US2521556A

US2521556A - Magnetron

Info

Publication number: US2521556A
Application number: US710962A
Authority: US
Inventors: Donald A Wilbur
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1946-11-20
Filing date: 1946-11-20
Publication date: 1950-09-05
Anticipated expiration: 1967-09-05
Also published as: FR999345A; GB655041A

Description

D. A. WILBUR Sept. 5, 1950 MAGNETRON Filed Nav. 2o, 194e /4 IIIIIIIIH MIN HN 2 im Il!! 24 Il! Inventor: Donald AVVHDMP7 b9 wy/AMM Hi Attorneg.

Patented Sept. 5, 195() MAGNn'raoN Dumm aiwiibumriey, N. in, assignent Generar Electric'einpa'ny, aborpcration'of New 'Yorkv4 Applicaticn November, 1946,' SeriaiNo. '710,962

poses, the art of -magnetron*` discharge devices has in recent yearsdevelcped .extensively in the direction of multi-electrode constructions because of their inherent high frequency characteristics In one type of construction, generally termed the resonant anode or cavity resonatortype, a

plurality.; of` resonant cavities are disposedthroughout an annularA block anode surroundingl the periphery of a rotating space charge, the cavities communicating with the'space charge in such mannerthat highfrequency oscillations are inducedin them bythe rotation of the space 19 Claims.' (El. Z50-275) charge. Principal among the advantages of such constructions have been the higher frequencies and higher efliciencies obtainable. However, they also exhibitv certain inherent drawbacks which limit their usefulness. Among these maybe mentioned: the troublesome tendency of individual cavities having small yunavoidable differ-- ences in dimension to oscillate at different natural frequencies whereby vthe ensemble produces a narrow band vof frequencies where but `one is desired; the fact that the ensemble, because of itsgeometry, may oscillate in any of several different modes .having closelyrelated frequencies which' fact vintroduces frequency stability problems; the difficulty in devising satisfactory tuning systems to tune all cavities rin unison when itis desired to change the operating frequency of the oscillator; and the manufacturing diiculti'es involved in lthe highly precise dimensioning of the large number of cavities.

In another type of magnetron construction which'ha's been widely used, a pair 'of mutually spaced anode members is provided lwith opposed semi-cylindrical' surfaces 'which define a generally cylindrical boundaryicnflnin'g the rotating space lcharge of the device.' Conductors of'a parallel "wiretransmission 'line areY connected, respectively, with the anode members,` and' entend through theenvelope` of' the device'to external circuits. In 'such constructions'the operating frequency is 'determined not by the electrode structure, Vas in the cavityresonator typefbut ratherv by`r the"parametersy of 'the"tra`nsmission lineconstitutingfqan external tuningk circuit forv the' magnetron.- The loperating frequency is,

' partly'in section,HA of a magnetronfdischargefd 2 therefore readily' controlled by adjustingith positionl of a short circuiting conductor con nl'ect'ingtlfieV conductors ofthe tran'snfiissionlv 1i l Magnetrons having the last lmentioned co`n` struction have been referred toas the split/ anode` type.' These devicesrhave been satisfactoryv-fo-r many applications and the simplicity of thetuning arrangement Ahas been an outstandingad-ivantage Yover magnetrons of the cavityresonaton type. Its general "constructional'simplicity has been a further advantage from the viewpoint manufacture', Howevenit has exhibited the ung desirable feature that its frequencyrangeis-in generai considerably lower than that` 'of-the cavityresonator type, this because of the fact that it' has fewer electrodes to sustain oscillation;Av Heretofore, because of structural complexity, ft. has not been practicable fto Ieffect any .substan--` tial "increase inthe'number of anode elements for the purpose of raisingthefrequency range.:-

IY have found, however, that by certain confstructional features to behereinafter described; it `is `pcssible to construct a magnetroniofethis general type which retains the simplicity ofthe split anode type particularly with respect .to tuning and manufacture and *.Which, `atl the same time, provides to' asubsrtantial ndegreejthe higher frequency characteristics of the cavity resonatorg type. At the same time,` it avoids to -asubsta tial degree certain undesirable-characteristics` of the cavity resonator type, such asthese aforementionedl v K v i Accordingly, it is the general object ofrny inL vention to'provide a new andimproved nag-g; netron construction characterizedby highergoperating frequencies,` fewer undesirable modcsi-of-- oscillation, higher eiiiciencies, and-simplicity/off constructionand operation.

