US2679007A - Variable inductance circuits - Google Patents

Description

May 18, 1954 w. F. SANDS VARIABLE I NDUCTANCE CIRCUITS Filed July 15 INVENTOR Wilma EJ123011 ATTORNEY Patented May 18, 1954 VARIABLE INDUCTANCE CIRCUITS William F. Sands, Haddonfield, N. J assignor to Radio Corporation of America, a corporation of Delaware Application July 15, 1949, Serial No. 104,984

14 Claims.

This invention relates to signaling circuits which are to be mutually coupled and which include inductances of different magnitudes having a predetermined ratio relative to one another, and particularly, to coil structures providing the inductance in the respective circuits which may be simultaneously varied by means of a movable core without varying substantially the predetermined inductance ratio of the two circuits.

In signaling systems, such as those included in radio receivers and the like, there are many cases where two inductive circuits are to be mutually intercoupled and in which the magnitudes of the inductances included in the respective circuits are different and yet have a predetermined ratio with respect to one another and in which the inductance ratio must be maintained substantially constant. Where circuits of the character described are to be variably tuned, or at least are to be controlled in such a manner that corresponding variations in the respective inductances are to be effected without changing the ratio of the inductances in the respective circuits, it has previously been necessary to provide the inductive devices in the respective circuits with individual movable core elements. This has been necessary when using coil structures of conventional design by reason of the fact that in structures of this character a single movable core element necessarily affects one inductive device differently from the other.

There are various types of circuits which are to be coupled and variably adjusted in the manner described wherein it is desirable to effect the necessary corresponding variations in the inductances included in the respective circuits by means of a single movable core element. One type of circuit wherein such a facility may be advantageously employed is the coupling between the output and input circuits of an oscillator. Another such circuit is one in which an impedance transformation is desired. In oscillators, particularly, which operate by reason of an inductive coupling between the input and output circuits thereof, it generally is necessary, in .1

order to effect the proper coupling, that a coil structure be provided in which one winding has a somewhat different inductance than another winding. For example, in the well known type of Hartley oscillator, there is employed a coil cillators operate in a satisfactory manner, it is necessary to maintain a substantially constant ratio between the inductance of the portion of the coil structure included in the anode or output circuit of the inductance and that portion of the coil structure included in the control grid or input circuit. When using coil structures of conventional design, it is not possible to employ a single movable core element for varying the frequency of the oscillator without effecting a greater change in the inductance of one of the coil structure portions than in the other. As a consequence, the ratio of the inductances in the respective circuits does not remain constant with the result that the operation of the oscillator is adversely affected.

Similarly, it is frequently desirable to employ an inductive device which is tapped at a point closer to one end than the other in a circuit wherein an impedance transformation is to be made. For example, where a relatively high impedance circuit is to be coupled to a relatively low impedance circuit, it is a common expedient to couple a coil structure having an inductance of a relatively large magnitude across the high impedance circuit, one terminal of which is common to a terminal of the low impedance circuit. By providing a tap on the coil structure at a point which is closer to that end thereof which is connected to the common terminal and by connecting this tap to the other terminal of the low impedance circuit, only a fractional part of the total inductance of the coil structure is effectively included in the low impedance circuit. However, where such a coupling device is to be variably tuned, a movable core element in conjunction With a conventional coil structure is unsatisfactory for the reason that a movement of the core element effects a substantially different change in the inductance of one portion of the coil structure from that efiected in the other portion, with the result that the ratio of inductance of the two circuits is not maintained constant.

Accordingly, it is an object of the present invention to provide an improved variable inductance apparatus for coupling two inductive circuits having respectively different inductances in a predetermined ratio, whereby the respective inductances may be varied without varying the inductance ratio.

Another object of the invention is to provide an improved variable inductance coil structure for coupling two inductive circuits having respectively different inductances in a predetermined ratio, whereby a movable core element may be variably positioned within the coil structure to effect a. variation in the inductances of the respective windings without substantially changing said inductance ratio.

Another object of the invention is to provide an improved coil structure for use with an osci1-- lator as the coupling means between the output and input circuits thereof and which comprises portions having different inductances in the respective circuits in predetermined ratio, whereby a single movable core element may be variably positioned within the coil structure for effecting inductance variations in the respective coil portion in substantially the same proportion.

