US2375053A - Signaling system - Google Patents

Description

H. E. VAUGHAN SIGNALING SYSTEM Filed Aug. 19, 1943 May 1,1 1945.

3 Sheets-Sheet 1 k .235 auf of /NVENTOR By HEI/A UGH/1N I A TTQRMED May 1, 1945.l

H. E. VAUGHAN SIGNALINGI SYSTEM Filed Aug. 19, 194s 3 sheetssneet 2 ,EN v oww l Qn, I l. x Rw Nm Bm gd d aw 351:2

. gUNK. Susu m25 .5E A v N U`U` ATTORNEY.

Mayl, 1945.. H. E. vAUGHAN. "2,315,053

SIGNALING SYSTEM 5 Sheets-Sheet 3 Patented May 1, 1945 UNITED STATES g PATENT OFFICE l. avants` I" f SIGNALING SYSTEM Henry E. Vaughan, Valley Stream, N. Y., `assigner to Bell Telephone Laboratorien` Incorporated,

New York, NQY., a corporation of New York Application August 19, 1943, Serial No.` 499,185

13 Claims. (Cl. 179`27) This invention relates to signaling systems and particularly to telephone systems in which voice frequency currents are used for` transmitting signals. Y I

Objects of the invention are the provision of' more reliable signal transmitting` means in systems employing voice frequency signals and particularly the provision of improved signal transmitters arranged to transmit signals` consisting of impulses of current of different frequencies in succession. Other objects of the inventionare a minimization of transient eifects, a reduction of the time required for changing from .one frequency to another, compensation for delay distortion in the line, and the modiiication of the per cent break of outgoing dial impulse signals to compensate for signal distortion. This invention is an improved signaling system comprising a signal transmitting circuit `arrangement adapted to transmit signals consisting of a plurality of impulses of current in succession, each impulse consisting of current of only one of a corresponding plurality ofsignaling frequencies; in which system, according to one feature of the invention, the necessary timing for the transmission of dial impulses is .obtained by the use of a low-pass delay network and in which, according to another feature, varistor bridges eiiect the starting and stopping of the current impulses and the low-'pass network is effective to shape the envelopes of the signallinpulses to minimize the production of transients at the beginning and end of each signal impulse. Otherl features are the use of crossed coils in combination with the varistor bridges for connecting the sources of signaling `current to the line over which signals are to be transmitted, the biasing of the bridges to control the time required for transition from one frequency to the other, the provision `of means for delaying an impulse of one frequency more than animpulse of the other frequency so as to modify the time relationship ofthe envelopes of the signal impulses to compensate for ldelayfdistortion in the line, and the provision of meansfor modiiying the per cent break of the dialimpulsesignals. Y v

The invention and `its various features will be further described by considering the specific embodiments of the invention shown in the drawings which form a part of this specification. The invention isnot limited in its application to the particular system shown but is applicable generally to any telephoneor other signaling `sys'- tem employing voice frequency currents for signaling.

Referring to the drawings: Figs. 1 and 2 show in a toll oiilce an operators position OP, an outgoing trunk circuit OTI, a two-waytrunk circuit TWTI, a .toll` selector TSI, `a signal transmitter STI, a signal receiver SRI, and a toll line circuit TLCI, all individually associated with an intertoll trunk line TLI Fig. 1 further shows an incoming trunk circuit ITI connected to a. set of terminals in the bank of toll selector TSI and Fig. 3 shows `another toll line circuit TLCZ, a signal transmitter ST2 and signal receiver `SR2 individually associated with an intertoll trunk line TL2 and adapted for use with the trunk and selector circuits shown in Fig. 1.

The system represented` in the drawings includes a plurality of toll offices, each `of which comprises a toll board with operators positions and cords for answering calls incoming from local, manualand dial offices, from toll subscribers lines, andfrom other toll oiiices, and for extending incoming calls to switching trunks leading tocalled local manual and dial offices, to intertollltrunks and to toll subscribers lines-` "Outgoing jacks are connected to trunks to local oces, to toll subscribers lines and to intertollv trunks. Answering jacks are directly connected to recording trunks, to toll subscribers` lines, to intertoll trunks and to incoming trimk circuits terminating in the banks of toll route selectors. Toll route selectors and auxiliary tollroute selectors are provided for useon calls incoming overintertoll trunks to extend connections to called toll, local or communitydial oflices.`

