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July 30, 1946. 2,404,773 R. H‘- BROWN APPARATUS FOR COMMUNICATION SYSTEMS Filed Aug. 30, 1945 ' IN VENTOR. . ?obem‘EBmwzz. Hrs Arron/var.‘ Patented July 30, 1946 2,404,773 UNITED STATES PATENT OFFICE 2,404,773 APPARATUS FOR COMMUNICATION SYSTEMS Robert H. Brown, Westview, Pa., assignor to The Union Switch & Signal Company, Swissvale, Pa., a corporation of Pennsylvania Application August 30, 1945, Serial No. 613,627 10 Claims. (Cl. 179-2) 1 2 My invention relates to apparatus for com munication systems, and more particularly to of my invention is the provision of novel and calling apparatus for telephone systems. improved calling apparatus for communication systems. A single pair of line wires is often used as a Another feature of my invention is the provi transmitting channel common to several differ sion of improved voice operated calling apparatus ent communication systems. For example, a two for a telephone system. wire line circuit along a railway may be used for Again, a feature of my invention is the provi a. dispatcher’s telephone circuit, a commercial sion of calling apparatus of the type here in telephone circuit, a telegraph circuit and a cen volved incorporating novel time delay means tralized tra?ic control circuit. In each such cir 10 which may readily be adjusted as to its delay cuit, the current is of a characteristic individual period, and which means requires that the calling to the system and in the centralized trafiic con current be continued during the full delay period trol system, coded current of a time code type may and if there is an interruption in the calling cur be used. Thus, an operator at an oi?ce governs rent, the full delay period is reestablished and through the centralized traffic control circuit the lo the timing cycle must be restarted. position of train signaling devices located at dif Still another feature of my invention is the ferent stations along the railway, current pulses provision of calling apparatus of the type here of a preselected code being used to select a de contemplated incorporating novel means to pro sired station and _a particular signaling device at vide an answer back to the person calling. the selected station. Also, in such centralized 20 Another feature of my invention is the provi traf?c control systems coded current is transmit sion of improved calling apparatus of high sensi ted from the different stations to the cf?ce to in tivity and which is readily adjustable as to its dicate the positions of the signaling devices and sensitivity. the positions of trains in the vicinity of such sta Other features, objects and advantages of my tions. In such a centralized traffic control systems telephone communication is usually provided ' invention will appear as the speci?cation pro gresses. The foregoing objects, features and advantages between each of the stations and the office, so of my invention are accomplished through an that a train crew member or a maintainer may omce calling unit which incorporates a power converse with the office operator in matters per 30 pack, a detecting means, a time delay means and taining to train movements or the condition of an answer back device. the train signaling devices. In such telephone The power pack is adaptable of converting en facilities it is advantageous to have apparatus ergy from a single alternating current source into wherewith a person at any one of the stations can direct current and alternating current of volt call the o?ice operator. Such calling apparatus 35 ages suitable for tne different elements of the ap cannot readily be of the common form for tel ephone calling apparatus because of the inter paratus. The detecting means includes a receiving or ference with the codes of the centralized traf?c selector circuit, a ?rst electron tube and a ?rst control system. Also, this calling means must control relay. The selector circuit is coupled to 40 be free from interference with the currents used the line circuit and is tuned to pass a preselect by the other facilities of the line circuit. Again, ed band of voice frequencies. The energy passed the calling indicators effected by the calling by this selector circuit is applied to a control means must be of a distinctive character because electrode of the ?rst electron tube which is pref at such an o?lce there are usually several differ erably of the controlled ionization type and the ent indication bells and lights to indicate train " tube is tripped in response to such energy and movements and other operating conditions. when thus made conductive the ?rst control re Furthermore, this calling means must be of rel lay is energized by the conduction current. Thus atively high sensitivity to provide reliable oper this detecting means is responsive to energy of ation through a relatively long line circuit and because of wide variations of the voltage of the 50 the calling current frequency regardless of the origin of such energy. A biasing voltage obtained usual power source. At the same time the calling from the power pack is applied to the ?rst elec means must be free from operation by conver tron tube and it is made adjustable and the call sation and noise energy that appears on the line ing can be set for high sensitivity. Also the sen circuit. ' In View of the foregoing conditions. a feature 55 sitivity of the calling unit can be adjusted in the 2,404,773 3 ?eld to the condition of the particular line cir cuit with which it is connected. The time delay means includes a capacitor, 2. second