US2367885A

US2367885A - Signaling system

Info

Publication number: US2367885A
Application number: US499184A
Authority: US
Inventors: Orlando J Murphy; Neal D Newby
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1943-08-19
Filing date: 1943-08-19
Publication date: 1945-01-23
Anticipated expiration: 1962-01-23

Description

553C SE xx:

3 Sheets-Sheet 1 O. J. MURPHY EI'AL SIGNALING SYSTEM Filed Aug. 19, 1943 v3 3 qt I1U|D U 1 vi) U l Win n 2 CK x Co ($9 3$9$ 9 302:0

0. J MURPHY INVENTORS a NEWB),

ATTORNEY Jan 23, 1945.

Jan. 23, 1945.

o. J. MURPHY ETAL SIGNALING SYSTEM 3 Sheets-Sheet 2 Filed Au .Fan. 23, 1945.

o. J. MURPHY ETAL SIGNALING SYSTEM I Filed Aug; 19, 1943 s Sheets-Sheet s mad.

ATTO RN V Patented Jan. 23, 1945 2,367,885 ICE SIGNALING SYSTEM Orlando J. Murphy, Summit, and Neal D.-Newby,'

Leonia, N. J assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y.,- a; corporation of New York Application August 19, 1943, Serial-No. 499,184

- Claims.

This invention relates to signaling systems and particularly to telephone systems in which voice frequency currents are used for transmitting sig-- nals.

Objects of the invention are the provision of more reliable signal transmitting means in systerns employing voice frequency signals and particularly the provision of signal transmitting means arranged to reduce the possibility of false operations.

This invention is an improved signaling system comprising means for transmitting signals each of which consists of a plurality of current impulses in succession, each of a different frequency, the transmission of the impulses being effected by the balancing and unbalancing of varistor-resistor bridges, one for each frequency under thecontrol of signal responsive relays. Means comprising a low-pass filter delays the transition from balanced to unbalanced and unbalanced to balanced condition of the bridges responsive to operations of the signal receiving reays.

A feature of the invention is a signaling system comprising. a signal transmitter arranged to transmit signals each consisting of an impulse of current of one frequency followed by an impulse of current of a different frequency and arranged to control the initiation and termination of the signal impulses by controlling the balance of' varistor-resistor bridges through which thesignal sources are individually coupled to the line over which signalsare to be transmitted. The production of transients upon transition from one frequency to the other is minimized by a condenser-resistance network connected across thebridges.

A further feature of theinvention is a signaling system comprising a-signal transmitter arranged to transmit signals consisting of a plurality of current impulses'each of different frequency, the transmission of the impulses being controlled by gas-filled tubes in combination with varistor-resistor bridges, the balance of which is controlled by signal responsive relays.

The invention and its various feature will be further described by consideringthe specific embodiments of the invention shown in the drawings which form a par-tof'this specification. The invention is not limited in its application to the partcular systemshown but is applicable generally to any telephone or other signaling system employing voice frequency signaling.

Referring to the drawings,

Figs. 1 and 2 show in a-toll'ofiice, anoperators position OP, an outgoing trunk circuit O'Il, a

' two-way trunk circuit TWTI, a-toll selector TSl,

a toll line circuit 'ILCI, a signal transmitter STI and a signal receiver SRI; all individually associated withatoll'line TLI ,Fig. 1 further shows an incoming" trunk" circuit I'I'Iconn'ectedto a set of terminalsinthe bank of selector TSI; and- Fig. 3 shows another toll line circuit TLC2,- signal transmitter STZ and signal receiver SR2 adapted for use with the trunk and selector circuits shown in Fig. 1.

