US2515267A

US2515267A - Grid structure and method of fabrication

Info

Publication number: US2515267A
Application number: US740560A
Authority: US
Inventors: Frederick L Salisbury
Original assignee: Sperry Corp
Current assignee: Sperry Corp
Priority date: 1947-04-10
Filing date: 1947-04-10
Publication date: 1950-07-18
Anticipated expiration: 1967-07-18

Description

y 1950 F. SALISBURY 2,515,267

GRID STRUCTURE AND METHOD OF FABRICATION Filed April 10, 1947 2 Sheets-Sheet l i 17 3Z 4 E \1 I y i I INVENTOR FREDERICK L S04 ISBl/R Y Y ATTORNEY July 18, 1950 F. L. SALISBURY 2,515,267

GRIDSTRUCTURE AND METHOD OF FABRICATION Filed April 10, 1947 Sheets-Sheet 2 INVENTOR FmspER/c/r 1, .5/71 lJBU/PY ATTORNEY Patented July 18, 1950 I GRID: STRUGTU-RE AND. METHODJOF* FABRICATIQN;

Frederick.L.. Salisbury, Lynbro'ok;',N. Y;, assignor to. The 'Sperry Corporation;- acorporation of Delaware Application April .10, 1947,;Sfil'ia1iN0m74m560h 1.? This, invention relates. to, improved electrode structures for electron discharge, devices, and

more particularly relates. to grid Lstructures and the method. oimaki'ngsame.

In. many typespf electron discharg tubes it is desirable to control the. flow of an electron beam. by a grid structure through which the elec-' tron beam passes. By varying the magnitude of an electric. potential between. the grid" and an other el'ectrod'e',such.as a source ofelectrons it is possible as is we1l,known, to accelerate or decelerate the electrons depending upon the polarity of the potential. It is generally desirable in such tubes, that the grid. structure intercept a. minimum ;.numberof electrons. This permits the maximum number of controlled electrons to pass through the grid to be utilized in the space beyond. Furthermore, the collisionof the intercepted electronswitlrthe grid causes the elements making up the grid to become heated. The grid may eventually be destroyed if its temperature rises't'oo'high;

It is also verydesirable in gridstructures that the elements making up the grid-be quite; rigid and free from mechanical vibrations: Otherwise, the motionof the grid elements mayeffect the controlled electron beam causing the tubes electrical output to be very unstable.

Finally, it isdesirable that the grid beeasy and economical to manufacture; This is especially true-if thegridi is to bemade in large quantities in a short length of time.

' In= oneembod-iment ofa grid constructed in accordance with the present 'inventionall the elements making up-the-grid are formed from a single ribbonof metal; This ribbon has-a series of spaced-V-shaped indentations formed in it"by suitable dies. Theinden-tat-ions are formed in groups, all those of any; one group being of the same size and shape. The indentations of each successive group are'made progressively smaller.

The ribbon, after the indentations, are formed therein; isthen wound in a spiral fashion so that the indentations of each succeeding "WIa'p' nest in spaced fashion within the indentationsof-the preceding wrap. The'wound ribbon is; then placed ina retaining member which is shaped to lit the aperture-inwhich it is-desired to place the grid. The wound ribbonand the retaining member are then placed in a brazing; furnace or treated in'someother-fashion so 'asto form them into a unitarygrid-structure.

By -making the grid elements ribbon -shaped; with the'narrow edge facing theelectron'source,

the numberofinterceptedelectrons-is reduced.

The cross-sectional'area ofjthe grid elements is still"lar andthereiore the gridwill' be easily able to conduct away undesirable heat, eliminating thedanger ofhigh' temperature destruction. The mechanical rigidity of such a grid structure is quite high; This is true since none of the grid elements haveunsupportedends, all of the-elements being formed from a continuous ribbon. Also since the cross-section of the grid elements is long andthinwith the long axis of the crosssection parallel to th-e' electron beam, the grid will have considerable'resistance to vibration along the electron beam axis. This mechanical rigidity reducesmicrophonics and. tends to make the output of the tube much more stable.