Reference may be had to the patent to R. E. King et al., No. 2,209,777, granted July 30, 1940, `for a detailed disclosure of the cord and operators position circuits provided in each of the toll oices for interconnecting calling and called lines and trunks and controlling the completion of toll calls` The outgongtrunk circuit DTI, incoming trunk circuit ITI, two-way trunk crcuit TWTLfand toll selector TSI are all similar to corresponding circuits disclosed in the aforementioned King etal. patent. The operators positions'are provided with dials for use in controlling the operation of incoming and auxiliary toll route selectors in other toll oilices and for controlling the operation of selector and connector switches in local or community dial ofces. l

I'hetoll line circuit TLCI comprises a repeating coil 2I2, directionally selective coils ZI I and a cut-off relay 2|0 for disconnecting the talking conductors of outgoing trunk circuit OTI from, and connecting the associated signal transmitter STI to, the intertoll trunk line TLI. The directionally selective coils 2| I transmit signaling current incoming over line TLI through conductors 2I3 and 2|4 to the signal receiver SRI without substantial attenuation and transmit voice currents and signaling current from signal transmitter STI to the repeatingr coil 2|2 without substantial attenuation. The coils 2| I reduce the portion of the signal energy transmitted by the transmitter STI which passes into the signal receiver SRI, thereby t reduce the possibility of false operations and reduce interference with the response of the signal receiver SRI to incoming signals.

The signal transmitters STI and ST2 are arranged to transmit seizure, selective, ringing and disconnect signals on outgoing calls and to transmit oli-hook, all-trunks-busy, called-line-busy and on-hook supervisory signals on incoming calls.

The signal transmitter STI comprises relays 230 and23| which are operatively controlled by relay ||0 of the outgoing trunk circuit OTI on outgoing calls and controlled by relay |24 of twoway trunk circuit TWTI on incoming calls. The signal transmitter STI further comprises relays 232 and 236 controlled by relay 23|, a delay network 240 connecting the contacts of relay 230 to varistor bridges 250 and 260, a source 25| of current of frequency FI and a source 26| of current of frequency F2. Relays 232 and 236 control the operating circuit of cut-off relay 2|() of toll line circuit 'ILCI Relay 232 is a slow-to-release relay which is operated responsive to the seizure of trunk circuit OTI and line TLI and remains operated `until the line TLI is released. The transmission of signaling current from source 25| through transformers 252 and 253 is controlled by varistor bridge 250 and the transmission of signaling current from source 26| through transformers' 262 and 263 is controlled by varistorA Veffect a substantial reduction in the resistance of the bridge only when relay 230 is normal; and

' the rectiflers of bridge 26|I`are so poled that the direct current therethrough is of the required polarity and amplitude necessary to effect a substantial reduction in the resistance of the bridge only when relay 230 is operated. Each operation and each release of relay 23|) is effective to reverse the current through the windings of the network 24U to control the resistance of the bridges 25|! and 26|). When the bridge 250 is low in resistance, due to relay 230 being normal, and relay 2 I0 is released as hereinafter described, signal energy from source 25| is transmitted through transformers 252 and 253 and bridge 250 to intertoll line TLI; and when the bridge 260 is low in resistance, due to relay 230 being operated, and relay 2l|l is released, signal energy from source 26| is transmitted through transformers 262 and 263 and bridge 260 to line TLI. The lowpass network 240 delays the change in the resistance of bridges 250 and 260 under the control of relay 236 for about .035 second.

The sending of the voice frequency current impulses by means of varistor bridges, the resistance of each of which is controlled by a direct current circuit which includes a low-pass delay network, is effective to shape the envelope of the impulses The rectiers of bridge 250 are so and thereby minimize the production of transients at the beginning and end of each impulse. Since the directionally selective coils 2| do allow some energy to pass from the signal transmitter STI to the signal receiver SRI, it has heretofore been known to insert a band-pass lter between the transmitter and the directionally selective coils and to insert a band-pass filter between the directionally selective coils and the signal receiver. The use of a 10W-pass network in the direct current circuit which controls the resistance of the varistor bridges is'eiective to delay the build-up and decay of the current through the bridges thereby to shape the envelope of the alternating current impulses transmitted over the line TLI. The reduction in the transients produced at the beginning and at the end of a voice frequency signal impulse, due to the shaping effected by the low-pass network, gives about the same protection against false operation of the signal receiver and against interference with the response of the signal receiver to incoming signals as is obtained by the use of a band-pass lter between the signal transmitter and the directionally selective coils. The use of varistors in place of relay contacts eliminates the possibility of failure to transmit the current impulses, which possibility exists when relay contacts are used to transmit the impulses, and makes possible the elimination of any interval between the end of an impulse of current of one frequency and the beginning of the succeeding impulse of another frequency. The interval of time between such impulses is herein referred to as travel time even though relay contacts are not used to control the current impulses. By introducing a biasing voltage, for instance a battery 241, in series with the bridges 250 and 260, succeeding impulses of the two frequencies may be overlapped to eliminate all travel time; or, if the biasing voltage is of opposite polarity, the travel time may be increased. If the character of the line is such that considerable delay distortion is produced, the impulses of different frequencies Will overlap if sent out in one order and will be separated by an interval if sent out in the reverse order. The overlap is the equivalent of a gap if the signal receiver makes use of volume limiting to protect against operation in response to speech currents; and any gap is undesirable because it requires a longer enablement or preparatory signal. To avoid these diiliculties, a condenser such as the condenser 254 may be connected in parallel with the bridge through which one of the frequencies is transmitted. When the polarity of the voltage applied across the bridge changes in one direction, the charging of the condensen delays the reduction of the resistance of the bridge for a desired interval; and

