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Oct. 8, 1946. F. R_ slAs HAL 2,409,073 CAPACITOR FUEL GA'UGE Filed March 7, 1945 2 Sheets-Sheet l Figl. TEHPER’Q TURE COMP IVJAT/D/V I . __._L. FULL sous * ADJUSTMENT + m’” ' Fig.2. ~14; 22 /1/3-l/7 s ~—f/;’ \ Inventors: ‘Frederick R. Sias, Donald B. Pearson, Their" Attorney Oct. 8, 1946. > F_ R_ 5M5 ETAL 2,409,073 ‘ CAPACITOR FUEL GAUGE Filed March ‘7, 1945 2 Sheets-Sheet 2 Inventors: Frederick R. Sias, Donald. B. Pearson, by Wé? M44 Their Attorney. Patented Oct. 8, 1946 2,409,073 UNITED ‘STATES PATENT OFFICE 2,409.01: CAPACITOR FUEL GAUGE Frederick R. Sias, Lynn?eld Center, and Donald . Pearson, Marblehead, Mam, assignors to General Electric Company, a corporation of New York Application March "I, 1945, Serial No. 581,494 1 6 Claims. Our invention relates to improvements in liquid fuel measuring apparatus and is suitable for the remote measuring of the quantity of gasoline in the tanks of aircraft and indicating such quantity in the pilot's compartment. Our invention relates (Cl. 177-351) 2 outer tube may conveniently be grounded to the metal tank i. The side wall of tube 4 is per forated at least at the top and bottom as indi cated at 5, so that if there is gasoline in the tank, it will fill the tube 4 to the level of the gasoline in the tank as represented. By restricting the ex tent of such perforations, there is a desirable damping action to the flow of gasoline in and out of the tube, and hence, a more uniform level to capacitor type measuring apparatus where the liquid .being measured is made the dielectric of a condenser and has a diil'erent dielectric constant than that of air such that by the displacement of the liquid fuel by air, the capacitance is 10 when the tank is tippedabout violently. It is changed and measured in terms of liquid quan also noted that the condenser is positioned in the tity. The capacitor type liquid fuel measuring center of the tank so that, ordinarily, tipping of apparatus idea is not new. Such apparatus is the tank will have as small e?ect as possible upon disclosed in British Patent No. 441,576 of 1936, for example. Our invention pertains to improve 15 the length of tubing which is ?lled with gasoline. It is evident that the dielectric between the ments in such apparatus and inparticular to fea condenser plates constitutes air above the liquid tures of the measuring circuit which make good ' level and gasoline below the liquid level. In the accuracy and reliable apparatus of this type prac case of gasoline it has approximately twice the ticable for installation and use on aircraft. The features of our invention which are be lieved to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of our invention, reference is made in the following description to the accompanying drawings in which Fig. 1 shows a preferred form of capacitor gauge measuring circuit suitable for use where a 110-volt source of direct current sup- ' ply is available. Fig. 2 shows modi?cations in the dielectric constant of air so that as the gasoline level rises from bottom to top of the tank during filling, the capacitance of the condenser increases by a factor of 2. Hence, we have here a variable condenser, and the means for varying the con denser is the change in level of the gasoline. This change in capacitance may be measured in terms of liquid level or gallons or, more accu rately, as we will point out, in weight of the liquid fuel in the tank. Thus there is provided a liquid level measurement transmitter having no moving parts, such as ?oat levers, gearing, electrical con circuit as adapted for use where the direct cur rent source of supply is of the order of 27 volts. Figs 3 and 3a are diagrams representing flux rela tacts, and the like. Also, while the condenser tions in the instrument of Fig. 2. Fig. 4 illus plates are exposed directly to the gasoline, there trates certain advantages of employing the ca is no danger of sparks such as would ignite the pacitor type gasoline gauge transmitter where gasoline. will explain in connection with Fig. odd-shaped tanks are encountered. Fig. 5 illus 85 4 that theWe condenser may be adjustable inde trates parallel condensers located outside of a ' pendently of changes in dielectric. tank. which is subject to considerable tipping ac The condenser structure may be secured se tion. Figs. 6, 7, and 8 showiforms of condensers curely in the tank and, in fact, the outer tube having a varying capacitance over their length. Fig. 7a represents a cross section of the structure 40 may serve also as a strengthening center brace between the top and bottom of the tank and may ‘of Fig. '1. Fig. 9 illustrates how a tank side or be welded, bolted, or threaded thereto. The mag .baiiie may conveniently be made one of the con nitude of the capacitance depends upon the area denser plates. Figs. 10, 11, and 12 are cross sec tions of adjustable forms of condensers. Fig. 13 of the adjacent condenser plate surfaces, their represents a tank with condenser