US2500355A - Electron discharge tube - Google Patents

Description

March 14, 1950 p, HAAS ELECTRON DISCHARGE TUBE 3 Sheets-Sheet 1 Filed NOV. 14, 1944 r .DI- 6 l DI. 0 l a y/ M F Q 7 8 .v 5 2 2 L; ff

@Mw Wm March 14, 1950 P. HAAs ELEcTRoN DISCHARGE TUBE 3 Sheets-Sheet 2 Filed Nov. 14, 1944 PAUL /74/15 INVENTOR WM M/ Y E N R O T T A March 14, 1950 P, HAAS ELECTRON DISCHARGE TUBE 3 Sheets-Sheet 3 Filed Nov. 14, 1944 INVENTOR B%/A/ ATTORNEY PA z/L HAM Tzlll Patented Mar. 14, 1950 ELECTRON DISCHARGE TUBE Paul Haas, Emporium, Pa., assignor to Sylvania Electric Products Inc., Emporium, Pa., a corporation of Massachusetts Application November 14, 1944, Serial No. 563,348

16 Claims.

This invention relates to electron discharge tubes and more particularly to discharge tubes with substantially planar parallel electrodes.

It is a principal object of the invention to provide an electron discharge tube for ultra high frequency use in which the electron transit time is reduced to a minimum by extremely close spacing of the tube electrodes.

It is another object of the invention to provide an electron discharge tube of the planar triode type, all electrodes and lead-in` conductors of which are adapted to form parts of co-axial cavity resonators to be externally disposed and used as input and output circuits of the tube.

According to one object of the invention, means are provided for establishing separate parallel channels for the high frequency components on one hand, and for the low frequency and direct current components on the other hand of the return paths of anode and grid currents in close proximity to the emitting surface of the cathode.

According to another object of the invention, means are provided whereby the cathode-to-grid spacing in a planar parallel electrode mount is carried out subsequent to all glass and `metalto-glass sealing operations.

A feature of the invention relates to the completion of the vacuum-tight connection between the cathode and the rest of the tube assembly at a location remote from the active cathode surface, whereby any danger of oxidation or deterioration of the active cathode surface due to the heat required for obtaining the vacuum-tight connection is completely eliminated.

While the invention is particularly useful for triodes with planar parallel electrodes oi rotational symmetry, which during operation utilize an axial electron stream either as negative grid or grounded grid triodes in conventional circuits, it may also be used in connection with velocity modulated tubes or for any type of electron gun, wherever an extremely close spacing between a flat cathode and a cooperating electrode is desirable.

For purposes of illustration, the invention will be described in connection with a planar triode. In the drawing,

Fig. 1 is a schematic view of a tube mount having axial symmetry, with planar parallel electrodes according to the invention.

Fig. 2 is an enlarged longitudinal section oi the grid-anode assembly of the tube shown schematically in Fig. 1.

Fig. 3 illustrates the method of winding the grid electrode.

(Cl. Z-27.5)

Fig. 4 is a sectional View of the header of the tube.

Fig, 5 is a sectional View of the heater-cathode assembly according to the invention, which is inserted as a unit into the completed grid-anode assembly of the tube.

Fig. 5A is a sectional view of Fig, 5 taken along line 5A-5A.

' Figs. 6, 6A, 6B and 7, show jigs used for achieving the cathode-grid spacing.

Figs. 8A and 8B are views oi the cathode coating trimming jigs.

Fig. 9 is a sectionalized view of a coaxial transmission line connector arrangement for the various electrodes of the tube.

The tube shown in Fig. 1 ,comprises a tubular A,glass envelope l consisting of cylindrical sections tially disk-shaped metal piece with a tray-like curved inner portion il surrounding the central flat anode portion l2. The peripheral part I3 of anode 9 is located externally with respect to the tube envelope, forming a ring-shaped leadin seal, to constitute the end surface of an external tubular conductor of a co-axial resonance cavity. Grid disk it has an external ring-shaped portion provided with apskirt Hi, adapted to be tted into the cylindrical end of a metal pipe forming another part of the external cavity. A central aperture l5 is provided in the flat portion I6 ofv grid disk Iii in which aperture may be an annular frame 6G by which the grid wires are supported.

