US2455846A - Coaxial leadin for electron discharge devices - Google Patents

Description

Dec. 7, 1948.

J. w. WEST COAXIAL LEADIN FOR ELECTRON DISCHARGE DEVICES 2 Sheets-Sheet 1 Filed Oct.

lNl/ENTOl? J. W. WEST :4

A 7' TORNE v Dec. 7, 1948. .1. w. WEST 'COAXIAL LEADIN FOR ELECTRON DISCHARGE DEVICES 2 Sheets-Sheet 2 Filed Oct. 18, 1946 //Vl/EN7'OR J. W. WEST A T TOR/V5 V fiatented Dec. 7,

COAXIAL LEADIN FOR ELECTRON DISCHARGE DEVICES John W. West, Jackson Heights, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application October 18, 1946, Serial No.704,075

This invention relates to electron discharge devices and more particularly to such devices for generating high power energy in high frequency transmission systems.

In certain applications of power generatmg '5' electronic devices, the operating voltages for the electrodes are supplied through coaxiallines to obtain high efficiency and low loss in .the coopcrating circuits coupled to the devices. Since the conductors of the devices are mutually coupled to the coaxial lines to simulate connective continuations of the lines to the electrodes of the device, certain mechanical strain difiiculties usually arise in the hermetic seals of the tubular conductors of the device connected to the internal electrodes. These difiiculties occur particularly in the seals for high temperature electrodes, for example, the cathode, which is usually composed of a plurality of filamentary sections heated to flow in the operation of the device.

One object of this invention is to prevent deleterious temperature strains in the conductor seals of high power discharge devices.

Another object of the invention is to improve 25" vthe coupling of the electrodes in the device to coaxial lines to reduce high frequency current losses in the propagation of the energy supplied to the device.

A further object of the invention is to rigidly mount the electrodes directly on coaxial conductors sealed in one end of the device for coupling to coaxial lines with low capacity interconnections.

Another object of the invention is to facilitate the mounting of the filamentary cathode in the device to provide coaxial terminals coextensive with the terminals of the other electrodes.

A further object relates to relieving temperature stresses in the joints between metal and glass portions of the evacuated vessel whereby a long operating life is assured.

These objects are realized in a typical embodiment of this invention in an external anode type discharge device including internal cooperating electrodes which are concentrically mounted within the anode. The internal electrodes are individually connected to external circuits by metallic sleeve terminal conductors ofithe coaxial type which are sealed in nested relation to telescopic insulating sections disposed intermediate the con--' ductors, and the conductor assembly isjoined to the anode by an insulated housing portion to segregate the low potential conductors from the high potential anode.

The feature of this construction is the reduction 13 Claims. (Cl. 250--27.5)

of the length of the device for a predetermined power output while maintaining the desired insulation resistance between adjacent conductors. This is accomplished by mounting the coaxial sleeve terminals in stepped relation about a central terminal and connecting adjacent terminals together by elongated insulating sections sealed adjacent opposite ends of the respective metallic walls of .the sleeve terminals to provide the required insulation paths between the nested terminals.

Another feature of the invention relates to the mounting of the electrodes on the sleeve terminal conductors to provide a rigid assembly wherein 1 the internal electrodes are held in proper concentric relation and substantiallybraced against misalignment by external shock or vibration. In a specific embodiment, a pair of cylindrical electrodes extending into the anode and uniformly emission temperature to supply copious electron 20 spaced therefrom are individually joined to the outer pair of sleeve conductors by metallic conical shields which form barriers between the low potential conductors and the portion of the high potential anode extending within the housing of the device.

Important features of the invention relate to the mounting of the filamentary cathode in the device and the construction of the terminals therefor so that the high heating temperatures inherently encountered in the operation of the cathode do not adversely affect the composite hermetic seals at .the junction of the metal terminals and insulating sections. In this construcn tion the cathode terminals comprise a central plug member or standard surrounded by a metallic cup member having the closed end spaced concentrically from one end of the plug by an internal telescopic insulating wall section which is hermetically sealed to the inner end of the cup and the outer end of the plug member. The closed end of the cup member is provided with spaced radial slots to permit the closed portion of the cup to resiliently yield under expansion and contraction clue to temperature changes so that 3 the device by reason of the coaxial position of the sleeve terminals on the end of the device. The coaxial sealing of the terminals in the insulating sections of the vessel insures coaxial symmetry of the individual electrodes supported thereon in relation to the surrounding electron receiving surface of the external anode.