when the polarity of the voltage changes in theA opposite direction, the discharge of the condenser delays the increase in the resistance of the bridge. Thus, the impulse of this frequency is delayed without distorting its length or its shape.

By changing the relative value of the resistors 24| and 242 at the input end of the delay network 24U, the relative duration of the impulses of the two frequencies is changed a, definite amount, the change in the percentage break depending upon the dial speed. For instance, if resistors 24| and 242 are 1,600 ohms each in one case'and are 1,000 ohms and 800 ohms, respectively, in another case, the impulse of one frequency may be lengthened .005 second and the impulse of the other frequency shortened .005 second; if the dial speed is 10 impulses per second, this change in the per aarzaosc i 3 cent break is 5-per cent, but if the dial speed is ,201 impulses per second, this change in the percent break is IOpercent.

f -The signal transmitter ST2 is similar to the signal transmitterfSTI except that the resistances rif-bridges" 350 and- 35Ilare controlled bya direct current`network which-includes a biasing battery 349 and the associated resistors341 and 348. The resistance of each lof resistors 34-I and 348` is high so that these resistors do not constil tute a substantial load across 4theoutput of network 340. Like elements in the transmitters STI and ST2' are identified by reference characters hav-ing the same tens and units digits.

' The signal receivers SRI and SR2 may be simil5 non of the calling p1ug CPI of a cord `CDI` into 20 jack J I' of outgoing trunk circuit ATI, as described in detail in the aforementioned King 'et alpatent, it closes a circuit through conductor II5 for operating relays 23|]` and 23| of signal-transmitter STI. mally closed circuitl through the upper winding of relay 236 and closes a circuit for operating the slow-to-release relay 232. Relay 2'36 is held nor-` mal by current charging condenser 235 for Aabout .l second after relay 23| operates. of relay 232- opens the normally closed circuit for operating the cut-off relay 2I|I of line circuit TLC I. The release of relay 2|0 initiates the transmission of current of frequency -FI from source 25| through transformer-252, bridge 250, 35

transformer 253, conductors 255`and 256, back contacts of relay 2`I'Il, coils `2I I and repeating coil 2'I2 to toll line TLI. When the resistance of bridge 250 has been substantiallyincreased and the resistance of bridge 26|] has been substantially 40 decreased due to the delayedreversal of the direct current through conductors 243 and 244 `responsive to the operation of relay 230, the transmission of current of frequency FI is ended; and

the transmission ofV current of frequency F2 from 45 source 26|, through transformer 262, bridge 260 and transformer 263 to toll line TLI is initiated. When relay 236 operates about .1 second afterrelay 23| operates, it connects ground to conductor 2I6 toeffect the reoperation of relay 2I0, thereby 50 frequency F2 for about .050 second` is transmitted 55 over toll line TLI as a seizure signal responsive to insertion of plug CPI in jack JI of outgoing trunk circuit OTI When the calling operator dials the various l digits'of a called toll route code, the digits of a 60 called local office code, and the digits of a called subscribers number on calls to local or community dial offices, relay IIDis alternately released and reoperated responsive to each dial impulse, thereby causing like operations of relays 230 and 23|. The release of relay 23|) effects the delayed reversal of the current through conductors 243 and 244 and the release of relay 23| causes the release of relay 236. The release of relay 236 causes the release of cutoff relay 2I3, thereby to 70 initiate the transmission of current of frequency F2 over toll line TLI. When the resistance of bridge^26|l is increased and the resistance of bridge 250 is decreased by a reversal of the cur-- rent through conductors 243 and 244 about .035