Cathode disk il which carries on its exposed face the electron-emitting coating ila is held in close proximity to grid-disk i6 and is carried by heater-cathode assembly 8. The unit consisting of anode 9, grid lll, envelope l and header 5 carrying tubular metal sheath 'i will be referred to herein as the envelope mount. Unit 8 will be referred to as the heater-cathode assembly.

The spacing between grid and anode obtainable by sealing the parts together in a glass lathe can be kept within the desired tolerances. Experience has, however, shown that it is very difficult to obtain uniformity of the extremely close cathode-grid-spacing needed for eliminating electron transit time effects at several thousand megacycles. This spacing must be of the order of a few thousandths of an inch, and it has been found that such spacings cannot be kept within the desired tolerances by known glass sealing methods.

In particular, it is dilicult to combine this critical. grid-cathode spacing with the further requirement, that the cathode lead-in should form a part of a co-ax'ial cavity comprising the input circuit of the tube, and to obtain minimum highfrequency losses in this lead-in conductor.

This is accomplished according to the invention by designing the cathode-heater assembly unit B as a substantially rigid insert, encased in metal sheath 1. Metal sheath 1 yforms, according tothe invention, a part of the envelope mount. The

lower part of this envelope mount, comprising header 5 with sealed-in metal sheath 1 is shown in enlarged form in Fig. 5. Heater-cathode assembly 8 is here shown inserted into metal sheath 1. It consists of an elongated angularly bulged metal tubing i8, of an outside diameter slightly smaller than the inside diameter of sheath tube 1. It is provided near its ends with two annular swelling's or bulges I9 and 2|), which act 'as guide rings for unit 8 within sheath 1. Cathode disk i1 is provided with a skirt portion 2|, and is held in spaced relation from, and attached to, the upper rim 22 of bulged tube 8 by a plurality of cathode support wires, two of which 23 and 24 are shown in Fig. 5. Skirt 2l is thus supported coaxially with respect to metal sheath 1, and separated by a very narrow annular space from it, which forms a capacity high enough to conduct the ultra high lfrequency currents passing through the cathode and to metal sheath 1 with no apnreciablc impedance. g

This arrangement has various advantages. In theiirst place, it establishes metal sheath 1 as a perfectly cylindrical lead-in conductor for cathode disk Vi. Sheath 1 is therefore adapted to form part of an internal conductor of a co-axial input cavity resonator for operation of the tube in an ultra high frequency circuit, whose outer conductor may consist of a metal pipe 25 (Fig. 9) attached to the skirt |4 of grid disk |U, for which cathode disk l1 and grid disk lll form the capacity gap between the end surfaces of this concentric line resonator cavity. No wires of small diameter are thus involved in making the electric connection between cathode disk l1 and the external and inner conductors Vof the input resonator cavity.

On the other hand, only a minimum of heat conduction from cathode skirt 2| to tubular sheath 1 takes place, because the annular space between them excludes a direct metallic contact. Metallic heat conduction from cathode skirt 2| to the bulged metal tube 22 is also minimized, by supporting the skirt 2| by a plurality of thin metal wires, of which 23 and 24 are shown in Figs. 5 and 5A.

Flat helical or spiral metal lament 25, which serves as heater for the cathode, is attached at its outer end 2@ to cathode skirt 2|, and at its inner end 21 to iilament lead 28. Filament lead 28 is held axially and concentrically with respect to bulged tube IB by glass bead 29, which is sealed to both, tube I8 and lead 28, in a vacuum-tight manner.

To -provide further heat conservation, a heat reector Sil is provided at the lower end of cathode skirt 2|, and the axial Vend portion of iilarnent 25 is threaded through 'a central hole in the heat reflector by means of ceramic bushing 3|, at-

tached to the heat reflector and to lament lead 28 by means of wire springs 32, 33.

Fig. 5 shows the heater-cathode unit 8 soldered into metal sheath 1 at its lower end by an annular solder ring 34. It also shows crimpings 35, 36, at the lower end of bulged tubing IB, with inserted bakelite spacers 31, 38 and metal base pin 3S, forming a lower plug-in connection for the heater current supply.

This plug-in contact pin for the heater current supply is formed after the tube l has been exhausted. The lower portion of bulged tubing I8, below bulge i8, is at that stage uncriinped, and does not contain the Bakelite spacers 31, 38, and metal base pin 39, when the heater-cathode unit 8 is being telescopecl into metal sheath 1.

