US2118919A

US2118919A - Testing arrangement for electron discharge devices and methods of operating same

Info

Publication number: US2118919A
Application number: US36463A
Authority: US
Inventors: Danforth K Gannett
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1935-08-16
Filing date: 1935-08-16
Publication date: 1938-05-31
Anticipated expiration: 1955-05-31

Description

' .IIVPUT May 31; 1938. D. K. GANNETT 7 2,118,919

TESTING ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES AND METHODS OF OPERATING SAME Filed Aug. 16, 1935 2 Sheets-Sheet 1 INVENTOR D. K. GANNETT gnaw; GM

ATTORNEY y 1, 1938. D. K. GANNETT 2,118,919

TESTING ARRANGEMENT FOR ELECTRON DISCHARGE DEVICES AND METHODS OF OPERATING SAME Filed Aug. 16, 1935 Y 2 Sheets-Sheet 2 L1 is, SIGNAL I 62 SIGNAL T as 64-;-

INVENTOR By D./(.GANNETT ATT ORNEY Patented May 31, 1938 UNITED STATES PATENT OFFICE TESTING ARRANGEMENT FOE, ELECTRON DISCHARGE DEVICES AND METHODS OF GPERATING SAME York Application August 16, 1935, Serial No. 36,463

19 Claims.

This invention relates to testing arrangements for electron discharge devices and methods of operating same and more particularly for testing the operating efficiency of such devices.

In communication translating systems, such as transcontinental or transoceanic telephone or telegraph systems, or in fact any other type of equivalent or continuously operating relatively long system, amplifiers or repeaters are essential in relaying the speech or signal currents in the system and these amplifiers or repeaters employ electron discharge devices as the amplifying medium. In some types of systems, electron discharge devices are also used as modulators, demodulators and oscillators. The efficiency and stability of the particular system is dependent, in a large measure, upon the operating characteristics of the discharge devices and a controlling characteristic is the electron activity of the cathode or the ability of the cathode to emit a sufiiciently copious supply of electrons across the discharge space between the cathode and other electrodes associated therewith, such as an input electrode or grid and an output electrode or anode.

In such a system which is in continuous operation for twenty-four hours a day it is important to ascertain the operating condition of the various discharge devices at regular intervals by suitable testing circuits, to determine whether any of the devices cause the eiiiciency or stability of the system to depreciate to a critical operating level. Furthermore, it is highly desirable to perform the testing without removing the devices from the repeater circuit and without interrupting communication service in the system. This is particularly important in multi-channel carrier systems where removal of the device would require interrupting a number of communication channels.

One object of this invention is to expedite th determination of the degree of activity of the cathodes in electron discharge devices in communication translating systems whereby the probable failure of the devices may be predicted and interruption of service in the system prevented.

Another object of this invention is to enable such determination without the removal of the discharge devices from the system, and, hence, without appreciably affecting service in the system.

In accordance with one aspect of the invention, an indicating meter relay is connected in the cathode circuit of a discharge device to register the normal voltage drop which is proportional to the space current flowing in the cathode circuit. A switching mechanism is provided for either disrupting the energizing circuit of the cathode or merely reducing the heating current to a lower value to cause the cathode to cool suflicientiy for the purposes of the test. Since there is a definite relation between temperature and time in a cooling cathode for any given type of device, it is proposed to measure the elapsed time required to produce a given change in space current. When the cathode heating current is lowered a timing element is energized to record the elapsed time for the space current to decay in the cathode circuit due to the cooling of the cathode. When the cathode has cooled sufilciently to cause the space current to decay to a predetermined point, say 10 to 50 per cent less than the normal operating value, the meter relay automatically controls the reestablishment of the cathode energizing circuit and discontinues the operation of the timing element. The more active the cathode, th further its temperature may fall before the space current will decay to the predetermined value, and the longer will be the elapsed time. The elapsed time is therefore a measure of the activity of the discharge device and if the time is less than a chosen critical value the device is determined to be sufficiently inactive to require replacement.

