US2640167A - Envelope for electron flow device and glass-metal seal embodied therein - Google Patents

Description

May 26, 1953 2. J. ATLEE ETAL 2,640,167

ENVELOPE FOR ELECTRON FLOW DEVICE AND GLASS-METAL SEAL EMBODIED THEREIN Filed May 29. 1947 Z50 A7255 & t/OSEPH Z5. G SL/A'G Patented May 26, 1953 ENVELOPE FOR ELECTRON FLOW DEVICE AND GLASS-METAL THEREIN SEAL EMBODIED Zed J. Atlee, Elmhurst, and Joseph B. Gosling, Chicago, 111., assignors to General Electric Company, a corporation of New York Application May 29, 1947, Serial No. 751,234

24 Claims.

The present invention relates, in general, to electronics, and has more particular reference to electron flow devices, such as X-ray generator tubes.

X-ray generator tubes and other electron flow devices commonly comprise an anode forming an electron target, a cathode structure embodying electron emission means, and a sealed enve" lope enclosing the anode and cathode and supporting the same in spaced apart, operative relationship therein. Such devices are operated by suitably exciting the cathode for electron emission. Electrons thus emitted at the cathode may be caused to travel toward and to impinge upon the anode target by applying electron driving potential between the cathode and anode.

Since it is thus necessary to apply electron driving potential between the anode and cathode, the same have to be electrically insulated the one from the other, such insulation being commonly fl provided by the envelope structure itself, as by forming at least portions of the envelope of insulating material, such as glass, in position electrically insulating the anode and cathode structures. To this end, it has been the practice to select glass for envelope purposes having such high electrical resistance as to substantially in hibit the flow of electrical current therein at the electrical potential applied between the anode and cathode for the operation of the device,

whereby the glass portions of the envelope provide virtually perfect insulation between the anode and cathode structures. Accordingly, in the fabrication of X-ray generator tubes and other electron flow devices, it has been common L and usual to seal the glass portions of the envelope to metal parts, such as electrode supports for the anode and cathode structures, in order thus to seal and mount the operating elements of the tube, within the envelope, and to 0011- h stitute such glass sealed metal parts as portions of the sealed envelope and also as electrical conductor means for connecting the enclosed operating elements in operating circuits outwardly of the envelope.

In sealing glass envelope material to metal parts. it is desirable to employ glass having substantially the same coefiicient of thermal expansion as that of the metal to which the glass is sealed, in order to aid in the provision of an effective seal between the connected glass and metal. parts. As a consequence, envelope glass desirably embodies a coefficient of thermal expansion of the order of 4.7:tD.3 1O cm. per cm. per degree 0., within the temperature range 2 between 100 C. and 300 C., the same being substantially equal to that of metals of the sort commonly employed in mounting and sealing the operating elements of the device in and on the glass portions of the envelope. Such metals commonly comprise alloys including nickel as an essential component, the most commonly employed alloys including steel containing 42% nickel, Kovar comprising 29% nickel, 17% cobait and 0.3% manganese, and Fernico comprising 54% iron, 28% nickel and 18% cobalt.

Other desirable characteristics of X-ray tube envelope glass include annealing temperature of the order of 500:50 0.; softening temperature of the order of 700: :50 0; strain point of the order of 480i30 C.; power factor of the order of 0.03i0.02, at sixty cycles and 100 0.; dielectrio constant of the order of 6:2, at sixty cycles and 100 C.; dielectric strength of the order o1 450:50 volts per mil, at 25 C., and not substantially less than 100 volts per mill at 200 C.; and X-ray transmissivity, per mm. of thickness, of less than the transmissivity of 0.55 mm. thickness of aluminum.

In the operation of X-ray generator tubes embodying glass envelopes, a readily apparent phenomenon is the progressive discoloration which tends to occur in the glass envelope. Where exposed to X-rays, glass tends to develop a characteristic amber discoloration which gradually deepens to a relatively dark brown shade, as a result of continued exposure to Y-rays. Such discoloration is undesirable, not only because of its unclean appearance, but also because the discoloration makes inspection of the interior of the generator, visually through the envelop wall, progressively more difficult during the service life of the device. The ability to inspect the operating parts of the device, freely, is particularly desirable toward the end of its service life. Accordingly, an exceedingly desirable characteristic, "for glass used in X-ray tube envelopes, is the ability of the glass to resist discoloration upon exposure to X-rays.