After the heatercathode assembly is completed in the form ready for being telescoped into metal sheath 1, the accurate cathode-grid spacing operation must be carried out. This is accomplished by gauges comprising two jigs 48 and 4| as will now be described in connection with Figs. 6, 6A, 6B, and Fig. 7.

Calibratihgrod 42, with calibrated length L between its end surfaces (as shown in Fig. 6A), serves as one `of tw'o auxiliary gauge means for adjusting the cathode grid spacing. The envelope mount above described is inserted and clamped horizontally in the left-hand end piece 43 of jig 40 `as indicated in Fig. 6, in which the outlines of part of the envelope anode 9, grid ring il] and metal sheath 1 are indicated. Calibrating rod 42 of length L is slid into micrometer screw arrangement 44 (shown in Fig. 6B), telescoping the straight end of rod 42 into metal sheath 1 on the jig. Micrometer screw 44 is now turned, the 'rod moving with it axially to the left, until its lefthand end touches the flat portion i6 of the grid inside the mount assembly. This contact is preferably indicated by a contact indicating lamp 45 as shown schematically in Fig. 6. The michrometer screw is now locked, and rod 42 removed yfrom the jig. It will be understood that part 43 is insulated from base 46, and part 44 by insulation 41, this insulation being bridged by lamp 45 and battery 48 in series.

Heater-cathode assembly 8 is now placed into a second jig 4|, whose length L' is shorter than length L of Calibrating rod 42 by the desired cathode-grid spacing. Assembly 8, being much shorter than either L or L', must be supple- 'mented by a second, shorter Calibrating rod 49, provided with a pin vise 5G, into which the end portion of l'ament lead 28 is clamped, so as to obtain the correct length L from cathode disk |1 to the Calibrating shoulder 5| of rod 4S.

The entire unit of Length L', consisting of heater-cathode assembly plus the vised-on supplementing calibrator rod 49 is now removed from jig 4|, and slid into jig 4l), telescoping the heatercathode assembly through block 52 and metal sheath 1, until shoulder 5| contacts the locked micrometer surface. The grid-cathode distance obtained in this manner is obviously L minus L', vi'z., the specified spacing.

In this position, the heater-cathode assembly 8 is soldered or brazed at 34 (Fig. 5) to metal sleeve 1, and the tube is ready for exhaust and processing. After termination of this step, the external parts of the tube are cleaned and silverplated, the metal base pin 39 and Bakelite spacers 31,38, are introduced, and the end of bulged tubing |8 is crimped at 35 and 35, as previously described in the nished mount of Fig. 5.

The tube is now ready for use, and can be mounted into an appropriate ensemble of metal pipes and 53 (Fig. 9) completing the co-axial cavity resonators to be connected to the external rings of anode 9, grid l0 and metal sheath 1. The heater voltage can be supplied conveniently by the base pin 39 of the cathode-heater assembly.

The various conductors and tube elements particularly those which are sealed into the glass, should preferably be made of an alloy such as Sylvania #4 alloy consisting of from 38 to 44% nickel, '4 to 8% chromium, the balance substantially iron, plus the addition of a small percent of another metal such as aluminum, zirconium, calcium, beryllium and the outer surface of the conductors should be coated with a low resistance metal such as copper. I have also found that in view of the critical and precise dimensioning that is required for tubes of the type disclosed, the thickness of the cathode electronemissive coating l'la must be accurately gauged. For this purpose, the heater-cathode unit 8 may be supported in a V-shaped jig block G0 (Figs. 8A and 8B). This block has a V-shaped groove 6l to receive the heater-cathode assembly and is provided with a stop plate 62 against which at one end the heater-cathode unit 8 abuts as indicated in Fig. 8B. The cathode coating Ila. then projects slightly beyond the opposite end of the groove 6|. If desired, the groove 6I in block 6l) may be provided with recesses to receive the bulged portions I9 and 20. In coating the cathode, the initial coating is preferably overcoated so that a certain thickness of this cathode coating projects beyond the flat right-hand end of block $0. This' 'overcoating can then be removed by shaving as indicated by the blade 63, the shaving edge 64 of which abuts against the end $5 of the jig,`:and by moving the blade 63 vertically upward and downward the excess emission coating is accratey removed. This shaving operation not only eliminates the ever-present problem of an uneven coating of uncertain thickness, but in addition it insures a smooth even surface which will be absolutely parallel to the grid lll even though the disk I1 itself is out of alignment and not perpendicular to the longitudinal axis of the cathode assembly.