A similar test may beperformed on two or more devices in a single amplifier circuit where the energizing circuit of the various cathodes is arranged in series relation. In one form of the invention, a resistance is substituted in the energizing circuit of the device under test to avoid affecting other devices in the same energizing circuit in which the emission may be lower than the device under test and may fall so low, if the energizing circuit were opened for the test period, that service in the system would be interrupted.

In accordance with another aspect of the invention the tests may be performed by matching two or more devices in a communicating system having their energizing circuits in series. In this embodiment an indicating meter relay is connected to each-cathode circuit of the multi-tube system with a single switching mechanism for controlling the timing element and the energizing circuits of the tubes. The rate of cooling of the cathodes will be registered on the relays and the relay associated with the cathode of lesser activity will close its contacts when the space current fails a predetermined amount, Whereupon the energizing circuit will be reestablished and the timing element stopped, to indicate the elapsed time of the space current decay in the least active tube. In addition to this arrangement, a signaling device is associated with each relay and is operated upon the closing of the contacts thereof, to indicate the tube which is at fault and that tube should be replaced if the emission is below a specified standard determined by the elapsed time registered on the timing element.

In accordance with a modification of the invention relating to the multi-tube testing arrangement, the timing element has a fixed time constant and the signaling device associated with each relay is a gas-filled trigger tube which flashes to indicate the defective device if the relay operates prior to the termination of the time interval established for the timing element.

The various arrangements and the several features of the invention will be understood more clearly by referring to the following detailed description taken in connection with the accompanying drawings, in which:

Fig. 1 is a diagrammatic view of the simplest form of the invention and shows the testing circuit including the indicating relay, timing device and switching mechanism to the right of the drawings, while the discharge device to be tested in a typical amplifier circuit, modified in accordance with this invention, is shown to the left of the drawings;

Fig. 2 shows the same testing circuit slightly modified for testing a group of discharge devices having their energizing circuits in series;

Fig. 3 is a diagrammatic view of another arrangement for. simultaneously testing a group of discharge devices and determining the least active device in the group; and

Fig. 4 shows a modified circuit to be substituted for the apparatus situated to the right of vertical lines XX in Fig. 3 and in which flashing gas discharge tubes indicate the defective device in the system under test.

Referring to Fig. 1 specifically, the testing arrangement of this invention consists essentially of an indicating or registering means I0, such as a meter relay having an adjustable contact II a switching mechanism or key I2, a timing element 13, such as an electric clock with its associated energizing source 54, an electromagnet or relay I5 controlled by the contacts of the indicating relay Hi and two connecting plugs I6 and H. The discharge device which is to be tested for its operating efficiency is shown in a typical amplifier circuit having an input transformer l8, one side of the secondary being connected to a control electrode or grid IQ of the discharge device and the other side being connected to ground. A cathode 20, of the equi-potential type is connected to ground 29 through a resistance 28. The cathode has associated therewith a heater element 2! which is energized by a battery or other energizing source 22. An output transformer 23 is shown on the other side of the device with one end of the primary winding connected to an output or plate electrode or anode 24 while the other end of the primary winding is connected to the positive side of a battery 25 having its negative side grounded. A pair of dual screen electrodes 26 surround the anode and are connected to the battery 25 at a lower positive value than the anode 25. The respective electrodes of the discharge device are enclosed in an evacuated vessel 2! and are so spaced with respect to each other that the cathode, when it attains a stable temperature under operating conditions, emits a copious supply of electrons across the discharge space between the electrodes, the electrons being drawn toward the plate electrode 26 and the flow of electrons being governed by the control electrode I9. The resistance 28 in the cathode circuit is of such value that the potential drop across it is a measure of the flow of space current in the tube 21. One of the contacts of a jack 3B is connected to one side of the resistance 28 and another jack 3I is provided with its main contacts connected to

conductors

32 and 33 forming the supply circuit for the heating element 2 I, the jack 3| having transposed inner contacts 34 to form a closed circuit for energizing the heater element 2I through the battery 22, This arrangement forms a typical set-up for an amplifier in a communication translating system in which the discharge device is continuously operated. The only additional equipment added to the amplifier circuit are the jacks 3B and 3|.

