USRE24267E - Electronic displacement measuring means - Google Patents

Description

Jan. 22, 195? H. J. OSTERMANN ETAL ELECTRONIC DISPLACEMENT MEASURING MEANS 5 She e'cs-Sheet 1 Original Filed Sept. 14, 1944 INVENTORS H N. J. "TIRMANN ARNO G WILKlNS BY (MARI-I15 3.3Ry-AN AT RNEyS- Jan. 22, 1957 H. J. OSTERMANN ET AL 24,257

ELECTRONIC DISPLACEMENT MEASURING MEANS Original Filed Sept. 14, 1944 5 Sheets-Sheet 2 Jan.22, 1957 H. J. OSTERMANN ETAL 24,267

ELECTRONIC DISPLACEMENT MEASURING MEANS Original Filed Sept. 14, 1944 5 Sheets-Sheet 3 -VOLT$ 12345678 a/smwcs-g y/a OF AN INCH GALLONS PER-HOUR B 8 INVENTORS HANS J- @O'FIRMANN Mme a K- -Knrvs /5' MlLl-IAMPERES.

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Jan. 22, 1957 H. J. OSTERMANN ET AL 24,267

ELECTRONIC DISPLACEMENT MEASURING MEANS Original Filed Sept. 14, 1944 5 Sheets-Sheet 4 F o m E E E 5 .5 E N (L 0 FORCE MEANS INVENTORS reRnmum NERO 0. K. wn-Ksns BY CHARLES 3. BR AN H. J. OSTERMANN ET AL ELECTRONIC DISPLACEMENT MEASURING MEANS Original Filed Sept. 14, 1944 5 Sheets-Sheet 5 MILLIAMFERES ANGULAR DEGREES DISPLACEMENT OF TORQUE- BEAM RT INPUT END.

ANGULBR BIS PMCE'MI'NT OF TOKQUE BEAM.

I N VEN TORS Inns 4. OSTERMANN Amvo G- K- waunlus CHARLES :BHBRYAN United States Patent 24,267 ELECTRONIC DISPLACEIVIENT NIEASURING MEANS Original No. 2,593,339, dated April 15, 1952, Serial No. 554,128, September 14, 1944. Application for reissue April 1, 1954, Serial No. 427,403

(Filed under Rule 47(a) and 35 U. S. C. 116) 13 Claims. (Cl. 340-187) Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

This invention relates to electro-mechanical means including electronic transmission system for measuring displacements representative of variables to be measured such as pressures, temperatures, fluid flow and the like.

The basic object of this invention is the provision of an apparatus by means of which the physical displacements of a member in response to temperature and pressure changes, the rate of flow of fluids and similar variables may be converted by means of an electron transmission system into electric currents or voltages directly proportional to such displacements and in turn converted into accurate visible indications of the quantitative values thereof or employed through control devices to maintain them "at desired values. V

The general combination of this invention includes:

12 A primary responsive element adapted to respond to a variable to be measured;

2. A variable gap-control device operated by said primary responsive element;

3; A gap-controlled current or voltage source having an output variable in response to said gap control;

4. A coupling network adapted to feed the output of said current or voltage source to a plurality of electroresponsive devices, in predetermined magnitude and phase relations;

5. An electro-responsive device associated with said output network and operatively connected to said control gap so as to superimpose its influence on said control gap and said primary responsive elements in a predetermined ratio; and

6. A plurality of electric meters, recorders or control devices, calibrated-in terms of the input variable of said primary responsive element, connected to said output network.

By means of such a combination constructed in accordance with the invent-ion herein disclosed, it is possible:

1. To indicate the magnitude of the variable being measured at a plurality of points at different locations;

2. To use a plurality of ditferent types of standard meters, recorders and the like;

3. To obtain a plurality of simultaneous separate indications as required;

' 4. To operate selectively a single indicator or recorder by means of a plurality of transmitters built in accordance with this invention;

5. To multiply, as required, the size of the indications representative of the variables thereby obtaining greater accuracy of indication;

6. To use standard electric meters or recorders to accurately measure any type of mechanical variations;

7. To produce sufilcient power in the indicating trans- Re. 24,267 Reissued Jan. 22,

mission lines to operate a relay or other control device without impairment of meter readings taken from the same transmission lines; and

8. To employ indicating meters having linear scales.

Other and more detailed objects of the invention will be apparent from the following disclosures of several embodiments or applications thereof as illustrated schematically in full detail in the attached drawings.