The featuresof theinvention desired .to beprotected herein are pointed out infthe appendedclaims. The invention itself, together with tsv further objects and advantages Fmay? best be understoodiby reference to the foll Awing descrip?. tiontakenin connection-with the accompa' ing drawinfrfin whichfiig. l vrepresents avi "embodying the principles ofk the inventi Fig. 'represent'sa side view of Figlj Fig representsa"perspective' view' 'of tlfe electr e structure of Fig? 1,-1Figi4 represents' 'a further embodiment of' 'the invention 'incorporfat'inglceri tai'nadditional features ;r and litig. 5 represents ,a perspective" vieu/"of the electrbde'structure. Fig, e. Like numerals have been' used-t des?" ignate" like 'parts' throughout -the drawing.u f

Generally speaking, l. accomplish the object of my invention by mounting the anodes in angular radial distribution along the inner surface of a spiral tubular coil, the ends of which adjoin one end of the transmission line forming the tuning and output circuit of the magnetron. By electrically connecting the alternate anodes to appropriate different points on the spiral it is possible to obtain the proper distribution of high frequency potentials among the anodes which is necessary to initiate and sustain oscillations of the desired frequency.

Referring now to Figs. l, 2 and 3, there is shown a magnetron comprising generally an hermetically sealed envelope I, enclosing a part of a resonant circuit such as a transmission line formed by the conductors 2 and 3 extending through the envelope I and a two turn spiral coil 4 which terminates the conductors. Within the spiral coil 4 there are provided a plurality of anode electrodes 5, B, I and 8, each of which is conductively supported from a different point on the inner periphery of the spiral 4 as by welding thereto. As is seen from the drawing, the anode electrodes are positioned in a generally cylindrical conguration by virtue of their attachment to the spiral. Centrally of the spiral 4 and therefore likewise centrally of the anode electrodes there may be provided a thermionic cathode of any suitable type. In the drawing the cathode is shown as comprising a spiral tungsten coil 9 which may be coated with a suitable thermionic emissive material of the types well known in the art.

Conductive connections may be made to the spiral 4 through the envelope I by means of any suitable hermetic glass to metal seals such as the seals ID and Il surrounding the lines 2 and 3 and permitting them to pass through the envelope wall. Similarly, conductive connections may be made to the cathode coil 9 by the leads I2 and I3 which pass through the envelope wall at similar glass to metal seals I4 and l5. The cathode coil 9 may be rigidly attached to the lead I2 at one end and at the other end to a spring tension member I6 rigidly secured to the lead I5. It will be understood that the spring member I6 will hold the cathode coil 9 taut and ilxed in its position relative to the spiral and anode structures.

For the purpose of precluding excessive destructive bombardment of the envelope I and the lead-in seals by electrons escaping from the generally cylindrical space charge region dened about the cathode by the anode electrodes, there may be provided a cathode end shield I1 having ilat annular end members I8 and I9 surrounding the cathode at one end and juxtaposed to the respective end faces of the anodes. The members I8 and I9 may be joined by an integral cross bar 28 for rigidity and for support. The entire structure may be xedly positioned and supported within the envelope by means of a support 2| welded to the lead-in member I3 and to the member I9. An additional end shield 22 may be provided on the lead I2 and the upper end of the spring member I8 may be shaped to effect a similar result at the other end of the cathode if desired. A suitable getter may be provided on a getter support 23 welded to the lead I2or to the shield 22. The getter may be flashed in the well known manner during the evacuation process. V

