, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK CARRIER-CURRENT TELEGRAPH SYSTEM Application filed March 7,

This invention relates to telegraph systems, and more particularly to telegraph systems which employ so-called carrier currents for transmitting signals, and discloses 5 novel means applicable to such-systems whereby the carrier current is transmitted directly to the subscribers stations to operate telegraph means thereat without recourse to direct current signaling'over the subscribers loo circuits which is the method now in usel s is well known, a carrier current telegraph system comprises means whereby a single telegraph line extending between two distant points or cities may be adapted to furnish a plurality of distinct telegraph channels such that a number of telegraph stations located in one city may communicate simultaneously with other stations individual thereto in the second city without interference between the various communications. At each city, the carrier telegraph line terminates in a central office and is connected thereat through certain well-known filter arrangements to a number of loop circuits extending to the various subscribers stations. Telegraph signaling is accomplished through the medium of pulses of alternating current of a distinct frequency for each channel, transmittedunder control of sending means at a subscribers station. The received pulses of carrier current in turn operate suitable receiving means at the proper subscribers station. The filters associated with the loops serve the purpose of directing the various signaling frequencies along their proper channels. In accordance with the present practice, the carrier signaling currents comprising the pulses. of alternating current are transmitted only over the telegraph line proper which extends between the central offices. Signaling over the loop circuits has in the past been accomplished by means of wellknown direct current operated telegraph equipment. The change from carrier current to direct current signaling, or vice versa, at the central stations requires the use of a considerable amount of expensive equipment which could be eliminated by transmitting the carrier current'over the loop circuit di- 1929. Serial No. 345,057.

rectly to the subscribers station to operate suitable signaling means thereat.

In the present invention, a number of novel circuit arrangements are disclosed in which the carrier current signals are transmitted to the subscribers station and control the operation of a suitable alternating current relay thereat. The source of-carrier current is, in each case, located at the central station, but is controlled from sending means located at the corresponding subscribers station.

The various modifications disclosing the invention are best explained by reference to the drawings, of which:

Figure 1 shows a full duplex arrangement of a four-wire carrier current'telegraph system in which the carrier current is transmitted directly between the subscribers stations over the loop circuits. In this modification, each subscribers station requires separate sending and receiving loops.

Fig. 2 shows a second modification employing alternating current signaling on the loops as applied to a full duplex four-wire, system. In this case, the sending and receiving loops are combined in a three-conductor circuit.

Fig. 3 shows a half-duplex arrangement for a two-wire system. A single subscribers loop-is usedfor transmitting and receiving. In this modification, the carrier current comprises two distinct frequencies which are demodulated at the receiving station and the beat note utilized for operating the receiving relay.

Fig. 4 shows a half-duplex, two-wire system utilizing a three-conductor loop.

Fig. 5 shows a full-duplex, four-wire system which utilizes a three-conductor loop.

Fig. 6 shows a system applicable to a fourwire telegraph line, which may be operated full or half-duplex, as desired, by operating a key switch. In I this modification, direct current signalingis utilized for the sending ioop and alternating current for the receiving oop.

In all of the drawings, the same designation has been utilized for a given element throughout insofar as this is possible.

Referring to Fig. 1, a portion of a fourwire carrier telegraph line extending to a distant point is shown to the left of the line AA. The portion of the drawing between lines AA and BB represents a central ofiice at which the near end of the carrier telegraph line terminates, and shows the manner in which the carrier line is connected through certain intermediate apparatus to a loop circuit extending to a local subscribers telegraph station. The loop circuits comprise the portion between lines BB and CC, while the subscribers station .is shown to the right of line CC.

