US2575448A - Electron discharge device - Google Patents

Description

NOV. 20, 1951 s 2,575,448

ELECTRICAL DISCHARGE DEVICE Filed Aug. 22, 1945 4 Sheets-Sheet l 0 =5 gig i \l 4a r INVENTOR. 7 \6 PazaZ Haafi An mme%% [12$ fllfiorwey Nov. 20, 1951 P. HAAS 2,575,448

ELECTRICAL DISCHARGE DEVICE Filed Aug. 22, 1945 4 Sheets-Sheet 2 Nov. 20, 1951 P. HAAS ELECTRICAL DISCHARGE DEVICE 4 Sheets-Sheet 15 Filed Aug. 22, 1945 INVENTOR.

Nov. 20, 1951 P. HAAS 2,575,448

ELECTRICAL DISCHARGE DEVICE Filed Aug. 22, 1945 4 Sheets-Sheet 4 HVVENTDR.

By Paul l /dafi and z W4 H15 flttorney Patented Nov. ED, 1951 UNITED STATES PATENT OFFICE 2,575,448 ELECTRON DISCHARGE iinvlcs Paul Haas, Emporium, Pa., assignor to Sylvania Electric Products, Inc., a corporation of Massachusetts Application August 22, 1945, Serial No. 612,029

13 Claims.

eectrodes in the form of discs or rings to a glass envelope portion to provide a composite glassmetal body structure with the metallic parts,

, spaced in a predetermined manner and to which other metal parts may be attached subsequent to such sealing. I

The terms disc or ring, as used in this case, are merely descriptive to the extent that these electrode parts need not be fiat but may be of various shapes, the fundamental requirement being that each has an annular portion equal in outer and inner diameters to those of the glass tubing which forms the envelope, such annular portions conforming to the abutting ends of the tubing which, preferably, are fiat. The discs or rings are sealed into the envelope with their axes coincident with and at right angles thereto, their outer periphcries being external of the envelope while adjacent ends of the tubin sections abut and are sealed to the conforming annular portions of the metal.

In electron discharge tubes of the character described and disclosed in my co-pending application S. N. 563,348, filed November 14, 1944, it

is often desired to have two electrodes closely and precisely spaced along the axis of the tube, and to have these electrodes of different shapes. The conventional method of assembly has been to make the seals one at a time. First, two sections of tubing are held with adjacent ends against the opposite sides of the fiat annular portion of an electrode which latter is then heated through the medium of a surrounding heating element until the glass melts and the seal is made. Then a second electrode and a third section of tubing are sealed to one of the first two-sections of envelope, and so on.

Among the difiiculties encountered in this method, particularly when the electrodes are closely spaced, is the problem of relative adjustment of the work and the heating means so that when the second seal is made the other electrode will not absorb enough heat to destroy the first seal. Even when the first seal withstands the reheatingit is subject to expansion and distortion, thus limiting the precision of alignment and spacing of the electrodes particularly where high melting point glass or other vitreous material is used. It is therefore difl'icult to obtain the same characteristics in successive tubes manufactured by this process. Heretofore it has not b p sible to make both seals simultaneously because the electrodes, being inherently of different sizes and shapes, absorb the heat at different rates. Thus the electrodes do not reach the sealing temperature at the same time, and the quick-heating electrode cannot be held at the sealing temperature while the other electrode comes up to temperature.

As hereinabove stated a principal object of the invention is to provide an improved method of sealing these disc or ring electrode whereby two or more electrodes, held precisely in position, are simultaneously sealed into the envelope, thus avoiding the disadvantages resultingfrom reheating the completed seals as each successive seal is made.

The invention accomplishes this object by using a method in which each electrode is composed of one or more pieces; the parts, one from each electrode, which are to be sealed to the glass are substantially identical in size and shape or have substantially the same heat absorption characteristics. Another advantage of using rings of substantially the same size is that the seals have approximately the same strain characteristics, thus assuring an hermetic or vacuumtight seal for all the rings. These similar parts of the electrodes are held, together with the glass tubing, in precise alignment and spacing and the seals made simultaneously, the parts absorbing heat at the same rate. After the seals are completed the remaining parts of the electrodes are fastened by suitable means to the parts which have alread been scaled to the glass.