The system represented in the drawings in cludes a plurality of toll offices each' of which comprises a toll board with operators positions and cords for answering call'sincoming from local manual and dial ofiices, from toll subscriber's" lines, and from other toll ofiices and for extending incoming calls to'switching' trunks leading to called local manual and dial offices to intertoll trunks and to called toll subscribers lines. Out going jacks are connected to trunk to local of-- fices, to toll subscribers lines and to intertolltrunks. Answering jack are directly connected to recording trunks, to toll subscribers lines, to

intertoll trunks and to incoming trunk circuits detailed disclosure of the cord and operators position circuits provided in each of the toll offices for interconnectingcalling' and called lines and trunks and controlling the completion of toll calls. The outgoingtrunk circuit OTI, incoming trunk circuit ITI, two-way trunk circuit TWTI, and toll selector TSI are all similar to corresponding circuits in the aforementioned King. et al. patent. The operators positions are provided with dials for use in controlling the operation of incoming" and auxiliary toll route selectors in other toll oflices and for controlling the operation of selector and connector switches in local or community dial ofilces.

The signal transmitter STI comprises

relays

230 and 231 which are operatively controlled by relay III] of the outgoing trunk circuit OTI on outgoing calls andcontrolled by relay I24 of twoway trunk circuit TWTI on incoming calls. The signal transmitter ST I further comprises relay 250 controlled by relay 230 and

relays

232 and 236 controlled by relay 2 31. Relay 23B is a polarized relay having a biasing-winding which normally maintains closure of the right contact. Relay 250 is a polarized relay having. three windings, the topwinding being a biasing winding, the bottom winding being a lockingrand biasing winding and the middle winding being an operating winding which is connected loy the delay network 240 to thecontacts of'relay 23D. Normally the middle winding of relay 25B is energized in the same direction as the biasing winding thereby to maintain closure of the right contact of this relay. I When relay 230 o'p'erates, the current through the middle winding. of" relay 250*is re versed thereby to effect closure ofthe left contact of relay 256, the reversal being delayed by the network 246 for about .025 second. The signal transmitter STI further comprises a source Gl of signaling current of frequency Fl, a source G2 of signaling current of frequenc F2, transformers 252 and 251 and

rectifiers

253 and 254 for transmitting current of frequency Fl to toll line TLl and

transformers

262 and 261 and rectifiers 263 and 264 for transmitting current of frequency F2 to toll lin 'ILl. The

rectifiers

253 and 254 and rectifiers 2'63 and 264 are so poled that there is normally a direct current through all of these rectifiers which reduces their impedance. The circuit for this direct current is traced from the positive pole of battery B2 through a simplex circuit path including both upper windings of transformer 252,

rectifier

253 and 254 and both lower windings of transformer 251 to conductor 266, thence through a simplex circuit path including both lower windings of transformer 261, rectifiers 263 and 264, and both upper windings of transformer 262 to th negative pole of battery Bl, the negative pole of battery B2 and the positive pole of battery Bl being connected to ground. With the impedance of

rectifiers

253 and 254 reduced, the bridge comprising these rectifiers and the

resistors

255 and 256 is unbalanced so as to allow the transmission of current from source Gl through transformers 252 and 251 to line TLI if and when the cutofi relay 2l6 of line circuit TLCl is released. With the impedance of rectifiers 263 and 264 reduced the bridge comprising these rectifiers and

resistors

265 and 266 is balanced so as to prevent the transmission of current from source G2 to line TLI. When relay 256 operates as hereinafter described, ground is connected through the top winding of relay 2% to conductor 25l thereby reducing the direct current through

rectifiers

253, 254, 263 and 264 below the critical value so as to increase the impedance of these rectifiers. The

bridge comprising rectifiers

253 and 254 and

resistors

255 and 256 is thereby balanced to prevent the further transmission of current from source GI and the bridge comprising rectifiers 263 and 264 and

resistors

265 and 266 is thereby unbalanced to allow the transmission of current from source G2 to the lin TLI if and when the cut-off relay 2| 6 is released. The energizaito-n of the top winding of relay 256 produces an additional bias which, in combination with the energization of the bottom winding, results in the desired percentage break for dial impulse signals. The circuit for operatively energizing the cut-off relay 2l6 is controlled by

relays

232 and 236 as hereinafter described.

The signal receiver SRl may be similar to that disclosed in the patent to J. G. Walsh, No. 2,366,- 236, granted December 22, 1942.

