US2421767A - Electrode structure - Google Patents

Description

June 10, 1947. s. F. VARIAN 2,421,767

ELECTRODE STRUCTURE Filed Nov. 30. 1945 2 Sheets-Sheet 1 INVENTOR ISIGURD F VQR/fl/V BYE/ ATTORNEY June 10, 1947. s. F. VARIAN 2,421,767

ELECTRODE STRUCTURE Filed Nov. so, 1945 2 Sheets-Sheet 2 Lilith/ I! lli ll INVENTOR ATTORNEY Patented June 10, 1947 ELECTRODE STRUCTURE Sigurd F. Varian, Garden City, N. Y., assignor to Sperry Gyroscope Company, Inc.,' a corporation of New York Application November 30, 1945, Serial No. 632,032

14 Claims. 1

electrodes was encountered. The development of the bombarded type of cathode aided in meeting the problem of supplying a large number of electrons. In this type of cathode a potential difference is applied between a filament and an emissive element so that those electrons that are emitted by the filament are attracted to the emissive element, striking it with sufficient force to raise it to a temperature of efficient electron emission. The electrons which are then emitted by the heated element are utilized inany desired manner in the discharge tube. In order to prevent high voltage flashover between lead-in pins, great care and thought were used in the spacing of base pins and in selecting better insulating material in the construction of the base. However, in attempting to solve the problems of both large currents and high voltages, other difliculties were encountered. It was found that the bombarded type of cathode structure was not sufiiciently rigid to prevent microphonics. Furthermore, the large cathode and heater currents that were used caused severe overheating of the base of the tube, which resulted in cracking of the glass envelope near the base, especially where the lead-in-pins were sealed to the glass envelope. Also, as better insulation was developed, the voltage at which the tube was designed to operate was increased, with the resulting increased danger of flashover reoccurring. Another disadvantage that was encountered was the modulation of the electron beam due to the voltage in the bombarded cathode circuit induced by the magnetic field of the alternating current which was usually supplied to the filament. The electrode mounting and cathode construction forming the present invention overcomes these disadvantages of prior devices while retaining large current and voltage capacity.

It is, therefore. an object of this invention to provide an electrode mount which is rigid and substantially free from microphonics.

It is another object of this invention to provide an electrode mount especially useful as a cathode construction, which can handle large currents and yet effectively dissipate much of the undesired heat produced.

It is still another object of this invention to provide an electrode mounting structure which can handle large voltage differences between elements without flashover.

Still another object of this invention is to provide a cathode construction which effectively re- 2 duces the magnetic field of the filament so that little or no voltage of the filament supply frequency is induced in the cathode circuit.

Still another object of this invention is to provide an electrode-supporting structure capable of handling large currents and having a lead-in connection of suflicient cross-section so as to prevent overheating.

In accordance with the present invention, there is provided an electrode-supporting structure especially useful in cathodes, of novel design which offers great mechanical rigidity and increased heat dissipation. In this structure the two filament leads are brought out through the base of the tube respectively by means of a central rod and a tube coaxial with the rod. A sec. ond outer tube, also coaxial, provides connection with and support for the bombarded cathode button. These three coaxial members serve to provide both electrical connection and heat conduction for the members to which they are attached and which they support. Each member is rigidly joined to a flat disc at the tube base. The three discs are supported by and sealed to a tubular glass member providing a rigid structure. In addition to its rigidity, the structure offers the advantage of improved heat conduction and in creased electrical insulation between members. The structure also provides a large lead-in contact area which is an advantage in making electrical connection and preventing local heating in the glass-to-metal seal.

The invention in another of its aspects relates to novel features of the instrumentalities described herein for achieving the principal objects of the invention and to novel principles employed in those instrumentalities, whether or not these features and principles are used for the said principal objects or in the said field.

A further object of the invention is to provide improved apparatus and instrumentalities embodying novel features and principles, adapted for use in realizing the above objects and also adapted for use in other fields.

Although in the following detailed description of a preferred embodiment of the invention, the improved electrode mount is shown as used in a bombarded cathode construction, it will be understood that this is for illustrative purposes only, and that the invention is not limited only to cathodes.