The sealed portion of each electrode need consist only of the opposing annular surfaces required for scaling to the adjacent sections of the tubular envelope and whatever additional area is required for attachment thereto of its respective other electrode part or parts. The more complicated parts of the electrodes are made separately. This independence of construction of the electronically active parts of the electrode and the disc or ring members required for sealing has other advantages in addition to allowing more than one seal to be made at the same time.

The manufacture of parts is simplified because it is unnecessary to provide an annular sealing surface in the same piece with the more complicated electronically active parts. It is thus also possible to use more complex electrode structures than can be used with electrodes made as units from a single piece of metal.

The cleaning of the electrodes after sealing is simplified. Since the sealed parts of the electrodes may be small and simply shaped, they do not become so heavily oxidized during sealing.

and their surfaces are more readily accessible for the application of cleaning fluids.

Another object of the invention is the incorporation of a simple way of providing electrical coupling between the electrodes or between resonant cavities which may be associated with the electrodes. The rivets or other devices used to fasten the active parts of one electrode to the sealed disc may be allowed to extend axially down the envelope into proximity with another electrode or through an opening in another electrode into the adjoining cavity.

A further object of the invention is in the fabrication of the discs or rings; to be sealed to the glass tubing, the provision of a series of apertures therethrough whereby such parts cooperate with an assembly fixture as means for holding themselves in true concentricity, and with stop members to limit their movement toward each other to a predetermined and exact parallel spacing as the glass tubing under end pressure becomes shorter during the sealing operation.

And, still another object of the invention is to make further use of certain of the apertures in the said fabricated discs to align the sealed assembly on a second fixture and to employ the others as rivet or screw holes in the fastening of other parts to complete the electrode assemblies.

My invention will be more fully set forth in the following description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out in the claims annexed to and forming a part of this specification.

Figs. 1 and 2 are enlarged end view and elevation, respectively, of the tubular glass sections which form the envelope.

Fig. 3 is a greatly enlarged detail in elevation showing the long and short rivets.

Fig. 4 is a plan view of one of two similar discs used in that embodiment of the invention illustrated in Figs. 9, 10, 11, 11-A, 13, 16, 17, 18,

21 and 24.

Figs. 5 and 6 are detail views in plan and elevation, respectively, of the cup-shaped anode used in both illustrated embodiments of the inventions.

Figs. 7 and 8 are views similar to Figs. 5 and 6 of the grid frame.

Fig. 9 is a longitudinal section of the disc assembly after sealing.

Fig. 10 is a view similar to Fig. 9 but showing the cup-shaped anode and the grid frame riveted to their respective discs.

Fig. 11 is a longitudinal sectional view drawn to a still larger scale and illustrating the disc assembly fixture and work pieces thereon before sealing.

Fig. ll-A (Sheet 4) is a view similar to Fig. 11 but drawn to a smaller scale and showing the parts in the relative positions they assume after sealing.

Fig. 12 is a fragmentary view similar to Fig. 11 but with the anode and grid discs shown as formed of thinner metal, the anode disc having a concentric annular, reinforcing depression while the grid disc has both a depressed central portion and a downwardly turned annular flange.

Fig. 13 shows in elevation a sub-assembly of a work piece and fixture part as will be explained. Fig. 13-A is a top plan view drawn to a smaller scale and illustrative of any suitable removable means for centering the intermediate glass section.

Figs. 14 and 15 are top plan views of the two disc members seen in Fig. 12.

Fig. 16 is a view partly in front elevation and partly in vertical section of the heating apparatus for effecting the disc seal, the work and weights thereon being shown in full lines before sealing and in dotted lines after heat has been applied.

Fig. 17 is a fragmentary view of parts seen in Fig. 16 with the heating coil shown in electrical diagram.

Fig. 18 is atop view of the heating coil with the work positioned therein.

Fig. 19 is a side view of the rack and gear mechanism seen in Fig. 16.

Fig. 20 is a fragmentary detail of the coil supporting member as viewed from the rear wit respect to Fig. 16.

Fig. 21 is a view partly in elevation and partly in vertical section of the riveting fixture and the work positioned thereon the grid frame being shown as the part to be riveted.

Figs. 22 and 23 are opposing end views of the movable punch and the fixed rivet holding member, respectively, of the riveting fixture.

And, Figs. 24 and 25 are views in elevation of the completed tubes illustrated in the two embodiments of the invention, part of Fig. 24 being shown in longitudinal central section. In these views the extra anode ring is shown as added which operation is done after sealing.