arranged to 45 spacing and the dielectric. Hence, the inner plate may be tubing or a rod, and the outer tube may compensate for tipping errors. have any desired thickness and outer diameter. Referring now to Fig. 1, I may represent a gas Thus the condenser may be made to have what oline tank ?lled to a level 2. The tank contains ever mechanical strength and stiffness as are de a condenser with its plates extending from top sirable to enable it to withstand bending and to bottom of the tank. This condenser may take 50 other forces to which it may be subjected with a variety of forms as will hereinafter be explained. out restricting the choice of condenser value. In Fig. 1 the plates of the condenserare repre The variable capacitance above described is sented as a central rod or tube 3 extending connected in the input of a. vacuum tube oscil through a tube 4. These plates are insulated lator having tuned grid and plate circuits induc from each other and are made of metal. The M tively coupled together. The ungrounded plate 2,409,078 4 I of the measuring condenser is connected to the grid 0! the vacuum tube 6, and the grounded plate 4 is connected to the cathode of tube 8. In parallel with the measuring condenser is a coil 1 cooperating with a coil 2 to provide inductive coupling between the plate and grid circuits. This coupling is preferably initially adjustable as indi cated by movement of the coils relative to each The indicator has three separate windings; one winding l3 supplied from the oscillator is in the same axis as a winding i1 connected across the direct current supply through an adjustable cali brating resistor it. The windings l3 and I‘! are connected in bucking relation. The reason for this is that when the tank I is empty, the direct current from the plate circuit of the tube is not zero. By the use of the bucking winding ll, the other. In the grid circuit is a grid leak compris ing a resistance 0 in parallel with a condenser 10 empty tank condition current in winding II can be bucked out and a longer indicator. scale ob ll. In addition to its grid leak function. the re tained. The third winding l9 also connected sistance 2 is designed with just the proper tem across the direct current supply through a call perature coe?icient of resistance to compensate brating resistor 20 has its flux axis 90 degrees the circuit for temperature changes as will be explained below. In the cathode circuit are a 15 from the axis of windings l3 and H and provides the controlling torque of the instrument in much variable resistance ii and an inductance l2. The the same way as a control spring does for a variable resistance Ii is adjustable for the pur D’Arsonvai instrument. The instrument thus has poses of full scale adjustment. Adjustment of no control spring. The armature consists of a this cathode resistor changes the grid bias of the lightweight polarized magnet 2i. The armature tube in proportion to the cathode current and may be bar-shaped or may comprise a cylindrical permits a full scale measurement current adjust magnet polarized across a diameter. The pivoted ment that does not appreciably affect the current armature carries a pointer 22 cooperating with in the measurement circuit at the zero point, as a stationary scale 23. The circle 2| surrounding the resistor has little eilect when the current the armature represents a, damping conductor. through the tube is low. The inductance i2 in structurally this instrument is quite similar to the cathode circuit serves as a stabilizer and that shown and described in United States Patent makes the relation between the measurement ca No. 2,354,555, July 25, 1944. pacitance value and plate circuit output more By adjusting the resistor I! in series with the nearly linear. It also assists in making the full bucking winding H, the midpoint of the scale scale adjustment independent of zero adjustment may be made to correspond to an armature posi The output or plate circuit of the tube contains tion in line with the axis of winding l9. The the coils ll of the direct current receiving instru resistance 20 in series with the winding as may ment, the inductance coil 0, and the 110-volt then be adjusted to expand or contract the scale. direct current supply. The plate circuit is tuned by the use or condensers II and it connected 3 Increasing the torque produced by winding is decreases the scale length. Thus the indicator across the inductance coil 8 through the con is provided with a center scale adjustment and denser lt. The condenser I5 is adjustable and a scale length adjustment. A scale length of is used to change the natural frequency of oscil approximately 110 degrees is feasible. The in lation of the plate circuit for~ zero scale adjust ment. Zero and full scale adjustments are desira so strument as thus designed and connected is ble in case it becomes necessary to change the tube at I. It is seen that both the input and output circuits of the oscillator tube are tuned and that these circuits are