The method of making an activity test on the discharge device in the amplifier circuit heretofore described consists of two simple operations, namely, inserting the plugs I6 and ii in the respective jacks and 3| and then depressing key I2 whereupon the remaining operations are performed automatically by the associate apparatus in the testing circuit.

The detailed operations of the testing circuit will now be described. When the plug I6 is inserted in jack 3!] the meter relay I0 is connected across resistance 28 and the pointer 35 of the relay indicates the voltage drop across the resistance 23, this voltage drop being proportional to the space current flowing in the cathode 26 so that the meter indicates the normal space current of the discharge device. The meter relay II] is a microammeter having a high resistance 35 connected between the tip contact of plug I6 and one side of the meter, while the other side is connected to ground so that the meter operates as a voltmeter. A variable resistance 31 is connected in shunt to the terminals of the meter 16 to provide a full scale deflection of the pointer 35. A suitable meter relay for the purpose of this invention is commercially known as a Sensitrol Relay, Model 705. It is a microammeter relay having a scale of 10 microamperes for indicating the current measurement. The adjustable contact II is a small permanent magnet while the movable contact is an iron rider mounted on the pointer 35 which travels over the scale. The operating torque moves the iron rider into the magnetic field of the adjustable contact and the rider is drawn firmly against it. This insures perfect contact, prevents chattering and permits a considerable amount of energy to be safely controlled. The adjustable contact may be moved to any desirable operating value. The contact II and an index are fastened to a common arm which may be moved to a desirable operating point by means of a knob on the front of the dial casing of the relay. After the space current is indicated on the meter relay I 8, and resistance 37 has been adjusted so as to obtain a full-scale defiection of pointer 35, the plug I! is inserted in jack 3| and the key I2 is depressed, to disrupt the energizing circuit including battery 22 of the heater element 2| of the discharge device through the open contact 38 of key I2. In accordance with this method of operation the bridging resistance may be omitted. Simultaneously with the opening of contact 38 of key I2 the lower contact 39 is closed to start the timing device which is an electric clock energized by the 60 cycle source 1 4. When the contact 38 of key l2 isiopened to disrupt theenergizing circuit, the cathode 2!! begins to cool, due to the absence of conduction heat from the heater element 2i. This results in a decay of the space current in the cathode due to the diminishing of the emission of electrons and consequently the same effect is produced in the resistance 28. The cooling of the cathode and'the decay of current inv the resistance 28 necessarily causes the pointer 35 on meter relay Hi to indicate the decreasing voltage drop in the resistance 28. The iron rider on the pointer 35 then engages the contact H which may be adjusted to a position such as 20 per cent below the normal value of space current in the discharge device so that the discharge device is not completely inactive during the test and may continue to amplify the signal and speech currents passing through the amplifier circuit.

When the falling space current as indicated on the meter relay I 0 causes the rider on the pointer 35 to engage the contact II a circuit is completed for relay l and this relay is energized by battery 49 through the contacts of meter relay l0 and the ground connection of the meter. Relay E5 in energizing automatically closes the energizing circuit of heater element 2| through the upper spring and contact 4|, to restore the discharge device to normal operation. At the same time the relay l5 opens the circuit of the timing device l3 through the lowermost spring and contact 42 and establishes a holdingcircuit for relay through the lower inner spring and contact 43 and the contact and spring 44 of key l2 to ground. The holding circuit is provided to prevent any unavoidable deenergizingof relay 15 through the opening of the contacts of meter relay Hl. Instead of completely disrupting the energizing circuit of heater element 2 l, as previously described, it may be preferable to merely lower the heating current to 50 per cent instead of reducing it to zero during the test. In this case a resistance 45 may be connected across the conductors connected to the tip and ring of plug ll, to produce the required reduction in the heating current. As previously described, the operation of relay i5 causes the timing circuit to be disrupted and the interval of time recorded on the electric clock l3 will be a measure of the activity of the cathode 20 of the discharge device. If the elapsed time is less than a chosen critical value, the. discharge device is determined to be sufficiently inactive to require replacement. The testing circuit is returned to normal by releasing key l2 and removing plugs 16 and H from the jacks 39 and SI, respectively.