In the drawing similar reference numerals refer to similar parts throughout the several views.

Referring first to Figs. 1 to ll-A, inclusive, 13, 13-A and 16 to 24, inclusive.

At I, 2 and 3 are shown the upper, lower and intermediate sections, respectively, of glass tubing which when the electronic tube is completed form the envelope 4, in this instance such sections being of different lengths. Whenever the term glass is used it should be understood that any glass like insulating material may be used which is capable of being softened by heat to form a seal with metal.

Spaced apart by the intermediate length of tubing 3 and in abutment with opposing ends of the sections I and 2 are two similar discs 5 and 6 through each of which surrounding a central opening I are a plurality of holes 6 of one diameter and a like number of holes 9 of a smaller diameter, all arranged with their axes in a circle concentric with the central opening 1. Six of these holes are shown, three of each size, regularly spaced and alternately arranged. The discs,

} it will be observed, are of greater diameter than the gass tubing to provide an annular exterior portion while there remains between the central opening and the wall of the tubing an exterior portion in which the concentric circle of holes is located and providing a supporting area to which other parts, as will be explained, are attached. The discs 5 and 6, in this modification, are shown as flat although for the purposes of the invention the only flats need be the annular surface portions engaged by the abutting ends of the glass tubing in order that seats be provided for the tubing and whereby the discs are spaced in true parallel relation.

For the purpose of assembling the parts thus far described and as a means for supporting them in assembled relation during the sealing operation, there is mounted upon a suitable pedestal ill, see Fig. 16, shown as of ceramic material, a vertically extending post turned down to provide a short base I I, a shoulder l2 beyond which is an elongated upper extension l3, and a reduced lower stem l4 which is anchored in the pedestal and threaded to receive a clamping nut I5.

Slidably fitted to the post extension II are two similar opposing members l6 and Il comprising fiat rings from each of which extend parallel to the post a plurality of pins I8 and a plurality of tubes IS, the pins of one ring slidnbly engaging in the tubes of the other see Figs. 11 and ll-A. These pins and tubes are also regularly spaced and arranged in a circle of like diameter as that in which the holes 6 and 6 are located, the pins also having a sliding fit in the disc holes 9 while the tubes are likewise fitted to the holes 6. An adapter or centralizing sleeve 26 for the tubing 2 is slidably fitted to the base member ll of the center post. This member, of ceramic material, rests upon the pedestal Ill and is slightly shorter than the height of the shoulder i2. A similar but longer sleeve 2i is slidably fitted v to the post extension l3 as a means for centralizing the tubing I. This adapter sleeve is of a length to extend above the end of post extension I! but stops short of the upper end of the tubing l.

Assembly of the parts for sealing and thus providing what is hereinafter termed the composite glass-metal envelope and body structure of the tube, while clearly illustrated in Figs. 11 and13, will be briefly described as follows:

With adapter sleeve 20 positioned on the base ll of the center post and resting upon the pedestal Hi, the bottom section 2 of glass tubing is placed thereover to also rest upon the pedestal. Ring I1 is then dropped over the post with its pins l6 and tubes l9 extending upwardly and rests upon the shoulder i2. Disc 6 is placed on the tubing 2 and centralized therewith by the pins and tubes i8 and i9 engaging, respectively, through the holes 9 and 8. Next the intermediate tubing section 3 is placed on the disc 6 substantially in alignment with section 2. Preferably the disc is then slipped onto the pins l8 and tubes IQ of the ring I6, as shown in Fig. 13 as a sub-assembly, after which these parts are positioned as shown in Fig. 11 with the disc 5 spaced from disc 6 by the glass tubing 3 and with the pins iii of one ring slidably engaging a short way into the tubes [9 of the other ring as indicated. .The adapter sleeve 2| is then placed over the post extension l3 to rest upon the ring i6 and the tubing section is slipped over such sleeve into abutting engagement with the disc 5.

As a means for precisely centering the intermediate glass section 3 any suitable removable mechanical device may be employed such, for instance, as that illustrated in Fig. 13-A and diagrammatically in Fig. 11. This device, which may be any simple contrivance adaptable for the purpose is shown as a pair of pliers 22 having a pivotal connection at 23 while the nose pieces, when closed upon the work, have a bore 24 the wall of which engages and. centralizes the glass tubing 3, the latter being located in relation to the peripheral edge of one of the discs 5 or 6 by engagement therewith of the wall of a concentric counterbore 25 in the centering tool.