In accordance with the present invention, there is provided apparatus for coupling two inductive circuits having respectively different inductances in a predetermined ratio relative to one another which comprises a multi-iilar coil structure having at least three windings arranged with ad jacent corresponding turns. At least one of the windings is connected in one of the circuits and a different number of the windings is connected in the other of said circuits. A core element also is included and is mounted for movement within the coil structure so as to simultaneously vary the inductances of the respective windings, all in substantially the same proportion whereby the ratio of the inductances remains substantially constant.

In accordance with another feature of the invention, there is provided a coil structure substantially of the character described for use in coupling the output and input circuits of an oscillator. At least one of the windings is includedin the output circuit of the oscillator and a greater number of windings is included in the input circuit of the oscillator. In a particular form of oscillator all of the windings are connected in series relationship. In another form of oscillator the windings associated respectively with the input and output circuits are not conductively coupled and the output circuit winding is centrally disposed relative to the input circuit windings.

The novel features that are considered char acteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization andmethod of operation as well as additional objects and advantages thereof will best be understood from the following description taken in connection with the accompanying drawing.

In the drawing:

Figure 1 is a circuit diagram of that portion of a superheterodyne radio receiver embodying the present invention wherein a Hartley oscillator is employed as the local frequency generator in a frequency converter stage, whereby the received radio frequency signals may be changed into intermediate frequency signals;

Figure 2 is a diagrammatic representation of a tri-filar winding together with its associated magnetic core element by which it may be tuned in accordance with the invention;

Figure 3 is another circuit diagram of a reaction type of oscillator embodying the invention in conjunction with a triode electronic tube;

Figure 4 is a further embodiment of the invention as it may be used in an impedance transformation circuit; and

Figures 5 and 6 show alternative embodiments of the invention.

Having reference now to Figure 1, there is shown the radiant energy receiving and frequency converting portion of a superheterodyne radio receiver. An antenna I9 is coupled by a capacitor H to the signal input grid [2 of an electronic tube l3 which as illustrated may be a pentagrid tube such as an RCA type 6SA'7. The antenna circuit is resonated by an inductor i l and a shunt-connected capacitor 15. The low potential terminal of the inductor i4 is bypassed to ground by a capacitor it so that a connection of this terminal may be made to a conventional AVC circuit as indicated. Tuning of the antenna circuit is effected by varying the inductance of the inductor M by means of an associated movable core element ll which, it will be understood by those skilled in the art, preferably is positioned within the convolutions of the inductor and is axially movable 'elative thereto as indicated by the double-headed arrow.

The frequency converter tube it also is provided with a cathode 18, an oscillator grid IS, a pair of space charge grids 29 and 2!, a suppressor grid 22 and an anode Space current and the necessary operating potentials for the tube iii are derived from a unidirectional power supply such as indicated by the battery 24. The two space charge grids 2G and 2! are connected together as shown and are supplied with suitable voltage of positive polarity by means of a connection including a resistor 25 to the positive terminal of the battery 24. The anode 23 of the converter tube is coupled through a parallel resonant circuit 23 comprising an inductor ill and a resonating capacitor 28 and through a resistor 2f: to the positive terminal of the battery 2 whereby space current is furnished to the tube l3 and also whereby signal-modulated intermediate frequency energy is developed in the circuit 2% which is tuned for resonance at the intermediate frequency. A capacitor 3%] provides a bypass to ground from the resonant circuit 2% for radio frequency currents. The suppressor grid 22 of the tube 13 is connected directly to ground as shown.

oscillator rid is of the tube iii is coupled by a capacitor 3i to one terminal of a multifilar coil structure 32, which is resonated by a capacitor 33. A resistor E i provides a leak to ground for the couplin capacitor 3 l. The multifiiar coil 32 as shown comprises three substantially similar windings 35, 3t and 37 connected in series aiding relationship to ground. The ungrounded terminal of the winding til is connected directly to the cathode it of the frequency converter tube i l. The multi-filar coil it! is provided with a movable core element 38 which, as will be more fully described hereinafter, is intima ely and substantially equally associated with each of the windings and 3?. Also the core 33 is arranged to be axially movable as indicated by the double-headed arrow, whereby to effect the desired variation in the inductance of the coil 32 to suitably alter the operating frequency of the oscillator for tuning purposes. The tuning cores 6'. and preferably are arranged. fcr concurrent movement and, accordingly, are suitably linked mechanically as indicated by the dash line 39.