coupled together not largely compensated for voltage changes. chances in supply voltage. value by selecting a resistor 9 in the grid leak of What little voltage error remains is greatest at full scale and may be further reduced by the use of a pull-off magnet which opposes the voltage only by the intercapacity eilects of the tube elec 45 error and has its greatest torque in the full scale position. Thus at 23' we have shown a small trodes but also by the inductance coils ‘I and 8. permanent magnet suitably fixed to some sta After calibration the natural frequency of oscilla tionary part of the instrument and positioned to tion of the plate circuit remains fixed and the cooperate with the permanent magnet armature natural frequency of oscillation of the grid circuit varies with the capacitance oir the measurement so 2| to rotate the latter to a position where the pointer 22 is of!’ scale in case the power supply to condenser. Since both circuits are coupled, they the instrument fails, thus preventing a false indi oscillate at a resultant intermediate frequency cation and giving warning of such failure. The which varies with the amount of gasoline or other pull-oi! magnet 01' course has some small torque dielectric ?uid being measured. Assuming a con stant voltage supply. the direct current fed to 55 effect on the armature when the instrument is energized, and this torque effect has greatest con instrument coils it changes with frequency, and trol when the instrument voltage is low and when hence, can be a measure of the fuel in tank I. the pointer is near the up-scale end of its deflec A high degree of stability and low distributed capacity is desirable with respect to the grid and tion range. Hence, the pull-off magnet in addi plate circuit elements. Relatively adjustable 50 tion to its usual function is used to still further reduce the voltage error of the instrument. The parts should be so designed and constructed as not to change their adjustment when subjected instrument as thus constructed and used has a voltage error of less than 1% for supply voltage to vibration and temperature and humidity varia tions. The condensers l0 and it are preferably variations of 20%. silver plated. mica condensers so as to have high 65 It was found that when reading mass of fuel without temperature compensation and when stability. Shielding of circuits should be used using low temperature coefiicient resistors, the where necessary. system had a positive temperature error; that is, As mentioned above. the resultant frequency the receiver instrument reading was high at high of oscillation changes with the fuel level. This results in a change of grid current which is ac 70 temperatures and low at low temperatures. This companied by a change in D.-C. plate current. is an over-all temperature error and is probably The plate current is used to operate a ratio type the resultant of a plurality of temperature errors direct current instrument. A ratio type instru in various parts of the system. It was further ment is desirable to minimize errors due to found that this could be reduced to a negligible 5 9,409,078 the oscillator tube which had the proper tem-~ perature coe?icient of resistance, which was not necessarily the same for different installations, tubes, etc. If the resistor at 0 is all copper, the system will generally have a temperature error opposite to that mentioned above. We have found that the 6 Fig. 2 shows the capacitor type gasoline gauge circuit adapted for use on av 24-volt direct current source of supply 20. Parts similar to those of Fig. 1 are designated by like reference characters. In this case we ?nd it desirable to employ a double triode 21 as the vacuum tube unit of the oscillator. The two plates of the tube are connected in parallel to one side of inductance coil 8. Temper ature compensating grid leak resistors of the temperature coeiilcient of resistance of the re sistance at 9 should be reduced toward a zero value, and that the desired results can be arrived 10 same type as used in Fig. 1 are provided in the at by using a resistor partially of copper and arid circuits, and the grid circuits are connected partially of a material having a zero or negligible in parallel beyond the grid leaks to the un temperature coe?lcient connected in series, and adjusting the relative values of each until com grounded plate 3 of the variable measurement condenser and to one side of inductance coil 1. pensation for the average conditions encountered 15 Separate grid leak resistors of equal value pro is obtained. In general, the temperature coei’? duce better operation than does a single grid leak cient to be used here was found to be nearer zero for both grids. The resistances used in these than that of copper but that it varied somewhat grid leaks may vary over a considerable range. with different systems. . A resistance value of 4000 ohms is very satisfac Without intending to con?ne our invention to 20 tory. The zero scale measurement current ad any particular values of circuit constants em justment is the same as in Fig. 1 by means of the ployed, it'may be ‘stated that the following may condenser II. be employed together to give satisfactory results. In Fig. 2 the cathodes are connected in series Use a variable measurement condenser in the across the source of supply 26 and it is not tank I having a capacitance of 150 m. m. f. with 25 easily possible'with this type of tube to provide tank empty and 300 m. m. f. with tank full. full load adjustment by means of a cathode re Condenser l0 _____ __> ___________ __m. m. f.