Still another feature of, the assembly relates. to the combination of the external anode with a large air-cooled heat radiator fixedly joined to the anode and forming a protective enclosure for the sealed exhaust tip on the end of the anode. This is accomplished by enclosing the anode in a metallic casing which provides a cavity or space for the sealing tip on one end of the anode and securing the anode in the radiator casing by fusible cadmium metal which also embeds the sealing tip in the metallic fusible mass in the cavity. The tip is, therefore, completely protected from injury and the metallic filling forms an ikltermediatemass of low resistance and high thermal conductivity which dissipates the heat ener y expended in the anode and efficiently transmits it to the radiator.

These and other features of the invention will appear from a consideration of the following ole-- tailed description when taken in connection with the. accompanying drawings which illustrate the construction of one embodiment of the invention. In the drawing:

Fig. 1 is an elevational view in cross-section of a typical construction of the device showing the assembly of the elemental parts;

Fig. 2- is'aplan view of the device of Fig. 1, with av portion only of the air-cooled radiator shown in full lines. showing the concentric arrangement of the conductors;

Fig. 3 is anenlar ed perspective view of the inal; portion; of the assembly with sections 1 e-seals broken away to illustrate the detailed con truction;

Fig. 4 is a planviewof the cup terminal detail employed in the device of Fig. 1- illustrating the configuration. of the closed end of the terminal cup;

5 is a similar plan view of the cup terminal in combine-t n with the central plug: terminal on the line 5- .,5 of Fig. 1 to show the connection. of the, filamentary assembly to these ermina and Fig. 6 is a perspective view of the filamentary cathodea-sd. terminal assembly em o ie in e device pith-is invention illustrated in Fig. 1.

R fer in to the drawings and particul rly to Fig. .1. the high power electronic ischarge device of-this invention s embodiedin any external anode air-cooled type of device having a heavy copper sleeve. or cylindrical portion 10 which forms the major section of the enclosing vessel and for convenience of, assembly is provided, with a separate closure plate i], also of copper, which is brazed to. the bottom. of the cylinder by a high melting point solder. The closure plate H is provided with a central aperture to receive a metallic .exhaust-tubulation I 2, the tubulation having a. flared; inner edge brazed to the inner surface of the closure and the outer surface of the closure being brazed to the tubulation, where it projects therefrom, by solder seal to prevent at the joints between these members. The:tubulationv permits the pumping and exhaust of: the internal discharge space of the device after complete assembly to secure a high vacuum. Th tr bulation may be sealed oif at l3 after the desiredlow pressure is secured in the device.

4 The opposite end of the anode is machined on the outer surface to slightly reduce t e outer diameter to provide an internal tapered edge and a positioning seat adjacent the outer boundary of anode for a metallic flanged ring 14 which is brazed to the copper anode at the outer surface of the anode. The ring 14 is preferably formed of steel having a. copper plating applied thereto and the flanged ring supports a flared sleeve l5, preferably of a low expansion metallic alloy having the composition of approximately 28 per cent nickel, 18 per cent cobalt and the remainder principally iron. The sleeve I5 is mounted coaxially about the reentrant portion N5 of the anode and the flared portion extends divergently with respect to the tapered rim of the anode. In the assembly of these parts to the anode section, it is preferable to secure the sleeve l5 tothe flanged ring i4 prior to mounting the ring on the anode. This is accomplished by inserting a copper wire ring within the sleeve at the junction of the sleeve to the ring and heating the copper wire to fusion temperature to flow copper into the joint between the sleeve and the flange of the ring. The ring and sleeve assembly may then be brazed to the anode body. The nickeliron cobalt alloy is preferred for the sleeve since this metal has a coefficient of expansion similar to glass or vitreous bulbous portion I! of the enclosing vessel which is hermetically sealed to the flared rim of the sleeve IS. The bulbous section 17 is preferably formed of a low expansion,

- mitti-ngenergy supplied by the'device, it is essential that the couplingyof the device to these lines have a minimum energy loss component and avoid. the introduction of extraneous capacity and impedance components so that the characteristics of the coordinatedlines and device may be maintained at prescribed values. This is accomplished y f rming the electrode terminals of the device separate sections of the coaxial lines and mounting these. sections in cooperative relation to the internal electrodes of the device and the insulating portions of the vessel so that these t rminal. sections intcrmesh with the lines and, in ef ect, form direct .continuations of the lines to the electrodca. However, in order to achieve this result, it is essential that the interrelation of the terminals with the device produce the required insulation resistance between the several electrodes to derive efiicient coupling and avoid theintrod-uction of temperature and mechanical strains or stresses in the terminal seals between the'conducti-ve and insulative .parts of the device.

