US2362908A - Cathode ray tube - Google Patents

Description

Nov. 14, 1944. v G, LIEBMANN 2,362,908

CATHODE RAY TUBE Filed Nov. 10, 1943 2 Sheets-Sheet 2 `vide meansl to @grid thickness, `down constancy yof clearances4 r.which 'Patented Nev. 414, 1944 Gerhard-Liebmann,

to Cathodeon Limited,

, British company The presentl inventionrrelates to tubes. -The invention isparticularly applicable iQPathOde my wbes in. which everytime im@ is required and Where themodulatingirequency is very. high, althoughit;l can 21S/, be V.applied to i other cathode ray tubes. l

,l l cathode Iv'ray ingfthe cathode or a.vcro ssover,v ofthe electron.

beam situated ,near the ,cathode'felectronf'opf tically on tov the screenrv yspot vis modulated by applying'a suitable voltage between the modulating .electrode and thera- .ihode-'v If a Very ,nefspt is requirdiihe-siiemeter ofthe crossover'V must be very'smallfwhich tube the Spotf'rflilege traceonthe screenis usuallyl generatedbyimag @mede-fie? The intensityoi the in turn'necessit tes a' small `diameter..,of the'hole inv thenidulating electrode, It is- "found,` experimentally that the currentdelivered intolfthe elec tron beam reduced by a-,factoroftapbroximately Q'fthe diameter V'of the hole inthe y strong to deliver'. ani-adequate' e, electron current `finto: the b'eam. 'Anotherfobject is'jtol provide' a simple andY` sturdy 'structure by"which n 't' essary ne dimensions and VinterelectrodefV pacf ings may be readily; achieved and accurately maintained, andf which has fthe further" vah'antube is to be modulated at vavery .high frequency.

',Although the structureaccordng to'uthis injvention is'. especiallyy designed andjiound par'- ticularlyuseful for .constructing cathdefray tubes producing'a `ery .fine trace and operating very high frequency modulation,` it' hasjfadvantages in tubesfof otherety'pes andwith less with lis ythe .change of field penetration modulating electrode is reducedby ya fact-,croi 2,

y cathodejfray tube.' n2.5

Cambitiene;'England'assignerA Cambridge? Euganea f .1 f f- Figs. i and`f5farel respectively alplan viewa'nd a sidefvievv f -thev..cathode-grd assembly; f Fig`.`6 is a fragmentary viewlpa'rtly section, showing taining elettrone systememoodying the infv rference to'fthe'anode assembly', Figi `@showing a plan'view of a cathode-grid'ass'embly andFig. showing als'ectin'on ther line 1- -T of Fig.8 With an anode added; v, l

VFigs. 9,"10'and 11 are similar {cross-sectional views Aillustrating mainly lmodications inthe grid form..v l. -f 5 For clearn'ess'the elements are showny greatly enlarged "n''all the g'uresexcept Fig. 6 in which the parts are. showniapproximately one and ahali times fullsize.l ,In all the gures, like reference characters designate identicalr or lsimilar parts... 4

` Referring to Fig.V 1, theelectrode system. therein depicted is mounted on aglass stem or pinch' I0 for insertion and, sealing .inthe neck'of a I The cathode is in theform `rof',` a nickel cylinder or tub'efI-I which is open at its lovverjendf'and has aplane'closed top yend I2 coated kwith a ysuitable'ell'ect'ro'nemissive mate-r rial forming; the cathode 'proper'. vThe `lcatho'de is'k indirectly heate'dfb'y' an internal heating fila- Yment,(no'tshbv/n) lWithin the cathode tube I I. .Afdisc I3 ,of 'insulatiompreierably ceramic, and

. Specifically porcelain, although' another4 suitable tage of 'a'verysmall .ca'pewityv between the modu-- v lating `electrode and all other electrodes-"winch .is a very important propertyiik the cathode` 'ra'y small electrode l, clearances,v particularly where L understood, reference is vmadev the following description tov the4 accompanying A ,draguzings, ,in

y Fig.` 4lfis a view of" an electrodelfstructu'remade in accordance with the invention, icertaun partsv being shown brokenaway `and shovvn'in v'section for clarityoi illustration;` l Figs. 2 and'.3..are two views takenglatright l,angle'sto eachother of [a -modiilcatonbf the I structureo'f Fig. 1;V