The invention and the above-noted and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawings in which:

Fig. 1 is a longitudinal view partly in cross-section of an electron discharge tube suitably embodying the present invention;

Figs. 2 and 3 are detailed perspective views ii and [1.

partly in cross-section of subassemblies o! the cathode structure shown in Fig. 1;

Fig. 4 is a cross-sectional view of the complete cathode structure of Figs. 1-3;

Fig. 5 is a plan view partially cut away looking in direction of arrows H of Fig. 4;

Fig. 6 is a cross-sectional view or. an alternative cathode structure according to the present invention; and Pig. 7 shows a longitudinal view partly in crosssection of another embodiment of the invention.

Referring in particular to Fig. 1, there is shown an electron discharge tube ll of the cathode ray" type employing an embodiment of this invention. At the base of this tube there is shown a bombarded cathode structure using the improved electrode support 0! the present invention. The filament leads ior the cathode are brought out through the base of the tube respectively as a central rod ii and a tube l2 coaxial with and surrounding the rod. A second coaxial outer tube It surrounding tube i2 provides connection with the bombarded cathode, which is supported inside the metal cylinder ll, whose diameter may be the same as that of the outer tube II. This metal cylinder I4 is rigidly supported on and spaced along the axis from the outer tube II by means of metal V-shaped members II, which are fastened to the racing peripheries or the cylinder I4 and the outer tube ll, as shown more in detail in Fig.2. The three coaxial members II, I! and it to serve to provide both eirective electrical connection and efficient heat conduction and dissipation for the member to which they are attached and which they rigidly support. For this reason it is desirable that they be iormed'oi a metal such as copper which has good thermal as well as electrical conducting pr p rties. The two tubularmembers l2 and II are rigid y Joined respectively to annuli or discs it and I1, in the base of the tube. Discs II and 11 may be made of Kovar" or some other metal readily scalable to glass. The central rod I I is fastened to the center or a similar disc it. The discs II and II are sealed vacuously to an insulating tubular glass member I! which rigidly spaces and connects these discs. A similar tubular glass member is sealed between and serves as a spacer for discs The main glass envelope II of the electron discharge tube in is sealed vacuously to the remaining side of the disc H. In manufacture it has been found that an emcient method oi sealing the glass members I! and 20 to the discs i8, i1 and I8 is by the known electronic induction method. Slots ti and 33' running longitudinally from the end of tubes I2 and il where they connect to the discs II and H are provided for the purpose of preventing buckling oi the tubes during the electronic sealing process. These slots also prevent similar damage during normal operation, should excessive heating be encountered.

The discs il, l! and II serve in making electrical connection to the tube. Their large area makes such electrical connection emcient, especially in view oi! the large currents which they are adapted to carry. Also their circular shape allows clamps to be used on the end or the leadin wires which fasten to these discs, thus making a positive and safe electrical connection. Since the discs are relatively thick (from 1*; to inch) they also serve as excellent heat conductors to the outside air, keeping the metal-to-glass Joints at a safe operating temperature and preventing cracking of the glass envelope.