By formingallof the elements from" a-single continuous element, the problems of manufacturing agrid are reduced and-an economical mthod of fabrication is achieved:

Itis therefore an object of the present invention to provide a grid structure in which the elements makin up the grid are formed of a continuous metallic filament or ribbon.

Another object 'ofthe present invention is to provide a grid structure in which the elements have no free unsupported ends.

Still another object of the present inventionis to provide a grid structure in which the grid elements have a large cross-sectional areawithout having a largeelectron beam obstructing area.

A further object of the present invention is to provide a method-of fabricating grid structures in which the grid elements are formed from-a continuous metallic filament or ribbon.

Other objects and advantageswill becomeapparent from the followingspecification; taken in connection with the accompanying drawings wherein.

Fig. 1 is -a-perspective=view of a gridstructure having independent grid elements formed of metallic filaments,-

, Fig. 2'is aperspective view ofanother grid structure having independent grid elements formed of metallic ribbon- Fig. 3 is a perspective viewofa metallic ribbon having -v-shaped-indentationswhich serve as grid elements,

Fig. 4i'saplan view of a grid structure formed from the indented ribbon Shown-in'Fig; 3 and Fig. 5' is the planview-of another grid structure formed from an indented ribbonsimilar to the one-showninFig. 3".

Referring now to- Fig. 1, a peripheralring- Ill isshown which may easily beformed by'joining the ends of a metallic ribbon at joint ll. Joint ll may be made in any conventional manner as I by lapping the ends of the ribbon and brazing or welding them together, or by making a con- Grid element I2 is formed by bending'a length .1

equal-length legs I4 and I5. The length of filament I3 is made slightly less than theidianieterf of ring it. The free ends [6 and l! of L-shaped element l2 are then rigidly fastenedrin any conventional manner, such as brazing,*tothe ='inner-- surface of ring Ill, with the bend 30 of element-I2 directed toward the center of ring l0. Since legs l4.and l5. are of the same lengthand ring I0, is circularinshape, the legs l4 and"! of element 12 arefrespectively parallel to, and slightly dis.- placed fror'na pair of orthogonal diameters of 'rin'g'lfi.

a p elements

23, 24 and are similar'to grid element 42. They are formed into L,-shapes and arerig'idly joinedto the; inner face of'ring 16 all orthebleg sof the elements being located in the same plane. The spacing of the L-shaped grid elements i2, 23, 24 and 25 is: made regular so th at thefacing legs of adjacentlpairs of grid ,elementsare parallel to each other. :Asecond set of L-shaped gridele nents-ZG,

21. 28 and 29 is formed in the same manner as the elements l2, 23,24 and 25, making up the first set, but the length of the-filaments irom which the elements of the second set are formed is shorter than that of the, filaments making up the elementsof the first set. The free ends 30 and 3i of; element 26 are fastened to the inner surface of ring 16 in a manner similar .to the free ends 15 and ll of element l2, the bend- 32 of .element'zii being positioned along the same radius oi ing was the bend 30 of element l2. Thus, L-shaped element 26 nests Within L-shaped ele ;mentl 2, corresponding legs of eachelement being parallel to each other but spaced slightly apart. In similar fashion, element 21 is anchored to 1ring-. ill in a spaced-nesting relationship with element 23, element 28 with element 24 and-element 29 with element 25. a ,1 1

The resulting unitary grid structure presents a grid-like ,face to the electron beam which it is to control, the grid elements presentinga very small obstructing area to the. passing electron beam. Nevertheless, the elements have suflicient crosssectional area so, as to be able to conduct away the heat caused by unavoidable collisions of elec; trons against the electron-beam facing area of the grid elements. This resulting high heat conductance keeps the operating temperature of the grid elements from reaching a dangerous value and possibly destroying the grid structure.