This invention resides substantially in the combination, construction, arrangement, relative location of parts, steps and series of steps, all as will 'be described in detail below.

In the accompanying drawings:

Figure 1 is a schematic and diagrammatic illustration of an application of the principles of this invention to a fluid pressure measuring system;

Figure 2 is a longitudinal, central, cross-sectional view through another form of the invention as applied to an instrument for measuring the rate of fluid flow through a conduit;

Figure 3 is a diagrammatic circuit illustration of the electronic transmission system used with the structure of Figure 2;

Figures 4 and 5 are charts of several characteristics of such a flowmeter;

Figure 6 is a diagrammatic and schematic illustration of another form of the invention as applied to measuring the angular displacement of a lever;

Figure 7 is a diagrammatic illustration of the circuit portion of the apparatus of Figure 6; and

Figures 8 and 9 are charts of some operating characteristics of the system of Figure 6.

The subject matter of the invention herein disclosed is illustrated in Figure 1 as applied a system for continuously measuring fluid pressure variations. The source of fluid pressure, not shown, will be connected by means of a pipe to the coupling and supporting element 2 of a Bourdon spring 1. The free end of this spring is connected by a leverage system 3 through a spring '4 to a coil 5 mounted on pivots 6 in suitable bearings, not shown. Another spring 7 is connected to the opposite end of the coil 5 and anchored on a fixed support 25 so that rotational movement of the coil 5 on the pivots 6 is controlled by the light hair springs 4 and 7. The coil 5 is, a shown, rotatatively mounted in the field of a magnetic system which, in the case illustrated, includes the permanent magnets 8. It is apparent that the structure 58 and its associated parts comprises a common form of galvanometer well known in the electrical arts.

Connected to the movable coil 5 is a metal vane 9 which rotates with it and when in central normal position is separated by a small gap from coil 10. This coil is a load across the input grid circuit of a Hartley oscillator including the pentode 11. The heater 12 for the cathode 14 of the pentode is supplied with energizing current from the power line 13. The inductor 10 and capacitor 17 constitute a series tuned load circuit on the oscillator system consisting of tube 11, grid inductor 16, blocking capacitor 21 and plate inductor 19. The resistor 162. provides a proper D. C. return for grid 15. The connection including the condenser 20 places the lead from 24 to the junction at 19--21 at ground radio frequency potential. The oscillator illustrated is but one form of many well known vacuum tube oscillator circuits suitable for the purposes of this invention. The output circuit is coupled by means of connections, as shown, including the screen grid 27 to a coupling network illustrated at 24. As shown, this network includes an inductor and a resistor in series in the circuit and a second resistor shunted across the circuit and connected to a tap on the inductor. This particular form of coupling network is merely illustrated by way of example since, as

will be apparent to those skilled in the electrical arts, there are many other suitable forms of coupling circuits for connecting the output of the oscillator with the instrumen't line 28;

For the purpose of illustrating "the adaptability of the invention, two D. C. voltmeters 29 and 30 are shown connected in the indicating line 28 as well as'a recording meter "31. These various meters may be distributed at any desired points with respect to the rest of the system so as to give a plurality of indications at relatively different points.

The coil 5 of the galvanometer is also connected in the output circuit of the oscillator across the coupling network 24'by means of the wire 22 including the fiexible connection 23 to the lever system 3. One terminal of the coil 5 is connected toQthe hair spring 4 in a well known mannerand the other-terminal is connected to the hair spring 7 in a well known manner and the circuit is completed through the anchor 25 for the hair spring and -a resistor 26 having a negative temperature coefficient back to the other side of the network 24.