-- 400 sistor, as this would change the cathode ?lament voltage and temperature. Provision is made as Condenser ll __________________ __msm. f.__ 250 by an adjustable resistance 29 in shunt to one Condenser l5 _______________ __m. m. i’.-- 4 to 30 Condenser I6 _____________________ __m. i’.__ .01 30 ?lament for obtaining a proper balance between Resistance 9 __________________ __ohms__ 14,500 the two cathode heating currents, and the heater Coil ‘I ___________________________ __turns__ 200 Col] 8 ____________________________ __do____ 200 current of both may be adjusted by a variable resistance 30 in series. The series reactance at 3| in conjunction with condenser 30 provides ?l Coupling spacing between coils ‘I and 8 with out iron core __________________ __inches__ 0.2 85 tering action, and at 32 we provide an iron ?la Coil I2____I _______________ __microhenrys__ 200 Frequency range-—240 kilocycles tank full to 280' kilocycles tank empty. In calibrating the apparatus, the zero adjust ment with condenser l5 should be made ?rst. Thus with the tank empty, adjust~condenser ii ment ballast lamp in the cathode heater circuit to compensate for variations in the voltage of the supply source. This is important where the voltage of the source 26 varies, since without it 40 the apparatus has an appreciable voltage error. With the lamp at 32 the voltage source may vary from 422.5 to 29.5, with a. practically constant cathode ?lament voltage and operation within Now with the tank full, adjust ll until the pointer acceptable voltage error limits over the entire 22 reads at the full scale point of the scale. The 45 load range. reason for this order of procedure is because the Full scale measurement current adjustment is full scale measurement current adjustment with provided for by an adjustable resistor 33 in the resistance ll does not affect the zero scale read plate circuit of the electronic oscillator leading ing, whereas this zero scale adjustment does in to the measuring instrument. While in Fig. 1 ?uence the full scale reading‘. These adjust GI the‘full scale current adjustment resistor ll ments should be made in conjunction with the might be placed at any point in the tube instru scale distribution adjustments with resistances l8 ment circuit, in Fig. 2 it may not readily be placed and 20. between the cathode and source of supply, be While the apparatus may be calibrated in gal cause then it would in?uence the heating cur lons or inches of fuel in the tank, there is some _- rent of the cathode ?laments. An inductance l2 advantage in calibration in terms of weight of may be desirable to improve stability. Also, in fuel because essentially that is what is measured the plate circuit, Fig. 2, is a ?ltering connection and the calibration will not change appreciably comprising an inductance 34 and a condenser 35 with different grades of gasoline or fuel in the which amists in screening the alternating cur tank. It was found that the dielectric constant 60 rent of the oscillator from the instrument circuit. of different grades of gasoline is substantially The instrument used is the same as in Fig. 1, proportional to the gasoline density. Hence, if although a somewhat different arrangement for calibrated in pounds or mass of fuel, changes in adjusting the length of scale distribution is pro the grade of gasoline used and changes in the vided. In Fig. 2 the potentiometer 36 adjusts the temperature of the gasoline with corresponding relative values of bucking ?eld current in wind changes in density will have little effect upon the ing l1 and the cross ?eld current in winding I9, calibration and, in general, such calibration will while the potentiometer 31 is in series with both be closer to the actual fuel value of the gasoline of these ?elds. used than if made on a volume basis. In Fig. 3 ‘let the vector In represent the mag In order to minimize radio interference, it is 70 nitude and direction of the ?eld of winding ill desirable to segregate the oscillator circuit by a of such polarity as to tend to pull the N pole suitable ?lter or ?lters which will substantially of the polarized rotor 2| in a corresponding di to cause pointer 22 to read zero on the scale. prevent undesirable frequency pulsations from passing in either direction therethrough. Such a ?lter is represented at 25 in Fig. l. rection. Then vector In correspondingly repre sents the bucking ?eld of ‘winding l1 and