The-realization of'these factors constitutes one feature of this invention. The terminal conductors of the device of this invention are grouped in coaxial relation on the insulating section I l of the vesselso that the coaxial lines may be readily combined with the device to couple the cooperatminals or conductor-s for the electrodes in the of J. E. Clark and V. L. Ronci.

device are a group of multiple, nested, sleeves or cylinders l8, I9 and 20 mounted in stepped and coaxial relation about a central terminal or standard 2|. The several conductors are insulatingly spaced in proper position by insulating walls which are mounted intelescopic relation between the sleeve terminals, to form a completely enclosed containing vessel for the internal electrodes. The sleeve terminal conductors are concentricallymounted both with respect to each other, with the axial standard 2| and the glass vessel section I! by hard glass bell-shaped sections 22, 23 and 24 which are interposed in telescopic relation between adjacent sleeve conductors I8 and I9 and 20 and between the interior of conductor 20 and the central standard 2 The glass bell-shaped sections and the sleeve conductors are arranged in nested relation to reduce the over-all length of the device, to provide internalcoaxial supports for the electrodes within the device and external contact portions for coupling to coaxial lines for supplying the operating voltages to the internal electrodes. In this construction, the sleeves progressively extend from the terminal end of the device in stepped relation so that the sleeve conductor l8 projects a short distance beyond the end of the glass vessel section H, the sleeve 20 extends the furthest from the end of the vessel and the sleeve l9 extends in a position intermediate the respective terminals on opposite sides thereof. The central standard 2| and the respective sleeve terminals Ill to 20 inclusive, are preferably formed of the nickel-iron-cobalt alloy similar to the sleeve l5 so that these terminals can readily be sealed to the borosilicate glass sections interposed therebetween and will have temperature coefiicients matching the coefiicients of the glass sections. The outer end of the glass section 24 is hermetically sealed to the cylindrical surface of the central column or standard 2| adjacent the outer end while the inner end of the section 24 is sealed to the inner wall of the sleeve 20. The latter is provided with a platform closure portion 25 having a central opening or aperture 26 through which the central standard 2| projects, the flared end of the glass section 24 being sealed to the inner wall of the sleeve 20 adjacent the platform 25. The bell section 23 is sealed to an intermediate portion of the outer wall of the sleeve 2|! and the flared end is sealed to the inner wall of the sleeve I9 adjacent the inner end thereof. The bell section 22 is sealed to the outer Wall of the sleeve l9 adjacent the outer end and the flared rim of the section is sealed to the inner wall of the sleeve |8 adjacent the inner end. The combined terminal assembly is sealed to the glass section H by joining this glass section to an outer intermediate wall portion of sleeve l8. The inner flared seals of the bell-shaped glass sections to the respective sleeve terminals may be performed in accordance with the method disclosed in the application Serial No. 434,520 filed March 13, 1942 The central standard 2| may be provided with a copper terminal head 21 which is brazed to the outer end of the standard by a high melting point solder, such as 20 per cent silver and 80 per cent copper which has a melting point of 910 C. the head being flush with the end of the sleeve terminal 20. It will be noted, in the nested combination of the insulated spacer sections and the sleeve terminals that the conductive sleeves form shield- .ing-members between successive insulating spacer section s,. not only to protect the insulating sec- '6 tions from conducting charges within the discharge space but also to protect the Sect n against mechanical injury from external causes.

In the device as shown in the embodiment of the invention, a pair of coaxial grid electrodes are mounted within the anode in coaxial spaced relation thereto and also to an enclosed electron emitting source in the form of a multi-strand filament assembly. The composite sleeve terminal assembly not only intermeshes with the coaxial lines of the system but also on the inner end rigidly mounts the internal electrodes in coaxial relation to the anode surface. The cathode assembly includes four strands 28, 29, 3|] and 3|, as shown in Fig. 6, which are formed of thoriated tungsten ofsufiicient diameter to form a self-supporting structure and capable of consuming the current supplied thereto to raise the strands to high temperature for the emission of electrons across the discharge space, to the anode. These strands are provided with gooseneck end portions 32 which are joined together at the center bya tungsten helix 33 which welds the four sections into a box-like frame to maintain the strands in uniform lateral relation.

