US2601528A - Structure and internal shielding of electron tubes - Google Patents

Description

June 24, 1952 STRUCTURE AND INTERNAL SHIELDING OF ELECTRON TUBES Filed July 13, 1946 H. K. ISHLER 2,601,528

2 SHEETSSHEET 2 INVENTOR- Harry/fenn el'b fshl Patented June 24, 1952 STRUCTURE AND INTERNAL SHIELDING OF ELECTRON TUBES Harry Kenneth Ishler, Floral Park, N. Y., assignor to Sylvania Electric Products, Inc., a corporation of Massachusetts Application July 13, 1946, Serial No. 683,302

31 Claims. 1

This invention relates to electron tubes, more particularly to the electrode assemblies thereof, and also to shielding arrangements for said electrode assemblies.

It is well known that in electron tubes, particularly multi-grid tubes adapted for operation at radio frequencies, the capacitances between electrodes of the device combine with circuit elements to which the tube is connected and that the influence of these capacitances must be considered in the successful application of an electron tube. Usually it is desirable to reduce these capacitances to the lowest possible values and thereby to minimize their effects on the operation of the tube. Various electrode systems and structures are used to reduce inter-electrode capacitances; in tetrode type tubes, for example, one of the functions of the screen grid is to reduce the capacitance between the anode and the control grid. Conductive shield members have also been used.

An object of the invention is to provide an improved construction and/or arrangement of the electrodes and shielding members in an electron tube.

Another object is to provide an electron tube with very small capacitance between closely spaced electrodes of the tube.

Another object of the invention is to provide novel structures for shields disposed between the anode and other electrodes of an electron tube.

A further object is to provide an electron tube with a shielding system consisting of several component members forming a unitary construction.

Other objects of the invention will manifest themselves as the description proceeds.

A particular embodiment of the invention is illustrated in the accompanying drawing, in which:

Fig. 1 is a side elevation of a complete electrode assembly;

Fig. 2 is a cross sectional view along line 2--2 of Fig. 3;

Fig. 3 is a sectional side elevation of an electron tube embodying the electrode assembly of Fig. 1;

Fig. 4 is a top view of an anode section;

Fig. 5 is a side elevational view of Fig. 4;

Fig. 6 is a bottom view along line 6-6 of Fig. '3;

Fig. 7 is a side elevation of the tube of Fig. 3 drawn to approximately actual size;

Fig. 8 is a greatly enlarged fragmentary view of an alternative construction of the lower end of the electrode assembly;

2 Fig. 9 is a sectional View on line 9-9 of Fig. 8; and

Fig. 10 is a perspective view, from the under side, of the lower mica and parts associated therewith.

The complete electron tube is shown in Fig. '7, and comprises generally an enclosing envelope I hermetically sealed at one end with header 2 through which pass leads 3, 4 5. and 6. It will be noted that leads 3, 5, and 6, Fig. l, and leads 5 and 6, Figs. 2 and 3, have portions adjacent the lower mica wafer (hereinafter described) broken away. This is to avoid confusion in reading the drawings that might result in placing too many lines in limited spaces. Side support rods or conductors are also sealed into the header 2 for supporting the interior electrodes. The other end of the envelope I tapers down to an hermetically sealed tip I through which passes a single lead 8. The interior structure of the tube may be more fully appreciated as viewed from three directions in Figs. 1, 2, and 3.

The illustrated embodiment is a pentode tube containing filamentary cathode 35 and the conventional group of three grids, i. e. control grid 9, screen grid It, and suppressor grid II. The grids may each consist of a helical winding supported on a pair of side rods whose ends extend through top and bottom insulating discs or wafers I2 and 13 respectively. The insulating disks are preferably made of thin refractory insulating material such as mica. The mica discs I2 and I3 are provided with apertures for receiving various parts of the electrode assembly.

The anode construction presently illustrated including two sections I4 and I5, which are, like the grids, supported between the top and bottom micas I2 and I3. Each anode member may constitute a section of a cylinder with a central longitudinal channel having a protruding tab on each end formed in the line of the channel and similarly shaped thereto. Figs. 4 and 5 clearly illustrate the formation of the anode members and their protruding tabs. More particularly, see Fig. 1 showing section I5 having tabs I6 and H, and section I4 provided with tabs I8 and I9.