The mechanical rigidity of this grid structure is quite high since each end of the grid element is firmly anchored in the peripheral ring. Thus, the grid elements will be little influenced by mechanical vibrations of the electron tubein which the grid structure is placed, and therefore such vibrations will not affect the stabilityof the tubes electrical output. 7

If it is desired to further decreasethe electron beam facing area ofthe grid elements without lowering the total cross-sectional area of each ele ment, the elements may be formed from lengths of metallic ribbon rather than filaments as shown in Fig. 1. Furthermore, the shape of the grid elements may be changed from the L-shape shown in Fig. l to other configurations which may permit a greater number of grid elements to be positioned aroundthe inner surface of the peripheral rin'gifi so'desired. Also, there is noilimitation in the number of grid elements which may be nested within each other. An embodiment having the elements formed in the shape of Vs rather .than Ls is shown in Fig. 2. The elements of this of metallic filament I3 into an L-shape having embodiment are formed from lengths of metallic ribbon. Peripheral ring 40 is formed similar to ring it ofFig. l by bending a metallic ribbon 4| "a the'shape 'of the aperture in which it is desired to place the grid structure, and joining the ends of ribbon 4| at joint 42 in any conventional manner, As in Fig. 1, peripheral element 40 is shown for convenience in description as formed intotheshape of a circle, although it may have thesh'a'pe of any closed curve. Grid element 45 isformed by bendinga length'of metallic ribbon int'o'a V-shape' having equal length legs 46 and disposed at anang'leslightly less than Thel'ength of thefmetallic ribbon fromwhich "giidiel'einent 45 is formed is "slightly less than the diameter of. ring 40 and the width of theribbon is preferably" the same'asthat of ring Ill. The free, ends 36 and 31 of

legs

45 and 41 are firmly anchored 'to the" inner wall or ring 40, the lines where these free ends 36 and'tl join ring I O'being parallel to the'axis thereof. ,The legs 46'and 4l are thus positioned along'a pair of, equal length intersecting chords of circular ring 4!]. Grid elements 50,1 5!,52 and 53 are formed in the same manner as grid element v45.1having the same size and shape and comprise along with element 45 what shall be call'ed the first set of gridelements'. These grid elements 50, 5!, 52 and 53 have their free ends" fastened to the inner wall of ring 40 in the same manner as the free ends 36 and 31 of legs 46' and 41 oi element 45. The angular position of-the five V-shaped grid elements 45, 5B,, 5!,52 and 53 making. up. the first set is made regular so that the spacing between facing legs of adjacent grid elements is maintained constant.

A second set of grid elements 60, BI, 62, 53 and 541s also formed by bending short lengths'of metallic ribbon into V-shapes having equal length legs. The length of the ribbons from which ,the elements of the second set are formed is slightly less than .the length of the ribbons forming the first set o grid elementswith the width preferably the same.- Theangle'between the legs of the V.-s haped members in the second set is not necessarily thesame as .the angle between the legs of the grid elements making up the first set, being preferably made smaller, although equal angles can also be used. In Fig. Zit can be seen that the angle between the legs 65.,and 66 of grid element 60 is slightly lessthan that of grid-element I45. The second set of grid elements are anchored to the inner face of; ring 40 in the same manner as ,the elements making up thefirst set, each of the elements of the second set being positionedin asymmetrical nested relationship with one of the elements of the first set. H g Q A third set of grid elements 'lll 'll, 12, I3 and 14 is formed by bending shorterlengths of metal-.- lic ribbon into ll-shapes in thesame manner. as the preceding two sets .of grid elements. ,The length of the. ribbons. making up the grid-ele merits in the third set is shorter than the length qfthe i nsmeki unihe elem n lithe e amaw 55 nd set and the width-of the ribbonsi'is.prefer= ably the same; Again; the anglebetwe'enthe'legs' of each V-shape members of the third seti'snot necessarily'the same'as theangle b'etweenthe legs of the preceding set of'grid elements and again the angle isarbitrarily.'made'smalleri This is" seen more clearly by'refer'ring togrid' element-i10 Where it can be-seen that the angle betweendegs l5 and IS-of this element issmaller-than the anglebetween the legs 65" and 66' of element 60-? As before, eachelement/making up the-thirdset of grid elements is rigidly connected to theinner face of peripheral ring-40 -in -asymmetrically nestedrelationship with the other two -sets=of grid elements, allmembers of each: set--beingecoaxialand co-directed. i