The internal structure of the cathode appears more clearly in Figs. 2, 3 and 4. Disc 17 is rigidly fastened to one end of outer tube ll. Metal cylinder I4 is supported by a plurality of V- shaped members ll irom the other end of outer tube II. In the embodiment shown, these supporting members II are formed of wire arranged in a V and having a small, circular cross section, but they may be of a variety of forms, such as small iiat rigid strips or metal or a plurality of parallel rigid filamentary rods, In operation they serve to support cylinder ll rigidly at a suitable distance from the outer tube II. This construction provides a good electrical and rigid, nonmicrophonic mechanical connection between members It and I4, while keeping the heat transfer therebetween at a small value, since the cross section of the metal is small. providing large resistance to heat flow therealong. The principal electron-emitting element 22, whichin this embodiment is in the shape of a circular plate or disc having a diameter slightly less than the inner diameter of cylinder II, is mounted concentricalLy inside metal cylinder ll by a plurality of small supporting members II. The supporting members 23 may be rods or strips which are fastened to the inner wall of cylinder II and are Joined to the edges oi emitter disc 22, keeping its edges spaced equidistant from the inner wall of cylinder I4 and at the same time providing a rigid mechanical support and an emcient electrical connection. Emitting disc 22 may be formed of tungsten, molybdenum or tantalum, or some other material capable of withstanding high temperatures and adapted to emit copious quantities oi electrons. It may be coated with an activating material such as thorium oxide for increasing the electron supply, if desired. Guard ring N, whose operation will be described later, i'smounted inside and near outer end of cylinder Fig. 3 shows the details of the filament structure. In this embodiment a bifllar spiral filament 2| has its ends 20 and 2! brought out normal to the plane of the spiral. These ends it and 21 terminate in conductive rods II and II. Rod ll extends down along the axis oi tube It terminating at disc II to which it is firmly attached, as by welding. Rod 28 is rigidly fastened to the inner side 01' tube II which in turn is attached to disc I. Spiral filament 2!, which may be constituted of uncoated tungsten or tungsten coated with an activating material such as thorium oxide, has its plane normal to the axis of tube II. By keeping the length of fllament ends 20 and 21 short, it is possible to provide a substantially rigid suspension or the filement 2!, thereby further aiding in minimizing troublesome microphonics. Metal cylinder 2!, whose diameter is preferably the same as that oi tube l2, surrounds filament II and is supported at a suitable distance from tube It by V- shaped members 30 in a manner similar to metal cylinder i. As stated before, the shape of these members is not limited to a V, but they may be parallel rods or strips. Mounted parallel to the plane of filament 2! and on the side nearer to disc ll are two flat metal semi-circular plates 3|. These plates II are supported rigidly to the inner wall of cylinder 2! by supporting members II, which may be rods or strips preferably I formed integral with plates II. The dimensions or these plates II are such that when they are mounted in place inside metal cylinder 2!, only a small gap 32 is left between their racing edges.

This gap 52, shown clearly in Fig. 5, permits filament ends 28 and 21 to pass through the plane of the plates II without causing an electrical short-circuit thereof.

Since the outer diameters of tube I2 and oi! metal cylinder 29 are less than the inner diameters of tube I9 and cylinder I 4, the filament structure can be mounted coaxially inside of the emitter support. In this manner, as is clearly seen by reference to Figs. 4 and 5, filament 25 is brought in cooperative relationship with the lower surface of emitting disc 22. Heating current is supplied tofilament 25 through discs I5 and I8 and conducting rods II and 28, the latter acting in cooperation with inner tube I2. Upon passage of current through the filament 25 and upon application of suitable potential between the filament 25 and the principal emitter 22, an electron discharge "-bombards emitter 22 sufilcient to raise and maintain the emitter disc 22 at a proper electron emitting temperature (say at 1600 C.).

In order tolmaintain the emitter disc 22 at this emitting temperature, it is necessary to minimize the loss of heat from the emitter support. For this purpose, metal cylinder 29 serves as a heat shield, as well as to direct the electron emission from filament 25 against the lower surface of emitting disc 22. Since the cylinder 29 is electrically, as well as mechanically connected to tube I2, it operates at the same uni-directional potential as the filament 25. Thus, there is no electrical field between the filament 25 and the cylinder 29. Instead the electric field which draws the electrons away from the emitting filament 25 toward emitter disc 22 is directed substantially along the axis of the structure.

It is the purpose of metal semi-circular plates 3| to further aid in directing the emission from filament 25 in the direction toward emitter 22. These plates 3| are also maintained at the same uni-directional potential as the filament 25 by their conductive supports 34. This arrangement succeeeds in reducing the flow of electrons from filament 25 in the direction toward disc I8. A further purpose of plates 3| is to act as a heat reflector; acting in thermal cooperation with cylinder 29, they succeedin keeping the ambient temperature high in the region of filament 25 and lower surface of emitter disc 22.

The V-shaped members 30 provide rigid support for cylinder 29 and in turn for plates 3|;

yet, their small cross section minimizes the thermal conduction from cylinder 29 to tube I2. At the same time, by virtue of the large cross-section of tube I2 and rod I I, relatively large heating currentsmay be supplied to the filament 25 without overheating these leads so as to impair the vacuum seal. Furthermore, discs I8 and I6, to which these leads are attached, also have large areas which aid in dissipating heat that may be developed in tube I2 and rod II. Thus, this construction succeeds in keeping the region of the glass seals relatively cool while permitting the cathode to operate at a high temperature.