Fig. 2 shows an arrangement for testing agroup of discharge devices individually, the devices being shown without the associated circuit apparatus to simplify the description. This figure shows the arrangement for two discharge devices, but the same arrangement may obviously be used for any number of discharge devices whose heaters are associated together in the same energizing circuit. The two discharge devices shown, 46 and 47, may be two separate amplifiers, two stages of a multi-stage amplifier or a single stage in pushpull relation with the heating elements '38 and 49 arranged in series through an energizing circuit which may be traced from ground 5%, battery 5i, conductor 52, heating element 48, conductor 53, upper contact and spring of jack 54, conductor 55, heating, element 4.9 of discharge device ll and uppercontactand spring of jack 5 6 to ground. It will be noted that the sleeves of both jacks 54 and 56 areconnected to the energizing circuit of the heater elements of the discharge devices to cooperate with the three-conductor plug 51 which is substituted for the plug H, as described in connection with Fig. 1. When the heater elements of two or more discharge devices are in series, it is obvious that if the test is performed on one device and it is allowed to control the time that the heater circuit is open, the electron emission or activityof another discharge device, which may be less active than the one under test, may fall as far during the. open period as to interrupt service inthe amplifier. In order to avoid this difiiculty a resistance 58 is connected to the sleeve contact of plug. 5? so that when the plug 51 is inserted in jack 54 the resistance 58 is substituted in the energizing circuit for the heater element 480i the discharge device 46 during the open period, thereby maintaining at approximately its normal value the current in heater 49 of the other discharge device 41 which is in the same circuit. When the plug 51 is inserted in jack 54 and the key [2 is depressed by the operator, the energizing circuit of heater element 48 is disrupted through the tip and ring contacts of plug 51, upper spring and closed contact of relay l5 and the spring, and open contact 38 of depressed key 12. The resistance 58 is substituted for the heater element 48 in the energizing circuit of discharge device 41 so that the heater element 49 will be subjected to the same heating current which it normally receives when heater element 48 is in series with it. This circuit may be traced from ground 50,

battery 51, sleeve of jack 54, sleeve contact of plug 51, resistance 58, closed contact 59 and spring of key 12 in its depressed condition, closed contact and upper spring 4| of relay 15 to the tip contact of plug 5?, upper main spring of jack 54, conductor 55, heater 49 and upper contact and'spring of jack 56 to ground.

' The remainder of the circuit is substantially the same as Fig.1 and the operations of the testing circuit are the same as described in conneccally operate relay 15, to reestablish the energi'zing circuit of the heater element 48 of discharge device 45 and disconnect the timing device 13 which indicates the elapsed time of the current decay which is a measure of the activity of the device' under test. The same operation is performed on discharge device 4? by inserting plug 16 in jack and plug 51 in jack 55, the cathode of discharge device 41 being connected to a grounded resistance 28' through the ground 29' .so that the insertion of plug H3 in jack 36' provides a connection for bridging the meter relay Id across the resistance 28 to obtain the space current flowing in the cathode. It will be obvious that during thistest the discharge device 46 is unaffected by the disruption of the energizing 7b circuit of the heater element of discharge: device is matched against the other and the device of lesser activity controls the operation of the testing circuit. The elapsed time of the lesser activity tube may then be checked with a standard critical value to determine whether the device should be replaced in the amplifier. In this arrangement the

heater elements

48 and 49, respectively, of the discharge devices 46 and 41 of the multi-stage amplifier are connected together in a series energizing circuit from ground 50, battery 5| to ground connected to the upper main spring and closed contact of jack 60. In order to perform the test according to this invention,

separate jacks

30 and 30 are connected across cathode resistances 28 and 28' and two meter relays IO and I 6' are respectively connected to plugs I6 and I6 for recording the normal space currents flowing in the cathodes of the respective discharge devices 46 and 41. Upon the insertion of plugs I6 and I6 in the

respective jacks

30 and 30 the normal space current flowing in the cathodes of discharge devices 46 and 4! will cause a deflection of the