I1: the 75 ?eld of measurement winding l3. The result 2,400,071; 7 ant o! the opposed ?elds I11 and In is their dif ference, or I17——Ils, and the resultant of all of the fields acting on the armature is R. This gen erally represents the conditions when the meas urement current In is low and the pointer 22 is near the zero or low end of the scale. It will now be evident that under such condi 8 from the vertical in proportion to the increase in tank area from point to point. At the smallest cross-sectional area with the lateral dimension II, which may be referred to as the unit’ area. the tubular condenser may be vertical. Where the area is doubled at the dimension 8|. the slope of the condenser is then made such that it has tions when In is greater than 11:. moving the double the length in rising a corresponding dis slider of potentiometer II to the right will de tance. Through the lower section having the crease In and increase 111. and Iva-In. and move dimension 20 and 4/3 the unit area, the con the pointer to a lower point oi’ the scale. Mov denser is sloped to have a length of 4/3 of that ing the slider of potentiometer ll upward will through the unit area section. increase both In and In, and also 111-113, since Through the upper section which has a uni In stays constant. While this will increase B. tormly increasing area from top to bottom with it will have a negligible e?ect on its vector direc 15 the top dimension II and the bottom dimension tion and the position of the pointer 22. On the II. the tubular condenser is curved and has an other hand. when In is large with respect to inclination varying from the vertical at the top In corresponding to the conditions represented to that corresponding to the double unit area in Fig. so. when the pointer 22 is near the upper at the bottom 0! this tank section, such that end of the scale. moving slider 0! ii to the right 20 its length per unit of rise is in the same pro decreases In-In and 11s. and ii the Proportions portion to the cross-sectional area of the tank are properly selected, will have a negligible e?ect at all points. It is further noted that the con on the direction of R, and the pointer position. denser is positioned centrally oi the tank so Moving slider of II upward will increase In and that if the tank tips, the level of the liquid being measured in the condenser will closely approxi In and decrease Iii-‘I1’! for the conditions repre mate that corresponding to a level tank condi sented in Fig. 3a, which will ,move the pointer 22 to a lower point on the scale. Thus potenti tion. The tubular condenser will of course have ometer I! may be used for adjusting the zero openings to the tank at least at top and bottom, end of the scale with negligible effect upon the and will be suitably braced in place and be upper scale distribution. and potentiometer Il 80 tween its insulated plates to remain in accurate working condition. may be used for adjusting the full end of the scale with negligible effect on the down-scale In Fig. 13. we have represented a tank with distribution. any desired shape of cross section, here repre This type of zero and full scale adjustment sented as circular, having a baille plate I! 7 may of course be used in Fig. 1 and is the meter 35 through its center which will be metallic and able arrangement in this respect. The other secured to the tank wall. Positioned closely resistances in the circuits of windings I1 and thereto but insulated from the tank and balls, II of Fig. 2 are voltage dropping resistors and is a metallic plate 43. Forty-two (l2) and 43 may or may not be needed. In case the current form the plates of the tank condenser and corre through any of the windings is too large in pro 40 spond to plates 4 and 3 of Fig. 1. This arrange portion to the current through other windings of ment will correctly measure the liquid in the the instrument. the excessive current may of tank for reasonable angles of tilt in any direction from the position shown. course be shunted by a suitable resistor. In any case where it is di?icult or undesirable Figs. 6, 7. and 8 show di?erent forms of tubu lar condensers having a varying capacity over for any reason to mount the measurement con denser or condensers within the tank. it or they their lengths. In each case the capacity per may be mounted outside as represented in Fig. 5. unit length increases from top to bottom; in Pig. 6 substantially uniformly; in Fig. 8 abruptly in Here a pair of tubular condensers are mounted at opposite ends or a tank at the centers thereof a step; and in Fig. 'l in a gradual step. The and connected thereto by conduits at top and 60 capacitance increases as the dimension between bottom so that any liquid in the adjacent end condenser plates decreases. It is‘ apparent that of the tank will flow into the condensers at the with the form of condenser 01 Fig. 7 the outer same level. I! the two condensers are connected pipe plate can be ?attened as indicated in Fig. ‘la in parallel in the input circuit of the oscillator, to vary the capacitance as desired within limits the parallel capacitance will correspond to the after the condenser is installed. 