The resulting. unitary grid structure is 1 thus; composed of three sets of-W-shaped grid elements, each set comprising fiveeleme'nts. Thefree ends.

of each member arerigidly connected -to theinner' Wall of the peripheral-ringzmembem.corresponde ing members of each set beingpositioned inua. symmetrical nested) relationship.v

Thus it canbe seenin. the: embodiment shown in Fig. 2 that the number of elements making up a set of grid members-can v-aryratwill, ascan the number of sets'used inia gridstructure. The electron-beam facing area. ofl'the: resulting. .grid structure can be kept-to avminimum while permitting the cross-sectional area, of the grid elements to be sufiiciently large so as to' conduct away the heat resultingirom the collision of electrons with the obstructing. facesof the grid structure. The rigid anchoring. of the free ends of each element, and the V--shape of the members contribute .to a grid. structure in which the elements have a high degree of mechanical stability and are little affected by vibrations-of the electron tube in which thegrid is placed.

The ribbon-like structure of thetgrid elements shown in Fig. 2 not only aidsin. minimizing the electron-beam facing area of the resulting grid structure, but aids in further increasing the mechanical rigidity of the resultant grid. This is true since the long. dimension. of the .crosssectional area of the grid members is. parallel to. the electron beam. This tends to reduce to: a minimum any bending. of the grid elements in a direction along the electron beam,, and thus minimizes instability effects due to motion of the grid elements in the direction parallel to the beam.

Although the grid elements making up the grid structures shown in Figs. 1 and 2are formed. from individual lengths of metallic filament or ribbon, all the grid" elements may be and 'preferably are easily and. economically formed from a single continuous filament or ribbon. This may be done by making properly spacedindentations in a filament or ribbon, winding the indented filament or ribbon in spiral fashion and then placing the wrapped or wound member in. a peripheral ring element, similar to that shown in Figs. 1 and 2. The spirall'ed' member and peripheral element are then treated in any conventional manner, such as brazing or welding, so as to form a unitary structure.

This procedure will be more clearly understood; by reference to Fig. 3. This figure shows a ribbon 80, which may be a filament, if desired, inwhich is formed a first set of equally spaced, V-shaped indentations 8|, 82', 83 and 84 separated by spacing

portions

85, 86, Bland 88. All'ofthe indentations of this set are of the same size and-shape.

A second set of equally spaced V'-'sh'aped' i'nd'eh tations 91, 92;? 93 and'94"'is*"fermed" in"tlie'ribbon 8'0 slightly spaced from"the-"first set by" spacing,-- portion 95; The" indentations oft'he' second set are-all of the-same size-and shape, but they are smallenthan' those of the firstset. spacingpor tions 96% 9"! and 98"separate the-indentations of this set. A third set of still smaller equally spaced indentations-- l Ill l 02, lli yand" W4- are formed in the ribbon '80'; slightly spacedf'ronrthe' second set by spacing'--port ion-' 4 052'" The indentations" of this set are separated by spacing portions- It 6; 1 0T and -l 08;

Thethree set of indentationsmay be -formed in ribbon 90 m any conventional manner. Qneway"-is'- by pressing ribbon Bli he-tween a suitably shaped'punch andmatrix.

The ribben fifl is then wound in spiral fashion with the'largeindentations of the' first set-com pletely occupying the inner-wrap. The flat spac ing portions sep'a-rating the indentations of the first -set are b'owed to givetli'e-required curvature ofthe-r-ibbon; Theindentations of tlt'e suceeed ing set are so spaced that; as-theribbomds wrapped closely aroundthe first or inner wra'm the indentations of each succeedingwrapposttion themselves in symmetrical, nesting-relatienship. with correspondi-ng- -indentationsxin the: preceding. wrap. The .bow'ede or arcuate spacing =p0rtiOIlSIOfi the-'ribboniin -eaclrwrap' between" in dentations iarein: complete: contactiwith-isimi Lr spacing portions in preceding :and succeedingwraps. i I