The intermediate frequency output signal is derived from the frequency converted by means of a secondary resonant circuit at comprising the parallel arrangement of an inductor 4! coupled to the primary inductor 2"! and a resonating capacitor by means of which the circuit 2-3 is tuned for resonance at the intermediate frequency. The terminals of the resonant circuit is may be suitably coupled to an intermediate frequency signal amplifier as indicated.

Having reference now to Figure 2 of the drawing, there is indicated diagrammatically the manner in which a multi-filar coil structure may be formed in accordance with. the invention. In the illustrative example, the multi-filar coil comprises three windings which may be supported upon a coil consisting of a hollow tube 43, preferably of insulating material. Since there are to be three windings forming the particular tri--filar coil shown for illustrative purposes, there are three conductors represented herein by the solid line 35, the dash line 35 and the dot-dash line 3? in order that they may be individually distinguished. It may be seen from an inspection of Figure 2 that the three conductors 35, 36 and 31 are interwound upon the tube 43 in such a manner that each turn or convolution of each winding is adjacent to the coresponding convolutions of the other windings. The magnetic core element 33 is inserted within the hollow tube 3 substantially as indicated. It may be readily seen from this figure that, irrespective of in what position the core 38 may be placed within the tube substantially the same number of turns of each or" the windings 35, 36 and 31 are magnetically linked thereby. Consequently, whatever change in the value of the inductance or" one or the windings may be effected by varying the position of the core 38 within the tube substantially a proportional change in the inductance of the other windings will be effected.

Bearin -1 mind the structural arrangement or" the Inuit) l coil and its cooperating tuning core shown in Figure 2, consideration should now given with reference to Figure l of the operation of the embodiment of the invention in conjunction with the oscillator stage of a superheterod ne radio receiver. As is well known by those skilled in the art, the pentagrid frequency converter tube It includes an oscillator comprising the cathode iii, the oscillator grid 59 and effectively the space charge grids 20 and 21 serving as the oscillator anode or output electrode. As in a conventional Hartley osci1- later, the cathode i8 is connected to the feedb ck inductor such as the tri-filar coil 32 at a point located approximately at one-third of the distance from the grounded terminal. Accordingly, a tri-filar coil such as the coil 32 serves effectively in an oscillator of this type by reason of the fact that the cathode is may be connected to one of the junction points between the windings of the coil. It readily be seen that, when the magnetic core elements ll and 38 are adjusted in position relative to their associated coils i l and to vary the tuning of the receiver, not only is the overall inductance of the coil 32 changed in the manner desired but also the inductance oi the winding i-i'i connected to the oscillator cathode it; is changed in substantially the same proportion. Therefore, the inter-relationship between the energy developed in the output circuit of the oscillator and the energy fed back. to the input circuit for producing sustained oscillations is substantially unaltered when the feedback coil. is a tri-filar one in accordance with the invention and the inductance thereof is varied for tuning purposes. The oscillator, as a result, performs with substantially equal facility irrespective of the frequency at which it is tuned.

Referring now to Figure 3 of the drawing, there is shown another form of oscillator which may advantageously employ an embodiment of the in- 6 vention. In this case, the electronic tube 44 is a triode having a directly heated or filamentary cathode 65 which may be suitably energized by a relatively low voltage unidirectional power supply such as indicated by the battery it. The anode t! oi the oscillator tube M is coupled through a centrally disposed winding id of a trifilar coil structure ill and thence, through a load resistor 56 to the positive terminal of a source of unidirectional space current such as a battery 5!. he resistor Eli and the battery 5i may be bypassed to ground by a capacitor 52.

The grid 5;- of the oscillator tube M is coupled by a capacitor 5% to one terminal of one end winding 55 of the tri-iilar coil structure t9, the other terminal of which is coupled in series aiding relationship to one terminal of the other end windin 55 of the coil structure. The other tenminal of the winding is connected to ground and the two windings and 5d are resonated by a capacitor 51. A resistor 58 provides a leak circult to ground from the grid or" the oscillator tube. The tri-filar coil structure is provided with a movable core element which, as indicated by the double headed arrow, be axially moved relative to the coil windings for tuning purposes.

The windings i8, 55 and 56 may be formed upon a suitable hollow tube in a manner substantially similar to that shown in Figure 2. Inasmuch as each convolution of each of the windings is wound upon the supporting form in Such a manner that it is adjacent to corresponding convolutions of each of the other windings, the position of the core 59 within the supporting tube will produce substantially proportional inductance changes in each of the windings.