As previously noted anode sections I4 and I5 are supported between the top and bottom mica disks or wafers I2 and I3, however the anode sections are further prevented from rotating and are held in the proper angular relationship to the grids by slots punched in the micas which slots conform to the shape of the protruding tabs of the anode sections. When the tabs are inserted in the mica slots, as shown in Figs. 1, 2, and 3, it will be seen that rotation of the anode sections in either direction is minimized.

The channels formed in the anode sections function as reinforcing ribs, thus preventing buckling or bending of the anodes if the tube is subjected to violent mechanical shock. By making the tab a continuation of the channel portion, and having the same curvature thereof, both the channel and the tab may be formed in one operation. As a result of this construction it is easy to manufacture uniform anode sections at high speed production.

The channels of the two anode sections, being curved inwardly, provide a convenient means in the tube for locating getter pellet 20 and 2 l, which may be secured in position by welding, as shown in Figs. 1, 2, and 3. In the present embodiment the getter pellets consists of a metal holder containing the getter material. While the envelope is being exhausted and the anode sections are being heated to drive out absorbed gases, the getter materia1 inside the metal tube is vaporized and is sprayed through a central aperture in the holder onto the interior surface of the envelope.

According to the invention, capacitance between the anode sections and the grids and filament is reduced by side shields 22 and 23, which prevent electrostatic lines of flux from leaving the outside of the anode sections and returning to the grids through the sides of the tube nearest the grid side rods. The side shields are held between the top and bottom micas and are welded to side rods 24, 25, 26, and 2'! which extend through holes in the micas. In this particular embodiment some of the rods are welded to leads sealed through the header which construction aids in supporting the entire electrode assembly. For instance the lead 4 to which rod 25 is welded extends entirely through the header and serves as a lead for making electrical connections to the side shield 22. Side rod 26 is connected to shield 23 by another shield in a manner which will hereinafter be described.

It will be observed that the shields 22 and 23 have approximately the same curvature as the anode sections l4 and I5 so that when these shield and plate elements are alternately disposed with respect to one another they form substantially a hollow conductive cylinder which surrounds the grids and filamentary cathode. In the arrangement illustrated in the drawing these shield and plate elements are just positioned and interposed providing a generally cylindrical shield of four sections, two oppositely disposed sections forming the anode and the other two sections forming the side shields.

The anode sections are connected together by connector 28 which may be a loop of wire with one end welded to tab l8 of anode section I4 and the other end welded to tab l6 of anode section [5. External electrical connections to the anode sections is made by lead 8 welded to the center of anode connector 28 and sealed through the tip 1 which closes the upper end of the envelope 1.

In order to minimize the capacitance between anode lead at the end and the grid I provide a top shield 29 between the anode lead and connector and the upper ends of the filamentary cathode and the grids which extend through the top mica wafer. As shown in Figs. 1 and 3, in this particular embodiment the top shield 29 is roughly a half cylinder welded near each corner to one of the rods 24, 25, 26, and 21 to which rods the side shields 22 and 23 are also welded, as noted above. The top shield 29 thus provides a connection between side shields 22 and 23, and constitutes, with the side shields, part of a unitary shield structure, all of which members are directly connected and of the same potential. It will be understood that any other suitable means may be employed for determining the potentials of the shield sections during operation of the device.

The shielding of the anode from the remaining portions of the electrode structure below the bottom mica I3 is providing bottom shields 30 and 3|. Bottom shield 30 is welded to rod 24 below the bottom mica, as shown in Figs. 1, 3,

and 6, and extends out into the space between tab I! on anode section l5 and the lower end of the grid side rods and the filament which parts project below the bottom mica, thus cutting ofi the electrostatic lines of flux which would otherwise pass directly between tab I! and those portions of the other electrode below the bottom mica. Some electrostatic flux would also pass through the bottom mica and to the upper portions of the grid and filament from tab l1, and these also are blocked by shield 3|], although the principal effect of shield 30 is between the tab and the lower ends of the other electrodes. Shield 3| is welded to rod 26 and performs the same shielding function for tab H] of anode section 14.