electron tub-e and a second control relay. The capacitor is connected to the power pack 4 centralized tra?ic control system are not shown. The type of centralized traffic control system is immaterial and it may be any one of several of the well known arrangements, The office of such a system is indicated by a dot and dash rectangle and is charged at a preselected direct voltage. OF and one station of the system is indicated by This capacitor is also connected to the second a dot and dash rectangle ST, only the one station being shown since the stations will have substan tially duplicate facilities, and this one station is prevents the tube from being normally tripped. When the ?rst control relay is energized, this 10 sufficient to illustrate the invention. The station ST is provided with a telephone capacitor of the time delay means is disconnected tube to provide that tube with a bias voltage that from the power pack and is connected to a dis charge circuit including a resistor and the capaci tor is discharged at a predetermined rate. When the discharge of the capacitor is carried far enough the bias of the second tube is reduced to a point where that tube is tripped and the second control relay is energized by the conduction cun rent of the tube. If for any reason the discharg ing of the capacitor is interrupted it is quickly recharged and the time cycle must be restarted. This arrangement provides a selective control against random noise and conversation of the calling current frequency band operating the sec ond relay, such noise energy and conversation energy of this frequency band being ordinarily of very short duration, so that the capacitor is set which includes a transmitter 2 and a receiver 3 of the standard form and which set is connected across the line wires through a connection which preferably includes a blocking capacitor 4. The of?ce GE‘ is provided with a telephone set TS, a source of alternating current, a calling unit indicated as a whole by a dotted rectangle CU, calling indicators comprising a lamp 5 and a bell 6, and a reset switch RS. The office telephone set TS may be of any suit able form and may comprise a standard hand set or a loud speaker or a combination of these devices. The office source of alternating current having terminals BX and CK may be of a usual com mercial frequency and this source is connected to terminals of the calling unit CU, terminal vBX of the source being connected to terminal "I of the second tube. The second electron tube is provided with a stick feature by which the tube 30 the calling unit and terminal CX of the source being connected to the two terminals 8 and 9 of is retained conductive regardless of the charge the unit, the connection to terminal 9 including of the capacitor once the tube has been tripped. a normally closed contact iii of reset switch RS, The second control relay controls a call indi as will be readily understood by an inspection cator, such as a lamp, and this visual indicator of the drawing. once established remains in effect until the op The calling unit CU includes a power pack erator acknowledges the call by operating a reset comprising a transformer Tl, a twin diode recti switch. The second relay also governs a novel ?er tube RT, and two potentiometers, one in circuit associated with the ?rst tube and through cluding a resistor R3 and the other including which novel circuit the first tube is transferred two resistors R13 and R5 in series; a detecting from the selector circuit to the time delay means means comprising a transformer T2, a ?rst elec and the ?rst tube is rendered conductive for a tron tube Vi and a first control relay A; a time predetermined time interval. A call bell is in delay means comprising a capacitor Cl, a second terposed in the anode circuit of the ?rst tube in electron tube V2 and a second control relay B; place of the ?rst control relay, and when the and an answer back device AB. ?rst tube is tripped for this time interval the The power pack of the unit CU is energized call bell is rendered active as an audible ‘call in from the of?ce source of alternating current and dication. > provides ?lament voltage from the tubes and a The second control relay when energized also direct voltage suitable for the biasing of the tubes. completes an energizing circuit for the answer A primary Winding H of transformer TI is con back device which is preferably of the buzzer nected to terminals l and s, and a ?rst secondary type, and this buzzer is operated ‘to supply to winding E2 of the transformer is connected to the line circuit an answer back current. the filaments of the tubes as indicated conven I shall describe one form of apparatus .em tionally in the drawing, a mid'terminal of second bodying my invention, and shall then point out :1 Lu ary winding l2 being connected to ground elec the novel features thereof in claims. trode IS. A second secondary winding M of The accompanying drawing is a diagrammatic transformer Tl is connected across the anodes view showing a form of apparatus embodying my of the two sections of the twin diode tube RT, invention when used in a centralized traffic vcon and the cathodes of these two sections of the tube trol system for railways. are connected in multiple to one terminal of the It is to be understood that the invention ‘is potentiometer resistor R3 and to one terminal not limited to a centralized traf?c control system of the potentiometer resistors R4 and R5. The in its use and this one use serves to illustrate the other terminal of resistor R3 and the other ter