Assume now that a call incoming to the operators position OP has been answered with answering plug APl of cord CDl, that the calling plua CPI of this cord has been inserted in jack J l of outgoing trunk circuit OTl, the call being one which may be completed through intertoll trunk line TLI. Relay H6 is operated responsive to insertion of plug CPl in jack J l in the manner described in the aforementioned patent to King et al. The operation of relay 6 closes a circuit for operating signal relays 236 and 23l of signal transmitter STI. The operation of relay 236 effects a reversal of the current through the middle winding of relay 256, but this operation of relay 256 is delayed for about .625 second by the delay network 246. The operation of relay 23| opens the normally closed circuit through the upper winding of relay 236, the closing of the right contact of relay 236 being delayed for about .100 second by current charging condenser 235. The operation of relay 23l closes a circuit for operating the slow-to-release relay 232 and relay 232 opens the normally closed circuit through the winding of cut-off relay 2l6 of toll line circuit TLCZ, thereby to effect the release of relay 2l6. Since the

bridge comprising rectifiers

253 and 254 is unbalanced at the time relay 2l6 releases, current of frequency Fl is transmitted through transformer 251, back contacts of relay 2l6, directionally selective coils 2H and repeating coil 2l2 to the conductors of toll trunk line TLI. When relay 256 closes its left contact responsive to the aforementioned reversal of the current through its middle winding, ground is connected through this contact and the bottom winding of relay 256 to conductor 25l thereby reducing the direct current through

rectifiers

253, 254, 263 and 264 below the critical value. whereby the resistance of these rectifiers is substantially increased. The

bridge comprising rectifiers

253 and 254 is thereb balanced to end the transmission of current of frequency Fl and the bridge comprising rectifiers 263 and 264 is unbalanced to initiate the transmission of current of frequency F2 over toll line TLI. The resistor 216 and condenser 21I bridged across conductors 25l and 26l prevent the change from one frequency to the other from occurring rapidly enough to produce transients which might effect the false operation of signal receiver SRl or interfere with the operation of receiver SRI in response to incoming signals. The operation of relay 236, about .075 second after the operation of relay 256, connects ground to conductor 2| 6 to effect the reo-peration of cut-off relay 2l6 and thereby end the transmission of current of frequency F2 to the conductors of toll line TLI. Thus the operation of relays 236 and 23l effects the transmission of a seizure signal over toll line TLl consisting of current of frequency Fl for about .025 second followed immediately by current of frequency F2 for about .075 second.

When the operator dials the digits of the toll route code, and the local office code and called line number if the called line is a dial office, relay I I6 is alternately released and reoperated responsive to each dial impulse in the manner set forth in the aforementioned King et al. patent. Each release and reoperation of relay H6 effects a corresponding release and reoperation of relays 236 and 23l. Each release and reoperation of relay 236 effects a corresponding release and reoperation of relay 256, the delay network 246 being effective to delay the response of relay 256 for about .625 second. Each release of relay 23l opens the circuit for operating relay 232; but, being slow in releasing, rela 232 does not release during the response of relay 23l to dial impulses. The release of relay 23l in response to the first dial impulse of a train closes the circuit through the upper winding of relay 236 causing the release of relay 236. The release of relay 236 disconnects ground from conductor 2l6 thereby releasing the cut-off relay 2l6. The release of relay 2| 6 again connects the upper winding of

transformers

251 and 261 to coils 2H and 2l2; and, since relay 256 is operated, the

bridge comprising rectifiers

253 and 254 is balanced and the bridge comprising rectifiers 263 and264 is unbalanced so that current of frequency F2 is transmitted over line TLI 'for about .025 second. When relay 256 releases responsive to a dial impulse, ground is disconnected from conductor 25I thereby unbalancing the

bridge comprising rectifiers

253 and 254 and balancing the bridge comprising rectifiers 263 and 264 so that the transmission of current of frequency F2 is ended and transmission of current of frequency FI is initiated. When relay 250 reoperates responsive to the end of each dial impulse, ground is -re-- connected to conductor 251 thereby balancing the