The operation `of relay `23| opens the nor- 25 The operation 30 second after `relay 230releases, the. transmission of current offrequency `F2.` ends andthe transmission `of current of 'frequency FI: isl initiated. The reoperationof relay 23| at4 the end of the first dial impulse of a. digit closeslthe circuit `for maintaining the; operative energization of slowto-release relay 232 andagain opens the circuit through the upperwinding of relay 236. Since the `deenergization of the `upper winding` of relay 236 responsive to each. operationof relay 23| is delayed for about .1 second by current charging condenser 235; relay 236remains normal following its release responsive to the first dial impulse of a digit until relay 23| has remained operated for about .1 second` at the end ofv the-last dial impulse of the digit. The reoperaton of relay 23B at the end of each dial impulseeffects the delayed reversal of current through conductors 243 and 244 thereby to end thetransmission of cur:- rent of frequency FI and initiate the transmission of current of frequency F2. Thus, an impulse of current of frequency FI is transmitted for the duration of each dial impulse and an impulse of current of frequency F2 is transmitted for) the duration of the interval between successive. dial impulses and for about .050 second at the end of the last dial impulse of a digit, `this impulse of current of frequency F2 being terminated bythe reoperation of relay 2IU responsive tothe reoperation of relay 236. Y `i The momentary release of` relay IIli of outgoing vtrunk circuit `O'II responsive to an operation `of the ringing keyy (not shown) associated with cord CDI causes `a corresponding momentary release of relays 23D and 23I1thereby causing` the transmission of a ringing signal, which is the lsame as that transmitted responsive to the dialing of the digit 1. i

The release of relay IIB, responsive tof disconnection of cord CDI from jack J I, causes the release of relays 230 and; 23|. The release of relay 23| causes the release of relay 236i thereby to release the cut-off relay 2|!) to initiate` the transmission of, current of frequency F2. The release ofrelay23|` also opens the circuit energizing the winding of relay 232, but relay 232 remains operated. `for about .6 second. The release of relay 23|) causes the delayed reversal of .the current through conductors 243 and 244, whereby the transmission of current of frequency F2 is ended about .035` second after relay 23D` releases and the `transmission `of current of, frequency-FI initiated. Since relays 230 and 23| do not reoperate; the release of relay 232 at the end of an interval of about .6 second causes thereoperation :of relay 2| 0 thereby terminating the transmission of current of frequency FI over toll line TLI. r

Assume now that the toll line TLI has been seized at its distant end and that the signal receiver has responded to the seizure signal to operate relay 220 and thereby connect ground to conductor ||4 to cause the operationiof. relays |20, I2I and |22 of two-way trunk circuit TWTI in the manner set forth in the aforementioned King et al. patent. When dial impulses are received, the selector TSI .is operated to select an idle set of terminals ina selected` level., As'- suming selection of the terminals `connected to the incoming `trunk circuit ITI, the` line relay I6I isk operated and answering lamp AL ener.- gized in the mannerv set forth in theaforementioned King et al. patent. When the call is answered, relay |64 is operated as described in the Kinget al. patent to connect ground to conductor |54 thereby to operate relay |24 of the two-way trunk circuit TWTI. The operation of relay |24 closes a circuit for operating relays 230 and 23| of signal transmitter STI, whereby an olf-hook supervisory signal is transmitted over line TLI in the same manner as a seizure signal on an outgoing call. When the answering operator disconnects; relay |64 is released causing the release of relays |24, 230 and 23|, whereby an on-hook signal is transmitted over toll line TLI, this signal being the same as a disconnect signal on an outgoing call. If the selector TSI encounters an all-trunks-busy condition or if a called line in a local office is busy, the alternate operation and release of relay |24 are effected as described in the King et al. patent; and the resulting alternate operation and release of relays 230 and 23| of signal transmitter STI effect the alternate transmission of off-hook and on-hook signals over toll line TL I.

The signal transmitter ST2 shown in Fig. 3 differs from the signal transmitter STI in that the biasing of the bridges 350 -and 360 is effected by connecting the biasing battery 349 through resistors 341 and 348 to the inner ends of the bridges 350 and 360 and through resistors 345 and 346 to the opposite sides of the delay network 340.