' As indicated more clearly in Fig. 3, the anode electrodes 5, 8, 1 and 8 are conductively supported from the spiral as by welding thereto at different points on its inner periphery. For example, it will be seen that the electrodes 6 and 8 are connected to the spiral at points displaced by from each other on the first turn of the `spiral while the electrodes 5 and 'l are similarly displaced from' each other by 180 on the second turn of the spiral, the anodes B and 8 being symmetrically interpositioned between the anodes 5 and 'l as shown. The electrodes are preferably positioned symmetrically about tlie axis of the envelope, i. e., they are positioned about 45 along the spiral from the intersection with the spiral coil of a plane through the axes of the envelope I and the spiral coil itself. Thus it will be seen that as one proceeds around the periphery of the anode configuration, alternate anode electrodes may be considered as connected to points which are on a lengthwise half of coil 4 on one side of its lengthwise mid-point 4a, while the anode electrodes intermediate between those alternate ones may be considered as connected to points on the lengthwise half of coil 4 on the other side of its lengthwise mid-point 4a. l'f. therefore, one lengthwise half of the coil becomes electrically positive and the other negative at any instant during high frequency oscillation of the tank circuit comprising the transmission line, then the electrodes 5, 6, 'l and 8 will be alternately positive and negative.

The spiral coil 4 and conductors 2 and 3 may advantageously be constructed of a continuous length of hollow metal tubing, for example, copper tubing. Thereby there is provided a channel through which a coolant such as water or equivalent uid may be circulated for the purpose of cooling the anode electrodes.

It will be understood that in operation a relatively high unidirectional voltage may be imposed between the cathode coil S and the anode electrodes 5, 6, 'I and 8, the cathode being negative with respect to the anode electrodesy and that a magnetic ield parallel to the cathode and anode electrodes will be provided by any suitable means not shown such as external magnets juxtaposed to the envelope surface. As will be well understood by those skilled in the art to which the invention pertains, the interaction of these electric and magnetic fields will induce a rotating electron space charge of the magnetron type in the generally cylindrical space defined by the inner faces of the anode electrodes. It will also be understood that the interaction between thel rotating space charge and the anode electrodes will set up high frequency oscillations and standing waves in the resonant transmission line oomprising spiral coil 4 and conductors 2 and 3. The frequency of oscillation will be determined primarily by the characteristics, for example the length, of the transmission line and may be controlled by any suitable means such as a tuning short 24 slidably positionable along the length of the external portion of the transmission line.

The foregoing construction obviously presents four gaps formed by the four electrodes and for normal oscillation of the tank circuit formed by the transmission line, the instantaneous high frequency voltages of alternate electrodes will be positive and negative. The position and direc-- tion of the elds in the gaps are thus the saine as those in a four gap magnetron and four gap action is obtained. Thus the employment of the four electrodes 5, 6, l and 8 will giverise to a considerably higher frequency than that afforded by the two anodes of the conventional split anode ,ascisse magnetron having juxtaposed anodes deilning a cylindricalsspace charge region' about aVV cathode, each anodebeing attached to onemember of 'a transmission line similar to 'that'sh'own Referring now to Figs. 4' and 5,' there' is 4vshown an' alternative embodiment of theMv invention employing but va single turn to Athe-spinal coil 3G and., in addition "to the four anode Aelectrodes 25, 26, 21 and 2t, a neutral electrode ii similar to that shown and broad-lyfcl-aimed in my U. S. Patent No."2,l62,698 issued `February 22,-1949, and assigned to the saine assignee as the present application. Electrode Eil-'may be conductiveiy connected to coil 3e as bywelding thereto,at the 'longitudinal mid-point 'or electrically -neutral point 38a thereof. It will benoted here that the anode electrodes '25 through 28 are 'positioned about the linner Vperiphery of `the spiral in the following manner: The electrodes 25and 28 are positioned 90 along-the periphery of the coil 30 from neutral electrodeil, one von either side thereof. Electrodes and 2i are positioned' 60` further alongl the peripheryofthe coil'l from electrodes 25 and '21, respectively.

thereby leaving 60 between electrodes'ZS and 21.