pointers

35 and 35, respectively, of the meter relays I0 and I0. Shunt resistances 3! and 31 are then adjusted so as to make the deflections of pointers 35 and 35' correspond to the full scale readings of meter relays I0 and ID. The next operation is to insert plug I! in jack 60. The depression of key I2 causes a disruption of the series energizing circuit of the

heater elements

48 and 49 or causes a specified reduction in the heating efiect of the energizing circuit due to the bridging resistance 45. At the same time the electric timing element I3 is connected in circuit through the lower spring and closed contact 39 of key I2, the circuit of the timing element being slightly changed in this arrangement due to the inclusion of associate relays 6| and 62 and

signaling devices

63 and 64. The energizing circuit for the timing element I3 may be traced from energizing source I4, closed contact 39 and lower spring of key I2, closed contact and upper spring 65 of relay 6I, closed contact 66a and upper spring of relay 62, timing element I3 to source I4.

With the insertion of the plugs in the respective jacks and the operation of the key I2, let us assume that the cathode of discharge device 46 is of lesser activity than the cathode of discharge device 41. In that event, as the cathodes cool, the space current of discharge device 46 will decay faster than the space current of discharge device 41, and pointer 35 of meter relay ID will engage its contact II before pointer 35 of relay I0 is ready to engage its contact I I'. It will be realized that the timing device is operating during this interval due to the operation of key I2 and when the pointer 35 engages the contact II of meter relay I0, relay I5 is operated to reestablish the energizing circuit through the left-hand spring and contact and establishes a holding circuit for relay I5 through the inner right-hand spring and contact of relay I5, closed contact 44 of key I2 to ground. Relay I5 also energizes relay 62 through the outerright-hand spring and contact 66 and the closed circuit including battery 67. The energization of relay 62 disrupts the timing circuit throughthe contact 66a to stop the timing clock I3 and closes a circuit through the lower spring and contact 68 to energize the signaling device 64 through a circuit including battery 69. The signaling device 64 may be a glow lamp or an incandescent lamp or possibly a hell or alarm to indicate the particular discharge device of lower activity. The

elapsed time indicated on the timing device will determine whether the operating discharge device is satisfactory for future service or should be replaced due to a loss of activity which might endanger the efiiciency of the communicating system in which it is incorporated.

If we assume that the discharge device 4! is of subnormal activity instead or discharge device 46, then the meter relay I0 will control the testing operation by the pointer 35' closing a circuit through the adjustable contact II' to operate relay I5' through the battery 40'. Then relay I5 will close its left-hand spring and contact to reestablish the energizing circuit of the heater elements of both tubes, establish a holding circuit through the contact 44 of key I2 and the inner right-hand spring and contact of relay I5 and also energize relay 6| through the outer righthand spring and contact of relay I5, whereupon the timing circuit of timing device I3 is disrupted through the spring and contact 65 of relay 6| and signaling device 63 is operated to indicate that discharge device 41 is of lesser activity than discharge device 46. Again, the elapsed time recorded on the timing device will determine whether the discharge device 41 should be replaced in the amplifying circuit.

While the timing device heretofore described has been specified as an electric clock, it is not essential for the purposes of this invention to confine the operation of the testing circuit to the specific use of an electric clock. Another arrangement including a timing circuit and two gaseous trigger tubes as the signaling elements of the testing circuit is shown in Fig. 4 and the circuit arrangement as illustrated may be substituted for the association of apparatus shown in Fig. 3 to the right of the vertical line X-X. In this arrangement a condenser-resistance dis-. charge circuit is employed as the timing element which is normally connected to the positive side of a 130-volt battery II. In order to evaluate the time constants of the timing circuit I0, say for about 10 seconds, as a measure of time in which a cathode of satisfactory emission is cooled when the energizing circuit is disrupted, the values of condenser I2 and resistance I3 may be 10 microfarads and 1 megohm, respectively. Instead of the timing circuit including the timing device I3 and energizing source I4 being connected across the lower spring and contact of key I2, a relay I4 is substituted as shown in Fig. 4. Relays I5 and I6 are substituted for relays 6| and 62 in the circuit of Fig. 3 and these relays control the gaseous trigger tubes 11 and I8, respectively.