The filamentary cathode (shown in broken lines Fig. 1) is secured between top and bottom supports 36 and 31, respectively, but only the bottom support 31 is shown in detail. (See Figs. 1, 2, 3, and 6.) The lower end of the cathode 35 is provided with a connector 38 which later engages a slot formed in the support 31. The support 31 is carried by lead wire 4 which is welded to the side rod 25 and is mounted below the bottom mica l3. The fact that the support 31 is carried by the rod 25, which rod forms part of an integral structure comprising the shielding system of the tube, incorporates the support 31 into said systems as a component member adapted to function as electrode support and also as a shielding element. Such an arrangement is of particular value where the electrodes of the tube are closely spaced as in the case of miniature tubes.

The side shields, the bottom shields, and top shield and cathode support are thus all directly connected together and form a unitary shielding arrangement, the potentials during operation of all parts being determined such as being at the same potential. To the extent that shielding at all sides is not necessary for a particular application the separate top, bottom, and side shields may be used individually if desired.

A structural feature of the illustrated embodiment of the invention is the manner in which the top and bottom micas are held firmly against the anode sections and the side shields. Thus the top shield 29 is welded to the support rods and is held firmly against the top mica so that after the welding process the mica is held between the top shield and the side shields. Similarly the bottom mica is held between the side shields and the bottom shields.

In some instances it might be preferred to bring the anode lead 8 out through the bottom of the tube instead of out through the top, as shown in Figs. 1 to 7. One such modification is illustrated in Figs. 8 to 10, and in order to effect this change in structure, slight variations in the i 24 and 2?, and E5 and 26, respectfully.

5, arrangement and design of tube parts were necessary, particularly those comprising the shielding system. For example, it will be noted that the header 2 is somewhat different in design from the header 2, this difference being dictated by the desire to retain the same or commensurate shielding efiects secured by the form of general construction shown in Figs. 1 to 7. Because of the general similarity between the two modifications the same reference characters indicate like or corresponding parts, and by comparison of Figs. 8 to 10 with Figs. 1 to 6, any difference in construction will be readily appreciated. It will be understood however that any structural changes made are limited primarily to the arrangement of the shielding system, and that the electrode assembly is substantially the same in both instances. The shield structure and supporting structure at the upper extremity of the tube formed by shield member 29 and connecting member 28 remains the same in both instances. However, in the embodiment shown in Figs. 8, 9, 10, the anode lead projects through the header rather than through the sealing tip.

It will be noted that the lead 8 which ccrresponds to the anode lead 8 of Figs. 1 and 3 is taken out through the header 2 A, center spud or lead 3* is connected by a strap 3 to the lead 3 and this arrangement constitutes a departure in the form of shielding of the control grid 9 from the anode lead a However, it will be observed that the center spud 3 is connected to the same lead for the side shields Z2 and 23; it therefore constitutes a part of the shielding system, and

shields the control grid lead b from the anode grid 5 is connected to the lead b by means of a strap 5. It may be pointed out that the bottom shield ilii is the same in both constructions, and functions to shield the tab ll of the anode section l5.

Another structural distinction in the present modification relates to the mounting of the bottom filament support 3%; it is attached to the header i by means of lead i instead of being fastened to the side rod 25 as in the case of the support 31, shown in Figs. 1 and 2.

Additional shield members ll and 52 are provided to supplement the side shields 22 and 23, said members being connected to grid side rods The principal function of the members ll and 32 is to shield the anode sections is and i l, respectively, more securely from the ends of the grid side rods 2%, 25, 26, and 23', which ends protrude or extend through the mica 13. By this arrangement more effective shielding is obtained, especially from the ends of the side rods of the control grid 9. Of course the principal object of the whole shielding system is to reduce to a minimum the capacity between the control grid and the anode members.

It will be understood that the rods on which i the shields are mounted may be attached to the header in any other suitable manner. Also, the shape of the anode sections and the shape of the top, side, and bottom shields may be varied to suit the requirements of a particular design, and

variations will occur to those skilled in the art while practicing the invention.

Therefore, although I have shown and described particular embodiments of my invention, I do not desire to be limited to the embodiments set forth. It is my intention to cover all such modifications as come within the scope of the appended claims.

What is claimed is:

1. An electron tube having a shield comprising first, second and third sections, electrode locating insulating wafers between said first and second sections and said second and third sections and shield portions extending through said wafers for'interconnecting said shield sections and for confining said wafers between said shield sections.