anda compact and sturdy, construction [are important or desirable.` HIn order that theinvention may be .morefully f cathodebygcq tact with theydiscgl aus 11101;v `sgmousv owing to thevejrylpoor thermal conductivity 60' material ;suchl,as"r`nica, alumina or'quartz 'may be employed, yhas'fa' central hole Il: Which're- .ceives the cathode tube. I I ,inserted therein such ,that the coated endlA I2r thereof isflfush or 'coplanar With the upper l more clearly shown in Figs. 4 and 5, .the cathode tube. Il lisiirged, very'nearvtoits -coated end, to the disci 1.3 by means. '.of. lmoi" [thermal conductivity; ttor n nicnrmezwhich, iabutson edge 'against the nn- .dersde off is vveldedtoitffThe endsv oif'theA strip I 5 are Welded'resp'ectively to the squashed and attened ends of two eyelets IG which are inserted through 'diametricallyopposed holes provided in the disc I3v nearv its`5outer' edge. The diameter of the cathode tube lfj and the sizef the disc I3 are such'that the cathode tube ts fairly tightly into. the hole very rigid way and once assembledv properly cannot move under A,heat vexpansion yas such expansion takes 'placef towards 'the "finden endof the tube v`I L] 'Although the `'loss of heat from lthe of-immaterial of the disc, this iosa of heat can the neck of a cathode ray tube conf face of the disc I3.. As

of a'strip I5 ofmetalV instance,V

vthe `disc .I13- and ,has @semi-,circular l loop'whchenibraces thef'cathodeftube. II and of the hole I4 in` I4, sd lthat the cathode tube is ixedfbythe described construction in a disc. This may be achieved by making the centre hole I4 of the disc of such a different shape from the cross-sectional shape of the cathode tube I I that contact is established only at a number of points; for instance, the centre hole I4 of the disc may conveniently be made a square one, as shown in Fig. 4, such that the sides of the hole form tangential planes to they cathode tube II, making a four-point contact.

`The modulating electrode or grid consists of a very thin metal disc II welded to and closing a circular hole I8 in a narrow supporting metal strip I9, which is laid at on the upper face of the discJ I3 so that the disc I'I faces away from the disc I3, and the strip I9 is fixed to the disc MI3 by eyelets 23 `in diametrically opposed holes provided in the disc and spaced at 90 to `the ,holes receiving the cathode fixing eyelets I6. The disc I'I constitutes the grid proper and has a circular hole 2`I` therein coaxial with the cathode tube `II andthe hole I8 in the supporting strip I9. The latter hole I8 is made slightly larger thanthe diameter of the cathode in order to provide an insulating margin between cathode and grid. The insulation can be improved, if desired, by 'chamfering the edge of thetop end 2 of the cathode tube and likewise the edge ofthe porting strip I9 is flat on the upper face of the discv ,I3 and the .grid proper I'I is flat on the upper face of, the strip I9, all of which parts are assembled together in invariable relation on the disc I3 as main support and carrier for the as sembly, it will be appreciated that the described structure ensures in a simple manner the maintenance of the clearance between cathode and grid at the pre-set value and provides a compact and sturdy structure.

To prevent spurious discharges and the deposition `of nickel from the cathode on to the disc hole I8 which faces the cathode. A further precaution, if desired, is to coat the disc I3 with a suitable material such as magnesium oxide.

The grid supporting strip I9 thus servesat the same time as a spacer to maintain the correct grid-cathode clearance. The thickness of the strip I9 is within close limits made identical with the grid-cathode clearance desired. The choice of this clearance depends upon the desired oper-l ating characteristic of the cathode ray tube in whichthe structure is employed, and is a very important design parameter. In modern cathode rayv tubes the range covered is approximately 'from .1 to '.5 mm. The thickness ofthe supporting strip I9 may, therefore, be anything between .1 and .5 mm., but usually a maximum thickness of` .2V to .3 mm. only can be allowed in order to provide a strong enough field at the cathode.

The diameter of the hole 2I in the grid disc I 1 and its thickness are chosen in accordance fwith' the requirements for the spot size or definition desired, the beam current, the length of the grid base, and the beam current "cut-01T point.