In-the-operation of this apparatus when the fluid under pressure is supplied to the Bourdon spring 1 its free end will move, causing a rotational movement of the lever system 3, and hence of the coil 5 through its hair spring connection with the lever system. The parts are arranged so--that rotation of the coil 5 in the structure, as illustrated, will be in a clockwise direction so that the metal vane 9 will approach the coil 10. This movement of the metal vane 9 in the field of the coil 10 will change its apparent inductance and thus vary the degree of resonance between the load and the oscillator, increasing the flow of current in the-tuned plate circuits thereof. This change of current in the plate circuit will appear as an indication on the meters 29 and 30 and a permanent record thereof will be made on the recording meter 31. A portion of the current flowing through the network 24 will f'ed'to the coil 5 creating a magnetic field therearound which will react the field of the permanent magnets 8 to' counterbalance the force applied to the coil by means of the'Bourdon spring 1. Thus for a particular fluid "pressure supplied to the Bourdon spring 1 the coi1j5 will "take a fixed position which means that the oscillator will continue to generate for that position of "the apparatus a'cu'rrent the strength of which will be a direcfmeasure of the magnitude of the pressure on the "fluid, and I hence the meters when properly calibrated will give a direct indication and make a record of the pressure orrthe fluid. The values of the fva'ri'ous circuit elements are sopropoitione'd that for each position of the g'alvanometenand associated parts a current of corresponding magnitude will be fed t'oithe' meters and, as stated, whenproperly calibrated will give a direct indic'ation of the fluid pressure applied to the Bourdon spring. The resistor 26 by reason of its well known characteristics will contribute'its function to cause an accurate position of the coil 5 and hence acorre'ct' balance' of {forces in the instrument under varying ambient temperature to insure an accurate indication on the meters. As previously stated, the parts may be proportioned so that the current flowing in the indication circuit 28 can be made of suflicient magnitude so that it is not only possible to operate a plurality of meters but to effect operation of control devices such as relays and the like without disturbing the accuracy of the meter readings. By way of further example, this system may be designed in accordance with known electrical skill so that it is possible to supply as much as 30 to 4 ma. to a load of 2500 ohms D. C., suflifficient power to opcrate 100 ordinary 30 ma., 50 mv., D. C. switchboard meters simultaneously or operate a lesser number of meters and a relay to control any other desired function without impairingthe meter reading p The flowmeter application as illustrated herein will now be described in connection with Figures 2 to inclusive. In this more specific application of the subject matter of this invention, the electromechanical force relationships involved will be described in greater detail. Briefly, however, the meter as illustrated is adapted to measure the rate of fl0w of any liquid and specifically, for example, for measuring the flow of gasoline in the fuel'line' of an airplane. I

The device includes a casting or housing 32 having a passage 35 therein to which a fuel supply line may bc connected at the threaded port 33,. This passage extends gradually into the supply pipe connection;34 extending beyond the meter as, for example, to a gasolineengine. Beyond the point of curvature of the passage 35 at a point of minimum turbulence in the fluid stream is mouriteda force plate or'disc 40. This disc is mounted upon a rod 39 extending axially of'the passage by any suitable means by which it may be lockedtheieon and the rod is provided with an adjustable counterweight 41. The rod 39 extends backwardly through a passage 36 which interconnects the passage 35 with a chamber 37 in the housing 32. Within the chamber 37 and attached to the rod 39 is a float 38 to the other end of whichis secured a soft iron core or armature 42.

The outer end of the chamber 37 is closed by means of atplate 43 having a passage therethrough into which the armature 42 extends. Mounted on an annular. shoulder on the outerface of the closure member 43 is a tubular housing 46 the outer face of which in turnv is closed by means of an apertured.plate'47. Extending between theplat'es 43 and 47 is a tube 46' into whichthe armature 42 projects and upon which is'mounted a solen'oid winding 44. Sealed on the outer end of the closure 47 is a, closed housing 48, the interior ofv which communic'ates with the chamber 37 by-means of the vpassages through the closure 43, the tube 46' and the closure .47, Attached to the end of the armature 42 by-rneans of a rod or shaft 49 is a metal disc 50 preferably of aluminum. This disc is normally positioned so as to be within the field of a winding 51 which may be termed a sensing winding. The chamber within the housing 48,may .be placed in communication with the passage 35 ;by, means of a venting pipe 200 which includes the adjustable valve The circuit; inwhich the solenoid 144 and the sensing coil51is included is shown in Figure 3. This circuit ins cl-udes an oscillator comprisinga pentode 52 having tuned'grid and tuned plate circuits. The tuned grid ,circuit is diagrammatically illustrated at 53 and the tuned plate circuit at 54. This circuit includes the sensing. coil 51. A power supply circuit 55 supplies the energizing current for the heater of the pentode 52 and the heater of the diode 56., The diode is connected in series with the circuit 55, the indicatingmilliammeter 57, the solenoid 44 and the tuned plate circuit 54. Here again it may be noted thatthe particular form of oscillator circuit is capablepf manyyariations and that shown in Figure 3 -msre y l st iye,

Before discussing the operation of the flow-meter .a brief -reference to-the theoretical principles involved in'its operation will be-helpful in understanding its full capabilities. By the laws of hydraulics and magnetism the force on the plate 40 in the stream of flowing fluid is proportional to the square of the velocity oi the fluid while the force exerted by the solenoid is proportionalto the square of theelectric current energizing it. By a proper design of the solenoid the force exerted by, it may be made independent of the position of the armature 42 oven the range of displacements encountered in its normal operation.