2. An electron tube having a cathode, at least one control electrode and an anode having at least one terminal projection, a pair of insulating Wafers terminated by said terminal projections and establishing the relative position of said electrodes a three-part shield assembly comprising a first, a second and a third shield portion, said shield portions being separated by said wafers and having portions extending through said wafers for electrical and mechanical interconnection, said portions being of such short extent, that the wafers are confined by the said shield portions.

3. In an electron tube, a shielding system including a plurality of shield components, one of said components comprising a pair of spaced apart side shields, a second component comprising a pair of bottom shields, and a third component comprising a top shield, all of said components being interconnected electrically and a pair of electrode locating insulated wafers, at the ends of said side shield and between said bottom and top shields.

4. In an electron tube having an anode element, a cathode and a control grid, a shielding system including a plurality of shield components of equal potential, one of said components comprising a top shield, a second component comprising a pair of side shields associated with the anode element, a third component comprising a pair of bottom shields, and a fourth component comprising a fixed radially extending support for the cathode, said support being adapted to shield the control grid from the anode.

5. In an electron tube, a cathode, and a shielding system comprising a plurality of component shielding members, one of said members constituting a cathode connector and support.

6. In an electron tube having a cathode and a control grid, a shielding system including a plurality of component shielding members, one of said members constituting a relatively fixed support for the cathode, said support also forming a shield for the control grid.

'7. In an electron tube, a cathode, a control grid encircling said cathode, a multi-sectional anode, multi-sectional shielding means adapted to shield said control grid, the sections of said anode and shielding means being mounted alternately in a compact and generally cylindrical shape.

8. An electron tube comprising an hermetically sealed envelope containing a plurality of electrodes including a cathode, a control grid encircling said cathode an anode and a shield for said anode, said anode and said shield comprising a generally hollow conductive cylinder divided into a plurality of sections, said anode constituting 7 r sections being connected together and said shield sections being connected together, said anode and shield sections being arranged in alternation.

9. An electron tube comprising an hermetically sealed envelope containing a cathode, a control grid encircling the cathode and a screen grid, a first anode member, a first shielding member, a second anode member, and a second shielding member, said anode and shielding members being sequentially spaced with respect to one another and surrounding said cathode and said grids, said shielding members being insulated from said screen grid.

10. An electron discharge device including a pair of apertured insulating wafers, a relatively long and narrow anode confined between said wafers, said anode being provided with a longitudinal strengthening rib terminating at each end in an extending mounting tab forming a continuation of said rib and passing through arcuate apertures in said wafers, the curvature of said apertures conforming to the shape of said tabs.

11. An electron discharge device including a pair of parallel spaced-apart insulating wafers, a pair of parallel spaced-apart anode sections perpendicular to said wafers and confined between said wafers, said anode sections having a single tab projecting from each end through apertures in said wafers, and a bridging connector for electrically and mechanically connecting the tabs of said anodes which project through one of said wafers.

12. An electron tube comprising a hermetically sealed envelope containing a plurality of electrodes including a grid and an anode member provided with a longitudinal reinforcing rib portion terminating at each end, and having projecting tab sections, insulating wafers for positioning said electrodes and apertured to retain said projecting tab sections in fixed position, and shield members between said tabs and said grid for shielding said projecting tab sections from said grid.

13. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including a sectional anode having a channel and end projections extending in axial alignment with said channel, said projections having the same shape as said channel, and an insulating member having apertures adapted to receive said projections, said anode sections being located in position with respect to the adjacent ends of the remainder of said electrode assembly by said projections engaging said apertures.

1%. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including a cathode and a plurality of grids, a first arcuate anode element, a first arcuate shielding element, a second arcuate anode element, and a second arcuate shielding element, said shielding and anode elements being alternately disposed around said cathode and said grids in cylindrical array, means connecting said anode e1ements together, and a third shielding member positioned between said connecting means and said cathode and grids.

15. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including a cathode and a plurality of grids, a first anode member, a first shielding member, a second anode member and a second shielding member, said members being alternately spaced around said cathode and said grids, means connecting said anode members, and a 8 third shielding member between said cathode and grids, and connecting means, said third shielding member being attached to said first shielding member and said second shielding member.