At the central station, an oscillator 10 is connected to the midpoints 31 of a-hybrid coil 9, to windings 27 of which is connected a sending loop llextending to the subscribers station. At the subscribers station loop 11 is terminated in an artificial line 13 which simulates the characteristic or surge impedance of the loop. The left windings 26 of the hybrid coil 9 are connected to a balancing network 8 which matches the impedance of the loop circuit 11 as viewed from the central ofiice, and with key 12 at the subscribers station open. A band-pass filter 7, adapted to pass the frequency generated by oscillator 10, is connected to the secondary windings 28 of the hybrid coil. The primary windings of the hybrid coil 9 are connected series-aiding for currents flowing as shown by the arrows. The secondary windings of the coil are connected series-aiding. The output terminals of filter 7 are connected through transformer 3 to line 1 of the four-wire telegraph line,

which constitutes the circuit for sending to the distant point, as indicated by the arrow.

The circuit for receiving from the distant point. comprises line 2 which is connected through transformer 4 to the input terminals of a band-pass filter 14 adapted to transmit a suitable frequency transmitted from an oscillator located at the. distant central oflice. The output terminals of filter 14 are connected to the receiving loop 15 which extends to the subscribers station. At the subscribers station, loop 15 terminates in a relay 16 which is operable by pulses of alternating current. Relay 16 controls the operation of receiving equipment at the subscribers station as is explained below.

The operation of the telegraph system is as follows: Associated with the sending loop 11 at the subscribers station is a sending key 12 which, with the key closed, connects a short circuit across the loop. When the key is open, however, the loop is terminated in its characteristic impedance by means of artificial line 13. Withkey 12 open, current supplied from oscillator 10 divides equally at terminals 31 with half of the current flowing into the network 8 and the other half flowing into the loop circuit 11. The equal currents thus flowing in

windings

26 and 27 set up equal and opposed fluxes with the result that no voltage is induced in windin s 28, and hence no current is transmitted rom oscillator 10 over line 1. Vith key 12 closed, the artificial line 13 is short-circuited and hence the loop impedance as seen from the terminals of windings 27 becomes considerably smaller than that of network 8. As a result, more current flows in windings 27 than in windings 26, thus producing a resultant effect in winding 28, which causes current of the oscillator frequency to be transmitted through filter 7 and over line 1 to the distant station.

At the distant point, line 1 will be terminated in a receiving circuit identical with that shown connected to line 2 of Fig. 1. It can be assumed, therefore, that current sent out as above over line 1 is received at the distant point over a line 2. The received current incoming over line 2 flows through transformer 4, through band-pass filter l4, and over the receiving loop 15, to the distant subscribers station. At the subscribers station, current received over loop 15 flows through the alternating current. relay 16 and operates the same against its marking contact M, as shown on the drawing. \Vhen no current is being received, due to the opening of key 12 at the sending station, relay 16 will be operated against its spacing contact S.

With relay lti in its marking position, as shown, a circuit is completed from battery 20 through the armature and back contact of relay 18. and through telegraph sounder 19 to ground, thus operating the sounder against its marking contact. When relay 16 is operated against its spacing contact S, current from battery 17 flows through the armature and spacing contact of relay 16 and the winding of relay 18 to ground, thus operating the latter. The operation of relay 18 disconnects battery 20 from sounder 19 and thus causes the sounder to be operated against its spacing contact.

From the above description, it is thus apparent that operation of the sending key 12, by unbalancing the hybrid coil bridge, causes pulses of alternating current to be transmitted over the telegraph line to a distant point and then over a receiving loop to a subscribers station to operate an A. C. relay thereat in accordance with the pulses of current received. The A. C. relay in turn controls the operation of D. C. telegraph means at the subscribers station.

In Fig. 1, only one subscribers station has been shown, having the loops associated therewith connected through

filters

7 and 14, respectively, to the telegraph line. It is to he understood. however, that a plurality of subscribers stations would be similarly connected to telegraph lines 1 and 2. The sending loops from the various subscribers stations would all be connected in multiple to conductors 5, while the receiving loops would be similarly connected to conductors 6. Each sending loop would be equipped with a source of oscillating current 10 having a different rraces frequency of oscillation for each such loop. A band-pass filter 7 would be inserted in each sending loop to transmit current of the frequency generated by the corresponding oscillator only. Each receiving loop would be equipped with a band-pass filter 14 adapted to transmit only the current transmitted from the distant sending loop associated therewith. Thus the filters divide thetelegraph system up into a number of distinct channels which use the telegraph line in common.