The operation of the testing circuit as modified with the substitution of the circuit shown in Fig. 4 is as follows: As previously described, the various plugs I6, I6 and II are assumed to be inserted in their respective jacks for indicating on the meter relays I0 and I6 the normal space current flowing in the discharge devices 46 and 41 of the amplifier circuit. When the key I2 is operated the lower contact 39 and spring are closed to energize relay I4 through battery I9. This operation transfers the 130-volt battery II from the timing circuit ID to the anodes or plates of the trigger tubes 11 and I8. With the removal of the l30-volt battery from the timing circuit I0 through the opening of the spring and outer contact of relay I4, the charged condenser I2 starts to discharge through the resistance I3 at such a rate that the residual potential across the condenser will decline to a value of about 50 volts in an interval of 10 seconds, due to the constants of the elements forming the timing circuit. The cathodes in the

gaseous trigger tubes

11 and 18 are energized by their associated heater elements, but in View of the connection of these cathodes to the intermediate point of resistance 86 which is in Shunt to the battery I! through the inner closed contact and spring of relay [4 this places a positive potential on the cathodes of approximately volts with respect to ground 8| and the grids of the trigger discharge tubes are normally at ground potential through the springs of relays l5 and 16 and are many volts negative with respect to their cathodes so that no current flows in the tubes. The operation of either relay 15 or 15 through the operation of relay IE or l5 due to the closing of the contacts of meter relay H! or ID, respectively, transfers either grid of trigger tubes T! or 18 from ground potential to the potential of condenser l2 at the moment. The grid of either tube, which is rendered positive with respect to its cathode, instantly causes a breakdown in the discharge path between the cathode and anode and the tube flashes to indicate which discharge device is at fault in the amplifying circuit. In order to prevent premature operation of the trigger tubes while the springs of

relays

15 or 16 are in mid-air, condensers B2 and 83 are connected to the grid circuits of the respective tubes and have a relatively small value, say .001 microfarad, so as not to discharge condenser 12 in the timing circuit appreciably. It is evident that if either relay #5 or 16 operates before condenser 72 discharges to a potential appreciably lower than that of the cathodes of the trigger tubes H and 78, respectively, one of the trigger tubes immediately flashes to indicate the discharge device of lesser activity. However, if the relay operation does not occur until later the condenser 12 will have discharged enough so that the grids of trigger tubes TI and 18, respectively, are sufficiently negative to prevent the tube from flashing. The flashing of either gaseous tube therefore gives a visual indication that the corresponding discharge device under test is inactive and should be replaced. At the conclusion of the test the release of key l2 interrupts the plate supply of

trigger tubes

11 and 18 through the deenergization of relay 14' whereupon the timing circuit 10 is recharged for the next test. The choice of the time constants of the timing circuit '10 and the potential difierence to ground furnished the trigger tube cathodes by the resistance 80 determines the critical time which will serve to indicate when a discharge device is sufficiently inactive to need replacing. It will be noted that the testing circuit is not critical with respect to the trigger-oftpoint of the trigger tubes. A variation of one volt in the trigger-01f point of the gaseous tubes 11 and i8 affects the time by only about 0.2 second.

The various arrangements heretofore described and embodying features of this invention provide an accurate and speedy test of the activity of dis charge devices Without removing them from service and without interrupting the service in the communicating system. The invention is sufliciently flexible to apply the testing arrangement to various types of discharge devices having different values of space current during normal operation. Furthermore, tests made with this invention indicate that a sufficient margin of discrimination is obtained to reject discharge devices on the basis of cathode activity. The time interval for the space current to decrease, for example, to half its value after the time the heater current is interrupted, ls approximately in the ratio of 2 to 1, comparing a satisfactory tube with an inactive tube. I

While the greatest utility of the invention is realized in testing discharge devices which are continuously operating in a communicating or translating system, it is, of course, understood that the invention is in no way limited to this field. The invention may also be applied to other types of systems where the discharge devices are not necessarily continuously translating speech and signaling currents, for example, in voice operated repeaters, or in carrier current systems. The invention may also be practised in other fields of use where discharge devices are employed, such as radio transmitters and receivers, where it is desired to maintain a high level of transmission efliciency in the system. Moreover, although in the illustrative multi-device circuits shown in Figs. 2 and 3, the heater elements are connected in series, it will be clear that'the invention may be utilized in circuits wherein the heater elements of a plurality of electron discharge devices are in parallel or certain of the heater elements are in parallel with one another and in series with other heater elements.