A Water-cooled high frequency, double wound induction heating coil 26 is shown, with particular reference to Figs. 16, 17 and 18, as horizontally disposed and supported by the terminal extensions 21 and 26 thereof being secured by clips 29 to a plate 30 and having couplings 3i and 32 with a source of water supply and a water outlot. This plate a is laterally adjustable on a bracket 56 slidable on a rod 34 extending vera hand wheel 44, a set screw '45 holding the rack in any desired position of adjustment. The electrical connections for the heating coil are illustrated diagrammatically in Fig. 13.

The rack is adjusted to raise or lower the pedestal l6 and thus move the work within the coil so as to bring the discs 5 and 6 into such position that the heating thereof will effect their simultaneous sealing with the glass. A weight is placed upon the upper end of the glass tubing i and comprises, in this instance, first a ceramic disc 46 and then the weight proper 41 of suitable heavy metal.

As the glass under heat becomes softened and begins to flow the sections I, 2 and 3 will become relatively shorter under end pressure of the weight acting against the fixed support. Consequently as the weight drops to the dotted line position indicated in Fig. 16 the discs 5 and 6 will also settle to the dotted line positions indicated in the same figure. In this dotted position the discs will have moved relatively closer together, the final spacing thereof being predeterminable by the length of the sleeve l9 which, it will be observed, act as stops by engaging the opposing inner faces of the discs 5 and 6 and at the same time act as leveling means to hold the discs in parallel relation to each other, as is shown in Fig. ll-A.

The seal having been made, weights 46 and 41 are removed and the work disengaged from the fixture. At this time the assembly is as illustrated in Fig. 9 and it will be observed that the surfaces are readily accessible for the application of cleaning fluids.

Further assembly of the tube is as follows:

In the illustrated embodiment of the invention the anode is made of three parts, viz., the disc or ring 5 sealed to the glass tubing sections l and 2, a cup-shaped anode proper 48 depending through the central opening I and formed with an outwardly turned annular flange 49 having three rivet holes 50 which register with the holes 5 and a like number of outwardly opening radial slots 5i which register with the larger openings 5, and, also, an outer anode ring 52 which is soldered to the disc 5 after sealing.

The grid is made of two parts, viz., the grid disc 6 sealed to the glass tubing sections 2 and 3 and the grid frame the latter comprising an annular ring 53 preferably of like outer diameter as the anode flange 49 with its center opening of like diameter as and registering with the hole I in the disc 6. This grid frame or ring carries the grid wires 54 and has a like number of rivet holes 55 and outwardly opening radial slots 56 as there are similar holes 50 and slots 5| in the anode flange 49, the holes 55 registering with the rivet holes 6 and slots 56 registering with the holes 6 in the disc 6.

The fastening devices employed in assembling the parts of the two electrodes are, in this instance, shown as short rivets 51 for the one and long rivets 58 for the other, the heads of the short and long rivets being provided with reduced pilot xtensions 59 and 58, respectively, which are coaxial with the shank portions. The short rivets 51 are used for the grid parts while'the longer rivets 58 are used for the anode, the extensions 58 of the long rivets extending into proximity with the grid or, as shown, through the holes 8 in the grid disc, thus providing electrical coupling between the electrodes or between resonant cavities which may be associated with the electrodes.

As shown in Fig. 21, the riveting jig or fixture is of simple construction and includes the two relatively movable members of a punch press. The press member is a post 58 upstanding from a base II and having an annular recess adjacent its upper end to provide a head portion 52 and a shoulder 63, the head being drilled to receive three vertically extending sleeves 64 which bottom on the shoulder 63.

' As with the assembly fixture hereinabove described, the holes 8 and 8 formed in the disc members and 6 in the fabrication of these parts permit these members to cooperate with the riveting device in centering and properly aligning the work for the riveting operation.

Preferably, the first operation is to attach the grid frame 53. To this end a short rivet is placed on the end of each sleeve 64 where it is held by its pilot 58 extending into the sleeve, as illustrated in Fig. 21. Then with the work inverted so that the tubular section I extends downwardly over the post 68 the sleeves 64 registering with the holes 8 in th disc 5 engage through such holes, the disc 5 resting on the rivet heads and the shanks of the rivets passing through the aligned holes 3 and 55 in the disc 6 and grid frame 53.