Likewise, since the grid electrode must be perfectly planar and parallel to the plate and cathode surface, it should be manufactured in the manner illustrated in Fig. 3. Each grid consists of a flat molybdenum disc 66 resembling a washer and with parallel tungsten wires 5l extending across the central opening. Fig. 3 shows the disc during the process of manufacture. The grids are made two at a time by sandwiching two bolybdenum washers 66 on opposite faces of a gauge block 58, the washers 66 being suitably clamped to the said opposite faces of the block. The ne grid wire 69 is then wound around the block with appropriate spacing between turns and then the wires are welded or gold-brazed to the iiat face of each washer preferably in a non-oxidizing atmosphere. The wound Wire may then be severed around the periphery of each washer thus leaving each grid in the form of a flat annular frame across one face of which the grid wires 6'! extend in spaced parallel relation. If desired, each of the washers 66 may be split and held in contracted position during the winding and grid-forming operation, so that when the finished grid is removed from the winding block 68, the washer 66 tends to expand and increase its circumferential size and thereby maintain thev grid wires completely taut. The finished grid electrode can then be centrally fastened to the grid disc lil by means of a series oi eyelets. By thus making the grid ring as a separate element, there is achieved a simplification in assembly in that it eliminates relatively complicated and less reliable supports for mounting the grid. Furthermore, by this arrangement, it is possible to make the disc assembly as a sub-assembly such as shown in Fig. 2, the three sections of glass and the two metal discs being assembled in a suitable ceramic jig and the sealing of the discs to the glass being effected by induction heating at high frequency.

In preparing the heater-cathode assembly (Fig. 5), the disk Il with its cylindrical wall 2| can be of nickel and its outer diameter should be slightly less than the inside diameter of the tube '1. This therefore leaves a small annular gap between the members 'l and I8 which serve as a capacity coupling means between the cathode Ila and the cathode metal sheath 1. This provides a sufciently low impedance return path for any high frequency currents which would otherwise tend to iiow through the support wires 23 and 2t. These latter wires may therefore, in accordance with the invention, be made of extremely small diameter, thus conducting a minimum of heat away from the cathode. In order further to corinne the heat within the cathode cap, the heat reflector 3i) is of course supported by the ceramic insulator tube 3l, so as to insulate it from the end 2l of the heater-cathode and the periphery of the said reflector 3U may be provided with three embossed points so that it contacts the lower edge of the rim 2l in only three very restricted areas.

What is claimed is:

l. An electronic control comprising an enclosing envelope, a plurality of metal electrodes having opposed surface areas in planar parallel relation, said electrodes having their peripheral .margins sealed into and extending through the wall of said envelope, a tubular metal member sealed through one end wall of said envelope, said 'tubular member having its inner end open and in spaced relation to one of said electrodes, a heatercathode unit including a metal sleeve carrying a cathode and telescoped into said tubular member, and means sealing the external end of said tubular member to said sleeve.

2. A device according to claim l in which one of Said plurality of electrodes has a dish-shaped central portion with a flat bottom, and another electrode is a grid electrode having a central window in which is fastened a substantially planar wire grid.

3. An electronic control device comprising a glass envelope formed of three tubular sections; a rst metal disc electrode having its marginal area sealed between the rst and second sections; a second metal disc electrode having its marginal area sealed between the second and third sections, a glass header extending transversely across the end of said third section, a tubular metal member sealed centrally through said header, a cathodeheater unit including a metal sleeve carrying a cathode cap and telescoped through said meinber, said cap carrying electron-emissive material,A

said cap being located with its iiat'face in closely spaced parallel planar relation to the center of said second electrode, and means sealing the external end of said member to said sleeve.

4. A device according to claim 3 in which the said first electrode has a central portion offset t0- wards said second electrode, the offset portion having a planar surface extending parallel to the plane of the said second electrode, and said second electrode'has a central window inwhich is rigidly mounted a grid unit comprising an annular metal member having a iiat face and having a plurality of spaced grid wires integrally united into the iiat face.