16. An electron tube comprising an hermetically sealed envelope containing a cathode, a grid encircling said cathode, an anode, and a shield connected to said cathode, said anode and said shield comprising a hollow conductive cylinder divided into four sections, said anode comprising two spaced-apart sections of said cylinder and said shield comprising the remaining two sections of said cylinder.

17. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including a cathode, a grid encircling said cathode, an anode, and a first shield connected to said cathode, said anode and said first shield comprising a hollow conductive cylinderdivided into four sections, said anode comprising two spaced apart sections of said cylinder and said first shield comprising the remaining sections of said cylinders, and an insulating disc at each end of said cylinder, projections on said sections comprising said anode extending through apertures in each of said discs the connection between said first shield and said first cathode being a fiat strip lying in a plane normal to one of said discs.

18. An electron tube comprising an hermetically sealed envelope containing a plurality of electrodes including an anode and a first shield for said anode, said anode and said first shield comprising a hollow conductive cylinder divided into four sections, said anode comprising two non-adjacent sections of said cylinder, said first shield comprising the remaining two sections of said cylinder, an apertured insulating disc at one end of said cylinder, tabs on said anode extending through apertures in said disc, conducting means connected to said tabs, a second shield between said conducting means and said insulating disc, and means including conducting members extending through said insulating disc for connecting said first shield and said second shield.

19. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including an anode and a first shield for said anode, said anode and said first shield comprising a hollow conducting cylinder divided into four sections, said anode comprising two nonadjacent sections of said cylinder, said first shield comprising the remaining sections of said cylinder, means connecting said anode sections, a second shield connecting said first shield and disposed between said anode connecting means and the remainder of said electrode assembly, means for supporting said anode sections including projections on said sections and an insulator having apertures adapted to receive said projections, and additional shielding means disposed between said projections and the remainder of said electrode assembly.

20. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including apertured insulating members for supporting the electrodes, an anode having projections extending through apertures in said insulating members, and conductive shield members disposed between said projections and the remainder of said electrode assembly.

21. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including a first and a second anode seccally sealed enveolpe containing an electrode assemblyincluding two anode sections, means connecting said anode sections, means including a conductor sealed through said envelope for making electrical connection to said means connecting said anode sections, and a shield member disposed between said means connecting said anode sections and the remainder of said electrode system.

23. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including an anode and a first shield for said anode, said anode and said first shield comprising a hollow conducting cylinder divided into four sections, said anode comprising two non-adjacent sections of said cylinder and said first shield comprising the remaining sections of said cylinder, a projecting tab on each of the ends of the two anode sections, top and bottom mica discs provided with perforations adapted to accommodate said tabs and support the anode structure therebetween, a connector having its ends attached to the tabs extending through the top mica, a lead-in wire sealed through the top of the tube envelope and attached to said connector, a cathode and a plurality of grids supported by said discs Within said cylinder, the ends of said cathode and said grids extending through the perforations in said top and bottom disc respectively, a second shield between said connector and said ends of said cathode and said grids, support rods extending through said top disc and serving to connect said first and second shields, additional shields between the tabs projecting from the bottom discs and extending beyond the ends of said cathode and said grids, and said support rods passing through the bottom disc for connecting said first shield and said bottom shields.

24. An electron tube comprising an hermetically sealed envelope containing an electrode assembly including an anode and a first shield for said anode, said anode and said first shield comprising a hollow conductive cylinder divided into four sections, said anode comprising two formed non-adjacent sections of said cylinder having longitudinal channels, said first shield comprising the remaining sections of said cylinder, a getter pellet fastened in each of said channels of said anode sections of said cylinder, a first slotted mica disc at one end of said cylinder, a tab on each section of said cylinder comprising part of said anode, said tabs extending through slots in said mica disc for locating said anode, a first conductive member having one end attached to each of said tabs, a second conductive member sealed through said envelope and attached to said first conductive member, a second slotted mica disc at the other end of said cylinder, a cathode and a plurality of grids supported by said discs within said cylinder, the ends of said cathode and said grids extending through apertures in said discs, a second shield between said first conductive member and said ends of said cathode and said grids, means including members extendin through said first disc for connecting said first shield and said second shield, tabs on the end of said anode extending through slots in said second disc, shields between said tabs and the ends of said cathode and said grids,

and members passing through said second disc for connecting said first shield and said last mentioned shields.