In mathematical terms:

whence KuI -Ki W I-V: i ma. Fu=magnetic force Fr=fluid force Ku=solenoid constant I=electric current Kr=constant a v=linear velocity of the fluid F constant m From this it is evident that an ordinary D. C. milliammeter inserted in series with the solenoidcoil as shown in Figure 3 could be calibrated to read fluid velocity directly on a linear scale. The housing 32 is so. constructed andthe force plate 40 is so positioned that turbulence in the flowing stream is kept to a minimum. This is accomplished by locating the force, plate in a straight section of:.the channel 35, by making the bend in the channel gradualand by locating the major part of the moving element in a duct where it does not interfere with the stream of fluid. I I I I The general law representative of the movement of the plunger of the elect-ro-magnet is that the force on the plunger is a function of the current squaredand of the position of the plunger with respect to the solenoid winding. The latter provision would prevent linear calibration of the flow meter scale and would cause the initial positioning of the plunger tobe a critical adjustment.

Therefore, the magnetic system should be developed in accordancewith known principles so that over the range of displacements used the force on the plunger is virtually independent of its position.

The electronic circuit shown:in Figure 3 includes a tuned plate tuned grid oscillator with the solenoid coil inserted ;-in series with the power supply. The coil 51 of the tuned plate circuit is a sensing coil which is loaded by inductive coupling with the metal, preferably aluminum, sensingdisc? 50 of the moving elementv of the meter. 'As the distance between the sensing disc and the sensing coil is varied by displacement'of the force plate .40 the loading of the oscillator is changed. Accordingly the oscillator draws a greater or lesser amount of power,

thiscurrent from the power supply changing the current in the solenoid coil 44. Thus the force applied to-the float system is balanced by the magnetic force of the solenoid 44 to hold the moving element of the system in each position corresponding to the particular rate of flow of the moment. 1 7

The float 38 is provided on the moving element to balance the effect of gravity permitting the flowmeter to be operated in any position without the introduction of errors due to theweight of the moving system. It also minimizes the pressure of the moving system on any hearing members that may be employed and consequent friction. The venting pipe 200 and valve 300 are employed as a bleeder connection from the sensing head to the duct to prevent the possibility of the formation of air pockets which would interfere with the accuracy of the device. It was found from actual experience that by varying the adjustment of valve 300 a variable hydraulic force was introduced into the system which opposed the main fluid force and reduced the flowmeter reading'for any given how. Thus this venting system would provide, if desired, means for calibrating the meter or in conjunction with a thermostat provide for temperature compensation.

Referring to the charts shown in Figures 4 and 5, curve Bv of Figure 4 is a plot of the magnetic pull of the solenoid 44 in grams at a constant current energization for displacements of the armature 42 in 16th of an inch. The central portion of this curve is substantially flat showing that when properly designed the field strength of the solenoid is substantially constant over. a'ruseful range of displacements thereof. Curve A of Figure. 4 is a graph of the effect of variations of the supply current voltage on the meter indications showing that when properly designed the flow meter may be made independent of all reasonable fluctuations of voltage in the supply system 55.

The graph of Figure 5 represents calibration curves of the device employing two difierent pentodes, that is two different tubes of the same kind, namely. 117 N7GT tubes. Thus it will be seen that the efiect of substituting one tube of the same kind for another is relatively unimportant on the accuracy of the meter.

From the above it will be apparent that a meter of this type may be readily construct-ed which will give a very accurate indication on a milliammeter properly calibrated of the rate of fluid flow through a pipe or other channel.

The subject matter of this invention is also illustrated in Figures 6 to 9 inclusive, as applied to an apparatus for measuring and transmitting torque and displacement values. An important advantage of a system of this kind is its ability to measure very small values of torque and displacement. This is accomplished in the system illustrated by translating the input variable of torque or displacement quantitatively into an electric current which can be measured to a high degree of accuracy onan ordinary D. C. meter. By the system illustrated, values of torque may be measured which are considerably smaller than those available from such devices as Bourdon tubes, diaphragms and the like.