As indicated by the drawing, the electrode'ZS is conductively connected to a point on the length wisehalf of 'coil Bil opposite the'lengthwisehali 'towhi'ch the electrode 25 is connected. vSim ilarly, the/electrode 2iis connected to a point on thelengthwisehalf of coil'til opposite the'length wise'ha'lfv towhich the electrode 23 is connected. The lengthwise halves referred tov are, ofcourse, 'those on nthe opposite sides of the point 353e. Similarly'to the arrangement oi Fig. 3, it will, therefore, be seenthat as one proceeds around the peripheryT of the anode configuration, alternatel members of the anode electrodes 25' through 428 may 'be considered as 'connected to 'points which are on'a'lengthwise half of coilS on one 'sideof its lengthwise midepoin't SELL, while the members of theanode electrodes intermediate between `those alternate ones maybe considered 'as connectedto point on the other lengthwise half of coil Sii on the other side of its lengthwise mid-point tta. Ii, therefore; one lengthwise'half .of the coil becomes electrically positive and the *other negative at any instant during high frequency oscillation of thetank circu-it comprising vvthe transmission line, then the electrodes 25 through 23 will be alternatively positive and negative. The five gaps formedbetween elec trodes 25 through 29 occupy the same relative positions and have high frequency vfields in the same directions as'would be'found in five consecutive gaps of'a siX'gap magnetron. The neutral electrode 29 will have no high frequency potential variations imposed upon it because itis `connected toa vpoint on the transmission line which has vzero `high frequency voltage during oscillation such as a nodal point ofthe standing potential "wave inthe'transmission line. Thus, inaccordance with'the principles set forth' in the aforementioned prior'application, its Vsole func- 'ti'on will beto maintain the unidirectional radial electric'eld through the entire 360 surrounding the 'cathode necessary for the establishment and maintenance of the rotating magnetron space charge around the cathode. As set forth in the aforementioned application, its usewill serve t0 .afford 'the lhigher frequency operation characteristic of multi anode constructions with a fewer `number of anodes and lconsequent structural simplicity.

In'FigqL the cathode islshown as comprising a straightmetallic wire, for example tungsten, although itmay be o'f-the'constructionsnownin Fig. 2. As With the coil 9 of Fig. 2,"it"'rl"xa:y"be coatedwith a suitable thermionically emisslve Icoating 'or left bare' fcoating asdesired.

From `the 'foregoing-*description iti is *apparent that magnetrons embodying-*my invention "provide a multianodefconstruction which 'is very' simple to fabricate. At the vsame time it Ais simple'jto operate, "principally because it may readily *be tuned in the same 'manner asconve'ntional' niagnetrons of split anode type'by simple meansexternalI to: the envelope. I`havefound"that"`these magnetrons also possess to'aingh degree'ithe'advantages `of the'resonant anode type "witnrefspeet'- to higher operating" frequencies for 1 a given magnetic eld'and `'this without `the resonant anode typesobjectionable characteristicof ai'plu-f rality of modes `of oscillation of 'c1os'ely`*relatd frequency. In my foregoing Aconstruction "the possible 'modes lof oscillation `are sufficiently widely separated in' frequency as 'to "obviateitlfie necessity of "strapping arrangements used'wth the `resonant Yanode "type, Moreover, there is ia substantial improvement 1in emciency of operation byv comparison withmagnetrons ofthe'split yanode'typeand atthe same time a substantial decrease in" the .objectionable backheating. ofi-'the cathode which -hasheretfore'been one .of the objectionable features ofthe split ano'de type.

It wiube understood itnat wn-ilernaveiii11us trated the invention' by. means of .embodiments en'lr'iloyingv speoic numbers of anodesand spiral turns, the principles of the'invention contemplate broadly .any number. of anodes and'. spiral- :turns desired. depending upon thefrequencyand power characteristics sought. In `gener-al, it ymaylbe stated that where-higher frequencies garer desired, a higher number of anodesshould-be used. :As an example, I have built and successfully tested magnetrons of the foregoing type operative in the frequency range above 700 megaoycles which six anodes have been used. In one-casera two-turn-coil havingsix anodes was successfully operated in the frequency'range from 700 to 1100 megacycles with one kilowatt ofv power' outputand efficiencies ranging from to 75 .per cent'ldee pending upon the particular frequencyof "op erationwithin that range. lIn another .caseta one and -a half turn coil with .six anodes-twas successfullyV operated .under substantially the same conditions.

.It will also be understood that while Ifhave illustrated the invention particularly in "connection -with magnetrons employed asoscillators, its principles are equally applicable to magnetrons employed. `fori-any other functions -which magnetron devices are capable of performing, sucha's those 'of an amplifier or reactance devices.