Prior to the conception of the present invention, various glasses having some of the foregoing desirable characteristics were known; but no glass containing all of theforegoing desirable characteristics was known or available. Consequently, it was the practice to employ localized bands orzones of glass, in X-ray tube envelopes, each zone comprising glass having selected characteristics, in order, somewhat ineiiectually, to obtain desired glass characteristics in the various localized sections of the envelope where such characteristics are desirable. Such multiple glass envelope technique is expen -ve because of the necessity of sealing the several envelope sections, one to the other, and also is generally v tisfactory because such multi le glass structure irnparts unwanted and uncontrollable mechanical strains, and consequent excessive fragility in the resulting envelope.

During the operation or" electron flow devices, including X-ray generator tubes, especially devices designed for operation at high voltage between anode and cathode, electrostatic charges tend to accumulate upon the glass envelope walls, as the result of impingement thereon of stray electrons which fail to reach the anode target. Such electrostatic charges may build up to high potential values upon the envelope walls, with consequent danger of perforation or rupture thereof and consequent failure of the envelope, if such potentials exceed the dielectric strength of the envelope materiel. Such envelope rupture commonly comprises a tiny perforation of capillary character seemingly burned through the glass envelope wall, at a location of minimum wall strength. such perforation apparently being 1 caused by the cumulative effect of repeated application of potential accumulations on the envelope wall during consecutive periods of operation of the device. The formation of the envelope rupturing perforation also appears to be due, at least in part, to the electrostatic interaction, through the envelope wall, of such repeated potential accumulations, with electrostatic objects, such as grounded metal equipment, located outwardly of the envelope.

The present invention contemplates the incorporation of electrical conducting characteristics in the glass portion of the envelope to an extent suificient to allow the relatively slow yet continuous drainage, from the glass, of the electrostatic voltage charges which accumulate thereon, to thereby remove the cause of envelope puncture, without, however, reducing the electrode insulating effect of the glass envelope portions to a degree permitting significant flow of electrical current through the glass between the anode and cathode structures. To this end, the present invention proposes the employment of glass having electrical resistance of the order of 10 ohms/cur, at 350 0.. structure of an electron flow device.

An important object of the present invention thus is to provide an electron flow device, such as an X-ray generator tube, having an envelope including a glass portion having electrical resistivity of an order low enough to allow for continuous drainage, from the glass envelope portion, of such electrostatic charges as may accumulate thereon, during the operation of the device, yet high enough to prevent significant electrical current flow in the envelope between the anode and cathode mounting stations there- Another important object is to provide an envelope structure for electron flow devices embodying glass envelope portions having optimum dielectric strength, thermal expansion characteristics rendering the glass readily scalable with metal, and. also an electrical resistance characteristic providing for the continuous drainage of electrostatic charges from the glass without permitting significant electrical current flow between the anode and cathode of the device.

Another important object is to provide a glass envelope for devices of the character mentioned,

the envelope L} including a glass envelope portion having high dielectric strength, relatively high electrical resistance sufficient to prevent significant electrical current flow in the envelope, between the anode and cathode electrodes of the device, yet low enough to permit drainage of potential accumulations from the glass envelope portions, and a coefficient of thermal expansion substantially identical to that of the metal alloys, such as Fernico and Kovar, containing substantial quantities of nickel.

Another important object is to provide for the fabrication of the glass portions of the envelope entirely from the same material, whereby the envelope has substantially identical characteristics throughout, thereby avoiding the necessity and expense of utilizing zones or bands of diiTerent glasses in the several portions of the envelope each for its particular desired characteristic, as has been necessary in the past, particularly in the fabrication of high voltage X-ray generators.

Another important object resides in providing an electron flow device including an envelope comprising a new glass-to-metal seal embodying a metal alloy including nickel as an essential component and a glass part sealed thereto and having substantially the same coefficient of thermal expansion of the order of 4.7iOBXlO- cm. per cm. per degree 0., within the temperature range from 100 to 300 C.; a further object being to utilize glass having a dielectric constant within the range of 4.0 to 8.0, at 60 cycles and 100 0., dielectric strength of the order of 100 volts per mil, at 200 C., strength at normal atmospheric temperature, 25 C., being of the order of 4:50:50 volts per mil; a still iurther object being to provide a seal of the character men tioned wherein the glass has electrical resistivity of the order of 10 ohms/cmfi, at 350 C, and a power factor, at 60 cycles and 100 C., of the order of 0.03i0.02.