The sending key 12 and the sounder 19 of Fig. 1 are shown merely for convenience. Any other type of telegraph sending and receiving means could, of course, replace them, such as a printing telegraph sending and re ceiving unit.

Relay 16 could, of course, be any suitable type of relay operable by alternating current of the carrier frequency. It is suggested, however, that a relay of the type disclosed in U. S. Patent No. 1,579,887, to E. J. Pratt,

April 6, 1926, or that disclosed in U. S. patent application, Serial No. 320,799, filed November 21, 1928, by E. J) Pratt, would be best adapted for use in the present instance. The relays disclosed in the references cited are very sensitive types of relays which are operable by alternating current within thevoice frequency range, and hence would fit in well with the well-known voice frequency carrier telegraph system.

The system disclosed in Fig. 1 operates fullduplex since for each station the sending circuit is independent of the receiving circuitr In Fig. 1, four conductors were required for each subscribers loop circuit. Fig. 2 shows a full-duplex system as applied to a four-wire carrier, telegraph line which requires only three conductors for connecting each 'subscribers station with the central oflice.

In Fig. 2, windings 27 of hybrid coil 9 are connected through transformer 21 to conductor 25, comprising one of three conductors 11 extending to the subscribers station. At the subscribers station, conductor 25 is connected through transformer 32 with sending key 12 and artificial line 13. Otherwise, the sending circuit is unchanged from Fig. 1.

The receiving circuit extends from line2 through filter 14, through balancing coil 22 and over

conductors

33 and 34 to the subscribers station, and thence through balanc-- ing coil 24 to the receiving relay 16. Conductor 25 is connected to the midpoints of

coils

22 and 24, as shown. e

The balancing network 8 is so designed that with key 12 open, equal and opposed currents flow from oscillator 10 into

windings

26 and 27, respectively, of coil 9, thus producing no resultant effect in the secondary windings 28. With key 9 closed, however, the current flowing in windings 27 becomes greater than in windings 26, thus inducing a voltage in windings 28.

The current flowing in windings 27 is transmitted through transformer 21 to lead 25. The current in lead 25 divides at the midpoints of

transformers

22 and 24, respectively, so that half of the total current flows through the upper portion of the primary winding of each transformer and half through the lower winding. As a result, no voltages are induced in the secondary windings to operate relay 16 or to flow back "through filter 14.

Signaling currents received over line 2 are transmitted through filter 14, transformer 22, over

loop conductors

33 and 34 to the subseribers station, and through transformer 24 to operate relay 16. The received signaling currents do not flow through transformer 21 into the sending circuit since conductor 25 is connected between equipotential points on

transformers

22 and 24. i

From the above, it will be seen that for a given subscribers station, the sending and receiving channels are independent of each other and hence the system operates full duplex.

Fig. 3 discloses a two-wire half-duplex system with a two-conductor loop extending from the central office to a given subscribers station. This system operates by modulat- Either current alone is too high in frequency to operate relay 16. Current of one frequency is supplied to the local relay 16 from the local oscillator while current of the second frequency is supplied from the distant oscillator.

Referring to the drawing, withkey 12 open, the hybrid coil 9 is balanced so that no currentfrom oscillator 10 gets out onto line 1, and also no currentfiows through the local relay 16 since its winding is in series with the contacts of key 12. With key 112 closed, current from the local oscillator is transmitted over line 1 and also through the winding of the local relay 16and the modulator 23 in series therewith. The current from the local oscillator alone and the modulated components thereof are, however, of too high a frequency to operate the local relay The system of operation can best'be explained by considering what is occurring simultaneously at both ends of a given carrier channel For example, assume that the local oscillator 10 associated with a given channel generates a frequency f while the corresponding oscillator at the distant station generates a frequency f With keys 12 at both stations open, relays 16 at both stations are operated against their spacin contacts since no current can fiow throng the winding of either due to the open circuit at the contacts of the corresponding key 12.