many places the apparatus is useful. minal of resistors R18 and R5 are connected to a Referring to the drawing, the reference char mid terminal of the secondary winding M. Con acters Y and Z designate line wires which extend sequently a direct voltage is created across these along a railway, and which are used as a circuit potentiometers due to the recti?ed current, the for different facilities, such as a dispatcher’s tele not discharged for the required period to trip value of such direct Voltage being predetermined according to the proportioning of the parts. A that the invention is not limited to a pair of line 70 by-pa-ss capacitor C2 is preferably connected phone circuit, a telegraph circuit anda central ized traffic control circuit. It is to be understood across the potentiometer-s to by-pass the alter hating current ripples of the recti?ed current. The detecting means is provided with a receiv communication only. In order not to complicate ing or selector circuit which comprises the two the drawing, these facilities except that of the elephone communication associated with the 75 parts of a primary‘winding l5 of transformer T2 wires used in several different circuits and the line wires Y and 2 may be used for telephone 2,404,773 5 connected in series through'a capacitor I6 and which circuit is connected to terminals I1 and I8 of the unit CU and which terminals I‘! and I8 in turn are connected across the line wires Y and Z in multiple with the o?ice telephone set. This selector circuit is tuned to pass a preselected band of voice frequencies which, for illustration, may contact 34 of relay A and back contact 3| of relay B to the other terminal of the capacitor. Thus capacitor CI is discharged at a rate predeter mined by the value of the resistor R6 and the set ting of the intermediate terminal 33. The inter mediate terminal 31:!v is preferably mechanically connected to a manually operable knob, not be taken as being of the order of 1000 cycles per second, but other voice frequencies can be used. shown, mounted on the calling unit for adjust I have found in practicing the invention that a 10 The electron tube V2 of the time delay means may be any one of several well-known ‘types and is shown as a hot cathode gas ?lled tetrode. Tube calling band produced by having the person call ing to whistle or blow into the telephone trans mitter is satisfactory. A secondary winding I9 of transformer T2 is ment of the discharge rate of the capacitor CI. V2 is provided with a connection to the capacitor CI to bias the tube according to the charge of connected to a control electrode of tube VI to 15 the capacitor. Normally, the positive plate of capacitor CI is connected to cathode 35 of tube control this tube in response to the energy passed V2 through wire 40 and the negative plate of by the selector circuit. To be explicit, a circuit the capacitor is connected to control electrode can be traced from the control electrode 20 of 36 of the tube through back contact 31 of relay tube VI, through resistor 2I, back contact 22 of B and resistor 38. Thus control electrode 35 control relay B, secondary winding I9, interme is negative in potential with respect to the oath diate terminal 23, of the potentiometer resistor ode 35 by a voltage predetermined by the charge R3, a portion of resistor R3 and wire 24 to cath of capacitor CI. Tube V2 is provided with an ode 25 of the tube VI. It is clear that the con anode circuit receiving energy from the alternat trol electrode 29 is provided with a bias voltage ing current source, the circuit extending from from the resistor R3, the control electrode being terminal BX of the source through terminal ‘I negative in potential with respect to the cathode of the unit CU to a winding of relay B, anode by a voltage predetermined by the setting of the 39 and tube space to cathode 35 of tube V2, intermediate terminal 23. I preferably mechani Wires 40, 24, 29 to terminal 9 of unit CU and cally connect the intermediate terminal 23 to a manual operable knob, not shown, mounted on 30 terminal II! of reset switch RS to terminal CX of the alternating current source. the calling unit for ready adjustment of the bias The parts are proportioned for the voltage ap voltage for the tube VI . This negative bias volt plied to the anode of tube V2 to be ine?ective to age for the tube VI is opposed by the positive half trip the tube when the control electrode 36 is cycles of the electromotive force induced in the winding I9 due to the current passed by the se 35 biased by the voltage of capacitor CI, but when capacitor CI is discharged to a preselected point lector circuit. ' the tube V2 is tripped and relay B is energized. Tube VI may take any one of several types and Relay B when energized to close front contact is disclosed as a hot cathode gas ?lled tetrode. 