The foregoing and numerous other important objects, advantages, and inherent functions of the invention will become apparent as the same is more fully understood from the following description, which, taken with the accompanying drawings, discloses the present invention and the manner of practicing the same.

Referring to the drawings:

Fig. is a sectional view taken lon tudinally through an tube made in acco nce with the present invention,

2 is an enlarged view of portion of the device shown in Fig, 1.

To illustrate the invention the drawings show an X-ray tube H comprising a cathode structure l2 and a cooperating anode l2 enclosed in a glass envelo i i, the envelope ing hermetically sealed a: the anode and rode supported in operative position therein by scans of end seals !5 disposed at the opposite ends of the envelope.

The cathode structure i2, as shown, an electron emission element supported on and insulated from a cup-shaped. member, which in is n ounted on a stem 20 having an end g in and opposite end projec out rardly of the envelope. The anode l3. shown, comprises a body of metal disposed within the envelope formed with a cavity opening toward the cathode, the anode providing an electron target at the bottom of the cavity. The anode also includes an extension forming a stem 20' extending outwardly or" the envelope.

The seals l5, as shown, each comprise a cuplike metal sealing member it having an annular, preferably feathered, edge I! sealed upon the end of a preferably re-entrant sleeve-like glass envelope portion l3, the terminal annular edge of which forms a sealing bead IS in which the feathered edge ii is sealingly embedded. The anode and cathode supporting stems 20 and 20 extend outwardly of the members it through openings 2i formed therein, the stems being sealed in said openings, as by brazing or welding, to hermetically seal the stems in said openings.

The cathode supporting seal element l6 may also be fitted with seals 22 through which conductors 23 may extend for electrically connecting the electron emission element of the cathode in electrical operating or control circuits out wardly of the envelope. The outwardly extending end of the stem 20 may, of course, be utilized for effecting the connection of the anode in an external circuit. The seals 22 may comprise metal grommets 20 sealed, as by brazing or welding, in openings formed in the element Hi. The conductors it and the grommets 24 are preferably made of metal, such as iron alloyed with nickel and cobalt, adapted to seal readily with glass. A conductor 23 is disposed in position extending concentrically through its associated grommet 24 and may be hermetically sealed in the grommet by means of a glass globule 25, which is adhered to the grommet and the wire in position closing the space around the wire in the grommet.

Considerable difficulty has been encountered, in the past, in providing electron flow devices, particularly X-ray generators, with glass envelopes, because of the substantial difference in the co-eflicient of expansion of suitable envelope glasses and metal seal elements. metal scalable glasses that are suitable from the standpoint of some of the desired characteristics, such as dielectric strength, power factor, dielectric constant, and the like, have heretofore been lacking in one or more and usually several of said desired characteristics. As a consequence, attempts have heretofore been made to fabricate glass envelopes for electron flow devices by joining several different glasses in order to meet various requirements as to metal scalability, transparency, dielectric strength, ability, and other desirable electrical characteristics in the various portions or zones of the envelope.

The necessity of thus building composite glass envelopes has been not only excessively expentric strength, and desirable annealing and softening temperature, strain point, power factor and dielectric constant characteristics.

The present invention further proposes the incorporation of an additional characteristic in the glass portions of the envelope, in order to Furthermore,

insulating inhibit envelope puncture due to the accumulation, on the envelope walls, of high potential electrostatic charges during operation of the electron flow device. This is accomplished by making the glass electrically conductive to a degree sufficient to allow for the continuous drainage of electrostatic charges from the envelope glass, at a regulated, relatively slow rate, the electrical resistivity of the glass, however, being maintained high enough to prevent significant electrical current flow between the anode and cathode of the electron flow device.

The envelope l4, according to the present invention, comprises glass having relatively high electrical resistivity of the order of 10 ohms/cm. at 350 C., and also having a coefiicient of thermal expansion of the order of 4=.'7 0.3 10 centimeters per degree C., within a temperature range from 100 C. to 300 C. The glass envelope material also has relatively high dielectric strength of the order of 100 volts per mil, at 200 (3., the dielectric strength at normal atmospheric temperature being of the order of 450-550 volts per mil. The glass preferably also has a dielectric constant, at sixty cycles and 100 0., within the range of from 4 to 8. Its power factor, at sixty cycles and 100 C., is preferably within the range from 0.01 to 0.05.