To further aid in maintaining the region of filament 25 and lower surface of emitter disc'22 at a high temperature, metal cylinder I4 is mounted in the same manner as cylinder 29. The V-shaped members I5 provide a rigid support for cylinder I4 and tube I3, yet their small crosssection minimizes the thermal conduction from cylinder I4 to tube I2. Not only does thi construction aid in maintaining a high temperature for emitter disc 22, but also, by virtue of the low 6 a thermal conduction of members I5, tube I3 operates without overheating and endangering the glass seal to disc II.

This is true even though the cathode currents are extremely large. Tube I3 not only conducts heat readily to disc I1, which is then dissipated into the outer air, but by virtue of its large sur/ face area, acts as an eificient radiator of heat. This radiation efiect may be improved by sand blasting the outer surface of the tube and coating it with "aquadag or other carbon black.

Since the distance between the cathode leadin disc I! and the nearer filament lead-in disc I I5 is determined by the length of tubular glass member 20, the problem of fiashover between the usual cathode pin and filament pins is eliminated; that is to say, the length of the glass tubular member 20 can be increased in manufacture, if the particular tube in which this cathode structure is to be used is to have a very high potential between the cathode and the filament.

In most applications of bombarded cathodes, it is highly desirable that the emitted stream of electrons be not modulated by the filament current. This modulation can occur in several ways. For example, the magnetic field of the alternating current which usually energizes the filament may induce a voltage in the cathode circuit. As another source of such modulation, some of the electrons which are emitted from the filament may actually find their way into the main electron beam. In the present construction the coaxial relationship of the filament supply leads II and I2 confines the magnetic field entirely within the tube I2. Thus, no voltage is induced into the cathode lead by the filament leads. To prevent electrons emitted by the filament from passing by the edge of the emitter disc 22, a guard ring 24 is provided. This is a ring which is fastened inside cylinder I4 just beyond the emitter disc 22. Its inner diameter is slightly less than the diameter of the emitter disc 22. Thus, electrons which might tend to pass beyond the edge of emitter disc 22 are intercepted by the guard ring 24, keeping the main electron beam free from those electrons emitted by the filament 25 and thus preventing undesirable modulation.

From the foregoing, it is seen that this construction provides a cathode which is capable of handling large cathode currents as well as high potentials between the cathode and filament Without many of the former shortcomings of such a cathode. The coaxial construction provides a method of supplying large currents to the oathode and filament without overheating of the leads and lead-in connection, thus eliminating the possibility of cracking the glass envelope. At the same time, the mechanical rigidityv of this construction eliminates microphonics by keeping the vibrations of the filament 25 to a'minimum. The large area and substantial thickness of the leadin discs I6, I! and I8 not only provide a method of efficient heat dissipation, but at the same time provide a large contact area for making electrical connection, which is an advantage when handling large currents. By separating the leadin discs I6 and I! an appreciable distance, large potential differences may be applied without the usual danger of high voltage fiashover.

It is readily seen that the concentric type of lead-in construction which is the subject of this invention offers a veryrigid mounting for the filament and emitting surface. If a structure is desired that is even more rigid than the above, the construction of Fig. 6 may be used. This structure employs the same three lead-in discs l6, H and i8 and cylindrical members II and I: which are fastened respectively at one end to discs I6 and IT. The inner conductor H in this construction is an aluminum rod which rests snugly inside tube 12 which is also preferably formed of aluminum. Before assembly these aluminum members II and I! have their surfaces anodized. The anodized layer thus serves as effective insulation between rod H and tube l2 across which is impressed the filament heating voltage, but this insulation in no way affects the snug fit between the two members. The filament 25 has its ends fastened respectively to the end of rod ii and to tube II, as before.

In a manner similar to the cathode structure of "Fig. 2, the emitting disc 22 is connected to cylinder 14 by supporting members '23. Cylinder i4 is mounted on outer tube II by the struts or filamentary members l5.

Thus, this arrangement offers a very rigid mount for the filament and emitting surface, and

reduces microphonics to a point where it is not troublesome. At the same time it retains many of the desirable features of the construction shown in Fig. 4, such as efficient heat dissipation, which removes the danger of cracking the glass envelope; large area of the lead-in discs, which allows for good electrical contact to the lead-in wires; and elimination of the voltage fiashover problem.