4I completes a stick circuit connection between Tube VI is provided with an anode circuit receiv ing energy from the alternating current source, 40 control electrode 36 and cathode 35 and the tube retained conductive due to control electrode 36 the terminal BX of the source being connected and cathode 35 being at substantially the same through terminal ‘I of the unit to a winding of potential. That is to say, the tube V2 is re the control relay A and thence through resistor tained conductive until such time as the anode 25, back contact 21 of relay B, anode 28 and tube circuit is opened due to the operation of the reset space to cathode 2-5 of tube VI, wires 24 and 29, switch RS in a manner to appear later. terminal 9 of the unit CU, and contact II) of the With relay B picked up to close front contact reset switch RS to the other terminal CX of the 42 the call indicator 5 is energized by current alternating current source. The parts are so pro flowing from the top terminal of the secondary portioned that the bias negative voltage applied to control electrode 20 from resistor R3 prevents 50 winding I2 of transformer TI through wire 45, front contact 42 of relay B, terminal 46 of the the anode voltage from tripping tube VI but a unit CU, lamp 5, and ground electrodes 41 and relatively small voltage of secondary winding I9 I3 to the mid terminal of secondary winding I2. That is to say, the call indicator lamp 5 is illumi nated as long as the control relay B is energized. Relay B on picking up to close front contacts 43 and 48 completes circuits by which tube VI is the relay energized during the negative half cycles disconnected from the receiving or selector cir of the alternating current when the tube is non cuit and is connected to the'capacitor CI andby conductive. Capacitor CI of the time delay means is con 60 which the relay A is disconnected and bell 6 is interposed in the anode circuit of tube VI. Since nected across resistor R4 through a charging cir relay B is not energized until the capacitor CI cuit which can be traced from the top terminal of is substantially discharged, the bias voltage ap resistor R4 as viewed in the drawing, through plied from capacitor CI to control electrode 20 capacitor CI, back contact 3| of relay B and back contact 32 of relay A to the junction terminal of 65 of tube VI through the connection including front contact 48 of relay B is low and tube VI resistors R4 and R5, and the capacitor CI is is tripped and anode current ?ows through the charged at a direct voltage equal to the voltage bell 6 from terminal BX of the current source drop across resistor R4, When relay A is picked to terminal 49 of unit CU and thence through up in response to the tripping of the electron tube V I of the detecting means, the condenser CI is 70 resistor 50, front contact 43 of relay B, anode 28 and tube space to cathode 25, wires 24 and disconnected from the charging circuit and is 29, terminal 9 and contact IQ of switch RS to connected to a discharging circuit and which dis terminal OK. The bell 6 is thus sounded as an charging circuit can be traced from the top termi audible call indicator while tube VI is retained nal of capacitor CI through a portion of resistor R6, an adjustable intermediate terminal 33, front 75 conductive by this circuit. Relay A when deen to oppose the bias voltage causes the tube to be tripped and relay A to be energized by the cur— rent passed by the tube. A capacitor 30 is con nected across the winding of relay A to retain 2,404,773 8 ergized to ‘close back contact 32 completes the charging circuit for vcapacitor CI, the charge be the call hell 6 rings for a preselected ‘period of say two seconds. The picking up of control relay B also ener gizes the answer back device AB and the device charging circuit due to relay B picking up to Cl AB is operated to supply current to the line cir ing built up on the capacitor at a predetermined rate because resistor R2 is now included in the open back contact 3|. After a preselected time interval of say 2 seconds, the charge on capacitor CI builds up to a voltage su?icient to prevent tube VI from tripping during the positive half cycles of the alternating current and tube VI is deionized and thebell? ‘stops ringing. The picking up of relay B to close front con tact 44 completes a circuit from the answer back device AB, which device is energized and oper ated to ‘supply a current to the line circuit. The device AB may be of ‘any one of several different forms, ‘and is shown as a buzzer ener gized by alternating current from secondary winding 52 of transformer TI, the circuit ex tending from the top terminal of secondary wind- 1 ing I2, through wire 45, front contact 44 of relay B, secondary winding ii ‘of transformer T2, winding of the buzzer AB, back contact 52 of the buzzer, and ground electrodes 53 and, I3 to the mid terminal of secondary winding I2. With the buzzer thus energized, current is transferred from winding 5I of transformer T2 to winding I5 and from winding I5 current is applied to the line wires for operating the telephone receiver at the station ST. The parts are so proportioned that . this answer back current thus applied to the line circuit would produce a distinctive tone in the telephone receiver at the station ST. In describing the operation ‘of the apparatus I shall assume that ‘power is applied to the unit CU and no energy is received from the line cir~ cuit. Under this normal condition, the tubes VI and V2 are biased to a non-conductive condition by voltages supplied from the power pack and re lays A and B are d‘eenergized so that the call indicators 5 and I3 are inactive. Assuming that a person at station ST wishes to telephone to the o?lce operator, the telephone set at the station is removed from the hook and the calling person either whistles or blows, as prearranged, into the transmitter at the station. This action creates a calling current of a pre selected frequency band, and which calling cur rent is applied to terminals I? and I8 of the o?ice calling unit and is passed by the selector circuit. .