In addition to the foregoing characteristics, the glass preferably has an annealing temperature of the order of 500:50" C.; a softening temperature of 700i50 0.; a strain point at 480:30 0.; X-ray transmissivity, per mm. thickness, within the k. v. p. range, between 30 and 90 k. v. p., of an order equivalent to the transmissivity of aluminum of less than 0.55 mm. in thickness; and the glass shows substantially no discoloration as the result of extensive exposure to X-rays.

The foregoing characteristics have never been utilized in combination in X-ray tube envelopes, nor in any combination sealed to metal. More particularly, the combination of high dielectric strength and resistivity within the stated range allows the fabrication of the envelope M as an element of uniform consistency throughout, and entirely eliminates the necessity of utilizing zones or hands of different glasses in the envelope of the tube.

In order to accomplish the foregoing purposes, the envelope I4 may be made from a glass mix or melt comprising mainly silica and boric oxide together with the oxides of sodium, iron, aluminum, cerium, and antimony. More particularly, the glass mix may comprise silica, about 64%; boric oxide, about 23%; sodium oxide, about 7%; iron and aluminum oxide, about 5%; cerium oxide, about 0.4%; and antimony oxide, about 0.1%; the remainder comprising impurities, and the proportions given, of course, being the relative weights of the several components.

Glass having the desired characteristics has been made from a mix comprising the following specific component quantities:

Per cent Silica 64.90 Boric oxide 22.50 Iron oxide .a 0.08 Aluminum oxide -e 4.90 Sodium oxide 7 Cerium oxide 0.50 Antimony oxide 0.11

It will, of course, be recognized that the proportions specified may be varied, within reasonable limits, without impairing the desired electrical and mechanical characteristics of the resultant glass. In this connection, the permissible variation in the amounts of the glass constituents is, generally speaking, inversely proportional to the quantity of the constituents, as stated above, for an iueal mix or melt; that is to say, the smaller the proportional amount of a stated ingredient in the ideal mix, the larger may be the permissible variation in its relative proportion in the mix, without altering the desired characteristics in the resultant glass. Accordingly, the relative proportions of such constituents as antimony, iron and cerium oxides may be increased or decreased in the mix by as much as 50% or more of the quantities thereof, without materially afiecting the characteristics of the resultant glass. Likewise, the amounts of sodium and aluminum oxides may be increased or decreased to a lesser extent, as up to say 30% of the quantities stated for the ideal mix; while the preponderant constituents, namely silica and boric oxide, may be increased or decreased up to say 10% for silica and 20% for boric oxide. Thus, the mix o n. ise silica lrom to '70 by weight: boric oxide. Irom 15 to 30% by weight; sodium oxide from to 09 3 by weight; aluminum oxide from 4 to 6% by weight; iron oxide from 0.04 to 0.12% by weight; cerium oxide from 0.3 to 0.5% by weight; and antimony oxide from 0.05 to 0.18% by weight.

The resulting glass not only has the desirable characteistics heretofore mentionec, but it also has a relatively low softening temperature of the order of 700 C., or less, and consequently is readily workable and lends itself to the fabrication of containers, envelopes and the like. Its softening temperature, however, is not so low as to impair the mechanical strength, of X-ray tube envelopes formed therefrom, at the temperatures at which generators and other electron flow equipment are expected to operate. The glass has a co-efiicient of thermal expansion that is substantially identical to that of nickel alloys, including steel containing 42 nickel, and is, therefore, readily scalable directly to metal elements of the character mentioned. More particularly, the glass, for example, makes an exceedingly effective seal with ferrous metal alloys, such as, Kovar, consisting of iron with about 29% nickel. 17% cobalt, and 0.3% manganese, and Fernico, consisting of iron with about 28% nickel, and 18% cobalt.

X-ray generators built in accordance with the teachings of the present invention may comprise a glass envelope !4 of uniform texture and consistency hermetically sealed by the readily attached nickel alloy elements IS. The cost of fabricating the device is unusually low, not only ecause of the resulting simplification in the steps of assembling and sealing the anode, cathode and other elements in the envelope, but also because oi the considerable cost reduction accomplished by eliminating the necessity of maintaining stocks of several difierent glasses and fabricating the same into composite envelopes.