Although the embodiment of the cathode structure shown in Fig. l is sealed to a tube ll having a glass envelope, this type of cathode construction may be used equally well in metal envelope tubes. As shown in Fig. 7 the cathode lead-in disc I! may have a tubular glass member 35 sealed to its upper end. To the opposite end of glass member 35 is sealed a metal annular member 36 which has a side wall 31 and flange 38. The metal envelope 39 of the discharge tube 40 may then be welded vacuously to flange 38. In this manner, this cathode structure, with all its inherent advantages, may be built as a unit and later joined to the metal tube in which it is to operate.

In the above embodiments of the invention a bombarded cathode structure was used to illustrate the many advantages of this type of improved electrode mount. It is understood, however, that the invention is not limited to cathodes. This type of electrode mount could be used advantageously for supporting any other type of electrode, such as plate, grid, etc. In such a construction, those features which were found to be an advantage in the cathode example would be equally desirable.

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 in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A cathode structure comprising a pair of coaxial members having a pair of corresponding adjacent ends. a filament having its ends supported respectively from said member ends, said filament extending beyond said members, whereby said coaxial members may serve as leads for supplying current to said filament, a cylindrical member mounted in coaxial relationship with and surrounding said coaxial members and filament, an emitting surface supported by said cylindrical member and adapted to be heated by said filament, whereby said cylindrical member may serve as a lead for supplying current to said emitting surface, a hollow metal cylinder mounted concentrically to said coaxial members and within said cylindrical member and surrounding said filament, whereby electron discharge from said filament in a radial direction to said cylindrical member is prevented, a metal disc mounted perpendicularly and concentrically to said coaxial members and on the side of said filament 0990- site from said emitting surface, whereby electron discharge from said filament axially in a direction away from said emitting surface is minimized.

2. A cathode structure comprising a pair of coaxial members having corresponding adjacent ends, a filament having its ends supported respectively from said member ends, said filament extending beyond said members, a tubular member mounted in coaxial relationship with said coaxial members, an emitting surface supported by said tubular member and adapted to be heated by said filament, a hollow metal cylinder mounted concentrically to said coaxial members and surrounding said filament, and a metal disc mounted perpendicularly and concentrically to said coaxial members and on the side of said filament opposite from said emitting surface, whereby the electron discharge path from said filament is confined to an axial direction and .directed toward said emitting surface.

3. A cathode structure as in claim 2, further comprising means for rigidly maintaining said coaxial and tubular members in coaxial relation, said last means comprising three spaced metallic discs rigidly connected respectively to said coaxial and tubular members and perpendicularly to the axis thereof, and insulating means sealed to said discs for maintaining said discs insulated from one another and in fixed relative positions.

4. A cathode structure comprising a pair of coaxial members having a pair of corresponding adjacent ends, a filament having its ends supported respectively from said member ends, said filament extending beyond said members, whereby said coaxial members may serve as leads for supplying current to said'filament, a cylindrical member mounted in coaxial relationship with and surrounding said coaxial members and filament, an emitting surface supported by said cylindrical member and adapted to be heated by said filament, whereby said cylindrical member may serve as a lead for supplying current to said emitting surface, and means for prohibiting an electron discharge from said filament to said cylindrical member, said means comprising a hollow metal cylinder supported coaxially between said filament and said cylindrical member and surrounding said filament, and a metal disc supported concentrically to and near one end of said hollow metal cylinder on the side of said filament opposite from said emitting surface, whereby the discharge path from said filament is confined to an axial direction and is directed toward said emitting surface.

5. A cathode structure comprising a pair of coaxial members having two corresponding ad- J'acent ends, a filament having its two ends connected respectively to said two adjacent ends of said coaxial members, whereby said coaxial members may serve as leads for supplying current to said filament, a tubular member mounted in coaxial relationship with said coaxial members, an emitting surface supported at one end of said tubular member adjacent to said filament, whereby said tubular member may serve as a lead for supplying current to said emitting surface, and

means providing for support of said coaxial members and said tubular member with substantial heat transfer from said coaxial members and said tubular member, said means comprising three metal discs mounted in spaced parallel planes perpendicularly to the axis of said members, said discs being rigidly connected respectively to the other ends of said coaxial members and said tubular member, and hollow insulating members mounted between said metal discs and sealed thereto for maintaining said discs and their connected member in rigid relationship.