-Moreover, it willv be obvious to those kskilled-lnv the art that various other changes and modifications may be made without departing frommy invention in its broader aspects. Itherefore'aim in the appended claims to cover all changes 'arid modifications as fall within the true spirit 'and "scope of-my invention.

riphery ofl said spiral coil, each of said lelectrodes being conductively connected to a different point on said spiral coll.

2. An electrical discharge device of the magnetron type comprising a spiral coil constituting a portion of a resonant circuit, a cathode positioned centrally within said spiral coil, and a plurality of electrodes positioned adjacent to the inner periphery of said spiral coil in generally cylindrical configuration about said cathode, each of said electrodes being conductively connected to a diierent point on said spiral coil.

3. An Velectrical discharge device of the magnetron type comprising a spiral coil constituting a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality of electrodes positioned adjacent to the inner periphery of said spiral coil, each of said electrodes being conductively supported from a different point n said spiral coil.

4. An electrical discharge device of the magnetron type comprising a spiral coil constituting a portion of a resonant circuit, a cathode positioned centrally within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil in generally cylindrical configuration about said cathode, each of said anodes being conductively supported from a different point on said spiral coil.

5. An electrical discharge device of the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors, a cathode positioned within said spiral coil, and 'a plurality of anodes positioned adjacent to the inner periphery of said spiral coil, each of said anodes being conductively connected to a different point on said spiral. 6. An electrical discharge device of' the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors, a cathode positioned within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil in generally lcylindrical configuration about said cathode, each of said anodes being conductively connected to a different point on said spiral coil.

7. An electrical discharge device of the magnetron type comprising a spiral coil constituting a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality of electrodes positioned adjacent to the inner periphery of said spiral coil, each of said anodes being conductively connected to a different point on said spiral coil, alternate anodes being connected lengthwise of said spiral coil to points on one side of the longitudinal mid-point thereof and the anodes intermediate between said alternate anodes being connected lengthwise of said spiral coil to points on the other side of said mid-point.

8. An electrical discharge device of the magnetron type comprising a spiral coil constituting a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality .of electrodes positioned in uniform angular distribution around and adjacent to the inner periphery of said spiral coil, alternate -members of said electrodes being conductively connected longitudinally o f said spiral coil to different points on one side of the longitudinal mid-point atenerse thereof, and the intermediate electrodes between said alternate members being conductively connected longitudinally of said spiral coil to different points on the other side of said mid-point.

9. An electrical discharge device of the magnetron type comprising a spiral coil constituting at least a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil, each of said anodes being conductively connected to a different point on said spiral coil, alternate anodes being connected lengthwise of said spiral coll to points on one side of the longitudinal midpoint thereof and the anodes intermediate between said alternate anodes being connected lengthwise of said spiral coil to points on the other side of said mid-point, said points being distributed about said periphery symmetrically with respect to said mid-point.

l0 An electrical discharge device of the magnetron type comprising a spiral coil constituting at least a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality of anodes conductively supported from and positioned adjacent to the inner periphery of said spiral coil, each of said anodes being supported at a different point on said spiral coil, alternate anodes being supported lengthwise of said spiral coil at points on one side of the longitudinal mid-point thereof and the anodes intermediate between said alternate anodes being supported lengthwise of said spiral coil at points on the other side of said mid-point, said points being distributed about said periphery symmetrically with respect to said mid-point.

11. An electrical discharge device of the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors whereby said spiral coil constitutes a closed conductive extension of said conductors, a cathode positioned within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil, each of said anodes being conductively connected to a different point on said spiral coil, said conductors and said spiral coil being formed of a continuous length of hollow tubing whereby a coolant may be circulated therethrough.

l2. An electrical discharge device of the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors whereby said spiral coil constitutes a closed conductive extension of said conductors, a cathode positioned centrally within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral in generally cylindrical configuration about said cathode, each of said anodes being conductively connected to a different point on sain` spiral coil, said conductors and said spiral coil being formed of a continuous length of hollow tubing whereby a coolant may be circulated therethrough.