bridge comprising rectifiers

253 and 254 and unbalancing the bridge comprising rectifiers 263 and 264 so that the transmission of current of frequency Fl is ended and the transmission of current of frequency F2 is initiated. After the release of relay 236 in response to the first impulse of a train created by the dialing of one digit, this relay remains normal until all of the impulses in the train have been received. When relay 236 reoperates, about .100 second after the reoperation of relay 23! at the end of the last impulse of a train or dial impulses, it reconnects ground to conductor 2l 6 to effect the reoperation of relay 2H] and thus end the transmission of current of frequency Fl. In this manner each train of dial impulses is transmitted over toll line TLI.

If the calling operator operates the ringing key (not shown) relay Illl is momentarily released in the manner described in the aforementioned King et al. patent thereby effecting a momentary release of relays 236 and-23I so as to cause the transmission of a ringing signal comprising current of frequency F2 followed by .4

frequency F2, the same as for a single dial impulse representing the digit one.

When the plug CPI of cord CDI is removed from jack J I, the release of relay H is effected in the manner set forth in the aforementioned King et al. patent. The release of relay IIU causes the release of relays 236 and 23I. The

release of relay 23I causes the successive release h of relays 236 and 2H] whereby current of frequency F2 is transmitted to line TLI. When relay 250 releases, about .025 second after the release of relay 230, the

bridge comprising rectifiers

253 and 254 is unbalanced and the bridge comprising rectifiers 263 and 264 is balanced thereby to end the transmission of current of frequency F2 and initiate the transmission of current of frequency Fl. When relay 23'2 releases, about .6 second after the release of relay 23I, ground is reconnected to conductor 2l6 thereby reoperating relay 2IIJ to end the transmission of current of frequency Fl. Thus a disconnect signal, consisting of current of frequency F2 for about .025 second followed by current of frequency Fl for about .6 second, is transmitted over line TLI If now a seizure signal is received by signal receiver SRI responsive to seizure of the line TLI at its distant end, relay 220 is operated in the manner described in the aforementioned Walsh patent. The operation of relay 220 con nects ground to conductor II4 thereby effecting the operation of relays I20, I2I and I22 of twoway trunk circuit TWTI and the operation of the line relay (not shown) of selector TSI in the manner set forth in the Walsh patent. The selector TSI is selectively operated responsive to dial impulse signals received by signal receiver SRI; and on a call to the incoming trunk circuit ITI, relay ,I6I is operated to light the answering lamp AL in the manner described in the 7 5 Walsh patent. When the call is answered, the operation of relay I64 connects ground to conductor I54, thereby operating relay I24 of two- Way trunk circuit TWTI. The operation of relay I24 closes the circuit for operating relays 230 and 23I of signal transmitter STI whereby an answering or off-hook supervisory signal, similar to a seizure signal on an outgoing call, is transmitted over toll line TLI to the distant office. If the answering operator disconnects, relays I64 and I24 are successively released, causing the release of relays 23D and 23] whereby an on-hook supervisory signal is transmitted over line TLI, this signal being similar to a disconnect signal on an outgoing call.

If an all-trunks-busy or a called line busy condition is encountered on an incoming call, the alternate operation and release of relay I24 is effected in the manner set forth in the aforementioned Walsh patent, thereby causing the alternate transmission of off-hook and on-hook signals over toll line TLI The signal transmitter STI of Fig. 1 may be modified so as to normally balance both of the varistor-resistor bridges by reversing the polarity of the elements 263 and 264 and reversing the polarity of the battery BI. When so arranged and relay 256 is normal, the voltage impressed across the bridges is equal to the voltage of battery B2 minus the voltage of the battery BI and is of such a polarity that the resistance of

rectifiers

253 and 254 is low and the resistance of rectifiers 263 and 264 is high; whereby the

bridge comprising rectifiers

253 and 254 is unbalanced to transmit current of frequency Fl and the bridge comprising rectifiers 263 and 264 is balanced ,to prevent the transmission of current of frequency F2. When relay 256 operates, the voltage impressed across'the bridges is that of the battery BI and of such a polarity that the resistance of

rectifiers

253 and 254 is high and the resistance of rectifiers 263 and 264 is low; whereby the

bridge comprising rectifiers

253 and 254 is balanced to prevent the transmission of current of frequency Fl and the bridge comprising rectifiers 263 and 264 is unbalanced to transznit current of frequency F2.