Wit key 12 closed, say at the local station,

' and open at the distant station, relays 16 rethrough the relays 16 at both stations, and simultaneously therewith, current of frequency f from the distant station flows through both relays 16. The frequencies 7, and f are thus intermodulated by modulating elements 23, and the resulting difference frequency f =f f flowing through the winding of each relay 16 operates the same against its marking contact M.

In a specific case, the frequency of the local oscillator might be 4,000 cycles and that of the corresponding distant oscillator 1,500 cycles. In this case, the difference frequency tlvhich operates the relays 16 would be 500 cyc es.

The system necessarily operates halfduplex since the position of relays 16 at both the local and distant stations depends on the operated condition of the sending keys 12 at both stations. Either station may break by holding key 12 open.

Fig. 4 shows a two-wire half-duplex system utilizing three-conductor loops to the subscribers stations. The oscillator 10 is connected through transformer 21 to the central lead 25, which latter is connected to the midpoints of the primary windings on

transformers

22 and 24, respectively With key 12 closed, no current is transmitted to the local relay 16 or over line 1, due to the balanced bridge arrangement of

transformers

22 and 24. Assuming key 12 likewise closed at the distant station, both relays 16 thus are operated against their spacing contacts. WVith key 12 open, the bridge arrangement becomes unbalanced and currents are induced in the secondaries of

transformers

22 and 21, the one being transmitted to the distant station over line 1 and the other operating the local relay 16 against its marking contact M. At the distant station, the

current incoming over line 1 flows through transformer 22 and over loop conductors 38 and 34 and through transformer 24. to operate the relay 16 thereat against its marking contact. The received signaling current produces no effect in the oscillator circuit since conductor 25 is connected between equipotential points on

transformers

22 and 24. If keys 12 are open at both stations, current from the local and distant oscillators are transmitted to each of relays 16 operating the same against their marking contacts. The system is, of course, half-duplex since the opening of either key 12 breaks the message being transmitted.

Fig. 5 shows a full-duplex arrangement as applied to a four-wire system. The arrangement of the subscribers loop is somewhat similar to Fig. 4. In Fig. 5, key 12 is connected in lead 25. The oscillator 10 is connected directly to filter 7 the upper conductor including the subscribers loop circuit'in series therewith. With key 12 closed at the local station, current from oscillator 10 flows out to the distant station over line 1. This current flows in the loop circuit at the local station but it does not affect the operation of the local relay 16 or cause current to be transmitted over line 2 due to the balanced bridge arrangement of

transformers

22 and 24.

The current thus transmitted from the local station comes in over a receiving circuit 2 at the distant station and flows through filter 14 and over the loop circuit thereat to operate relay 16 against its marking contact, as shown. When key 12 at the local station is opened, it disconnects the oscillator from line 1, thereby interrupting the current transmission to the distant station which causes relay 16 thereat to move against its spacing contact. In the same manner, the operation of key 12 at the distant station controls relay 16 at the local station. Transmission in one direction is thus independent of that in the other and the system thus operates fullduplex. The received current produces no effect in the oscillator circuit since the latter is connected between equipotential points on

transformers

22 and 24.

Fig. 6 shows a four-wire system which may be operated half-duplex or full-duplex, as desired, by suitably operating key 37. This system utilizes direct current telegraph equipment for sending over the loop circuit, but on receiving the carrier current is transmitted directly to the subscribers station as before.

\Vith key 37 in the position shown, the system operates half-duplex. -With key 12 open. local oscillator 10 transmits current to the distant station to operate relay 16 thereat against'its spacing contact. At the same time, with key 12 at the distant station open, current is transmitted therefrom which flows in over loop 11 and operates the local relay 16 against its marking contact.