65 amount of ?uid in the tank even though the In Fig. 9 we have represented cross-sectional tank tips as represented. or at right angles to views of condenser plates Ill and N where II; that illustrated. is a piece of angle metal and ‘I one corner oi’ In Fig. 4 we have represented an odd-shaped the tank or a baille in the tank. At II and ll tank in cross section. To‘ simplify the illustra-— liat plates are represented, one being a baille tion. the tank is assumed to have a unilorm 60 and a wall of the tank. dimension in the direction perpendicular to the Figs. 10 and 11 represent forms or condensers plane of the drawings and to have four equal which may be varied as a whole or from end to height sections of diilerent lateral dimensions end by changes in the separation of the plates. or shapes in the plane of the drawings. such rela Fig. 12 represents a variable condenser compris 65 tive latter dimensions being marked thereon. A ing two slit tubular portions one within the other. tubular condenser l of uniform cross-sectional The condenser as a whole can be accurately ad dimensions may be mounted in the tank from top justed by turning one tubular part on its axis. to bottom, as represented, and give a capacitance while the condenser may be varied irom end to variation which is linearly proportional to the 70 end by an eccentricity adjustment. It is under amount 0! ?uid in the tank in spite of the fact stood that in Figs. 6 to 13, inclusive, the dielectric that the dimensions and cross-sectional capacity used is a combination oi’ liquid to be measured of the tank diiler materially from top to bottom. and air, as explained in connection with Fig. 1. This is accomplished by simply changing the It will be observed that with a measurement scope or inclination oi the tubular condenser 75 condenser like that shown in Fig. 1 or 4, for 9 2,400,078 10 example, the outer tube of the condenser is in - put circuit so as to respond to variations in the effect a miniature tank as well as a condenser plate, so that ii’ such condenser were included as a part of a pipe line at any convenient point, measurement condenser with minimum disturb ance caused by variations in the voltage oi said source oi swim a grid leak for said vacuum tube one could tell by observing the measuring instru— ment whether the pipe line contained liquid or containing a resistance having a temperature co emcient of resistance selected to compensate tor not. ' the over-all temperature errors of said measur The apparatus can also be used as a pressure ing system, inductance included in the cathode indicator by the simple expedient oi’ iorcinga connection of said tube for stabilizing the opera dielectric liquid up in a tubular condenser struc 10 tion of said system, and ?ltering means for seg ture in respect to pressure instead of liquid level regating the oscillation and measuring instru and calibrating the measuring instrument accord ment circuits; , " ingly. 4. An oscillation type measuring system com What we claim as new and desire to secure by prising a vacuum tube having cathode, control Letters Patent of the United States is: grid and plate electrodes, a variable measurement 1. An oscillation circuit measuring system com condenser connected in a tuned input circuit be prising a vacuum tube having cathode, control tween the cathode and grid, an output circuit grid and plate electrodes, a tuned input circuit containing a direct current source of supply con connected between the cathode and grid contain nected between the cathode and plate, means for ing a variable measurement condenser, a tuned inductively coupling said input and output cir output circuit including a direct current source of cuits, a ratio type measuring instrument having supply connected between the cathode and plate, bucking windings in one axis, one winding being said tuned circuits including a coil connected supplied from said source and the other from said across the input circuit and a coil connected in output circuit, a winding in an axis at right the output circuit for coupling said circuits, a angles to the bucking windings also supplied from measuring instrument responsive to changes in said source, said instrument responding to current in the output circuit caused by variations changes in the current in the output circuit of the measurement condenser in the input cir caused by changes in the capacitance of said cuit, variable resistance means in the output cir measurement condenser, means for tuning the cuit for obtaining full scale instrument adjust 30 output circuit for zero scale adjustment of said ment, variable capacitance connected across the instrument, resistance in the output circuit for coil in the output circuit for varying the tuning full scale adjustment oi said instrument, and of such circuit and obtaining zero scale instru means for varying the current in both or the ment adjustment, and a grid leak in the grid con- , measuring instrument windings which are sup nection oi said vacuum tube including resistance plied irom said source, relative to each other and having a temperature coefficient of resistance se relative to the current in the winding supplied irom the output circuit. 