The strands are interconnected in parallel relation to the central standard 2| and the platform 25 of the cup-shaped sleeve 2|] by a pair of molybdenum studs 34 which are welded in sockets 35 in off-center relation in the top surface of the standard 2|. Another pair of molybdenum studs 36 are mounted in off-center relation on the platform 25 in apertures 37 of the platform and are reenforced at the junction with the platform by nickel eyelets 38, to improve the welded joint at the thin wall of the platform 25. The filament strands are joined to the stud rock by embracing molybdenum collars 39, to rigidly support the frame filament assembly from the platform and central standard.

A cylindrical control grid surrounds the filament assembly and is formed of upright spaced molybdenum wires 40 having a uniformly pitched helix 4| of small diameter molybdenum wire wound around the support wires and welded thereto to form a cylindrical grid structure. The lower end of the grid structure is reenforced by a heat shield or disc 42, for example, of tantalum, andthe opposite ends of the support wires 40 are welded to a nickel collar 43which is attached to the inner surfaces of the support wires and forms a base ring support for the grid structure. The control grid assembly is mounted in coaxial relation to the cathode assembly by a truncated conical shield 44 of nickel-iron-cobalt alloy having a turned ring portion 45 at the smaller diameter end which embraces the terminal end of the grid assembly and in conjunction with the ring 43 welds the grid frame to a rigid mounting assembly. The shield 44 is also provided with a turned rim 46 at the large diameter end" which extends around the inner rim of the sleeve terminal |9 and is Welded thereto.