If, now, local key 12 is closed, it completes direct current paths to ground at the key contacts traced over the loop circuit to operate

relays

35 and 36. The operation of relay short circuits the output of local oscillator 10 and thus interrupts the flow of current to the distant station, thereby causing relay 16 thereat to move against its spacing contact. At the same time. the operation of relay 36 short circuits the input to loop 11, and thus interrupts the flow of current transmitted from the distant station to the local relay 16 thereby causing this relay also to move against its spacing contact. In a similar manner, operation of key 12 at the distant station controls the relays 16 at both stlations. The system thus operates half-dup ex.

With key 37 operated, relay 36 is disconnccted from the ocal key 12 and hence no longer operates in accordance therewith. Thus, local key 12 now controls only the operation of the distant relay 16, while key 12 at the distant stations controls only relay 16 at the local stations. The system thus operates full-duplex. I

WVhat is claimed is: i

1. In a telegraph system, the method of telegraph communication which consists in controlling from a branch station the transmission of alternating signaling current from an adjacent central station to a distant station byunbalancing the loop circuit, and utilizing the signaling current received at the distant station to operate the telegraph receiving means thereat.

2. In a carrier current telegraph system including a source of alternating signaling current, the method of carrier current telegraphy which consists in controlling, from any station the transmission of said alternating signaling current to a distant central station, transmitting said alternating signaling current through said distant central station to a branch station adjacent thereto, and utilizing at said distant branch station the received alternating signaling current to operate telegraph receiving means thereat.

3. A telegraph system comprising in combination, a line extending between two telegraph stations, a telegraph transmitting and receiving means at each station, a source of alternating signaling current associated with said line at' a point adjacent to each station, means responsive to the sending means at either station for transmitting said signaling current from the source adjacent thereto to the distant station through the point adjacent to saiddistant station, and means at the distant station responsive to said received signaling current to operate said telegraph receiving means thereat.

4:. A telegraph system comprising in combination, a line extending between two telegraph stations, telegraph transmitting and receiving means at each station, a source of alternating signaling current associated with said line at a point adjacent to each station, means responsive to the sending means at either station for transmitting said-signaling current from the source adjacent thereto to both said stations, and means at the distant station responsive to said received signaling current to operate said telegraph receiving means thereat.

prising a transmission circuit connecting a plurality of central oflices, a plurality of loop circuits associated therewith at each oflice I with each loop extending to a station equipped with telegraph sending and receiving means, filter means associated with said loop circuits for providing a plurality of electrically distinct telegraph channels extending between stations, alternating signaling current of a distinct frequency individual to each loop circuit at a central station, means responsive to the sending means at any said station for transmitting said signaling cur rent associated therewith over the corresponding telegraph channel, and means at thereto at said point and extending to a station equipped with telegraph sending means, means controlled over said loop responsive to said sending means for transmitting said signaling current to said transmission circuit, a relay responsive to alternating signaling current connected to said loop at said station, and telegraph receiving means thereat responsive to said relay.

,8. A telegraph system comprising a transmission circuit, a loop circuit connected thereto at a certain point and extending to a telegraph station, balancing means interposed between said loop and said circuit, a source of alternating signaling current associated with the neutral points of said balanc ing means, telegraph sending means at said station for varying the impedance of said loop circuit whereby said signaling current may be transmitted to said transmission circuit, a relay responsive to alternating signaling current connected to the loop at said station, and telegraph receiving means thereat controlled by said relay.

9. A telegraph system comprising a transmission circuit, a loop circuit connected thereto at. a certain point and extending to a telegraph station, a relay responsive to alternating signaling current connected to said loop thereat, telegraph receiving means at said station responsive to said relay, balancing means associated with said loop, a source of alternating signaling current connected to neutral points of said balancing means, and telegraph sending means at said station for unbalancing'said balancing means whereby said signaling current may be transmitted to said transmission circuit.

10. A telegraph system comprising a transmission circuit, a loop circuit connected thereto and extending to a telegraph station, electrical balancing means interposed between said transmission circuit and said loop. a source of alternating signaling current connected to the midpoints of said balancing means whereby normally no current is transmitted to said circuit, telegraph sending means associated with said loop at said station for unbalancing said balancing means to transmit said signaling current to said circuit, a relay connected to said loop at said station responsive to alternating signaling current, and telegraph receiving means there'- at responsive to said relay.