What is claimed is:

1. In a signaling system, a signal transmitter adapted to transmit 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, means comprising a pair of crossed coils and one of said bridges connecting one of said sources to the line over which signals are to be transmitted, means comprising another pair of crossed coils and the other of said bridges connecting the other of said sources to the line over which the signals are to be transmitted, and means including a delay network and the contacts f said relay for varying the resistances of said Varistor bridges to effect the transmission of signal impulses over said line.

2. In a signaling system, a line and a signal transmitter 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, two pairs of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said line only when the resistance of said bridge is low, and means for rendering the resistance of one of the bridges low while said signal responsive relay is normal and for rendering the resistance of the other of the bridges low while said signal responsive relay is operated.

3. In a signaling system, a line and a signal transmitter 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two Varistor bridges, two pairs of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said line only when the resistance of said bridge is low, and means including a low-pass network connecting the contacts, of said relay to said bridges for rendering the resistance of one of said bridges low when said signal responsive relay is normal and for rendering the resistance of the other of said bridges low when said signal responsive relay is operated, said network being effective to delay the change in the resistance of said bridges for a predetermined interval of time.

4. In a signaling system, a line and a signal transmitter 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, two pairs of transformers, means comprising one of said bridges and o-ne of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair of transformers connecting the other` of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said line only when the resistance of said bridge is low, means including a low-pass network connecting the contacts of said relay -to said bridges for rendering the resistance of one of said bridges low when said signal responsive relay is normal and for rendering the resistance of the other of said bridges low when said signal responsive relay is operated, said network being effective to delay the change in the resistance of said bridges for a predetermined interval of time, and means comprising a condenser connected across one of said bridges for increasing the delay in the change in the resistance of said one of the bridges.

5. In a signaling system, a line and a signal transmitter 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, two pairs of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit curbridges to decrease the overlap in the transmission of an impulse of current-of one frequency and the transmission of an impulse of `current of` the other frequency'r thereby to reduce the interval between the effective" portions of said impulses. i

6. In a signaling system, a line and a signal transmitter adapted totransmitisignals consisting of a plurality of impulses of current in succession, each impulseconsisting of Vcurrent of only one of two signalingA frequencies, `said transmitter comprising a signal responsive relay, a source of current of one of said frequencies, a source of current of theother of said frequencies, two varistor bridges, `two` pairs of y transformers, means comprising one of said bridges and` one of said pairsof transformers connecting one of vsaid sources to said line,` means comprising the .other of `said bridges and the other pair of transformers connecting the` other of said sources to said line, `the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said `line only l when the resistance of said bridge islow, means for rendering the resistance of one of the bridges low while .said signal responsive relay isnormal and forirendering the resistance ofthe other of thel bridges Alow while saidv signaliresponsive relay is operated, 'and means for biasing saidbridges to increase `the overlap in the transmission of an impulse of current of one frequency and the transmission of an impulse of current of ,the4

other frequency thereby to increase the interval between the effective portions of `said impulses.

7. In a signaling system, a line and a signal transmitter adapted to transmit, over said line signals consisting of a yplurality of impulses of current in succession, each impulse consisting of currentof only one of two signaling frequencies,

said transmitter comprising a signal responsive relay, a source of current of one Vof said frequencies, a `source of current of the other of' said frequencies, two varistor bridges, two pairs of transfcrmers, means comprisinglone'of said bridges and one of said pairsof transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair l' of transformers connecting the other of lsaid sources to'said line. the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said line only whenthe resistance of said bridge is low, means including a low-pass network connecting the contacts of said relay to said bridges for rendering the` resistance of one of said bridgeslow when said signal responsive relay is normal and for rendering the resistance of the other of said bridges low when said signal responsive relay is operated, said network being effective to delay the change in the resistance of said bridges for a predetermined interval of time, means comprising a condenser connectedacross one of said bridges for increasing the delay in the change in the resistance of said one of the bridges, and means for biasing said bridges to change the overlap in the transmission of current frequencies, two varistor bridges, two pairs `of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one `of said sources to said line, means comprising the other of said bridges and the other pair `of transformers connecting the other ofsaid Asources to" said: line, the connection4 between each pair of transformers andthe associatedv bridge being effective to: transmit current from `the associated `sourceto said line only when the resistance of said bridge is low, means `including :a low-pass network connecting the contacts of said relay to said bridges for rendering the resistance of Aone of said bridges low when said signal responsive relay isV normal and for rendering the resistance ofthe other of said bridges low when 'saidsignal responsive relay is operated, said network being `effective to delay the change :in the resistance of said bridges for a predetermined interval of time, a resistor connectedin series with the coils of said delay network when said relayis normal, and arresistor connected in"series with the coils'of said network when said relay is operated, the delay `in changing the resistance of each of said bridgesresponsive to the operation and release of said relay being dependent upon the value of the resistance which ,is connected in series with said coils.