The diameter of the hole 2| Ifay therefore, in practice, vary between .1 and 1.2 mm. and the thickness of the grid proper between .025 and .20 mm'. By way of illustration, three sets of values which have been used in actual tube constructions are:

Thickness of grid They cathode-grid clearance and the grid-anode clearance have, of course, to be taken into acwith the upper face of the disc I3, the grid sup- I 3, Which might impair the good insulation of the electrodes xed to that disc, a cathode shield 22 is provided surrounding the cathode and part of its heater and abutting 'against the underiace of the disc` I3.` This cathode shield may be in the form of a metal cylinder 22 which may be fixed to theassembly by Welding it; at its upper'end to the cathode fixing eyelets I6 vin any convenient manner, such as by Welding the cylinder to short pieces or brackets of stiff Wire which are welded in turn to the eyelets.V The upper edge of the cylinder is notched or recessed at 23 adjacent the eyelets L20 so as to provide suliicient insulating margin between them. Welded to the inside ofthe cylinder 22 are a pair of inclined side shields or plates 24 which are disposed so as to protect the disc I3 from cathodic deposition in the neighborhood of the eyelets 28.' Aligned inspection holes 25, 26 are provided in the peripheryof the cylinder 22 and in'one side shield 24 respectively. The shielding cylinder 22 is closed at/its lower end by a metal thimble 2l welded tothe cylinder, the thimble having an aperture 28 therein for passage of the cathode heater leads 29.

SometimesV only one high potential anode is employed which may consist of a conducting deposit von the glass wall of the cathode ray tube envelope. In othercases, one or more additional lanodes are used. In these cases, the first anode isl also included in the structure mounted on the disc I3. In the'embodiment shown in Fig. l, the rst anode `is in the lformof a flanged-rim metal ycup 30 of say .2 mm. thick material having a centre hole 3| inits bottom, which is mounted coaxially on the disc ,I3`by means of a metal claw ring 32 which surrounds the cup 3U with the interposition of a metal packing ring 33, of stouter material than'theanode cup, e. g., .5 mm. thick material, the packing ring 33 being welded to the anode cup 30 and the claw ring I32 being welded to the packing ring 33. The claws 34 oi 'ring 32 embrace the periphery of the insulating trodes. The two' wires 35 (which provide cathode leads) 'are weldedr to the closure thimble 21 of the cathode shield 22, the anode lead 36 is welded to theclaw'ring 32, and the grid lead 31 is welded to a tag 38 formed integrally at one end of the grid supporting strip I9.

V vThe' electrode assembly of Fig. 1 has specifically been applied to a two-anode cathode ray tube vwith magnetic deflection and focussing, the secrond anode being formed in conventional manner by a "conductive coating on the glass wall of. the

tube.

f quence, the grid lead 31 may,

v an eyelet 44 `disc I3 and anode 30a so as disc proper, the

made that of the "OrdeL l changing the clearancesI andy y mamans The `structure illustrated in' Figs'. 2- 'andv 3 'Jis similarftothat of Fig. -1 in vessentials as regards assembly, location Vand spacingotthe electrodes mounted "on the' .insulating disc 43, butdistinguishes from `it some particulars. structure is mounted vonr a cupfshaped kmoulded glass base 39` instead Vof a/stem or pinch. The glass base 39 hasf'lead-in A'pins'' sealed therethrough and arranged in a-'circle'near theperiphery of the bottom oi.the'cu1;;-shaped base.` This type ofv mounting enables the leads to be kept shortand widely 2spaced;-,thu`s making full` use ofthe reducedgridcapacity achieved by the described electrode construction and assembly-.on the disc I3.v Theus'e of la'moulded kglassbase The..

makes theassembly evenfmore compact as c ontrasted with that of Fig. 1.

- The cathode shield 4i Iis,-in as being of rectangular, box-like Aforminstead of the cylindrical-formv of the shield in Fig; '1,the bottom of theshield4l being, however,- closedV in similar fashion by ra rectangular closure 'plate 42 vwelded 'on' andy having an aperture` therein allowing theheaterleads 29 to passthrough. The shield' 4I does not, in this case, enclose the underside vof the eyelets 2` which fasten the grid supporting strip to the disc i3, in consequence oi which no side shields as provided yin the cylindrical'shield'are required. As a furtherconsethis case, be weldeddirectlyto one of the eyelets 20, as is shown in Figs. 2v and 3, and the previously described tag on the gridsupporting strip omitted. Of course, the connecr -ion o f' the grid lead to the strip may alternatively be made in the manner previously described with reference to Fig. 1.