It is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form. construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the form herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention is hereby claimed as follows:

A puncture resistant electron flow device comprising an anode and a cooperating cathode electrode, and an envelope of glass enclosing and supporting said electrodes thereon at spaced mounting stations, said glass having electrical resistivity of the order of 10 ohms/cm at 350 C., such resistivity being low enough to provide for drainage, at a predetermined rate, of 31 ch electrostatic charges as may accumulate on the envelope through electron impact thereon during the operation of the device, and high enough to prevent significant flow of electricity in the envelope between said anode and cathode mounting stations.

2. A puncture resistant electron flow device, as set forth in claim 1, wherein the glass portion of the envelope has dielectric strength of the order of 450250 volts per mil, at C.

3. A puncture resistant e ectron flow device. as set forth in claim 1, wherein the glass portion of the envelope has a dielectric constant at sixty cycles and 100 C., within the range from [our to eight.

4. A puncture resistant electron flow device, set forth in claim 1, wherein the power factor of the glass portion of the envelope, at sixty cycles and 100 0., is within the range from 0.01 to 0.05.

5. A puncture resistant electron flow device, as set forth in 1. wheein the annealing temperature of the portion. of the envelope is of the order of 500350 C.

6. A. puncture resistant electron flow device, as set forth in claim 1, wherein the softening temperature of the glass portion of the envelope is of the order of 700150 C.

7. A puncture resistant electron flow device, as set forth in claim 1, wherein the strain point of the glass portion of the envelope is of the order of 480:" C.

8. A puncture resistant electron flow device, as set forth in claim 1, wherein the X-ray transmissivity of the glass portion of the envelope, per mm. of thickness thereof, is of order less than the Xray transmissivity of 0.55 mm. thickness of aluminum.

9. A puncture resistant electron flow device, as set forth in claim 1, wherein the glass poi 'ion of the envelope has the following characteristics: a thern 1 expansion cceficient oi the order of i. 7i().3 10 cm., per c!n., per degree C.; dielectric strength of the order of 4501550 volts er mil, at 25 C.; a dielectric constant at sixty cycles and C. within the range from four to eight; power iactor within range from 0.01 to 0.05, at sixty cycles and 100 C.

10. {l puncture resistant electron flow device comprising an anode and a cooperatin: cathode electrode, an envelope enclosing said electrodes and comprising glass, and a metal seal member, forming a part of said envelope and having portrons forming a glass-metal seal with the glass material of said envelope, a said electrode bein mounted on and supported by said seal mambo; ts1ado%l1a(3sl ortr1age iaghaving elecstricel resistivity oi vide fo drainose t gg jv at 350 to her at DVQdEE-EI O H in r said metal seal meg p r .-c\. rate, of such electrostatic a es as may accumulate on the envelope through electron impact thereon during the 5 eratlon of the device. col r l-jri gl ll r liu'relogeasistilnt electron flow device electrode, and an envi lon 60011361?mg Pathode ro es and Con my -e encosing said elecmg a metal class a said electrode hav- 111g portlon forming a, glassmetal seal with the glass material of said envelope, said glass material having electrical resistivity of the order of 10 ohms/cmfi, at 350 C., to provide for drainage, at a predetermined rate, of such electrostatic charges as may accumulate on the envelope through electron impact thereon during the operation of the device, said glass also having a thermal expansion coefficient of the order of 4.7i0. X10 cm., per per degree C., for maintenance of the said glass-metal seal.

12. A puncture resistant electron flow device comprising an anode and a cooperating cathode electrode, an elongated tubular envelope enclosing said electrons and comprising a glass portion, and a metal upport member carrying a said electrode and having an annular skirt-like rim portion forming a glass-metal with the glass portion of said envelope, at an end thereof, said glass portion having electrical resistivity of the order of l ohms/cm at 350 C., to provide for drainage, a predetermined rate. of such electrostatic charges as may accumulate on the glass portions 01 said envelope through electron impact thereon during the operation oi the device, glass porti n also having a thermal expansion coefficient of the order of 47:50.3 X cm., per cm., per degree C., for maintenance of the said glass-metal seal, and a dielectric strength of the order of 450150 volts per mil. at C.