Furthermore, the various embodiments of the invention herein disclosed may be elaborated and modified as to the specific elements described and all such modifications are presumed to be within the scope of the invention as defined i the appended claims.

What is claimed is:

1. A testing circuit for electronic discharge devices having a cathode, comprising an indicating means connected to said cathode, timing means associated with said indicating means, means for altering the heating source of said cathode to register the decay period of said cathode on said indicating means, means for initiating said timing means at the start of the decay period, and means for restoring the heating source to normal operation and discontinuing the operation of said timing means.

2. A testing circuit for electronic discharge devices having a cathode, comprising an indicating means connected to said cathode, timing means associated with said indicating means, means for altering the heating source of said cathode to register the decay period of said cathode on said indicating means, means for initiating said timing means at the start of the decay period, and means under the control of said indicating means for restoring the heating source to normal operation and discontinuing the. operation of said timing means.

3. A testing circuit for determining the activity of an electron discharge device having a cathode and a heater element for said cathode, comprising a relay connected in circuit with the space current path of said device, indicating means controlledby said relay, a timing device associated with said relay, and switching means for simultaneously controlling said timing device and the heating current through said heater element.

4. In a system for testing the activity of any. one or more of a plurality of discharge devices, indicating means responsive to the space current of any one of said devices, switching means for altering the heating circuit of the cathode of said one device to cause a decay of space current in said device, timing means actuated at the instant of the altering of the heating circuit, and means actuated by said indicating means for restoring the heating circuit to normal operation and stopping said timing means.

5. A testing circuit for electronic discharge devices having a cathode, comprising a resistance connected in series with said cathode, means bridging said resistance to indicate the voltage drop therein, a timing device, means simultaneously disconnecting the heating source of said cathode and connecting said timing device to said bridging means, and means associated with said bridging means for reestablishing the heating source to normal operation and stopping said timing device.

6. A testing circuit for determining the characteristics of an electron discharge device having a cathode, comprising a resistance in series with said cathode, energizing means for said cathode, deflecting means connected across said resistance, timing means associated with said deflecting means, switching means for altering the effect of said energizing means and starting said timing means, and means for restoring said energizing means to normal operation and terminating the operation of said timing means.

'7. A testing circuit for electronic discharge devices having a cathode, a heater element for said cathode, and energizing means for said heater element, comprising an impedance in circuit with said cathode, means having an adjustable contact for indicating the voltage drop across said impedance, means for connecting said indicating means in circuit with said impedance, a time indicator circuit, switching means connected to said energizing means for simultaneously disconnecting said energizing means and energizing said time indicator circuit, and means controlled by said adjustable contact for simultaneously restoring said energizing means to normal and disconnecting said time indicator circuit.

8. The method of testing the activity of discharge devices which comprises measuring the space current in the discharge device, reducing the cathode temperature, recording the elapsed time for the cathode to cool to the point where it reaches a certain degree of inactivity, restoring the cathode temperature to normal, and terminating the time recording simultaneously with the restoration of the cathode temperature to normal.

9. The method of measuring the elapsed time required to produce a given change in space current in an electron discharge device without disrupting service in a translating system which comprises measuring the normal space currrent flowing in the cathode circuit, cooling the cathode to reduce the space current to a definite value, recording the elapsed time during the cooling period, and automatically restoring the cathode temperature to normal when the given change in space current is reached.

10. The method of measuring the elapsed time required to produce a given change in space current in an electron discharge device without disrupting service in a translating system which comprises measuring the normal space current flowing in the cathode circuit, cooling the cathode so that the space current falls to 10 to 50 per cent less than the normal value thereof, initiating a time record at the instant of cathode cooling, continuing said time record until the space current reaches a given lower value, and discontinuing the time record while simultaneously restoring the cathode to normal temperature operation, the elapsed time being a measure of the activity of the discharge device as compared to a chosen critical value.

