US2945327A

US2945327A - Method of manufacturing electric lamps or similar devices

Info

Publication number: US2945327A
Application number: US657774A
Authority: US
Inventors: Robert N Malm; Vodicka Vincent
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1955-11-28
Filing date: 1957-05-08
Publication date: 1960-07-19
Anticipated expiration: 1977-07-19
Also published as: GB843814A; FR1167850A; CH355216A

Description

July 19, 1960 R. N. MALM EI'AL 2,945,327

METHOD OF MANUFACTURING auac'rmc LAMPS 0R SIMILAR mavrcss Original Filed Nov. 28. 1955 lnvervtor'sz RObTt' M. MaLm, Vincervt Vodicka;

b9 l eir r ii eg. I:

2,945,327 Patented July 19, 1960 METHOD F MANUFACTURING ELECTRIC LAMPS OR SHWILAR DEVICES Original application Nov. 28, 1955, Ser. No. 549,424,

now Patent No. 2,904,716, dated Sept. 15, 1959.. Di- .vided and this application May 8, 1957, Ser. No.

57,774 4 Claims. 01. '4978) Our invention relates to electric lamps or similar devices and to a method of manufacture thereof. The invention is of particular utility in connection with the manufacture of electric lamps of the type having an external stem press through which the lead-in wires of the lamp are sealed. The present application is a division of our co-pending application Serial No. 549,424, filed November 28, 1955, which issued as Patent No. 2,904,716, dated September 15, 1959 and is assigned to the assignee of the present invention.

Electric lamps and other similar type devices commonly consist of an enclosing envelope or bulb having a glass stem press portion through which the lead-in Wires of the lamp extend and are sealed. These glass stem press portions are usually provided with an exhaust passageway extending therethrough which communicates with the interior of the envelope and through which the envelope is subsequently exhausted and, if desired, filled with a suitable inert gas at a suitable pressure.

The glass press portions are generally formed by the fusion of a glass exhaust tube to a surrounding tubular glass body such as a stem tube or the tubular neck portion of a lamp bulb. The lead-in wires are positioned and held in place between the two glass tubes, during the interfusion thereof, and the heated and softened glass is compressed by the closure of opposed jaws thereagainst to form a press portion through which the leadin wires extend and are hermetically sealed. This compression of the softened glass tubes, however, to form the stem press normally results in the closureof the passageway through the exhaust tube, a condition which of course would prevent subsequent evacuation of the lamp envelope or bulb. For such reason it is necessary, during the press-forming operation, to adopt additional procedures or steps which will insure the provision of an exhaust passageway through the completed stem press. In the usual case where the stem press is located internally of the lamp envelope or bulb, the exhaust passageway through the stem press is customarily produced by blowing a hole through the side of the pressed portion while the glass thereof is still in a heated and plastic state, the blow-out being accomplished by the force of a blast of compressed air introduced into the open outer end of the exhaust tube. disclosed in Mitchell and White Patent 1,423,956 and is the standard procedure employed in the manufacture of conventional type incandescent lamps having internal stem presses. However, such a procedure cannot be employed where the stem press is located externally of the bulb as, for example, in the case of the miniature bi-pin incandescent lamp recently introduced on the.

market by applicants assignee and commerciallydesignated as their No. 10 lamps. In suchcase, a side-opening exhaust passageway through the stem press would open to the outside atmosphere only, instead-of into the interior of the lamp envelope or bulb. As a result, there would be no exhaust passageway through the stem press This is the method which is leading into the interior of the envelope through which the envelope could then be exhausted.

For such reason, other procedures have been proposed and followed, in the case of lamps having external stem presses, to insure the provision of an exhaust passageway through the stem press communicating with the interior of the lamp envelope. Heretofore, such procedures have generally involved either the insertion of metal sleeves, springs or other similar liners in the exhaust tube, or the temporary introduction of a mandrel into the exhaust tube during the press-forming operation, for the purpose of maintaining the opening through the exhaust tube. However, these prior procedures for various reasons have not been entirely satisfactory. The

use of metal sleeves or bushings, for instance, in the exhaust tube, besides being rather expensive, also introduces strains in the glass stem press which frequently results in the cracking of the press, thus causing an undesirably high percentage of manufacturing rejects or so-called shrinkage. Likewise, the temporary insertion of a mandrel into the exhaust tube during the pressforming operation frequently results in the sticking of the plastic glass to the mandrel and consequent breakage of the glass press when the mandrel is subsequently withdrawn from the exhaut tube. It has also been pro-. posed, in the manufacture of vacuum or radio tubes, to employ a liner for the inner wall of the exhaust tube in the form of a graphite coating thereon. Such a material, however, is wholly unsuitable for use in tungsten filament incandescent lamps because of its deleterious effect on the lamp, the graphite reacting with the tungsten of the filament at the incandescent operating temperature thereof and causing embrittlement of the tungsten filament such as results in greatly foreshortened lamp life.