5. A device according to claim 3 in which said cathode-heater unit comprises an inverted metal cup having its flat surface coated exteriorly with electron-ernissive material, a heater nlament mounted within said cup,a heat coniining closure plate closing the open end of said cup, a tubular metal member telescoped into said metal sleeve, and a plurality of fine wires'supporting said cup: in spaced relation longitudinally from the end of said tubular metal member, the external diameter of said cup being slightly less than the internal diameter of the adjacent end of said tubular metal member providing a capacity between said cup and said tubular member which is of much lower impedance to ultra high frequency currents than the impedance of said line wires.

6. An assembly unit for tubes comprising a glass header having an .elongated tubular metal sleeve sealed centrally and transversely therethrough, another tubular metal sleeve of slightly smaller diameter than the first sleeve and telescoped therein, means maintaining said sleeves in fixed spaced relation, a cathode cap having a downwardly depending skirt telescoped into one end of the first-mentioned sleeve, and a plurality of thin support wires extending between the end of said second sleeve and said skirt supporting said cap in longitudinal spaced relation to the end of said second sleeve while maintaining said skirt in circumferential spaced relation within said end of said first sleeve.

7. An assembly unit according to claim 6 in which said cathode cap has its open end substantially entirely closed by a'separate heat reiiector disc, and a Vcoiled heater wire is 'mounted within said cap, said heater wire having one end fastened to the inner periphery of said skirt, and the other end extending centrally through said reflector disc but insulated therefrom, and a current supply conductor extending axially through said second sleeve and fastened to said other end of the heater wire.

8. An indirectly Vheated cathode for electron tubes comprisingan inverted metal cup having its iiat surface coated exteriorly with electronemissive material, a heater wire having a substantial planar spiral shape mounted in close spaced relation to the internal face of said flat surface, a heat reiiector disc over the open end of said cup, and means to support said disc so that it substantially entirely closes said cup.

9. An electronic control device comprising an enclosing envelope having a planar cathode surface, a planar grid electrode, and a planar anode, said grid and anode having integral flat peripheral disc portions sealed into and through the wall of said envelope and providing integral external annular flanges, and a fiat grid unit including a metal annulus carrying spaced grid wires with the annulus rigidly fastened to the margin of a central window `in said grid electrode.

l0. Apparatus for assembling the parts of a `tube of the type having planar cathode grid and anode; comprising means to hold a preformed assembly unit .consisting oi a tubular glass envelope containing kplanar anode, planar grid and a header with a central tubular metal sleeve extending exteriorly therethrough; means to support a mandrel of fixed known length in axial alignment with said sleeve andextending through said sleeve into abutting relation with the planar grid; an adjustable stop cooperating withV said mandrel for determining a fixed length between said grid and said stop; means to hold another preformed assembly unit consisting of a cathode cap and an elongated tubular metal member attached thereto and with a central rigid conductor; means to hold another mandrel in axial alignment with said central conductor, said other mandrel having an adjustable gripping member and a gauging stop whereby the overall length of said other assembly unit to said stop can be adjusted to a fixed length slightly less than the iirst-mentioned fixed length; said first-men tioned mandrel being removable and replaceable by the second-mentioned mandrel with its attached second assembly unit whereby a predetermined iixed gauged spaced relation is obtained between said planar grid and the end of the cathode.

11. An elongated unitary heatercathode assembly adapted to be slipped into a tubular metal sheath, comprising a piece of metal tubing having two narrow annular bulged portions whose diameters are slightly larger than that of the unbulged length of the metal tubing, a skirted cathode disc, means supporting the cathode disc in axially spaced and coaxial relation on one end of the metal tubing, said means including a plurality of stiff metal Wires, a straight filament lead disposed axially Within the metal tubing, means supporting and spacing the iilament lead in its axial position within and by the inner wall of said metal tubing, said means including an annular glass bead, heating means for the cathode disc including a flat spiral filament whose outer terminal is welded lto the skirt of the cathode disc, and whose inner terminal is welded to the axially disposed filament lead.