11. The method of detecting loss of activity in an electron discharge device which comprises measuring the space current in a resistance in the cathode circuit, disconnecting the energizing source of the cathode, recording the elapsed time for the current to decay in said resistance after the termination of the energizing source, and reestablishing said energizing source when the decaying current reaches a critical value.

12. The method of determining the degree of activity of electronic discharge devices which comprises measuring the space current of the device, reducing the heating current of the cathode to a subnormal value, timing the space current decay during the cooling of the cathode, and simultaneously restoring the heating current to normal and discontinuing the timing to indicate the time interval elapsed during the decay period.

13. A test circuit for determining the activity of heater type cathodes of electron discharge devices which comprises means for measuring the voltage drop in the cathode circuit of the device, means for reducing the heating circuit of the cathode to a subnormal value, means for initiating a timing element at the instant of the reduction of the heating circuit, signaling means associated with said timing element, and means associated with the measuring means for restoring the heating circuit to normal operation, disconnecting said timing element and operating said signaling means.

14. A testing circuit for differentiating between active and inactive electron discharge devices having a heater type cathode which comprises means for measuring the normal space current in the cathode circuits of the individual devices, a timing element, switching means for changing the value of the heating current in the cathode circuits to cool the cathodes to a subnormal value and simultaneously operating a timing element during the cooling interval of the cathodes, flashing devices individually associated with the respective measuring means, and means operated by each measuring means to operate the flashing device associated therewith when the cooling rate in one of the cathode circuits is greater than in another circuit, said means simultaneously restoring the heating circuits to normal operation and discontinuing the operation of said timing element.

15. A testing device for measuring the activity of multi-tube circuits having heater type cathodes and a common energizing circuit for said cathodes, which comprises means for measuring the voltage drop in the cathode circuit of either tube, an adjustable contact associated with the measuring means, means for connecting a series resistance in place of the heating circuit of the tube under test, an electric timing device, switching means for opening the heating circuit of the tube under test and simultaneously operating said timing device, and a relay operated by said adjustable contact to automatically restore the heating circuit to normal operation and terminate the operation of said timing device.

16. A testing circuit for a plurality of electron discharge devices having individual cathodes, a resistance in series with each cathode and energizing heater circuits in series, comprising an indicator for each device, means for connecting each indicator across each cathode resistance, a time recorder circuit, switching means for reducing the heating effect of said energizing circuit and initiating said time recorder circuit, means controlled by said indicators for restoring said heater circuits to normal and discontinuing the operation of said time recorder circuit, and signal responsive means for indicating the less active device under test.

17. A testing circuit for a plurality of electron discharge devices having individual cathodes, a resistance in series with each cathode and energizing heater circuits, comprising an indicator for each device, means for connecting each indicator across each cathode resistance, a time recorder circuit, switching means for reducing the heating effect of said energizing circuits and initiating said time recorder circuit, means controlled by said indicators for restoring said heater circuits to normal operation and discontinuing the operation of said time recorder circuit, and individual signal responsive means under the control of each indicator to designate the device of subnormal operation.

18. A testing circuit for a plurality of electron discharge devices having individual cathodes, a resistance in series with each cathode and energizing heater circuits in series, comprising an indicator for each device, means for connecting each indicator across each cathode resistance, a time recorder circuit, switching means for reducing the heating effect of said energizing circuits and initiating the operation of said time recorder circuit, means controlled by said indicators for restoring said heater circuits to normal operation and discontinuing the operation of said time recorder circuit, a signal device associated with each indicator, and a relay controlling each signal device and actuable by said indicator to designate the discharge device of lesser activity.

19. An activity testing arrangement for a group of discharge devices having equipotential cathodes, a heater circuit for each cathode, and energizing means connected in series with said heater circuits, comprising a resistance connected to each cathode, a voltage indicator for each device connected across each resistance, timing means, switching means for reducing the effects of said energizing means and applying said timing means to said devices simultaneously with the reduction of the energizing means, and signal responsive means under the control of the individual indicators to determine the least active device in the group.

DAN'FORTH K. GANNETT.