11. A telegraph system comprising a transmission circuit, a two-conductor loop circuit connected thereto at a certain point and extending to a telegraph station, a relay responsive to alternating signaling current connected to said loop thereat. telegraph receiving means associated with said relay and responsive thereto, impedance elements bridging said loop circuit at said point and at said station, respectively, a third loop conductor connecting the midpoints of said impedance elements, a source of alternating signaling current associated with said third conductor at said point, whereby normally no current from said source is delivered to said relay, and telegraph sending means at said station adapted to vary the impedance in one said loop conductor whereby current from said source is delivered to said transquency associated with said conductor at said central oflice, telegraph sending means at said station for openingone side of said loop between saidimpedance elements, whereby current from said source operates said relay and is delivered to said line, and filter means interposed between said loop and transmission circuit for passing the received and transmitted signaling current.

13. A telegraph system comprising a line extending between two points, and carrier current telegraph sending and receiving means connected to said line at each point,

said means at the first point comprising a loop circuit connecting said line with a telegraph station, telegraph sending contacts and a relay responsive to carrier signaling currents connected to said loop thereat, an impedance element shunting said relay at said station and a second impedance element bridging said loop at the first point, a third loop conductor connecting the midpoints of the respective impedance elements, a current source of definite frequency connected to said third conductor for transmitting signaling current to said line and said relay responsive to said sending contacts, telegraph receiving means at said station responsive to said relay. and filter means interposed between said line and loop for transmitting definite frequencies including that of said source.

14. A telegraph system comprising in combination, a line extending between two stations, telegraph transmitting and receiving means at each station, a source of alternating signaling current associated with said line, means responsive to the sending means at either station for transmitting said signaling current from said source to the distant station through a central office adjacent to said distant station, and means at the distant station responsive to said received signaling current to operate said telegraph receiving means thereat.

15. A telegraph system comprising in combination, a line extending between two stations, telegraph transmitting'and receiving means at each station, a source of alternating signaling current associated with said line at a point adjacent to each station, means responsive to the sending means at either station for transmitting said signaling current from the source adjacent thereto to said transmitting station and to the distant station through the point adjacent to said distant station, and means at said distant station responsive to said receiyed signaling current to operate said telegraph re ceiving means thereat.

16. A telegraph system comprising in combination, a transmission circuit extending between two central oflices, a loop circuit associated with said transmission circuit at each central oflice and extending to a station equipped with telegraph sending and receiving means, a source of alternating signaling current associated with said transmission circuit, means responsive to the sending means at either station for transmitting said signaling current from said source to the distant station through the central office adjacent thereto, and means at said distant station rcsponsive to said received signaling current to operate said telegraph receiving means thereat.

17 A telegraph system comprising in combination, a transmission circuit extending to a central station, a balanced loop circuit associated with said circuit at said central station and extending to a branch station, telegraph sending and receiving means at said branch station, a source of alternating signaling current associated with said loop circuit at said central station, means responsive to said sending means for unbalancing said loop circuit whereby said signaling current may be transmitted to said transmission circuit, and means at said branch station responsive to alternating signaling current for operating said receiving means thereat.

18. A telegraph system comprising incombination, a transmission circuit, a loop circuit connected thereto at a certain point and extending to a telegraph station, a relay responsive to alternating signaling current connected to said loop circuit thereat. telegraph receiving means associated with said relay and responsive thereto, impedance elements bridging said loop circuit at said point and at said station, respectively, a third loop conductor connecting the midpoint of each of said impedance elements, a source of alternating signaling current associated with said third conductor at said point \vherebv normally no current from said source is delivered to said relay, and telegraph sending means at said station adapted to unbalance said loop circuit whereby current from said source is delivered to said transmission circuit.