A screen grid or accelerating electrode coaxially surrounds the control grid and is formed of a plurality of upright support wires 41, preferably of molybdenum, supporting a uniformly spaced grid helix 48, also of molybdenum wire having the same diameter as the helix 4| on the control grid. The inner end of this grid is closed by a tantalum disc shield 49 and the supporting end carries a nickel sleeve or rim 50 on the inner surfaces of the support Wires 41 to assure the cylindrical alignment of the grid supports. The accelerator grid is coaxially spaced around the 7 ontrol grid and uniform]yspacedfrom the inner surface of'the-anode l by-a truncated Cb'n'i al shield of 'nickel 'iron -cbballt alley;"havln'gi rin'g'extens'ion 5-2 at the "smalle" diameter 'and embracing the" grid support To ds 4 "and the associated nickel ringfill "to whiclf'it vlrelded. The *larger diameter end (if the shield "is pro- "vided'with a flanged "lip 53 which "embraces the inner "end of the'sleev'e conductor ligand iswelde'd thereto. This arrangement prdvid'es eo'afiial sym' metry'for the mounting-of the interri'aie eetro'des in the anode andrigid'ly'maiii'taii'i's the elettrodes in their prescribed r'ela'tion' "regardless of tem: perature'variations in the d'eyl'ce; 'The fco akia-lly spacedeonio'a'l shields also' -forni su'dcessiy'ebarrier's between the tapered-edgeoithe anode-6X tending withinthe-vessel'hf the de'v'iean'd'the low potential conductors-of the Cathode assembly so thatarcin'g is prevented"aeross this fiatliwithrespect to the a-n'edesurfaee and the'rebymaf 1 terially prolongs the? operating -'-life of the device due to the absenceof 'sjhofts between the Closely spaced elect'rbdes within the enclesing*vessel. Anotherin ipoit'aht feature of the 1 "vention -rem the glass tofmetal seals at'the 'junct'inof the glass -section '24 and "the "cup' shap'ed terminal member in view O'f the high temperaturein posed on these joints; -partiulafly at'thje inner joint close "to the '='conduotingj studs of 'the'ffila' me'ntafy assembly. The stresses-and'stfairi's alttendant upon varying temperature'conditions at the seale'd jointare' overcomefin aocoldande ith a feature of this invention, b nna-5g thefli igtform substantially yieldable sotha'ttenipera' ture changes I are dissipated "in the ineta'l f'rather than being transmitted to the hermetic Seal" at the inner end of" the ten-sha d" ectio n za. The top of the p1atfo1"m"2'5 is firoirided withspaced circular openin'g st lfas shown in-Figsflifand '5 connected with the large di anieten'central'opening -rm by short slots vyfhih'xtehdf'iadi ally between the "larger diameter 'openi'n'gsin "the platform. In addition to-these" slots the outer boundaries of the apertures 54 are sl'ott d as shown at 56,'and theseslots; as sho uinfin F gi3 extend beyond the edge'o'f the pas oral and partially along" the "cylindiioal' sur'facie of the sleeve 'p'ei'tion' 20 of "the terminal. Thejradiai slots and 'spaced" apertures across the transverse dimension of the-platform 25 pro ide vi wable segments in the 'ciip 'shapd t'ermtnal 'f the cat'l'iode"assenibly"which expand and eont act with temperature changes in the high temperature structure'of the cathode and thereby p 'e vent such changes, ortheeffets of suoh changes, reaehing' the her netie seal adjacent the inner end] of the-Cup terminal. Henbejleaks' ot fiiaetures 0f the glass are eliminated and' thefjyacuilm type s'eal'of the-glass joint ispi e' s'eiived'to in: sure maximum operating life for'the "combihed structure; W v I I I The heat energy gneiated injthe anode due to the 1 high operating pote'ntial'itiesfof the device is efiicientlydissibated hy "air-eoolihg and thi s involves a large surfacej adiator assembly'xyhich completely encloses theouteranodeseetifin ofthe device. This radiato inclu'de s' a c'oppefisleeve casing -51 of slightly larger diametetfthan the ext'efnaldiamete'r of theandde; "The sl'eve pbr fiioiibfthe CaSiiig abuts againstthe flanged "TIM 7 M on the *anode', the sleeve being of sufitin't length to form a proteeting WalI arQuiid' the sealing tip l'2-'*of theanode; The' cas'ing i's"si1rmoundedty "a cylindrical "assembly" of "spaced coppefradiatihg fihs 58 "whiChmaybe seeui'e'd to thec'asing =51;in-accordanee with themetnods disclosed in' ihyp'rior'Patent 2,341,752," issued-Febru'ary" 15, 19441 The radiator assembly is s'e- Cured to the *a'nodeby a fusible"fi1lirig"of *i'pw thermal resistance "and high thermal conductivity metal 5 9 ,"suclras cadmium, haying a -i'xielt' mg "p'oint' of 1 approximately 320 0., which fidw's between 'the' walls of the casing "and Y anode to ri idly "affix theradiator tothe' a ode and forms a solidr'n'ass "of 'nie'tal"'aroiihd the sealing tip (2 'Whl'h 'is-embedde'd in the massand plotebted from" injury fr'om external causes; The bottom of thehasih'ginaybe closed by acoVerBOflQlso of "bpper; to "provide a finished appeal'anlfe for theassemb'ly. I

While "the invention "has been disclosed "with iSpet 0 2 513601116 mbfidili'iht' of related "E trades in theuevice; it is of c'ourseyun'derswod thatvari'ous' modifications may be 'li'nade in the number of electrodes combined in thejassembiy orih thenblnb'inati'on of the *detail'ed parts' of the construction Without departing from th'fscope of the "invention as defined in bhe appended cla ms;

'Wh'atis clair'n'edis: I H 1. A'n el'e'ctro'n dis(311ait??? 'uevic'e cdmfirlsinfg vessel elm-losing "a plurality'of eleotfodesfleono'ent'fi'c tubular 'ooi'iduetors 'ihsulitii'i'glftaild to said-"vessel; said electrodes being' eonnecte tbfsaid tors'and yieldable means extendin acf'dss one at "said cond'uetors to relieve tempeiiap r'e st'rain'sin the'sealed insulating j'ointadjaeeht the junotfon withsaidone'ndu'ctorg I 2. An." electron discharge "devit'e'combn'sing vessel enclosing plurality-0f electrodes,jconcng t'rictubular 'e oii'duct'ors insulatingly sealed to sf'ai d tresselffsaid electrodes being conneoted to said conductormnd the-inneftubu'iaic nductor? avi ing inte''ia'l transverse *pnrtion eateiqidi g mass-15h end, said transverse p 'tionfhajinga pluralit'y'of yielda hle' segniehts t i ete' ten pe1f+ atdie strains ih"the"sfea1'ed' insulat g 'j oi nt adj cent thefiunctio with saidinnei con ll'CtQIt, I i H ing said terminal to said anode portion, an electrode within said anode portion and supported by said terminal, a cathode axially mounted in relation to said electrode, a cup-shaped terminal coaxially within said tubular terminal, a central solid metallic terminal extending through said cup-shaped terminal, said cathode being connected to said cup-shaped and central terminals, and interconnecting insulating sections sealed between said central, cup-shaped and tubular terminals and surrounded thereby.