A peripheral: ring. memherwis: then fitted around the outer wrapzimalringicontact with theno'w ar cuate spacing. portions. of. the outer-wrap This ring member maintains-the;spirahshapez: The): assembled structure is .then given a .heat. treatment,. or theulike to .weldgLbraze, or .soldenor: otherwise: form together the; contacting. arcua-te portions of each Wrap... At the-same time the: retaining or peripheral: ring;,-member is similarlyfixed tothearcuateportions of-th'e outeir-wrapi of the spiral. As. a=result 0ft such heat treatment. the spiraled; indentedi ribboni andtthe penpherali retaining-- member unite toform a-..unitary:..grid: structure of great. mechanical rigidity.v

This construction isanioreleasily' understood. by reference to Fig-4;. In this figure-.ribboniliilrof Fig. 3 is wound into" a; sipira l and the-zspirzalledi. member is then fittedinto airing-.inember High Indentations 8b,; 82,5. 83 and 84- along with spacing; portions 86, 81- and; 88 form-:the:innerwrap of the spiral The second begins with spac ing. portion. 9-5 WhOSGriIHlGI'I' facea is incontact with: the outer faceof spacing. portion 35 of the inner wrap-.- Indentations -M, 92. 9'3:.and 9 i along withz spacing. portions 96, 81- and 98 complete this. wrap. Spacing portion. lfi5.whose= inner face is in. con-.- tact. with the. outer fa-cehof spacing: portionvfiiiw begins the third wnap of the spiral, This wrap-is; completed by indentations HM,- lll2, I03 and '04". along with spacing. permanent-86; till, 108 and M9. The inner face of outer ring; member till contacts. the outer-face of thespacing portions N35,, N18; 01, lfl8' .and. H19 of the. outer wrap. This. ring member I I'll maintains the. ribbon 8.6 in its spiral; shape until it. is formedlinto a-unitary structure. Thismay bedone in any conventional manner as, by placing. the ..assembled gridintowelding or brazing furnace and forminga unitary.gridstruce ture. 3

It'will' be noted, [by reference to Figs. 3' aridlf l... that the first i'ndentations of each set-ar.r.ange ..v

themselves in asymmetrically nested'relati'onship. Thus, after the brazin'gor welding or like operation, the corresponding-indentations of each set are similar to, the grid'elements shown in Fig. 2. However, by making the grid elements from a single ribbon rather than by forming a multiplicity of individual grid elements the resulting structure can be made more readily and easily. The inherent advantages of such a grid structure as shown in Figs. 1 and 2 are also realized in the continuous ribbon grid structure of Fig. 1. The additional advantage, however, of the continuous ribbon type structure is in the ease and economy of manufacture. Using the above suggested method of fabricating the grid structure,

there is considerable assurance of uniformity in the resulting products. This. is particularly advantageousin the mass production of electron discharge tubes using grid structures if the tubes are to be interchanged in operation. In such cases, there will be little or no change in the static and dynamic operating characteristics of the tubes due to non-uniformity of grid structures.

It will be understood that the apex angles of the V-shaped indentations of the successive sets may be the same or may get smaller toward the outer wrap. 1

A slightly different embodiment of the grid structure of Fig. 4 is shown in Fig. 5. In this embodiment, the shapes of the indentations form-.

of ellipses. Intermediate arcuate portions I34,} I35, I36 and I31 complete the second wrap. The

inner faces of the arcuate portions I34, I35, I36, I31 of the second wrap are in close contact with the outer faces of the arcuate portions I24, I25,

I26, I21 of the inner or first wrap. I

A third set of curved indentations I40, I4I, I42 and I43 are semi-circular in shape with equal length, parallel extending legs separated from each other by the length of the diameter of the semi-circular section. These indentations along with intermediate arcuate portions I44, I 45, I46 and I47 complete the third wrap of the spiralled grid structure. Arcuate portion I50, which extends from the last indentation I43 of the third set, makes a complete wrap around the spiral and has its free end joined to the point where its opposite end and the connecting leg of indenta tion I43 form bend II. By connecting the free end of outer Wrap I50 to bend I5I the spiralled shape of the grid structure is maintained, and the structure may then be. given a suitable treatment so that the close by contacting arcuate portions may be welded or brazed together to form a unitary structure.