The punch 65 is recessed to provide a head 66 and shoulder 61 similar to the corresponding parts of the press member. This head 66 is provided, however, with six regularly spaced holes in alternate ones of which are punch rods 68 formed to head the rivets against the grid frame 53 while in the other holes are guide rods 69 which register with and pass through the aligned openings 8 and slots 56 respectively, in the disc 8 and grid frame 53. A spring pressed pressure pad or stripper 18 is carried by the punch to yieldingly engage the work in advance of the punch rods.

To rivet the flange 49 of the anode to the anode disc 5 the operation is the same except that the long rivets 58 are used and the work is reversed so that the tubing section 2 extends downwardly over the part 60. In the final operation the header II and exhaust tube 12 are sealed in the usual manner and, with the glass sections 1, 2 and 3, give the complete envelope 4. It may here be stated that sealed into the header H, previously to its sealing with the tubular section 2, is the cathode sheath I3 which houses the heater-cathode assembly 14, the latter including the cathode grid 15, as shown in Fig. 24.

In some instances, as illustrated in Figs. 12, 14, 15 and 25, the anode and grid discs may be made of much thinner metal, and for the purpose of reinforcing such parts the anode disc 5 is shown as having an annular depression 16 as the bearing surface for the tubing sections l and 3 while the grid disc 6' is formed with a central depression TI, in which the holes I, 8 and 8 are located, and a depending annular flange 18 exteriorly of the tubing, the amount of metal in the parts of the two electrodes exteriorly of the glass tubing being substantially the same so as to allow substantially equal heating. Also, in this modification an outer ring 52 is soldered to the anode disc 5".

While I have shown-and described rivets as the means for rigidly securing the parts of the electrodes together it will of course be understood that other suitable fastening devices, as screws, may be used. In this latter event the plunger member of the final assembly fixture would be eliminated.

Furthermore, although I have shown but a single heating unit and manually operable parts associated therewith, it will also be understood that, for quantity production, and without departing from the spirit of the invention a number of the units for the assembly and sealing of the composite glass-metal body structure may be mounted on the intermittently rotatable tu'rret of a machine built for such purpose.

It will also be understood that features of my invention, such as the structural means for facilitating the alignment of the electrode parts may be employed to produce other suitable types of electrical devices.

Although I have shown and described particular embodiments of my invention, I do not desire to be limited to the embodiments described, and I intend in the appended claims to cover all modifications which do not depart from the spirit and scope of my invention.

What I claim is:

1. The method of fabricating a glass to metal article including the steps of assembling a plurality of metal articles having approximately the same heat absorption characteristics with glass members separating the metal articles, simultaneously heating all of said metal articles whereby the glass and metal attains the softening temperature of the glass, while allowing mutual approach between the metallic parts until a predetermined position obtains therebetween,

and then stopping relative movement between the metallic parts while allowing the glass to cool and seal with the metallic parts.

2. In the assembly and glass-to-metal sealing of a composite glass-metal body comprising a plurality of metal wafers substantially identical in size and shape and spaced apart in parallel planes by and between sections of glass tubing coaxially aligned therewith, the method of insuring precise, predetermined axial spacing of said wafers, true parallelism thereof and identical sealing-in conditions which comprises the steps end pressure, and mechanically arresting suchaxial movement of said wafers.

3. The method of making a composite glass metal envelope and electrode body structure for electronic tubes which comprises the steps of arranging a plurality of metal rings coaxially with and as separatin means between properly proportioned lengths of glass tubing, exerting end pressure against said tubing, softening the glass adjacent said rings by the application of heat from heating means surrounding said rings, guid ing the relative axial movement of said rings toward each other as said tubing sections shorten under heat and said end pressure, and stopping such movement with said rings in parallel relation and at a predetermined spacing, thereby effecting both said predetermined spacing and 9 a vacuum-tight sealing of the glass tosaid rings by a single heating operation. I

4. The method of making a composite glassmetal envelope and electrode body structure for electronic tubes which comprises the steps of arranging a plurality of metal rings coaxially with and as separating means between properly proportioned lengths of glass tubing, exerting end pressure against said tubing, softening the glass adjacent said rings by the application of 10 heat from a high frequency induction coil surrounding said rings, guiding the relative axial movement of said rings toward each other as said tubing sections shorten under heat and said end pressure, and stopping such movement with said rings in true parallel relation and at a predetermined axial spacing, thereby efi'ecting both said spacing and a vacuum-tight sealing of the glass to said rings by a single heating operation.