The essential parts of such. a system are illustrated in Figure 6. Rotatably mounted in a fixed bearing 59 is a shaft 58 to which a torque arm 60 is attached. For illustrative purposes the torque arm is shown connected to the piston of a cylinder 61 fed by pressure fluid through the pipe connection 62. It will be apparent that any other force to be measured can be applied in any suitable manner to the arm 60 and it will be understood, therefore, that the force means illustrated is given as an example only. A torque beam 63 is rigidly mounted in the end of the shaft 58. As illustrated, this torque beam is formed by bending a thin sheet of suitably resilient material such as, for example, beryllium copper 0.0015 inch thick, into a bar of T-shaped cross-section. This arrangement is more rugged than pivots and bearings and is less affected by dirt. Secured as, for example, by welding to the top face of the torque beam 63 near its free end is an arm 64 which terminates at one end in a metal plate 65 positioned between a pair of coils 68 and 71. Mounted on the other end of the arm 64 is a solenoid coil 66 through the center of which extends one leg of a permanent magnet 67. A D. C. millimmeter 70 is connected to the output of the oscillator 69 in series with the coil 66. The coils 68 and 71 are connected in the input and output circuits of the oscillator 69 as diagrammatically illustrated in Figure 6.

The oscillator is shown in greater detail in Figure 7. It includes a pentode having a tuned grid and tuned plate circuit as illustrated. The coils 68 and 71 are included in the tuned plate and tuned grid circuits, respectively, of the pentode, as shown, which coils, as previously mentioned, are in the control gap and positioned at opposite sides of the moving flag or vane 65. The tuned plate circuit includes the indicating milliammeter 70 and a diode 81 is connected in the power supply circuit which includes leads 82 and across which the heaters 90' and 81 for the pentode and diode, respectively are connected. Here again the specific circuit arrangements are immaterial since various equivalents thereofare well known in the electronic arts.

In the operation of this device, when a fluid under a particular pressure is supplied to the cylinder 61 through the pipe 62, the arm 60 is displaced an' amount proportional to the pressure thereof, causing rotation of shaft 58, beam 63 and arm 64. Movement of arm 64 displaces the vane 65 bringing it nearer to one of the coils"68 and 71 and moving it further from the other eifecting the output current of said said output current, and means also connected to said output circuit and calibrated in suitable units to indicate the values of said variable.

6. In an clectro-mechanical system for measuring displacement, the combination comprising means including a resilient member displaceable by an applied force, a current source comprising an electrical [a vacuum tube] oscillator having an output circuit, means calibrated in suitable units to indicate the output values of said oscillator connected to said output circuit, electro-mechanical means conductively fed by said output circuit to apply a balancing force to said member to oppose the displacement thereof, an inductor for varying the output of said oscillator, and a metal vane relatively movable with respect to said inductor and operatively connected to said displaceable member and displaced by movements thereof to vary the tuning of said oscillator, said inductor comprising a pair of pancake coils disposed in parallel relation and said metal vane being moved by said displaceable member along a path at right angles to the planes of said pancake coils.

7. In a control system, the combination including means having a planar control element displaceable in an amount proportional to the deviation of a variable from a preset condition, an electrical oscillator having a planar control inductor, movement of said control element in a line perpendicular to the plane of said inductor varying oscillator, an electromagnetic mechanical system energized by the oscillator output current and connected with said displaceable means for applying force to said displaceable means in opposition to the displacement of said displaceable means and proportional to the output current of said oscillator, and a work circuit energized by said output current.

8. In a control system, the combination of displaceable means including a resilient member and having a. planar control element displaceable in an amount proportional to the deviation of a variable from a preset condition, an electrical oscillator having a planar control inductor, movement of said control element in a line perpendicular to the plane of said inductor varying the output current of said oscillator, an output circuit including means energized by said output current and reacting on said resilient member for applying a force to said displaceable means in opposition to the displacement of said displacea'ble means and proportional to the output current, and a work circuit energized by said output current.

9. In a control system, the combination including means having a planar control element displaceable in an amount proportional to the deviation of a variable from a preset condition, an electrical oscillator having a planar control inductor, movement of said control element in a line perpendicular to the plane of said inductor varying the output current of said oscillator, an output circuit for said oscillator including an impedance, means connected across said impedance and energized by current flow therethrough for applying a force to said displaceable means in opposition to the displacement of said displaceable means and proportional to said output current, and a work circuit energized by said output current.