Thus, by extending one end of the ribbon I I5 sufliciently beyond the last indentation formed therein, it is possible to form the peripheral element of the grid structure from the same ribbon which forms the grid elements themselves. This permits the entire grid structure to be constructed from a single length of metallic ribbon. It will be understood that all the indentations I33 of the,

, or filament inv which a. series of suitable spaced indentations are formed it is possible to fabricate aunitary grid structure by winding the indented ribbon or filament in a. spiral fashion and suitably forming the resulting spiralled member 10,- into a unitary structure, as by a heat treatment in the above description or shown in the accaused by electron collisions.

which welds or brazes together the contacting portions of the spiralled member between indentations.- .The indentations are so spaced along the ribbon or filament so that those of succeeding wraps of the spiral nest together.

The spiral shape may be maintained during the fasteningoperation by placing the spiralled member into ,a peripheral, retaining member. If desired, an additional wrap may be made by the ribbon andthefree end of the ribbon fixed to thebeginningof this additional wrap so that the spiral shape will be maintained. The structure may then be given a suitable heat treating or Otheroper'ation so that a unitary structure results. 7

The resulting grid structure has a 10W electronbeam obstructing area, but the grid elements themselves have SllfiiCiGllt cross-sectional area so as to be able tosafely conduct away the heat The mechanical rigidity of this structure is quite high since none of the grid elements have free, unsupported ends. If the grid elements are formed from a ribbonlike element rather than a filament, additional rigidity is given along the axis of the spiral.

The indentations may be formed in the filament or ribbon by using dies. This permits a large-number of uniformly shaped grid structures to be made easily and quickly.

Therefore,'in accordance with the present invention it is possibly to easily and economically fashion a grid structure in which the elements making up the grid are formed from a continuous metallic ribbon or filament. The grid elements of such a grid structure have no free, un-

supported ends, which permits great mechanical rigidity. Furthermore, the cross-sectional area of the grid elements may be large while the electron-beam obstructing area can be kept quite low. Finally, such a grid structure may be produc'ed in great numbers easily and economically with a high degree of uniformity by forming the grid elements by dies or otherwise from a single elongated metallic member.

Since many changes could be made in the above construction and'many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained companying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A single piece spiral shaped grid structure comprising a ribbon having a first plurality of equal-sized V-shaped indentations therein spaced equally and having their apexes directed toward the center of said spiral, and having a second plurality of smaller equal-sized V-shaped indentations in said spiral spaced from, but in nested relationship with a corresponding indentation of said first plurality.

2. A grid structure consisting of a thin metallic ribbon mounted in a spiral having three wraps,

1.- said ribbon {having its innermost. wrap a first .-.series-;ofa:-egually sized N-shaped indentations separated by equal length spacing portions, a

I second-series of ,Veshaped indentations in the -middle.-,wrap .-.of 'saidlspiral separated I by equallengthspacingportions,rsaid indentations of said x-second series v beingtsmallerinsize than those of said first series and nestingawithin the larger indentations of ,said-first-,series, the equal-length spacing portions of said middle wrap being in contact with the spacing .portions of said inner wrap, and the outer wrap of said spiral being in contact -.with the equal length spacing portions of said middle wrap.

3. A grid "structure-for an electron discharge tube consisting of an elongated metallic member mounted .in spiral shape having a plurality of wraps, the. innermost wrap having a plurality of substantially equal-sized V-shaped indentations directedtoward'the 'center of said spiral, said indentations being spaced from each other by a plurality of equal-length member portions, succeeding wraps of said spiral each having an equal number of V-shaped indentations arranged in nested relationship with the indentations of the innermost wrap, and separated from each other by an equal number of member portions, the indentations of each succeeding outward wrap being of the same size but progressively smaller than the indentations of the preceding wrap, the member portions of each succeeding outward wrap being of the same length but progressively longer than and making contact with the member portions of the preceding wrap.