5. The method of making an electron dis- 2o charge tube which includes the steps of vacuumtight sealing a plurality of planar parallel metal rings coaxially with and to the wall of an open ended cylindrical glass envelope forming tube, said rings being spacers between sections of tubing forming said wall, inserting discharge electrodes into the inner peripheries of said rings, attaching said electrodes to their respective rings, sealing a header, including a sealed-in cathodeheater assembly unit, to one end of said wall, evacuating the tube at its other end, andsealing said evacuation end to completely form said envelope. a;

6. Apparatus of the character and for the purpose described including a support, a rod upstanding from said support, a shoulder on said rod. an adapter sleeve on said rod below said shoulder, a second adapter sleeve slidable on said rod above said shoulder, two rings slidable on said rod and interposed between said adapter 40 sleeves, the lower ring normally restin upon said shoulder, alternately arranged pins and sleeves parallel to said rod and extending from the opposing faces of said rings, said pins and sleeves being regularly spaced circumferentially and radially with respect to said rod and with the sleeves of one ringtelescoping the pins ofv said other ring, the free ends of said sleeves comprising both a stop for limiting axial movement of said rings toward each other and a support for maintaining said rings in parallel relation when in engagement therewith.

7. The method of making electron discharge devices comprising the steps of forming a pair of annuli with large and small holes, sealing the annulitoaxiallyalignedslasstubeswiththe small hoes on each annulus aligned with the large holes in the other annulus and with one 81855 b positioned between theannuli, and

securing an electrode to each of said annuli with the aid of pressure applied through the large holes in the opposite annulus.

8. The method of making an electron discharge devicecomprisingthestepsofformingapairot annuliwithholesnearthecentral hole ofeach annulus, sealing the annuli to axially aligned glass tubes while holding the holes of one annulus mpredeterminedrelationtotheholcsofthe, other annulus, and with one glass tube positioned betweentheannuli,andsecuringanelectrodeto 1 each of said annuli in predetermined relationship to said holes.-

9. The method of making an electron ischarge simultaneously sealing a device which comprises number of conductive annuli between. sections of u vitreous tubing in accurately controlled axial alignment and mutual spacing, and thereafter securing electrodes to said annuli.

10. The method of making an electron discharge device which includes assembling a numthe central apertures of ber of metal rings in alternating order between a number of sections of vitreous tubing, applying axialpressure to said sections of tubing, fusing the edges of said tubing to make joints with said rings, and thereafter securing individual electrode structures to said metal rings.

11. Apparatus for assembling plural metal discs to plural glass tubular sections in an alternating succession includinga post extending from a base member, a number of spacing sleeves having central apertures slidable over said post and having outer diameters such as to fit within said glass of spacing rings received on said post, interposed between said sleeves and each having stop members extending through apertures in said metal discs to engage another metal disc whereby the approach of said discs to each other is limited. 13. Apparatus for assembling plural metal discs to plural glass tubular sections in an alternating succasion including a post extending from a base member, a number of spacing sleeves having central apertures slidable over said post and having outer diameters such as to fit within said glass tubular sections whereb said glass sections are maintained in axial alignment and a number of spacing rings received on said post, interposed between said sleeves and each having tubular stop members extending through apertures in said metal discs to engage an adjacent metal disc whereby the approach of said discs to each other is limited, each oi said spacing rings having an extending pin passing through an aperture in one of said metal discs and received in said tubular stop member to maintain the relative orientation of said discs.

, PAUL HAAB.

narsnnucns map The following references are of record in the file of this patent:

v UNITED STATES PATENTS Number Name Date 1,805,828 Jordan May 19, 1931 2,089,795 Hodge Aug. 10, 1937 2,121,627 Donovan et al. June 21, 1938 2,125,316 Ronci Aug. 2, 1939 2,129,849 Laico Sept. 13, 1938 2,297,492 Michaelis Sept. 29, 1942 2,317,889 Dannemann Apr. 27, 1943 2,343,849 Binneweg Mar. 7, 1944 2,386,820 'Bpencer Oct. 16, 1945 2,462,205 Machlett et al. Feb. 22, 1949 2,488,302 Machlett et a1. Nov. 15, 1949 7 2,494,870 Greiner Jan. 17, 1960"

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