It is an object of our invention, therefore, to provide a novel method of forming the'glass stem press of an electrical device having an exhaust passageway extending therethrough.

Another object of our invention is to provide a novel method of sealing and compressing a tubular glass member around an inserted glass exhaust tube while preserving the opening therethrough to serve as an exhaust passageway.

Still another object of our invention is to provide a novel and inexpensive method of manufacturing an electric incandescent lamp or similar device having a stem press with an exhaust passageway therethrough.

Briefly stated, in accordance with one aspect of our invention, an electric incandescent lamp or similar device is constructed with a glass stem press having an exhaust passageway extending therethrough and maintained open, during the press-forming operation, by a coating on the wall of the exhaust passageway of a material having a melting point above that of the glass and chemically inert with respect to the filament at the incandescent operating temperature thereof. According to a further aspect of the invention, the coating material employed for'the above mentioned purpose is one which, under the conventionalhigh-frequency glow test to which vacuum type electric incandescent lamps-or similar devices are subjected in order to indicate the degree of vacuum in the device, will produce a bright and highly concentrated glow or spot of light'such as enables the ready and easy detection thereof and determination of the character of the vacuum in the device by the test operator.

- Further objects and advantages of our invention will appear from the following detailed description of species thereof and from the accompanying drawing,

I lathe drawing, Fig. 1 is an elevational view partly in section showing the component parts employed in the manufacture of an electric incandescent lamp according to the invention, the component parts being shown in assembled position in readiness for sealing the lamp mount into the lamp bulb and forming the glass stem press portion thereof.

Fig. 2 is a sectional view similar. to Fig. 1 and showingthe compression of the bulb neck to form the stem press of the lamp;

Fig. 3 is a sectional view on the line 3-3 of Fig. 2, and

Fig. 4 is an elevation, partly in section, of a completed electric incandescent lamp comprising our invention.

Referring to the drawing, the invention is there shown as embodied in an electric incandescent lamp of the type having an external stem press, such as the miniature bipin lamp recently introduced on the market by applicants assignee and known as theirNo. 10 lamp. It should be understood, however, that the invention is applicable as well to other types of electric incandescent lamps and similar devices such as those having re-entrant stems provided with stem press portions located within the envelope of the device.

As shown in Fig. 4, the particular electric incandescent lamp there illustrated comprises a sealed glass bulb or envelope 1 provided with an outwardly protruding pinched seal or press portion 2 through which a plurality (two in the particular case shown) of lead-in conductors -3, 3 are sealed so as to extend parallel to each other in side-by-side relation. The lead-in

conductors

3, 3 are of the multi-section pin type such as are commonly employed in miniature radio tubes and comprising rigid metal outer pin portions 4 and inner lead portions 5 which are hermetically sealed in the envelopepress 2, the two sections 4 and 5 of each conductor being butt-welded together in end-to-end relation. The outer pin portions 4 may be made, for example, of nickel wire having a diameter for instance of V inch. The inner lead'portions 5 may be made of conventional Dumet wire com monly employed in the lamp making art for sealing into glass. The metal outer pins 4 serve as terminal contacts for the lamp and for such purpose are embedded at their inner ends in the glass of the press 2 in order to firmly anchor or support the pins in place from the press. The inner leads 5 are connected to an electrical energy translation element such as a filament 6 comprised, for example, of a wire of tungsten or other suitable refractory metal in coiled or coiled-coil or any other suitable form. The envelope 1 is evacuated to the degree customary for conventional vacuum type incandescent lamps. The press portion 2 of the lamp envelope 1 is provided with an exhaust passageway 7 which extends through the press 2 at a point between the two

conductors

3, 3 and which serves as a means for evacuating the lamp envelope 1.

In accordance with the invention, the exhaust passageway 7 is constituted by the bore 8 of a glass exhaust tube 9 which is sealed inithe stem press 2 and the bore of which is maintained open during the press-forming operation. To this end, the end portion of the exhaust tube which is fusion-sealed into the stem press 2 is provided with a firmly adherent coating 10 of a suitable inorganic material which has a melting point above that of the glass of which the stem press is constituted and which is chemically inert with respect to the lamp filament 6 at the incandescent operating temperature thereof, the said material acting to raise the melting point of the glass underlying the coating during the press-forming operation. Alternatively, the coating 10 may be of a material which, when subjected to the heat attending the press-forming operation, will be transformed into an inorganic material having the above-stated properties. Since the glass lamp parts (i.e., the exhaust tube and the bulb or stem tube) from which the stem presses of electric lamps are fabricated, are customarily made of so-called sof type glasses having a melting point of around 650 C., the coating material 10 in such case should be one having a melting point above 650 C. The coating material 10 should also be one which is both physically and chemically stable at temperatures up to at least the temperature (for example at least 700 C.) which the glas ex,