10. In a control system, the combination including means having a planar control element displaceable in an amount proportional to the deviation of a variable from: a preset condition, means including a planar control inductor positioned adjacent said control element for developing an output current which varies in accordance with movement of said control element in a line perpendicular to the plane of said inductor, an output circuit including means energized by said output current for applying a force to said displaceable means in opposition to the displacement of said displaceable means and proportional to said output current, and a work circuit energized by said output current.

11. In an electro-mechanical system for measuring a variable, the combination including a displaceable memher having a control element provided with a planar control surface and displaceable in an amount propor tional to the change in a variable to be measured, torsion'ally resilient mounting means for supporting said member and including at least one element of L-shaped cross section, angular displacement of said member causing movement of said control element in a direction substantially perpendicular to the plane of said control surface, a control inductor so positioned adjacent said control element that movement of said element in said direction causes a variation in the inductance of said control inductor, a source of output current variable in accordance with changes in said control inductor and connected thereto, means energized by said output current for applying a force to said. member in opposition to the displacement of said member, and a work circuit energized by said output current.

12. In an electro-mechanical system for measuring a variable, the combination including a displaceable meanher having a control element provided with a planar control surface and displaceable in an amount proportional to the change in the variable to be measured, torsiona'lly resilient mounting means for supporting said member and including a mounting element having a thin resilient web portion and at least one thin resilient flange portion extending substantially at right angles to said web portion, angular displacement of said member causing movement of said control element in a direction Substantially perpendicular to the plane of said control surface, a control inductor so positioned adjacent said control element that movement of said element in said direction causes a variation in the inductance of said control inductor, a source of output current variable in accordance with changes in said control inductor and connected thereto, means energized by said output current for applying a force to said member in opposition to the displacement of said member, and a work circuit energized by said output current.

13. In an clectro-mechanical system for measuring a variable, the combination including a displaceable member having a control element provided with a planar control surface and displaceable in an amount proportional to the change in the variable to be measured, torsionally resilient mounting means for supporting said member and including at least one element of substantially T-shapcd cross section, angular displacement of said member causing movement of said control element in a direction substantially perpendicular to the plane of said control sur face, a control inductor so positioned adjacent said control element that movement of said element in said direction causes a variation in the inductance of said control inductor, a source of output current variable in accordance with changes in said control inductor and connected thereto, means energized by said output current for applying a force to said member in opposition to the displacement of said member, and a work circuit energized by said output current.

14. The combination as set forth in claim 13 wherein said displaccablc member is displaced in an amount proportional to the change in the variable to be measured by mechanical input means.

15. In an electro-mechanical system for measuring a variable, the combination including a displaceable member having a control element provided with a planar control surface, torsionally resilient mounting means for supporting said member and including at least one element of substantially T-shaped cross section, means for applying torque to said member in proportion to the variable to be measured, angular displacement of said member causing movement of said control element in adirection substantially perpendicular to the plane of said control surface, a control inductor so positioned adjacent said control element that movement of said element in said direction causes a variation in the inductance of said control inductor, an electrical oscillator connected to said control inductor and adapted to produce a direct current output which is variable in accordance with changes in ?1 1 1 2 'the inductance of vis'llidwontrol inductot, means energized 12,005,884 Bernarde 11111025,, 1935 bgi said'outputcurrentgforlapplyinga force to said member ,.2,103 ,741 :Bencowitz o Dec-:28, 957 in og'position to {said applied torque, and a work circuit 2,112,682 Ryder Mar. 29, 19,38 "e'nergized by said-outputcurrem. 2,117,894 Lenehan May .-17, 1938 V16. The combination of claim 15, wherein said force 5 2,154,260 :Brandcnburger Apr. 311, 1939 a lying meansincludes a, feedback coil on said displace- 2,212,085 Tate Aug. 20,' 194 0 able :m ember anicl positioned in a zmagnetic field, and 2,231,570 -Ryder Feb. 11,1941 means for supplying said output current to said feedback 2,238,380 Almen Apr. -15, 1941 coil. 2,320,881 NfiWtOll June ,1, 1 943 10 2,360,754 -Zieb0l-z Oct. ,17, 1944 References (ltiltled t he 1file t01. {h s p t n 2,371,040 Fisher et a1. Mar-6, 19 45 or e ongma pa en I -FOREIGN PATENTS UNITED STATES PATENTS 195,229 Great Britain Mar..29, ,19 23 'l,"1 9 l,4'16 Gibson Jul -1s, 1916 m 1,827,560 B'i'nkley Oct. 13, 1931

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