19. A telegraph system comprising in combination, a transmission circuit extendin between two central oiiices, a loop circuit associated With said transmission circuit at each central oflice and extending to a branch station, impedance elements bridging said loop circuit at each central office and at each branch station, a source of alternating signaling current associated with each loop circuit, sending means at each branch station for opening one side of the loop extending thereto whereby alternating signaling current is transmitted to said transmission circuit, and telegraph receiving means at each branch station responsive to alternating signaling current.

20. A telegraph system comprising in combination, a transmission circuit extending to a central ofiice, a balanced loop circuit associated with said transmission circuit at said central oflice and extending to a branch station, a source of alternating signaling current associated with said loop circuit. telegraph receiving means at said branch station, and telegraph sending means at said branch station for opening one side of said balanced loop circuit whereby alternating signaling current is transmitted from said source to said transmissioncircuit. r

' 21. A telegraph system comprising in coni-- bination, a transmission circuit extending between two central oflices, a balanced loop circuit associated with said transmission circuit at each central office and extending to a branch station, a source of alternating signaling current associated with each loop circuit, sending means at each branch station for unbalancing the loop circuit extending thereto whereby alternating signaling current is transmitted to said transmission circuit, and telegraph receiving means at each branch station responsive to alternating signaling current.

22. A telegraph system comprising in combination, a transmission circuit extending to a central oflice, a balanced loop circuit asso= ciated with said transmission circuit at said central oflice and extending to a branch station, a source of alternating signaling current associated with said loop circuit, telegraph receiving means at. said branch station, and telegraph sending means at said branch station for unbalancing said loop circuit whereby alternating signaling current is transmitted from said source to said transmission circuit.

23. In a telegraph system, the method of signaling which consists in controlling from a branch station the transmission of alternating signaling current from an adjacent central station to a distant central station, transmitting said alternating signaling current through said distantcentral station to a branch station adjacent thereto, and utilizing said received alternating-signaling current at said distant branch station for signaling purposes.

24. In a carrier current telegraph system, the method of signaling which consists in controlling from a branch station the transmission of alternating signaling current from an adjacent central station to a distant central station by unbalancing the loop circuit, transmitting said alternating signaling current through said distant central, station to a branch station adjacent thereto, and utilizing said received alternating signaling current at said distant branch station for signaling purposes.

25. In a telegraph system, the method of signaling which conslsts in transmitting an alternating signaling current from a central station to an adjacent branch station, controlling from said branch station the transmission of said alternating signaling current from said central station to a distant central station, transmitting said alternating signaling current through said distant central station to a branch station adjacent thereto, and utilizing said received alternating signaling current at said distant branch station for signalingpurposes.

26. In a carrier current telegraph system, the "method of signaling which consists in transmitting an alternating signaling current from a centralstation over a loop circuit to an adjacent branch station, controlling from said branch station the transmission of said signaling current from said central station to a distant central station byunbalancing said loop circuit, transmitting said alternating signaling current through said distant central station to a branch station adjacent thereto and utilizing said received alternating signaling current at said distant branch station for operating telegraph receiving means thereat.

27. A telegraph system comprising, in combination, a transmission circuit, a telegraph station associated with said circuit, telegraph transmitting means at said station, a source of alternating signaling current associated with said circuit at a point adjacent to said station, and means responsive to said transmitting means for transmitting said signaling current from the said source to said station and to said transmission circuit.

28. A telegraph system comprising, in combination, a central office, a transmission circuit associated with said central oflice and extending to a distant point, a loop circuit associated with said transmission circuit at said central ofiice and extending to a telegraph station equipped with telegraph sending means, a source of alternating signaling current associated with said loop circuit at said central ofiice, and means responsive to said sending means for transmitting said alternating signaling current from said' source to said sending station and to said transmission circuit. a

29. In a telegraph system the method of signaling, which consists in transmitting an alternating signaling current from a central station to an adjacent branch station, controlling from said branch station the trans;

mission of said alternating signalin current from said central oflice to a istant point, and utilizlng said alternating signaling current at said distant point for signaling purposes.

In testimony whereof, I have name to this specification this. 6th day of March, 1929.

WILTON-T. an}.

signed my