\ The resultant electromotive force induced in sec ondary winding I9 is supplied to the control elec trode 257 of tube VI to drive the control electrode in the positive direction with the result that the tube is tripped causing the control relay A to be energized. Relay A on picking up transfers ca pacitor CI of the time delay means to its dis charge circuit and the direct voltage across the capacitor is discharged at a rate predetermined by the setting of the intermediate terminal 33 of 60 the resistor R6 as described h'ereinbefore. Assuming the person calling continues his call for a preselected interval, the discharge of ca pacitor CI is carried to a point where the bias of tube V2 is reduced in voltage to a point that permits the tube V2 to trip and the control relay B is energized. With control relay 3 energized to close front contact 4|, tube V2 is sealed in by its stick feature and remains conductive re gardless of the charge on the capacitor Cl. The calling lamp 5 is now illuminated due to the clos ing of ‘front contact 42 of relay B and becomes a calling indication. Relay B on picking up to close front contact 43 and 48 brings the transfer circuits associated with tube VI into play and cuit to produce a tone in the telephone receiver at station ST, so that the person calling knows that his call has been properly received at the office. The office operator in acknowledging this call will take his telephone set to initiate communi cation with the calling person at the station ST. The operator also will operate the reset switch RS which opens the anode circuit for tubes VI and V2 and the calling unit is restored to its normal condition ready to receive another call. Assuming that energy of the calling current frequency appears in the line circuit due to some noise condition or due to a particular syllable or word in ‘the conversation, such energy is passed by the selector circuit, the tube VI is tripped, and the control relay A is picked up. This noise energy will ordinarily be of short duration and consequently the tube VI is deionized and relay A released before the capacitor CI is discharged to a point where the tube V2 is tripped. With relay A thus released, capacitor ‘CI is reconnected to its charging circuit and the charge on the capacitor is immediately restored and the full delay period of the device becomes reestablished. Thus, a call will not be made effective due to a succession of pulses of noise energy occurring in short intervals apart. Apparatus here disclosed has the advantages that it is adjustable for relatively high sensi tivity and a call can be accomplished through a relatively long line circuit. A relatively large variation in the voltage of the source of power does not greatly Vary the sensitivity of the unit or the established delay period. The sensitivity and the delay period of the device can both be ‘quickly adjusted to suit the operating conditions of the line circuit with ‘which the apparatus is associated. Successive pulses of current of the calling frequency due to noise energy or due to the conversation will not ordinarily establish a call because of the delay period of the device. Although ‘I have herein shown and described but one form of apparatus for communication systems embodying ‘my invention, it is understood that various changes and modifications may be made therein within the scope of the appended claims without departing from ‘the spirit and scope of my invention. Having thus described my invention, what I claim is: 1. In apparatus for communication systems, the combination ‘comprising, supply means to transmit a calling current, a selector circuit adapted to receive such calling current, a con trolled ionization tube provided with an anode cathode circuit including a power source and an ‘electromagnet and with a control electrode cath ode circuit including a capacitor, a charging cir cuit including a source of direct voltage, a dis charging circuit including a resistor, circuit means governed by said selector circuit and operable to connect either said charging circuit or said dis charging circuit across said capacitor according as calling current is not or is being received, said charging circuit to charge the capacitor at a voltage as required to bias said control electrode negative in potential with respect to said cathode to retain the tube deionized, said discharging cir Icuit to discharge said capacitor at a preselected 2,404,77 8 ' rate to permit said tube to be ionized and said electromagnet energized in a preselected time in terval, and a call indicator governed by said electromagnet. 2. In apparatus for communication systems, the combination comprising, means operable at times 10 of said relay to indicate the reception of such calling current,'and a manually operable reset switch to open said anode cathode circuit to de ionize the tube. '5. In apparatus for communication systems, the combination comprising, an oii‘ice and a sta to transmit a calling current of a given charac tion connected by a line circuit, station telephone teristic, a receiving circuit adapted to receive such means to supply at times to said line circuit a calling current, a controlled ionization tube pro calling current having a given voice frequency vided with an anode cathode circuit including a 10 band, an o?ice receiving circuit connected to said power source and a relay and with a control elec line circuit and tuned to pass said frequency band, trode cathode circuit including a capacitor, a charging circuit including a source of direct volt age, a discharging circuit including impedance, a circuit element operatively associated with said an o?ice gas ?lled tube provided with an anode circuit including a power source and a relay and with a control electrode circuit including a