Also, the anode cup and pa ferently prcportionedcoinpared with'those parts in Fig. 1. The anode cup 30a is vshallower and with a flangeless rim, and the packing ring 33a extends substantially above cup.v Another difference from thestructureY of Fig. l'appears in 43 between the grid andanode. strip of suitable material such as mica'xed by in one end to a support wire 45 welded to'the eyelet 44 and to the vcathode shield 4I, the strip 43 vextending between the insulating to overlie theg'rid thickness of the strip 43'being clearance between grid and an- The portion of thestrip 43-lying over the grid disc has a hole therein aligned with the 'grid and anode holes butlarger than them.` The provision'fof such an. insulating'spacer asv strip 43 radds more parts and assembly operations'to the constructionand also introduces increased capacity. On this account thej use'ofvsuch a spacer is'restricted -tofcases where the grid disc has to be extremely disc from vbucklingand so the plane-paralthe very thin grid lelity of the electrode surfaces. i Fig. 6 illustrates an application of the cathode-v grid-anode assembly structure according-to this of Figl as regards the cathhowadditional 'electrodes may tbe-included in the assembly in proper and constant location and spacing; In thisvcase, contrasted with the structhin such as 03 mm.thick. The purpose of thefspacerin this case is tokeep Figs. 2 and 3 shown cking ring are difthe rim oftheanode lthe' use of an insulating spacer This spa'cer is a suitable number,

port wires^50` for thesecond or focussing anode `5I. Alsin assemblin'g'the rstanode 30 tothe disc I3, another mica ring 52 is .secured tothe flrstjanode 30 by being held .at its inner periphery between the-upper edge Aof the packing -ring 33 and-the flanged rim/of the anode' cup, and this ring 52 is providedwith eyelets 53 through which the support wires 5U pass. second anode 5l is fitted with a mica ring 54similar to ring 52 andl in like manner furnished. with eyeletsl 55,- and another.` centering starring 56 with eyelets Slisilttedon the upper partof'the second anode 5I. The vwires k50 pass through' the eyelets and 51,1and aresuitably'secured as by yWelding tosuchvof the eyelets as are necessary to afford a'rigid location and spacingof the electrode 5I.y a lead to the second ranode 5|- through connecting wire or stripl 58. yThethircl4 anode is the usual rconductive coating 59 deposited on the glass Walla-L i i The location and spacing of the' anode 30 with 'referencev to the 'grid' I1 may be effected .in an alternative manneras'illustrated in Figs'? and 8 byeinploying a vmodified form I3a of the insulating supporting disc which is formed with'suitably shaped' bosses 60 which providesfseatings `for the anode 3U-and act as spacers tomaintain the anode at the required distance from4 the grid discVI'I. v Theanode 30, in rectly` secured to the disc i3d. by eyelets'BI in .holes 62 through the vbos'sesiiil'.` The cathode fixving eyelets I6`arein holes 63' Which,fin this-case, are. counter-sunk in the bosses so as vto afford a suiiicient insulating vmarginfbetween the heads of the eyelets I6 and the bottom of the anode cup 3D seating on the bosses 60. At the same time these bosses 60 may be shaped so las to locate between them theA grid supporting strip VI9' which,

l lfixing holes and eyelets the disc I3a a further o' inthe disc I3a. i "In the .cathode-grid-anode lassembly described,

yinvention toa three-anode cathode ray tube lwith electrostatic focussing. .l `The construction ofFig. 6 is similar to that y ode, 'grid and riirst anode assembly and illustrates Yslight'modification is also made in this case contrastedwith Fig. 4 in displacing the posicionar Y 'the cathode fixing holes and eyelets so as to obtain a wider and better spacing of the eyelet holes the electrode surfaces are plane-parallel, but the invention does not depend upon andis 4therefore vnot limited to this type ofdesign, since other .'shapesrnay be employed. The' upperface of the grid disc proper may, for example, vbe shaped to vform a spherical surfaceas indicated at I'Ia in klig.9, yor a conical surface as indicated' at |11)l in Fig. 10,01' in another alternative thegrid disc ,may beattachedto a shortcylinder forming a cup-shaped grid as illustrated at IIc in Fig. 1l. Alsopthe cathode need not always have .a hat;y top end as previously describedy but may besuitably curved as. `for, instance, f in a concave form as il- The lower part ofthe One of thesupport wires 5I) serves'as this case, may Abe dilustrated at 12a in yFigi). Further, the bottom of theanode 30 facing the grid may be tapered or shaped in a way such as to reduce the gridanode capacity to a minimum. An advantage of forms of grid plate such as are represented by I 1a and Hb in whichthe thickness of the plate decreases from its periphery to the hole therein, is that the accelerating field between the grid and the anode reduces in many cases the spread of the emerging electron beam, vand that the grid can be made extremely thin at the hole to suit the spot size4 or definition required while it may be made thicker elsewhere so as to render it rela- Y tively stout and able to withstand buckling.