13. A. puncture resistant X-ray tube for high voltage operation comprising an anode and a cooperating cathode electrode, an envelope of glass enclosing and supporting said electrodes thereon at spaced mounting stations, said glass having electrical resistivity of the order of 10 ohms/cmfi, at 350 C., such resistivity being low enough to provide for drainage. at a predetermined rate, of such electrostatic charges as may accumulate on the envelope througl'i electron impact thereon. during the operation of the tlevice, and high enough to prevent significant flow of electricity in the envelope between said anode and cathode mounting stations.

14.. A puncture resistant Xray tube, as set forth in claim 13, wherein the glass portion of the envelope has dielectric strength of the order of 4501*:50 volts per mil, at C.

15. A puncture resistant X-ray tube, forth in claim wherein the glass portion of the envelope has a dielectric constant at sixty cycles and 10. C., within the range from four to eight.

16. A puncture resistant X-ray tube, as set forth in claim 13, wherein the power factor of the glass portion of the envelope at sixty cycles and 100 C., is within the range from 0.01 to 0.05,

17. A puncture resistant X-ray tube, as set forth. in claim 13. wherein the annealing temperature of the glass portion of the envelope is of the order or" 5001:5'0 C.

18. A puncture resistant X-ray tube, as set forth in claim 13, wherein the softening temperae ture of the glass portion of the envelope is of the order of 700:t50 C.

19. A puncture resistant X-ray tube, as set forth in claim 13, wherein the strain point of the glass portion of the envelope is of the order of 480::" C.

20. A puncture resistant X-ray tube, as set forth in claim 13, wherein the X-ray transmissivity oi the glass portion of the envelope, per mm. of thickness thereof, is of an order less than the X-ray transmissivity of 0.55 mm. thickness of aluminum.

21. A puncture resistant X-ray tube, as set forth in claim 13, wherein the glass portion of the envelope has the following characteristics: a thermal expansion coefficient of the order of 4.7i0.3 10' cm., per cm., per degree C; dielectric strength of the order of 450:50 volts per mil, at 25 C.; a dielectric constant at sixty cycles and C. within the range from four to eight; power I actor within the range from 0.01 to 0.05, at sixty cycles and 100 C.

22. A puncture resistant Xray tube comprising an anode and a cooperating cathode electrode, an envelope enclosing said electrodes and comprising glass, and a metal seal member, forming a part of said envelope and having portions forming a glass-metal seal with the glass material of said envelope, at said electrode being mounted on and supported by said seal member, said glass material having electrical resistivity of the order of 10 ohms/cm. at 350 C., to provide for drainage, through said metal seal member, at a predetermined rate, of such electrostatic charges as may accumulate on the envelope through electron impact thereon during the operation of the device.

23. A puncture resistant X-ray tube comprising an anode and a cooperating cathode elec trode, and an envelope enclosing said electrodes and comprising glass, a said electrode having a metal mounting portion forming a glass-metal seal with the glass material of said envelope, said glass material having electrical resistivity of the order of l0 ohms/cm. at 350 C., to provide for drainage, at a predetermined rate, of such electrostatic charges as may accumulate on the envelope through electron impact thereon during the operation of the device, said glass also having a thermal expansion coeflicient of the order of 4.7; :0.3 l0 cm., er cm., per degree C., for maintenance of the said glass-metal seal.

.4 A puncture resistant X-ray tube comprising an anode and a cooperating cathode electrode, an elongated tubular envelope enclosing said electrodes and comprising a glass portion, and. a metal support member carrying a said electrode and having an annular skirt-like rim portion forming a glass-metal seal with the glass portion of said envelope, at an end thereof, said glass portion having electrical resistivity of the order of 10 ohms/cm. at 350 C., to provide for drainage, at a predetermined rate, of such electrostatic charges as may accumulate on the glass portions of said envelope through electron impact thereon during the operation of the device, said glass portion also having a thermal expansion coefficient of the order of cm., per cm., per degree C., for maintenance of the said glass-metal seal, and a dielectric strength of the order of 4501-50 volts per mil, at 25 C.

ZED J. A'ILEE. JOSEPH B. GOSLING.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,975,880 Ulrey et a1 Oct. 9, 1034 2,088,801 Hood Jan. 26, 1937 2,076,012 U'lrey Apr. 6, 1937 2,167,431 Bowie July 25, 1939 2,172,548 Schwarzkopf Sept. 12, 1939 2,405,477 Westendorp Aug. 6, 1946 2,564,950 Black Aug. 21, 1951

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