capacitor, o?ice means operatively associated with said receiving circuit to connect a source of circuit or said discharging circuit to said capac direct voltage or a resistor across said capacitor itor according as said calling current is not or is according as said calling current is not or is re being received, said charging circuit to charge ceived, said direct voltage source to charge said said capacitor to bias said tube as required to 20 capacitor to bias the tube as required to retain the retain the tube non-conductive, said discharging tube deionized, said resistor to discharge the circuit to discharge the capacitor to reduce capacitor‘ as required to trip the tube and ener the bias of the tube for the tube to be tripped gize said relay in a predetermined time interval, and said relay energized in a predetermined time an indication circuit including a front contact of interval, and a call indication circuit including a 25 said relay and an indicator to indicate reception front contact of said relay. of the calling current, an of?ce vibrator coupled 3. In apparatus for communication systems, the to said line circuit, and an operating circuit in combination comprising, means to supply at times cluding a front contact of said relay to energize a calling current of a given frequency band, a said vibrator to supply an answer back current to selector circuit tuned to receive current of said 30 said station telephone means. frequency band, a controlled ionization tube, said 6. In apparatus for communication systems, the tube having an anode cathode circuit including combination comprising, calling means operable a power source and a relay, a control electrode to supply at times a calling current, a selector cathode circuit including a capacitor for said circuit to receive such calling current, a ?rst con tube, a charging circuit including a direct Voltage trolled ionization tube provided with an anode source having connections to said capacitor, a circuit including a power source and a ?rst relay discharging circuit including a resistor having and with a control electrode circuit including an connections to said capacitor, said charging cir element coupled to said selector circuit, said anode cuit eii‘ective to charge the capacitor as required and control electrode circuits to ionize said ?rst to bias the tube to a non-conductive condition, 40 tube and energize said ?rst relay in response to said discharging circuit e?ective to discharge the reception of said calling current, a second con capacitor as required to trip the tube and ener trolled ionization tube provided with an anode receiving circuit to connect either said charging gize said relay in a preselected time interval, means governed by said selector circuit to close either the connections of the charging circuit or of the discharging circuit according as calling current is not or is being received whereby said relay is energized only when the calling current continues for said preselected time interval, and a call indication means governed by said relay ' and made active only when the relay is energized. 4. In apparatus for communication systems, the combination comprising, means to supply at times a calling current of a given frequency band, circuit including a power source and a second relay and with a control electrode circuit includ ing a biasing means, said biasing means effective to retain said second tube deionized, means in cluding a front contact of said first relay con nected to said biasing means to vary its effect at a preselected rate to ionize said second tube and energize said second relay in a predetermined time interval whereby said second relay is ener gized only when the calling current continues uninterrupted for said time interval, and a call indicator governed by said second relay. a receiving circuit tuned to receive current of said 55 7. In apparatus for communication systems, frequency band, a controlled ionization tube, said the combination comprising, transmitting means tube having an anode cathode circuit including a power source and a relay, a control electrode cathode circuit including a capacitor for said tube, a charging circuit including a direct voltage source having connections to said capacitor, a discharging circuit including a resistor having connections to said capacitor, said charging cir cuit effective to charge the capacitor as required to bias the tube to a non-conductive condition, said discharging circuit effective to discharge the capacitor as required to trip the tube and ener gize said relay in a preselected time interval, means governed by said receiving circuit to close to transmit from a station to an of?ce a calling current of a preselected voice frequency band, a selector circuit at said o?ice tuned to pass said frequency band and arranged to receive said call ing current, a first c-?ice gas tube having an anode circuit including a power source and a ?rst relay and a control electrode circuit including a wind _ ing coupled to said selector circuit, said ?rst gas tube normally non-conductive and tripped in re sponse to said calling current to energize said ?rst relay, a second o?ice gas tube having an anode circuit including a power source and a sec ond relay and a control electrode circuit includ either the connections of the charging circuit or 70 ing a capacitor, means including a back contact of the discharging circuit according as calling of said ?rst relay to charge said capacitor at a current is not or is being received, means includ direct voltage and said capacitor poled