The movable core structure 5E3, as used in this form of the invention, may consist of a ferromagnetic portion fill and a substantially nonmagnetic portion Bi. The non-magnetic portion may be formed by mounting upon an insulating rod 62 a series of finitely spaced non-metallic rings 63 which, for example, may be made or copper. Not only are the rings 83 finitely spaced with respect to one another, but there also is provided a finite spacing between the end ring 54 and the adjacent end of the ferromagnetic portion 56. The purpose of such a movable core element is to extend the range over which the tri-filar coil structure 49 may be tuned.

Briefly, the operation of the oscillator shown in Figure 3 consists of the development in the Winding 48 of output circuit energy which is inductively coupled in suitable proportion into the windings 55 and 56 so that there is impressed upon the input circuit of the oscillator tube by means of the capacitor 5 9, energy in suitable magnitude and phase to effect the generation of sustained oscillations. lI'he magnetic core element 59 may be varied in its position relative to the number of convolutions oi the windings 48, 55 and 56 which it links so as to suitably vary the frequency at which the oscillator operates.

During that portion of the movement of the core 59 wherein the ferromagnetic portion to only is associated with the windings oi"- the trifilar coil structure '49, the non-magnetic portion (ii of the core has no effect upon the coil structure. As the ferromagnetic portion 65 is withdrawn from the coil structure in a downward direction, as viewed in the drawing, there is er"- fected a continuous decrease in the value of the inductances of the respective windings it, 55 and '56,. As the downward movement of the core 5% is continued the copper rings of the non-magnetic portion 6| become associated with the windings of the coil structure it so that the respective inductances of the windings 48, and 56 are still further decreased as a result of the production of eddy current losses in the spaced non-magnetic rings 63. In other words, maximum inductance of the coil windings is achieved by maximum penetration of the coil structure by the ferromagnetic portion alone and minimum inductance of the coil windings is achievec when only the non-magnetic portion ii! is en tirely within structure. Thus, it may be that the additional decrease in the inductances of the respective windings, effected by means of the non-magnetic rings 655 after the ferromagnetic portion iiii has been entirely withdrawn from the coil structure, enables the tuning of the coil structure over a much wider range than would otherwise be possible.

By virtue of the separation of corresponding turns of the input circuit windings and of the tri-filar coil structure it, the capacitive coupling between the two portions of the input circuit windings is decreased thereby enabling more eiiicient performance of the oscillator.

Having reference now to Figure l of the draw ing, there is shown an embodiment of the invention for use in effecting an impedance transformation between two circuits which is desired to couple together and which it is to variably tune. The terminals 65 are nected to a circuit having a relatively high inn pedance. There is provided a inulti-fiiar coil structure 655 having in this instance three W ings 6i, E58 and G9. The multi filar winding to is formed substantially in the as at indicated in Figure 2. The individual winare connected in series aiding relation shown, for coupling across the terminals and are resonated by capacitor it, also, coupled across the terminals The multi-iilar coil structure 65 is provided with a movable core element H by which to correspondingly vary the inductances of all of the respective windi. thereof. The low impedance circuit which is to be coupled to the terminals 65 comprises the cathode circuit of an electronic tube control grid it of which grounded. cuit connected to the cathode it includes re tively low impedance resistor "iii which passed by a capacitor The output circuit of the tube '12 includes an anode l which is sup. lied with space current from a suitable source i cated at +33 through circuit Wi an inductor and a load resistor bypassed to ground by a capacitor The c Considering new r m in which multi-filar coil structure functions to e a satisfactory impedance transformation between the relatively, high impedance circuit coupled to the terminals and the relatively low iinped cathode input circuit of the tube terminating between the grounded grid '53 and the ungrounded terminal of the coil 69, it should be taken into account that these impedanoes reinai tially constant. Also, will be appras the core ii is variably positioned re veto the windings Bl, es and til, the inductance, therefore the impedance of the rec ings necessarily varies which, of course a variation of the total impedance of the multifiler coil structure. Consequently, in the present case, a compromise is necessary, whereby the impedances of the windings of the multifilar coil structure are chosen so that, at a pre determined intermediate point of the range of adjustment of the core 'H, a substantially exact impedance transformation is efiected. For example, the center point of the range, in most cases, will produce optimum performance. Obviously, for all other points of the range the impedance transformation will not be exact, but ordinarily is satisfactory. However, for all points in the range of adiusment of the core H, the impedance ratio of the two portions of the multi-filar coil structure Sit which are coupled respectively to the high and low impedance circuits remains substantially constant and substantially equal to the impedance ratio of the circuits. Accordingly, in this description and in the appended claims, it will be understood that the values given are those pertaining to the previously described intermediate point of the range of adjustment of the core H.