25. In an electron tube, a shield construction comprising a main assembly formed of a plurality of sub-assemblies, one of said sub-assemblies comprising a pair of side shields mounted between a first mica disc and a second mica disc, said side shields being secured to a plurality of support rods, a second sub-assembly comprising a top shield connected to a given set of said support rods, said first mica disc being confined between said top shield and said side shields, and a third sub-assembly comprising a pair of bottom shields secured to another set of said support rods, said second mica being confined between said bottom shields and said side shields, certain rods of the two sets being common to more than one of said sub-assemblies, whereby said sub-assemblies are interconnected forming a unitary structure.

26. In an electron tube, an envelope hermetically sealed at one end by a header, said envelope containing an electrode assembly and a shielding system therefor, said electrode assembly including an anode and a control grid, said shielding system including shield members for said anode and said grid, one of said anode shields comprising a shielding member carried by said header, and a lead-in wire sealed through said header and connected to said anode, whereby the anode lead is taken out through the bottom of the tube and is shielded in part from the control grid by the shielding member.

27. In an electron tube, an hermetically sealed envelope containing an electrode assembly and a shielding system including a plurality oi shield components, one of said components comprising a pair of side shields associated with a sectional anode, said anode section and side shields forming a generally hollow-cylinder, a lead-in wire connected to said anode and sealed through a header closing the envelope of the tube, and a shielding member supported upon said header and connected to said side shields, said shielding member thus forming a component of said shielding system and affording additional shielding means for shielding the control grid lead from the anode lead.

28. An electron tube comprising an envelope hermetically sealed at one end by a header, said envelope containing a plurality of electrodes supported between top and bottom mica discs provided with perforations, said perforations being adapted to accommodate the projecting ends of side rods to which the electrodes are secured, lead-in wires sealed through said header and connected to said electrodes, and means for shielding the ends of said side rods projecting through said bottom disc, said shielding means comprising conductive members carried below the bottom disc and interposed between the electrode leads and the lower ends of the support wires.

29. An electron tube comprising an envelope hermetically sealed at one end by a header, said envelope containing an electrode assembly including a cathode, an anode and a plurality of grids; a shielding system including a shielding component for each of said electrodes; lead-in wires sealed through said header and connected severally to said electrodes, said shielding com ponents being interconnected to a system of support rods, one of said shield components being anchored in the header and connected to the support rods for the cathodes, whereby the capacity between the control grid lead and anode lead is greatly reduced.

30. An electron tube having a central cathode, a control grid surrounding the cathode, and an anode, an insulated wafer for establishing the relative position of said electrodes, said electrodes having projections penetrating said wafers, an envelope enclosing said electrodes including a header having a cathode lead projecting therefrom adjacent the periphery of said header and an edgewise extending shielding plate interconnecting the cathode and said cathode lead.

31. An electron tube having a central cathode, a control grid surrounding the cathode, and an anode, an insulated wafer for establishing the relative position of electrodes, said electrodes having projections penetrating said wafers for said purposes, an envelope enclosing said electrodes including an end wall having a cathode lead adjacent the periphery, and an edgewise shield plate interconnecting the said cathode and cathode lead and positioned between said control grid and the anode.

HARRY KENNETH ISI-ILER.

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

UNITED STATES PATENTS Number Name Date Re. 20,545 Jarvis Nov. 2, 1937 1,700,945 Loppacker Feb. 5, 1929 1,913,427 Bond June 13, 1933 1,975,140 Eitel Oct. 2, 1934 2,017,549 Salzberg Oct. 15, 1935 2,051,950 Jobst et a1 Aug. 25, 1936 2,077,814 Taylor Apr. 20, 1937 2,115,934 Smith May 3, 1938 2,150,800 Jordan Mar. 14, 1939 2,213,162 Feindel Aug. 27, 1940 2,278,630 Winter Apr. 7, 1942 2,284,495 Rishell May 26, 1942 2,320,120 Depew et a1 May 25, 1943 FOREIGN PATENTS Number Country Date 448,101 Great Britain 1- May 27, 1936

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