to render ing a front contact of said relay to connect said said second tube control electrode negative in po control electrode to said cathode to retain the tential with respect to the tube cathode to re tube conductive, means including a front contact tain the tube non-conductive, means including a 11 12 front contact of said ?rst relay to connect a re sistor across said capacitor to discharge the ca— pacitor to a voltage where, said second tube is tripped in a predetermined time interval to ener gize said second relay, and a call indication cir cuit including a front contact of said second relay. quired to retain said second tube non-conductive, a discharging circuit including a front, contact of said ?rst relay and a resistor connected to said capacitor to discharge said capacitor at a rate as required to trip said second tube and energize said. second relay in a predetermined time inter val, a call. bell, means including contacts of said second relay to disconnect said ?rst relay and 8. In apparatus for communication systems, the combination comprising, transmitting means connect the call bell, in the anode circuit of said ?rst tube, means including a contact of said sec ond relay to interpose a resistor in said charging to transmit from a station to an oi?ce a calling current of a preselected voice frequency band, a receiving circuit at said o?ice tuned to pass said frequency band and arranged to receive said call ing current, a ?rst o-f?ce gas tube having an an ode circuit including a power source and a ?rst relay and a control electrode circuit including a ~ of said second relay to switch said capacitor to the winding coupled to said receiving circuit, said ?rst gas tube normally non-conductive and tripped in response to said calling current, to energize said control electrode circuit of the ?rst tube to trip thev ?rst tube and operate said call bell for said preselected time period, when said second relay is circuit to delay the charging of the capacitor for a preselected time period when the second relay is energized, and means including contacts energized. ?rst relay, a second of?ce gas tube having an anode circuit including a power source and a sec 20 10. In combination, a ?rst and a second. gas tube,‘ a ?rst and a second relay, a call bell, said ond relay and a control electrode circuit including ?rst tube provided with an anode circuit having a capacitor, means including a back contact of two paths one of which paths includes a current said ?rst relay to charge said capacitor at a. di source, said ?rst relay and a back contact of said rect voltage and. said capacitor poled to render second relay; and the other of Which paths in said second tube control electrode negative in‘ po cludes said current source, said call bell and a tential with respect to the tube cathode to retain front contact of said second relay; said second the tube non-conductive, means including a front tube provided with an anode circuit including a. contact of said ?rst relay to connect a resistor current source and said second relay, said ?rst across said capacitor to discharge the capacitor to a voltage where said second tube is tripped in 30 tube provided with a control electrode circuit having two paths one of which paths includes a a predetermined time interval to energize said back contact of said second relay, a winding and second relay, means including a front contact of a bias voltage source; and the other of which said second relay to connect said second tube con paths includes a front contact of said second trol electrode to the cathode. of the tube to retain the tube conductive, and a call indicator lamp 35 relay and a capacitor; said second tube provided with a control electrode circuit including said ca controlled through, a front contact of said second pacitor, a charging circuit including a back con relay. tact. of said ?rst relay and a voltage source con 9. In apparatus for communication. systems, the combination comprising, transmitting means nected to said capacitor to charge the capacitor to transmit, from a station to an o?ice a calling 40 to create a bias voltage for said second tube to current of a preselected voice frequency band, a receiving circuit at said of?ce tuned to pass said retain the tube non-conductive, a discharging frequency band and arranged to receive. said calling current, a'?rst of?ce gas tube having an lay and a resistor connected to saidv capacitor to discharge the capacitor at a given rate to trip said second tube in a predetermined time interval, anode circuit including a power source and a ?rst relay and a control electrode. circuit including a Winding coupled to said receiving circuit, said ?rst gas tube normally non-conductive and tripped in response to said calling current to, ener gize said ?rst relay, a second o?ice gas tube hav ing an, anode circuit including a power source and a second relay and a control electrode circuit including a capacitor, a charging circuit includ ing a back contact of said ?rst relay and a direct voltage source to charge said capacitor as re 55 circuit including a front contact of said ?rst re means including a contact of said second relay to interpose a resistor in said charging circuit when the second relay is energized to charge the capacitor in a preselected time period, and supply means to at times induce in said winding an elec tromotive force to trip said ?rst tube whereby said call bell is. sounded for said preselected time period when said electromotive force continues for said predetermined time interval. ROBERT. E. BROWN.