Assume now that the outgoing trunk, two-way trunk and toll route selector circuits of Fig. 1 are associated with the toll line circuit TLC2, signal transmitter ST2, and signal receiver SR2 shown in Fig. 3. Relays 336 and 33I, of signal transmitter STZ are controlled by relay I ll] of outgoing trunk circuit O-TI on calls outgoing over the associated toll line TL2 and are controlled by relay I24 of two-way trunk circuit TWTI on calls incoming over line TL2. Relays 332 and 336 of the signal transmitter STZ, relay 3I6 of the toll line circuit TLC2 and relay 326 of the signal receiver SR2 are similar to relays 236, 2I0

and 226 in Fig. 2 and perform like functions. The delay network340 performs a like function to that of the network 240 of Fig. 2 and the transformer 344 and gas-filled

tubes

345 and 346 perform like functions to that of relay 250 of Fig. 2. The transmission of current of frequency Fl from source 31I through transformers 312 and 311 is controlled by the bridge consisting of rectifiers 313 and 314and resistors 315 and 316, and the transmission of current of frequency F2 from source 38I through

transformers

382 and 381 is controlled by the bridge consisting of

rectifiers

383 and 384 and

resistors

385 and 386. With tube 345 deenergized, the resistance of rectifiers 313 and 314 is reduced by direct current in a circuit from the positive pole of battery B2, through resistor 35!, conductor 351, right windings of transformer 312, rectifiers 313 and 314, left windings of transformer 311, conductor 359, to the negative pole of battery BI. The bridge comprising rectifiers 313 and 314 is thereby balanced to block the transmission of current from source 31 I, When the equipment is placed in service and thereafter upon closure of the right contact of relay 330, an electromotive force is induced in the right windings of transformer 344, which becomes effective about .025 second after closure of this contact, to energize tube 345. The energization of tube 345 connects ground through resistance 341, to conductor 351 to reduce the direct current through rectifiers 313 and 314. By reducing the current through rectifiers 313 and 314, their resistance is increased to unbalance the bridge and thus render the bridge effective to transmit current of frequency Fl to the toll line T12 if and when the cut-off relay 3H) is released. With tube 346 deenergized, the resistance of

rectifiers

383 and 384 is reduced by direct current in a circuit from the positive pole of battery B2, through resistor 352, conductor 358, right windings of transformer 382,

rectifiers

383 and 384, left windings of transformer 381, and conductor 360 to the negative pole of battery Bl. The

bridge comprising rectifiers

383 and 384 is thereby balanced to block the transmission of current from source 38!. When tube 346 is energized by an electromotive force induced in the input windings of transformer 344 responsive to the reversal of current through the left windings of this transformer about .025 second after the operation of relay 330, ground is connected to conductor 358 through resistor 348, to reduce the current through

rectifiers

383 and 384. By reducing the current through the

rectifiers

383 and 384, their resistance is increased to unbalance the bridge and thus render the bridge effective to transmit current of frequency F2 to the toll line TL2 if and when relay 3H] is released. Condenser 349 is bridged across the anodes of

tubes

345 and 346 to effect the deenergization of tube 345 when tube 345 becomes energized and to effect the deenergization of tube 346 when tube 345 becomes energized. Resistor 353 and condenser 355 and resistor 354 and condenser 356 serve a similar purpose to that of resistor 210 and condenser 21! of Fig. 2. Thus the transmitter STZ effects the transmission of seizure, selective, supervisory, ringing and disconnect signals similar to those above described as being transmitted by the signal transmitter S'Il.