12. An envelope mount comprising a plurality of axially aligned disc-shaped electrodes sealed through and supported by the cylindrical wall of a tubular glass envelope closed on one end by a glass header through which is centrally sealed a tubular metal sheath axially aligned with said disc-shaped electrodes, said tubular metal sheath having a portion extending axially in the glass envelope into close proximity of one oi said discshaped electrodes and an external portion extending outwardly from the glass header in axial direction; and a unitary tubular heater-cathode assembly, telescoped into the tubular metal sheath, and comprising a disc-shaped thermionic cathode provided with a cylindrical iiange, positioned at the end of the tubular sheath adjacent to one of the disc-shaped electrodes, means supporting and radially spacing said cathode coaxially within the tubular sheath, said means including a second metal tubing to which the cathode is attached by wires and which is provided with a plurality of axially spaced peripheral annular bulges engaging the inner surface of the tubular metal sheath, a flat spiral iilament in close proximity to the cathode, a lament lead extending ,along the axis or the second metal tubing supported by and spaced from the inner surface of the second metal tubing by a glass bead forming vacuum-tight metal-to-glass seals with the axial ilament lead and the inner surface of the second metal tubing, and a. vacuum'- tight annular metal joint between the external end of the tubular metal sheath, removed from the glass header, and a ring-shaped portion near one of the peripheral bulges of the second metal tubing. l

13. The method of assembling a triode with planar parallel electrodes which comprises the steps of preparing an envelope mount comprising a tubular glass envelope, a disc-shaped anode, a disc-shaped grid electrode and a header carrying a tubular metal sheath, part of which extends into the envelope and having an end near the grid electrode, and another part of which extends outward from the envelope, having an external end, the two disc-shaped electrodes and the metal sheath being axially aligned; making a tubular heater-cathode assembly adapted to be telescoped into the metal sheath, and having a discshaped cathode on one end; telescoping the tubular heater-cathode assembly into the metal sheath; adjusting the desired cathode-to-grid spacing; and forming a vacuum-tight annular metal joint between the external end of the tubular sheath and a short peripheral section of the tubular heater-cathode assembly.

14. The method according to claim 13 in which the adjusting of the desired cathode-to-grid spacing is carried out by establishing externally to the envelope mount a fixed reference plane at a known axial distance from the grid on a calibrating xture to which the envelope mount is rigidly clamped, supplementing, in a second, separate Calibrating xture, the axial length of the heater-cathode assembly by an adjustably connectable and lockable auxiliary Calibrating rod, calibrating the overall axial extension of the heater-cathode assembly and the auxiliary calibrating rod to a length less than the known axial distance from the grid to the xed reference plane by the desired cathode-to-grid spacing, locking the heater-cathode assembly to the aux/iliary calibrating rod forming a rigid unit with it, telescoping this rigid unit into the tubular metal sheath with the cathode pointing toward the grid and terminating the telescoping motion when the Ifree end of the auxilary Calibrating rod coincides with the Xed reference plane.

15. An electron discharge tube comprising an envelope containing a plurality of cold electrodes and closed by a glass header through which a tubular cathode lead-in conductor is sealed, a disc-shaped thermionic cathode provided with a cylindrical peripheral flange of a diameter smaller than the inside diameter of the tubular cathode lead-in conductor, means adjacent said cathode for heating the cathode, and means coaxially supporting the ange of the disc-shaped cathode within the tubular lead-in conductor in radially spaced relation thereto.

16. An electron discharge tube comprising an annular condenser, constituting a low impedance for ultra high frequency currents, formed by the cylindrical ilange of a disc-shaped cathode and the inner surface of a tubular cathode lead-in conductor axially aligned and radially spaced with respect to and around said cylindrical cathode ilange, means supporting said cathode from the inner surface of the tubular cathode lead-in conductor, means connected to said cathode for conducting direct current from the cathode to the lead-in conductor, a at resistance heater spiral near the cathode, and coaxially disposed means within said conductor for supplying heater current to the flat heater spiral.

PAUL HAAS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,805,626 Jordan May 19, 1931 1,930,219 Zimber Oct. 10, 1933 2,091,443 Heintz Aug. 31, 1937 2,218,254 Wengel Oct. 15, 1940 2,225,853 Baker et al Dec. 24, 1940 2,280,980 Samuel Apr. 28, 1942 2,310,811 Schantl et al Feb. 9, 1943 2,335,818 Trumbull et al Nov. 30, 1943 2,353,742 McArthur July 18, 1944 2,353,743 McArthur July 18, 1944 2,367,331 Bondley Jan. 16, 1945 2,421,039 Segerstrom, Jr May 27, 1947

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