6. An electron discharge device comprising a hollow metallic anode portion, a tubular conductor, an insulating enclosing vessel portion joining said conductor to said anode portion, an electrode within said anode portion and supported by said conductor, a cathode axially mounted in relation to said electrode, a cup-shaped terminal coaxially within said tubular conductor, a central solid metallic terminal extending through said cupshaped terminal, said cathode being connected to said cup-shaped and central terminals, and interconnecting glass cup sections having opposite ends hermetically sealed to the respective adjacent walls of said conductor and terminals within and between which they are situated.

7. An electron discharge device comprising a vessel enclosing a plurality of electrodes including a multi-section filamentary assembly, concentric conductors insulatingly sealed to said vessel and supporting said electrodes, one of said conductors being an inverted cup-shaped member having a central aperture, alternate sections of said filamentary assembly being attached thereto, a central plug member extending through said cupshaped member and supporting the remaining sections of said filamentary assembly, and a vitreous seal portion extending between said cupshaped member and central plug, said cup having peripheral slots therein to relieve stresses in the vitreous seal adjacent thereto.

8. An electron discharge service comprising an enclosing vessel, a plurality of electrodes therein, concentric cylindrical conductors at one end of said vessel, said electrodes being individually supported by said conductors, and bell-shaped glass sections interposed between said conductors, the inner ends of said sections being sealed to the interior inner walls of said conductors and the outer ends being sealed to the exterior outer walls of adjacent conductors.

9. An electron discharge device comprising an enclosing vessel, an anode, a filamentary cathode within said anode,aterminal support forsaid cathode including a metallic cup-shaped member having a central aperture, a metallic conductor extending through said aperture, connectors joining said cathode to said member and conductor, and insulating sections forming part of said vessel sealed between said conductor and cup-shaped member and between said cup-shaped member and vessel, said cup-shaped member having spaced openings adjacent said central aperture and radial slots extending from said aperture and beyond the closed edge of said member.

10. An electron discharge device comprising an enclosing vessel, an anode, a multi-strand filamentary cathode within said anode, a terminal support for said cathode including a metallic cup-shaped member having a central aperture, a metallic standard extending through said aperture, insulating sections forming part of said vessel sealed between said standard and cup-shaped member and between said cup-shaped member and said vessel, and a pair of rigid supports extending from each said standard and cup-shaped member and joined to said multi-strand cathode.

11. An electron discharge device comprising an enclosing vessel having a hollow anode portion, a metallic sealing tip extending from said anode portion, cooperating electrodes within said anode portion, conductors sealed on one end of said vessel supporting said cooperating electrodes therein, a protective casing surrounding said anode and tip, a plurality of radiator fins mounted externally on said casing, and a solder filling joining said casing to said anode and embedding said tip within said casing.

12. An electron discharge device comprising an enclosing vessel having a hollow anode portion, an annular flange member afiixed to said anode adjacent one end, a metallic sealing tip extending from the opposite end of said anode portion, cooperating electrodes Within said anode portion, conductors supporting said cooperating electrodes therein, an insulating portion of said vessel sealing said conductors to said flange member, a metallic casing surrounding said anode and having a cavity enclosing said tip, said casing being in contact with said flange member, a filling of cadmium metal joining said casing to said anode and embedding said tip in said cavity, and a closure for said casing adjacent the filling mass of metal.

13. A connector for electron discharge devices comprising a plurality of coaxial cylindrical conductors disposed one within another and in laterally spaced relation, and a plurality of insulating spacers each disposed between two adjacent conductors, each of said spacers including an elongated cylindrical portion spaced from the respective adjacent conductors and end portions sealed hermetically to the respective adjacent conductors.

JQHN W. WEST.

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

UNITED STATES PATENTS Number

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