What is claimed is:

1. In a signaling system, a line and a signal transmitter associated with said line, said transmitter comprising a signal responsive relay, 2. source of current of one frequency, means including a varistor-resistor bridge connecting said source to said line, another source of current, the frequency of said other source being different from said one frequency, means including a varistor-resistor bridge connecting said other source to said line, each of said bridges being effective only when unbalanced to transmit current from the associated source to said line, means including a relay controlling the balance of said tridges, and means including said signal responsive relay for operating the last-mentioned relay to effect the transmission over said line of signals consisting of impulses of current from said sources.

2. In a signaling system, a line and a signal transmitter associated with said line, said transmitter comprising a signal responsive relay, a source of current of one frequency, means including a varistor-resistor bridge connecting said source to said line, another source of current, the frequency of said other source being different from said one frequency, means including a varistor-resistor bridge connecting said other source to said line, each of said bridges being effective when unbalanced to transmit current from the associated source to said line and effective when balanced to prevent the transmission of current from the associated source to said line, means including a relay controlling the balance of said bridges, and means including a low-pass network connecting the contacts of said signal responsive relay to the winding of the last-mentioned relay for controlling the operation of said last-mentioned relay, thereby to effect the transmission of signals consisting of impulses of current from said sources corresponding to the signals received by said signal responsive relay, said network being effective to delay the response of said last-mentioned relay to operations of said signal responsive relay for a predetermined interval of time.

3. In combination in a signaling system, an incoming signaling circuit, a signal responsive relay in said circuit, an outgoing signaling circuit, a source of current of one frequency, a source of current of another frequency, means including a normally unbalanced varistor-resistor bridge connecting one of said sources to said outgoing circuit, means including a normally balanced varistor-resistor bridge connecting the other of said sources to said outgoing circuit, each of said bridges being effective to transmit current from the associated source to said outgoing circuit When said bridge is unbalanced and to prevent the transmission of current from the associated source to said outgoing circuit when said bridge is balanced, means including a relay controlling the balance of said bridges, and means including said signal responsive relay for operating and releasing said last-mentioned relay thereby to effect the transmission of signals consisting of impulses of current from said sources through said outgoing circuit.

4. In a signaling system, a line over which signals are to be transmitted and a signal transmitter associated with said line and adapted to transmit over said line signals consisting of a plurality of impulses of current in succession, each impulse consisting of current of only one of two signaling frequencies, said transmitter comprising a signal relay responsive to signals to be transmitted over said line, a source of current of one of said frequencies, a source of current of the other of said frequencies, a normally balanced varistor-resistor bridge connecting one of said sources to said line, a normally unbalanced varistor-resistor bridge connecting the other of said sources to said line, each of said bridges being effective when balanced to prevent the transmission of current from the associated source to said line and effective when unbalanced to transmit current from the associated source to said line, a relay controlling the balance of said bridges, and means including said signal relay controlling said last-mentioned relay to effect the transmission of signals over said line.

5. In a signaling system, a line over which signals are to be transmitted and a signal transmitter associated with said line and adapted to transmit over said line signals consisting of a pluralityof impulses of current in succession, each impulse consisting of current of only one of two signaling frequencies, said transmitter Comprising a signal relay responsive to signals to be transmitted over said line, a source of current of one of said frequencies, a source of current of the other of said frequencies, a normally balanced varistor-resistor bridge connecting one of said sources to said line, a normally unbalanced varistor-resistor bridge connecting the other of said sources to said line, each of aid bridges being effective when balanced to prevent the transmission of current from the associated source to said line and effective when unbalanced to transmit current from the associated source to said line, a relay controlling the balance of said bridges, means including said signal relay controlling said last-mentioned relay to effect the transmission of signals over said line,

and means comprising a condenser-resistance network bridged across said varistor-resistor bridges for delaying the transition from one frequency to the other thereby to minimize the production of transients.