The total impedance of the serially connected windings fil, 53 and @9 is substantially equal to the impedance of the circuit connected to the terminals 65. The impedance of the winding 59 is substantially equal to the impedance or" the cathode input circuit of the tube '32. In the embodiment of the invention illustrated in Figure 4i, it is assumed that the impedance of the circuit connected to the terminals 65 is substantially nine times the impedance of the cathode input circuit of the tube '12. Since all of the windings 68 and E9 of the multi-fi1ar coil structure 66 are substantially identical and are substantially unity-coupled by virtue of the described nianner in which the coil structure 68 made, it is seen that the total impedance of the serially connected windings is substantially nine times the individual impedance of any one of the windings such as the winding Obviously, if the ratio of the relatively high pedance circuit to the relatively low impedance circuit is greater or less than nine, a difierent number of windings and/or different circuit connections thereto may he made within the scope of the present invention.

For example, in the event that the impedance of the circuit connected to the terminals bears a relationship to the cathode input impedance of the tube '52 such as substantially 9%, three windings may be employed in the niulti-illar coil structure 86 connected substantially shown in Figure 5 across the terminals 65. In this case, it will be noted that the cathode-connected re sister 55 is connected to the junction point between windings i5! and B8.

In the event that the impedance of the circuit connected to the terminals 55 is substantially sixteen times the impedance of the cathode input circuit of the tube i2, Figure 6 illustrates an embodiment of the invention for coupling two such circuits. In this case, a multhfilar coil structure 32 is provided consisting of four substantially identical windings 83, 8t, 35 and 86, all of which are wound upon a supporting form in a manner similar to that shown Fig re 2. The windings are con ected in series aiding relationship, the entire series arrangement being connected across the relatively high impedance terminals 65 The cathode resistor (not shown) is connected to the junction point between windings and 86. It may be seen, therefore, that by reason of the substantial identity of the individual windings, the 16:1 impedance ratio may 9 be readily effected by coil structure substantially as shown in Figure 6.

It is to be understood that coil structures in accordance with the present invention are not necessarily limited for use as impedance transformation devices wherein a relatively high impedance output circuit is to be coupled to a relatively low impedance input circuit as shown in Figures 4, and 6. Alternatively, it obviously is within the scope of this invention to employ coil structures of the character described herein for effecting impedance transformations between relatively low impedance output circuit a relatively high impedance input circuit. In

order to effect this type of impedance trans formation, it merely is necessary to reverse the connections of the coil terminals to their respective circuits. For example, in either of Figures 4, 5 and 6, the terminals 65 would be connected to the relatively high impedance input circuit, while the terminals shown in these figures as connected to the input circuit of an electronic tube would be coupled instead to the output circuit of an electronic tube, for example, the impedance of which would be less than the impedance of the input circuit of the following stage.

It should be clear from a consideration of the several illustrative embodiments of the invention disclosed herein, that the inductance or impedance ratios between the two circuits which are to be coupled by means of the improved coil structures determines the minimum number of individual windings comprising the coil structure, and also the respective number of windings included in the two circuits. For example, where the impedance ratio is 9:1 or its reciprocal 1:9, three windings or any multiple thereof are required in the coil structure as shown in Figure 4. In such a case, two windings or a multiple thereof are included in one circuit and one winding or a multiple thereof is included in the other circuit. Also, where the impedance ratio is 9:4 or the reciprocal thereof, the coil structure may comprise three windings or a multiple thereof connected as shown in Figure 5. Also, as one further example where the impedance ratio is 16:1, 16:9 or the reciprocals thereof, four windings or a multiple thereof are required in the coil structure. Typical circuit connections for a 16:1 impedance ratio are indicated in Figure 6. Further extensions of the principles underlying the present invention are considered to be obvious in view of the foregoing illustrative examples whereby coil structures in accordance with this invention may be provided for coupling two circuits having substantially any inductance ratio other than unity.

It will be seen from the foregoing disclosure of several illustrative embodiments of the invention that there is provided an improved coil structure for coupling substantially any two inductive circuits having difierent inductances in predetermined ratios relative to one another, both or which inductances are to be correspondingly varied by means of a core element movable relative thereto without substantially changing the inductance ratio. Consequently, coil struc tures of the type embodying this invention make it possible to simultaneously vary the inductance by a movable core means of numerous circuits which heretofore have required individual sepa rately movable core means.