5. An oscillation type measuring system com prising a vacuum tube having double cathode, lected to compensate said measuring system for temperature errors. - 2. An oscillation type measuring system com prising a vacuum tube having cathode, control 40 control grid and plate electrodes, an input circuit grid and plate electrodes, a tuned input circuit having parallel connections to the cathodes and connected between the cathode and grid and in grids, said input circuit containing a variable cluding a variable measurement condenser ior de measurement condenser which controls the tun termining the tuning of the input circuit, a tuned ing oi’ such input circuit, an output circuit con output circuit containing a source of direct cur 45 taining a direct current source of supply having rent supply connected between the cathode and parallel connections to the cathodes and plates plate, said tuned circuits including a coil connected said direct current source of supply being con across the input circuit and a coil connected in the nected in series between said cathodes, a ballast output circuit for coupling said circuits, a ratio lamp in said series connection, means for induc type measuring instrument having a winding energized from said source and a winding ener 50 gized from said output circuit and responding to variations in the capacitance of the measure ment condenser, variable resistance means in the output circuit for obtaining full scale instrument tively coupling said input and output circuits, a ratio type direct current measuring instrument having bucking windings in one axis and a third winding in a quadrature axis serving the purpose of a zero return spring, one of said bucking wind ings being supplied from said output circuit and adjustment, variable capacitance connected 55 the other two windings from said source of sup across the coil in the output circuit for varying ply, said instrument being responsive to the cur the tuning of the output circuit and obtaining rent in said output circuit in response to varia zero scale instrument adjustment, ?ltering means segregating the measuring instrument and oscil 60 tions in tuning of the input circuit, a variable resistance in the output circuit for full scale lator circuits and inductance in the cathode con instrument adjustment, adjustable capacitance nection of said tube for obtaining stable opera for tuning the output circuit to obtain zero scale tion of said measuring system. instrument adjustment, a potentiometer connec 3. All oscillation type measuring system com tion between the source of supply and the two in prising a vacuum tube having cathode, control 65 strument windings supplied therefrom whereby grid and plate electrodes, an input circuit be the relative magnitudes of the currents in these tween cathode and grid including a, variable windings may be adjusted, and an adjustable measurement condenser by means of which said resistance in series with both of the last-men circuit is tuned, an output circuit including a di tioned windings whereby their currents may be rect current source of supply connected between 70 varied relative to the current in the winding the cathode and plate, an inductive coupling be supplied from the output circuit. tween the input and output circuits, means for 6. An oscillation circuit measuring system com tuning the output circuit, a ratio type of meas prising vacuum tube means having cathode, con uring instrument having a winding supplied from trol grid, and plate electrodes, a tuned input cir said source and a winding supplied from said out 75 cuit connected between a cathode and a grid of 11‘ 2,409,073 12 said vacuum tube means and containing a vari scale instrument adjustment, variable capaci able measurement condenser, a tuned output cir cuit including a direct current source of supply connected between a cathode and a plate of said vacuum tube means. said tuned circuits includ ing a coil connected across said input circuit and a coil connected in said output circuit for coupling said circuits, a measuring instrument responsive tance connected across the coil in said output circuit for varying the tuning of such circuit and obtaining zero scale instrument adjustment, and a grid leak in the grid electrode connection of said input circuit including a resistance havin: a temperature coemcient oi’ resistance selected to minimize temperature errors in said measuring to chances in ‘ current in said output circuit system. caused by variations 0! the measurement con 10 denser in said input circuit. variable resistance meansinsaidoutputcircuitiorobtainingiuii FREDERICK R. BIAS. DONALD B. PEARSON.