13. An electrical discharge device of the magnetron type comprising an envelope, a spiral coil within said envelope constituting a portion of a resonant circuit, a cathode positioned within said spiral coil, and a plurality of electrodes positioned adjacent to the inner periphery of said spiral coil, one of said electrodes being conductively connected to the longitudinal mid-point of said spiral coil, alternate electrodes from said one electrode being conductively connected longitudinally of said spiral coil to points on one side of said mid-point, and the intermediate electrodes between said alternate electrodes being conductively connected longitudinally of said spiral coil to points on the other side or" said mid-point.

14. An electrical discharge device of the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wail of said envelope and a spiral coil Within said envelope having ends conductively connected each to one ci' said conductors, a cathode positioned centrally Within said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil, each of said anodes being conductively connected to a dierent point on said spiral coil, alternate anodes being connected to points on one side of the longitudinal mid-point of said spiral coil and the anodes intermediate said alternate anodes being connected to points on the other side of said mid-point.

15. An electrical discharge device of the magnetron type cen sing an a transmission line including a pair of conductors eX- tending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors, a, cathode positioned along the axis of said spiral coil, and a plurality of anodes positioned adjacent to the inner periphery of said spiral coil in generally cylindrical conguration about said cathode, each of said anodes being conductively connected to a diilerent point on said spiral coil, alternate anodes being connected to points on one side of the longitudinal midpoint of said spiral coil and the anodes intermediate said alternate anodes being connected to points on the other side of said mid-point.

16. An electrical discharge device of the magnetron type Comprising an envelope, a transmission line including a, pair of conductors extending through a Wall of said envelope and a spiral coil Within said envelope having ends conductively connected each to one of said conductors whereby said spiral coil constitutes a closed conductive extension of said conductors, a cathode positioned centrally within said spiral coil, and a plurality of anodes positioned in uniform angular distribution around and adjacent to the inner periphery of said spiral coil, alternate members of said anodes being conductively connected to diiierent points on one side of the longitudinal mid-point of said coil, and the intermediate anodes between said alternate members being conductively connected to diierent points of the other side of said mid-point.

17. An electrical discharge device of the magnetron type comprising an envelope, a transmission line including a pair of conductors extending through a wall of said envelope and a spiral coil within said envelope having ends conductively connected each to one of said conductors whereby said spiral coil constitutes a closed conductive extension of said conductors, a cathode positioned along the axis of said spiral coil, and an odd numbered plurality of anodes positioned adjacent to the inner surface of said spiral coil in generally cylindrical configuration about said cathode, one of said anodes being conductively connected to the longitudinal mid-point on said spiral coil, alternate anodes from Said one anode being conductively connected to said points on one side of the mid-point ci said coil, and the intermediate anodes between said alternate anodes being conductively connected to points on the other side of said mid-point.

18. An electrical discharge device of the magnetron type comprising a spiral coil constituting a resonant circuit, a cathode positioned within said spiral coil, and a plurality of electrodes positoned adjacent to the inner periphery of said spiral coil, each of said electrodes being conductively connected to a different point on said spiral coil.

19. An electrical discharge device of the magnetron type comprising an envelope, a coil within said envelope constituting a resonant circuit, a cathode positioned within said coil, and a plurality of electrodes positioned adjacent to the inner periphery of said spiral coil, one of said electrodes being conductively connected to the longitudinal mid-point of said coil, alternate electrodes from said one electrode being conductively connected longitudinally of said coil to points on one side of said mid-point, and the intermediate electrodes between said alternate electrodes being conductively connected longitudinally of said spiral coil to points on the other side of said mid-point.

DONALD A. WILBUR.

REFERENCES CTE) The following references are oi" record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,009,368 Usselman July 23, 1935 2,147,159 Gutton et al. Feb. 14, 1939 2,247,077 Blewett et al .lune 24, 1941 2,399,114 Hansell Apr. 23, 1946 2,462,698 Wilbur Feb. 22, 1949 2,463,416 Nordsieck Mar. 1, 1949 FOREIGN PATENTS Number Country Date 216,807 Switzerland Jan. 5, 1942 522,360 Great Britain June 17, 1940 522,905 Great Britain July 1, 1940 696,368 Germany Sept. 19, 1940