It, therefore, will be apparent to those skilled in the art that the invention may be embodied in various forms other than those specifically disclosed herein for illustrative purposes. Accordingly, the scope of the invention is to be determined particularly by reference to the appended claims.

What is claimed is:

1. Apparatus for coupling two inductive circuits having different inductances in a predetermined ratio relative to one another, said apparatus comprising a multi-filar coil structure having at least three windings arranged with adjacent corresponding turns, at least one of said windings being connected in one of said circuits, at dine-r nt number of said windings being included in the other of said circuits, and a core element adjustably positionable within said coil structure, whereby to simultaneously vary the inductances of said respective windings all in substantially the same proportion so as to correspondingly vary the inductances in said circuits while maintaining said predetermined ratio substantially constant.

2. Apparatus for coupling two inductive circuits as defined in claim 1, wherein said multifilar coil structure comprises three windings, two of which are included in one of said circuits and the third of which is included in the other of said circuits.

3. Apparatus for coupling two inductive circuits as defined in claim 1, wherein all of said windings are interconnected in series aiding relationship.

4. Apparatus for coupling two inductive circuits as defined in claim 3, wherein a tap is provided at the junction point between two of said windings so as to locate said tap at a point which is unsymmetrical relative to the series connection of said windings.

5. Apparatus for coupling two inductive circuits as defined in claim 1, wherein said multifilar coil structure comprises an odd number of windings.

6. Apparatus for coupling two inductive circuits as defined in claim 5, wherein a centrally disposed one of said windings is included in one of said circuits and the remainder of said windings are connected in series aiding relationship and are included in the other of said circuits.

7. Apparatus for coupling two inductive circuits as defined in claim 6, wherein said multifilar coil structure comprises three windings.

e. an osciliator including an electronic tube nput circuit electrode, an output circuit electrode and a third electrode common to both input and output circuits, a circuit coupling input and output circuits, comprising a multifilar coil structure having at least three mutually closely coupled windings in which each of the turns of each winding is adjacent to corresponding turns cf each of the other windings, circuit means connecting at least one of windings to output circuit electrode and a different number of said windings to said input circuit electrode, capacitance coupled to said coil structure for resonating certain ones of said windings at oscillator frequency deterrc ned by the values of said capacitance and the inductances oi respective windings, and a core element adjustably positionable within said c s" cture to vary the inductances oi all of dings substantially in the ea .c proportion, whereby to vary said oscillator f1 ,uency.

9. An oscillator as defined in claim 8, in which at least two of said windings are connected to one another in series aiding relationship.

10. An oscillator as defined in claim 9, in which two of said interconnected windings are coupled between said input circuit electrode and said common electrode.

11. An oscillator as defined in claim 10, in which a third one of said windings is coupled between said output circuit electrode and said common electrode.

12. An oscillator as defined in claim 8, in which at least one of windings is coupled in one circuit between said output circuit and said common electrodes, and the others of said windings are connected in series aiding relationship and are coupled in another circuit magnetically as sociated only with said one circuit between said input circuit and said common electrodes.

13. An oscillator as defined in claim 12, in which said multi filar coil structure comprises three windings of which two are connected in series.

14. An oscillator as defined in claim 8, in which said multi-iilar coil structure comprises three windings of which the central one is coupled between said output circuit and said com- 12 mon electrodes, and the two end ones of said windings are connected in series. aiding relationship and are coupled between said input circuit and said common electrodes.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,356,763 Hartley Oct. 26, 1920 1,723,485 Kummerer Aug. 6, 1929 1,942,385 Piety Jan. 2, 1934 1,943,799 Franks Jan. 16, 1934 2,127,334 Hathaway Aug. 11, 1934 2,190,048 Sinninger Feb. 13, 1940 2,343,101 Vogt Feb. 29, 1944 2,368,857 McClellan Feb. 6, 1945 2,431,438 Wentworth 1- Nov. 25, 1947 2,461,238 Schaevitz Feb. 8, 1949 2,489,114 Vladimir NOV. 22, 1949 2,491,486 Ewen Dec. 20, 1949 2,505,577 Rich Apr. 25,1950 2,525,438 Wuerfel -1 Oct. 10, 1950 2,568,587 Mac-George Sept. 18, 1951

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