9. In a signaling system, a lineiand a signal transmitter adapted to transmit over-saidzline signals consisting of afplurality off'impulses of current in succession,` each impulsev consisting `of current of only one of two signaling frequencies, said transmitter comprising a signal responsive relay, a source of current of one of said frequencies,a sourceiof current ofthe other of said frequencies, two varistor bridges, twopairs `of transformers, means comprising one of saidbridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of `said bridges and the otherpair of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source` to said line only when the resistance of said bridge is low, means including a lQWDss network connecting the `contacts of said relay to said bridges for rendering the resistance of oneof said bridges low when said signal responsive relay is normal and for rendering the resistance of the other of said bridges low when said signal responsive relay is operated, said network being effective to delay the change in the resistance of said bridges for a predetermined interval of time, a resistor-connected in series with the coils of said delay network when said relay is norsaid coils, and means comprising a condenser connected across one of said bridges for increasing the delay in the change in the resistance of said one of the bridges. i

10. In a signaling system, a line and a signal transmitter 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 responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, two pairs of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being eiective to transmit current from the associated source to said line only when the resistance of said bridge is low, means for rendering the resistance of one of the bridges low while said signal responsive relay is normal and for rendering the resistance of the other of the bridges low while said signal responsive relay is operated, and means for biasing said bridges to introduce a desired interval of time between the end of an impulse of current of one frequency and the beginning of an impulse of current of the other frequency.

11. In a signaling system, a line and a signal transmitter adapted to transmit signals consisting of a plurality of impulses of current in succession, each impulse consistingof current of only one of two signaling frequencies, said transmitter comprising a signal responsive relay, a source of current of one of said frequencies, a source of current of the other of said frequencies, two varistor bridges, two pairs of transformers, means comprising one of said bridges and one of said pairs of transformers connecting one of said sources to said line, means comprising the other of said bridges and the other pair. of transformers connecting the other of said sources to said line, the connection between each pair of transformers and the associated bridge being effective to transmit current from the associated source to said line only when the resistance of said bridge is low. means for rendering the resistance of one of the bridges low while said signal responsive relay is normal and for rendering the resistance of the other of the bridges low while said signal responsive relay is operated, and means for biasing said bridges to initiate the transmission of an impulse of current of one frequency a desired interval of time prior to the ending of an impulse of current of the other frequency.

12. In a signaling system, a signal transmitter adapted to transmit signals consisting of a plurality of impulses of current in succession, each of said impulses consisting of current of only one of a plurality of signaling frequencies, said transmitter comprising a signal responsive relay, sources of current, each of said sources being of a diiferent ione of said signaling frequencies, varistor bridges one foreach of said sources, means including said bridges connecting said sources to the line over which signals are to be transmitted, each of said bridges being adapted to transmit a signal impulse from the associated source to said line when the resistance of the bridge is low and to prevent the transmission of a signal impulse from the associated source to said line when the resistance of said bridge is high, means comprising said signal responsive relay. for controlling the resistance of each of said bridges to render each of said bridges low in resistance one at a time, and means comprising a low-pass network connected in series with each of said bridges for shaping the envelope of the impulses of current of each of said frequencies thereby to minimize the production of transients at the beginning and end of each impulse.

13. In a signaling system, a signal transmitter adapted to transmit signals consisting of a plurality of impulses of current in succession, each of said impulses consisting of current of only one of a plurality of signaling frequencies, said transmitter comprising a signal responsive relay, sources of current, each of said sources being of a dierent one of said signaling frequencies, varistor bridges one for each of said sources, means including said bridges connecting said sources to the line over which signals are to be transmitted, each of said bridges being adapted to transmit a signal impulse from the associated source to said line when the resistance of the bridge is low and to prevent the transmission of a signal impulse from the associated source to said line when the resistance of said bridge is high, means comprising said signal responsive relay for controlling the resistance of each of said bridges to render each of said bridges low in resistance one at a time, and means comprising a low-pass network connected in series with each of said bridges for delaying the change in resistance of said bridges a desired interval of time and for shaping the envelope of the impulses of current of each of said frequencies thereby to minimize the production of transients at the beginning and end of each impulse.

HENRY E. VAUGHAN.

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