6. In a signaling system, a line and a signal transmitter associated with said line, said transmitter comprising a signal responsive relay, a source of current of one frequency, means including a varistor-resistor bridge connecting said source to said line, another source of current, the frequency of said other source being different from the frequency of said one frequency, means including a varistor-resistor bridge connecting said other source to said line, each of said bridges being effective when unbalanced to transmit current from the associated source to said line and effective when balanced to prevent the transmission of current from the associated source to said line, means comprising two gas-filled tubes controlling the balance of said bridges, and means connecting the contacts of said signal responsive relay to said tubes to render one of the tubes conductive when said relay is operated and the other of said tubes conductive when said relay is released.

7. In a signaling system, a line and a signal transmitter associated with said line, said transmitter comprising a signal responsive relay, a

source of current of one frequency, means includ ing a varistor-resistor bridge connecting said source to said line, another source of current, the frequency of said other source being different from the frequency of said one frequency, means including a varistor-resistor bridge connecting said other source to said line, each of said bridges being effective when unbalanced to transmit current from the associated source to said line and effective when balanced to prevent the transmission of current from the associated source to said line, means comprising two gas-filled tubes controlling the balance of said bridges, means connecting the contacts of said signal responsive relay to said tubes to render one of the tubes conductive when said relay is operated and the other of said tubes conductive when said relay is released. and means effective upon energization of either of said tubes to deenergize the other of said tubes.

8. In a signaling system, a line and a signal transmitter associated with said line, said transmitter comprising a signal responsive relay, a source of current of one frequency, means including a varistor-resistor bridge connectin said source to said line, another source of current, the

frequency of said other source being different from the frequency of said one frequency, means including a varistor-resistor bridge connecting said other source to said line, each of said bridges being effective when unbalanced to transmit current from the associated source to said line and effective when balanced to prevent the'transmission of current from the associated source to said line, means comprising two gas-filled tubes controlling th balance of said bridges, and means comprising a low-pass network connecting the contacts of said signal responsive relay to said tubes to render one of the tubes conductive when said relay is operated and the other of said tubes conductive when said relay is released, said network being effective to delay for a predetermined interval of time the energization of said tubes responsive to operations of said signal relay.

9. In a signaling system, a line over which signals are to be transmitted and a signal transmitter associated withsaid line and adapted to transmit over said line signals consisting of a plurality of impulses of current in succession, each impulse consisting of current of only one of two signaling 0 frequencies, said transmitter comprising a signal relay responsive to signals to be transmitted over said line, a source of current of one of said frequencies, a source of current of the other of said frequencies, a normally balanced varistor-resistor bridge connecting one of said sources to said line, a normally unbalanced varistor-resistor bridge connecting the other of said sources to said line, each of said bridges being effective when balanced to prevent the transmission of current from the associated source to said line and effective when unbalanced to transmit current from the associated source to said line, means comprising one gas-filled tube for controlling the balance of one of said bridges, means comprising another gasfilled tube for controlling the balance of the other of said bridges, and means including said signal relay for effecting the energization and deenergization of said tubes.

10. In a signaling system, a line over which signals are to be transmitted and a signal transmitter associated with said line and adapted to transmit over said linesignals consisting of a plurality of impulses of current in succession, each impulse consisting of current of only one of two signaling frequencies, said transmitter comprising a signal relay responsive to signals to be transmitted over said line, a source of current of one of said frequencies, a source of current of the other of said frequencies, a normally balanced varistor-resistor bridge connecting one of said sources to said line, a normally unbalanced varistor-resistor bridge connecting the other of said sources to said line, each of said bridges being effective when balanced to prevent the transmissionof current from the associated source to said line and effective when unbalanced to trans mit current from the associated source to said line, means comprising one gas-filled tube for controlling the balance of one of said bridges, means comprising another gas-filled tube for controlling the balance of the other of said bridges, means including said signal relay for effecting the energization and deenergization of said tubes, and means comprising a low-pass network connecting the contacts of said signal relay to said tubes to delay for a predetermined interval of time the energization of each of said tubes responsive to operation or release of said signal relay.

ORLANDO J. MURPHY. NEAL D. NEWBY.