The shape of the grid supporting strip I 9 as shown in the drawings is thepreferred shape as an optimum compromise between maximum mechanical strength and minimum interelectrode capacity. However, other shapes are not excluded and the strip may, for instance', bestraightedged. i

While a number of specific embodiments of the invention have been shown and described and certainmodications therein have been particularly pointed out in this specification, it is to be understood that various other modifications and changes may be made without departing from the invention, the scope of which is limited only by the appended claims. l

I claim:

1. In a cathode ray tube, an electrode strucsaid hole in said strip, said strip having an eyelet fixing hole therethrough near each o-f its ends.

6. Amodulating electrode assembly as defined in claim 5, -wherein the portions of said strip between each fixing hole and the hole closed by said disc are of reduced width.

7. In a cathoderay tube, an electrode structure comprising a disc of insulation having a hole therethrough, a tubular cathode member secured to said disc in, said hole and having a closed end provided with an electron emissive surface arranged flush with a fiat surface of said ture comprising aninsulating support having an aperture in a surface thereof, a cathode secured to said support in said aperture so that an electron emissive surface of the cathode is fixed with respect to said disc surface, a narrow metal strip secured to said support and lying on'said surface thereof with a hole in said strip in line with said cathode surface, and a modulating electrode lying on and conductively ,securedv to said strip and closing said hole therein with a hole in said electrode in line with saidcathode surface, the thick- Vness of said stripv determining the spacing between said modulating electrode and said cathode.

2. In a cathode ray tube, an electrode structure comprising a disc of insulation having an aperture in a fiat surface thereof, a cathode rigidly secured to said discin said aperture and having an electron emissive surface flush with said disc surface, a fiat narrow metal strip secured on said f disc surface and having a hole therethrough which is in line with and larger than said cathode surface, and a modulating electrode having a flat portion with a hole therein conductively secured to said strip with said: flat portion closing the v hole in said strip and the hole in said flat portion in line with said cathode surface, said cathvode surface and said flat portion of said modulating electrode being'spaced by the .thickness of said strip. i

3. An electrode structure as defined in claim `2, wherein said modulating electrode is directly welded by its said fiat portion to said strip.

4. An electrode structure as defined inclaim 2,

wherein said modulating electrode comprises a thin metal disc having a flat face, and is Welded by its said face to said strip.

5. A modulating electrode assembly for a cathode ray tube, comprising a flat narrow metal spacer strip having a relatively large hole there- `through intermediate its ends, and a very thin metal disc constituting the modulating electrode proper having a flat face Welded to one face of "said strip so as to close said hole therein, said disc having a ne hole therethrough coaxial with disc, said tubular member projecting from the opposite face of said disc, al narrow. metal strip secured to said disc and extending flat on said flat disc, surface vacrossv said hole therein, said strip having a hole therethrough which is in line withbut larger lthan said cathode surface, a modulating electrode fixed on said strip and closing said hole therein, with a hole in said electrode inline with said cathode surface, said strip supporting and spacing said electrode from said cathode, and a tubular metallic shield secured to said opposite face of said disc and surrounding the cathode tubeprojecting from said face.

8. In a cathode ray tube, an electrode structure comprising an insulating support having an aperture in a surface thereof, a cathode secured to said supportin said aperture so that an electron emissive surface of the cathode is fixed with respect to said disc surface, a narrow metal strip secured to said support and lying on said surface thereof with a hole in said strip in line with said cathode surface, and a modulating electrode lying on and conductively secured to said strip and closing said hole therein with a hole in said electrode in line with said cathode surface, the thickness*v of said strip determining the spacing between said modulating electrode and said cathode, an anode` having a hole therein in line with the hole in said modulating electrode, and means securing said anode to said insulating support in `parallel spaced relation to said modulating elecvelectron emissive portion flush with said surface,

a narrow flat metal strip secured to said disc and extending flat on said surface across said aperture, said strip `having a hole therethrough in `line with and larger than said cathode portion, a modulating relectrode comprising av plate fixed `fiat on said strip andfclosing the hole therein,

with a hole in said plate in line with said cathode portion,- said strip spacing said plate from said cathode, an anode having a surface facing and parallel to said-` plate, with a hole in said anode surface in line with the hole in said plate, and means including a plurality of claws secured to said anode which. embrace and grip the periphery of said disc to fix and support said anode on said disc in spaced relation to said plate.