6. A cathode structure comprising a pair of coaxial members having two corresponding adjacent ends, a filament having its two ends connected respectively to said two adjacent ends of said coaxial members, a tubular member mounted in coaxial relationship with said coaxial members, an emittin surface supported at one end of said tubular member and adapted to be heated by said filament, three metal discs mounted in spaced parallel planes perpendicularly to the axis of said members and connected respectively to the other ends of said coaxial members and said tubular member, and hollow insulating members sealed between said metal discs, whereby said discs may serve as leads for supplying current to the respective members to which they are connected and to said filament and said emitting surface connected thereto.

7. A cathode structure comprising a pair of coaxial members having a pair of corresponding adjacent ends, a filament having its ends supported respectively from said member ends, said filament extending beyond said members, whereby said coaxial members may serve as leads for supplying current to said filament, a cylindrical member mounted in coaxial relationship with said coaxial members and surrounding said filament, an emitting surface supported by said cylindrical member and adapted to be heated by said filament, whereby said tubular member may serve as a lead for supplying current to said emitting surface, and a hollow metal cylinder mounted concentrically to said coaxial members within said cylindrical member and surrounding said filament, whereby electron discharge from said filament to said cylindrical member is p vented.

8. A cathode structure comprising a pair of coaxial members having corresponding adjacent ends, a filament having its ends supported respectively from said member ends, said filament extending beyond said members, a cylindrical member mounted in coaxial relationship with said coaxial members, an emitting surface supported by said cylindrical member and adapted to be heated by said filament, and a hollow metal cylinder mounted concentrically to said coaxial members and surrounding said filament.

9. A cathode structure comprising a pair of coaxial members having corresponding adjacent ends, a filament having its ends supported respectively from said ends, whereby said coaxial members may serve as leads for supplying current to said filament, an emitting element adapted to be heated by said filament, and means supporting said emitting element so as to minimize heat loss while providing sufficient electrical conductivity, said means including a tubular member supported in coaxial relationship to said coaxial members, a metal cylinder, a plurality of stiff filamentary members supporting said metal cylinder on and spaced from said tubular memher, and a plurality of thin metal strips extending from the inner wall of said metal cylinder and engaging the periphery of said emitting member.

10. A cathode structure comprising a pair of coaxial members, a filament supported by said coaxial members, whereby said coaxial members may serve as leads for supplying current to said filament, a tubular member mounted in coaxial and spaced relationship with said coaxial members and insulated therefrom, and an emitting surface supported by said tubular member and adapted to be heated by said filament, whereby said tubular member may serve as a lead for supplying current to said emitting surface.

11. A cathode structure comprising a pair of coaxial snug-fitting nested aluminum members, a filament supported by said coaxial members, a tubular member mounted in coaxial and spaced relationship with said coaxial members and insulated therefrom, an emitting element supported by said tubular member and adapted to be heated by said filament, and means for insulating said coaxial members from one another, said means comprising a layer of anodized aluminum on the outer wall of the innermost coaxial member of said pair of coaxial members'and an anodized layer of aluminum on the inner wall of the outer member of said pair of coaxial members.

12. A vacuum tube lead-in structure comprising a plurality of coaxially nested, snug-fitting cylindrical members, a corresponding plurality of axially spaced disc members disposed in planes perpendicular to the common axis of said cylindrical members and each rigidly connected to a respective cylindrical member, insulating means vacuously sealed between adjacent pairs of said disc members and maintaining said disc members and their connected members in rigid relationship, and means for insulating said coaxial members from one another, said means comprising a layer of anodized material on the surfaces of said coaxial members.

13. An electrode structure comprising a plu-' rality of coaxial nested radially spaced cylindrical members, said plurality exceeding two in number, a corresponding plurality of spaced disc members disposed in planes perpendicular to the common axis of said cylindrical members and each rigidly connected to a respective cylindrical member, insulating means vacuously sealed between adjacent pairs of said disc members and maintaining said disc members in rigid relationship, and a plurality of electrodes supported by said cylindrical members in aligned relationship.

14. A vacuum tube lead-in structure comprising a plurality of coaxial nested radially spaced cylindrical members, a corresponding plurality of axially spaced disc members disposed in planes perpendicular to the common axis of said cylhidrical members and each rigidly connected to a. respective cylindrical member, and insulating means vacuously sealed between adjacent pairs of said disc members and maintaining said disc members and their connected cylindrical members in rigid relationship.

BIGURD F. VARIAN.

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