4. A grid structure for an electron discharge tube comprising an elongated metallic member mounted in a flat spiral shape having a plurality of wraps. the innermost wrap having a plurality of spaced, equal-sized, inwardly directed indentations, succeeding wraps each having an equal number of spaced inwardly directed indentations arranged. in symmetrical nested relationship with the indentations of the innermost wrap, the indentations of each succeeding outward wrap being of the same size but progressively smaller than the indentations of the preceding wrap, the connecting portions of each wrap between the inwardly directed indentations thereof being in close contact with the connecting portions between the inwardly directed indentations of the next succeeding outward wrap, and a closed perimeter member positioned around the outward Wrap of said spiral, the connecting portions of said outer wrap between the inwardly directed indentations thereof being in close contact with said perimeter member.

5. A grid structure for an electron discharge tube consisting of an elongated metallic member mounted in a spiral shape having a plurality of wraps, the innermost wrap having a plurality of inwardly directed indentations separated by arcuate portions, each succeeding wrap of said spiral having a like plurality of inwardly directed indentations arranged in spaced, nested relationship with the indentations of the innermost wrap and separated from each other by arcuate portions, the indentations of each succeeding outward Wrap being progressively smaller than the indentations of the preceding wrap, the arcuate portions of each succeeding wrap making contact with the arcuate portions of the preceding wrap.

6. The method of forming an electron discharge tube grid structure comprising the steps of forming a first series 01 equal-sized V-shaped indentations in ae-thinlmetallic ribbon, the indentations .Joeing separated -by.-a first series of equal-length .ribbon.portions, forming asecond series of equalsized, -V-shapedgindentations in said metallic ribboni displaced from said first ,series, the indentations of said second series .being. separated by a second series of equals-length ribbonportions, the

: indentations of said second series being smaller .in gsize than [those of said first series land the equal-length ribbon portions -of sai'd first series being shorter than those of said second series,

wrapping-said ribbon into a.-spiral having two -wraps-o that the :indentationsof said second iseries nestwithin-the indentations 40f said first ,series bothtbeinginwardly directed, and so that the-ribbon portions ofsaidfirst series make con- .tact with the ribbon port-ions of said first series,

and joining said first series of ribbon ,portions to, saidssecondseries-where they make.- contact.

' Wardly directed and the first series of spacing portions making contact with the second series of spacing portions, fitting said wrapped member into a closed metallic peripheral member such that the spacing portions of said second series make contact with said peripheral member, joining together said first series of spacing portions to said second series where they make contact and said second series of spacing portions to said peripheral member where they make contact.

8. The method of forming a grid structure comprising the steps of forming a first series of indentations in an elongated metallic member, the indentations being separated by a first series of spacing portions, forming a second series of indentations in said metallic member displaced from said first series of indentations, the indentations of said second series being separated by a second series of spacing portions, wrapping said elongated member into a spiral having two turns so that the indentations of said second series nest within the indentations of said first series, both being inwardly directed and the first series of spacing portions making contact with the second series of spacing portions, and joining together said first series of spacing portions to said second series of spacing portions where they make contact.

9. The method of forming a grid structure comprising the steps of forming a first series of indentations in an elongated metallic member, the indentations being separated by a first series of spacing portions, forming a second series of indentations in said metallic member displaced from said first series of indentations, the indentations of said second series being separated by a second series of spacing portions, wrapping said elongated member into a spiral having three wraps so that the indentations of said first series and the spacing portions of said first series form the innermost wrap, the indentations of said second series and the spacing portions of said second series form the second wrap, and the extended free end of said elongated member makes one complete turn to form the outer wrap, the indentations of said second series nesting in spaced relationship within the indentations of said first series, the spacing portions of said first series making contact with the spacing portions of said second series, and said extended free end making contact with the spacing portions of said second series, and joining together said spacing portions and said extended free end when they make contact.

10. A spiral form grid structure comprising a continuous ribbon arranged to have a plurality of substantially radial portions of equal length and a second plurality of substantially radial.

continuous ribbon arranged to have a firstu plurality of substantially radial portions and a second plurality of smaller substantially radial portions.

12. A plane spiral grid structure comprising a continuous ribbon arranged to have a plurality of substantially radial portions and a second plurality of smaller portions substantially parallel to said first portions.

FREDERICK L. SALISBURY.

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

UNITED STATES PATENTS Mouromtsefi' et al. Feb. 5, 1946