'i l0. In a cathode ray tube, an electrode structure comprising a disc of insulation having an aperture in a fiat surface thereof, a cathode securedto said rdisc Withinsaid aperture and having an electron emissive portion flush with lsaid surface, anarrow flat metal strip secured to said disc and extending at on said surface across .flat disc surface across said having a hole modulating electrode, and meanssecuring said anode to said disc in parallel spaced relation to vaperture in a flat surface secured to said disc within said aperture'and hole therein, with a hole in said plate'in line with said strip spacing said plate therein, with a hole in said plate in line with said cathode portion, said strip spacingsaid plate kfrom'said cathode, a plurality. of spacer bosses upstanding from said flat surfaceof'said disc at opposite sides of said strip, an anode seated on said bosses and spacedthereby from said plate, with a hole in said anode in saidplate, and means securing saidl anode to said disc.

11. In a cathode ray ture comprising a disc of tube, an Velectrode strucvinsulation having a disc, said tubular member projecting from' the opposite face of saiddisc, a narrow'metal strip secured to said disc and extendingflat on said hole therein, said strip having a hole therethrough which is in line with but larger thansaid cathode surface, a modulating electrode fixed on said strip and closing opposite face of said disc. and surrounding kthe cathode tube therein in line with the'hole in said said modulatingA electrode. n

12. An electrode structure as defined in claim ll, and including an insulating ring around and secured to said tubular shield, a second insulating projecting from said face, an anode' line`=with thehole inl said cathode portion,

from said cathode, an anode having a flat surface 'facing and parallel to said plate, with a hole in said anode lsurface in line with the hole in said plate, a narrow insulating spacer strip extending from theperiphery of said disc between the opposed kfaces ,of said plate andy saidanode, said spacer strip having a hole therein in line with the holes in said plate and anode and larger than said holes, means securing said anode to said disc,

and means Asecuring said 1nsulating strip by itsy outer end to the structure.V y '15'. In a cathode ray ytube an electrode structure comprising a'diseofinsu'lation having an aperture :in` a flat surface thereof, a cathode vsaid hole therein,'with a hole in said electrode in linewith said cathode surface, said strip sup- A porting and .spacing said electrode from' said v cathode, a tubular metallic shield securedto said ring around and secured 'to said anode, each of said rings having a set of angularly spaced holes therethrough line with the -holes inthe other ring, parallel rigid support rods extending through aligned holes in said rings beyond said.

anode, a tubular electrode disposed between said rods beyond said anode, and means securing said.

tubular electrode to said rods in coaxial' spaced relation to said anode. l

13.` An electroder structureas defined in claim v8, and including an insulating spacer betweenk said anode and modulating electrode. f

.14.- In a cathode ray tube, an electrode structure comprising a disc of insulationhavingfan thereof,y a cathode having an electron emissive portion'flush with 'saidjsurfaca a narrow flat metal strip secured to a plate fixed flat on said strip and closing theA comprising a narrow over said metal disc with its open end facing away from said discs and having a flat discy secured to said disc within said aperture and a narrow flat metal strip secured to said disc and extending flat on said surface across said aperture, `said `strip having a hole therethrough in line with and larger than said cathode portion, a modulating electrode comprising a metal disc having a flat surface and xed with itssaid at surface flat on said strip and closing the hole therein so that said strip spaces said disc from said cathode portion, said disc having a small hole therethrough in line with said cathode portion and said disc tapering in thicknessfradially from its outer periphery to its hole.

16. In a cathode ray tube, an electrode strucsaid surface,

ture comprising a disc of insulating material hav-` ing a disc-shaped electron emissive surface in the plane of said disc surface, a modulating electrode plane-parallel metal supporting strip having a hole therethrough larger than said cathode surface and secured to said disc with one face of the strip contacting said flat surface of said disc and with the hole in said strip coaxial with'said cathode surface, and a metal disc constituting the modulating electrode proper secured flat on and closing the hole therein, with a centre hole in said metal disc coaxial with said cathode surface, said metal disc lying vparallel to said cathode surface and is separated therefrom by a spac-l ing determined by the thickness of said strip on which said metal disc is mounted, and a cupshaped anode supported by said insulating disc shaped bottom surface with a centre hole therein positionedparallel to and coaxial with said cathode surface and said metal disc. Y

GERHARD the other face of said strip

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