haust tube 9 will attain during the tipping-0d thereof, otherwise contamination of lamps will occur from the gases or other contaminating impurities introduced in the lamp envelope by unstable coating materials. While the coating 10 is preferably applied-on the internal wall of the exhaust tube 9 as shown, it may be applied instead on the external. wall thereof as shown in dotted lines at 10 in Fig. 1 or it may be applied on both the inside and outside walls of-the exhaust tube. The

coating

10 or 10 is applied to the exhaust tube 9 prior to the pressforming operation. During the heating and interfusion of the end of the exhaust tube 9 and the surrounding neck 11 of the' lamp bulb 1 to form the stem press 2, the coating 10 on the exhaust tube acts to maintain the bore of the exhaust tube open so as to leave an exhaust passageway 7 through the stem press. While the exact manner in which the coating 10 acts to accomplish this object is not known, it is believed that the coating acts either to raise the melting point of the glass of the exhaust tube so that it is at a temperature below that at which it is appreciably plastic, or to raise the surface tension of the glass so that the coated surfaces of the exhaust tube do not wet together and close oif the bore of the exhaust tube.

There are a number of materials which may he satisfactorily employed as coating materials 10 for the purposes of the invention. Thus, the coating 10 on the exhaust tube 9 may consist of aluminum or silver, or the oxides of aluminum, silver, zirconium, magnesium, titanium, zinc, tin, calcium, barium, and strontium, or refractory materials such as silicates, double silicates and titanates. It is' preferable to employ coating materials 10 such as those specified above which are light or white in color not only because of their better appearance and the ease of discerning whether lamp-contaminating impurities are present in the coating 10 but primarily because of the greater heat-reflectivity characteristic of such light-colored coatings 10 which therefore minimizes or lessens the softening and likelihood of closing of the glass exhaust tube during the press-forming operation. As a result, considerably greater latitude in the degree of heating of the glass parts, during the press-forming operation, is afiorded by the use of such light or white-colored coatings 10, as compared to dark colored coatings, without encountering closure of the exhaust tube. This means that considerably greater freedom is permitted in the setting of the gas fires which are ordinarily used to melt and fuse the glass parts. This comparatively wide range of permissible gas fire settings therefore renders the stem press making process according to the invention commercially practical for'high speed production manufacture since it results in -a very low percentage (2% or less) of production rejects due to closed exhaust tubes.

Of the various coating materials 10which have been found satisfactory forthe purposes of the invention, the oxide and silicates (both single and double) of zirconium are preferred for the reason that they perform the additional function of producing a bright and distinctive colored glow Within the lamp envelope 1 during the customary high-frequency glow testing of the lamp for the determination of the degree of vacuum in the lamp envelope. This glow of light, which appears as a green column of light withinthe exhaust passageway 7, is of particular utility in the case of lamps having envelopes or bulbs 1 of low light transmissivity such as where the bulbs are provided with enamel or other coatings. In such case, the color of the glow discharge ordinarily is not discernible in the absence of zirconium oxide or zirconium silicate coating material within the exhaust passageway 7. However, because of its brightness, the green glow of light produced by a zirconium oxide or zirconium silicate coating 10 within the exhaust passageway 7, dur ing the high-frequency glow testing of the lamp, still can be observed through a lamp bulb of low light transmissiyity. Moreover, even in the case of devices having bulbs which are completely opaque except for a protruding external stem press, the bright green glow of light produced within the exhaust passageway 7 of the stem press by a zirconium oxide or silicate coating therein, during high-frequency glow testing of the device, nevertheless is still readily observable through the protruding stern press. Of the zirconium-containing glow-producing materials which are suitable as coating materials 10, however, zirconium oxide is preferred because it produces the brightest light glow on high-frequency glow testing of the lamp. I

From the standpoint of their relative effectiveness in maintaining the exhaust tube open during the press-formirig operation, magnesium oxide and the oxide and silicates of zirconium have been found to be the most effective for such purpose. Thus, the use of such materials as the coating 10 on the exhaust tube 9 in the production manufacture of incandescent lamps has been found to produce less than /2 percent of production rejects due to closed exhaust tubes 9.

Where the coating 10 on the exhaust tube 9 consists of an oxide or a refractory material, it is preferably applied thereto as a powder coating. The powdered material is suspended in a suitable vehicle to form a slurry of suitable viscosity which, for the application of a coating to the inside wall of the exhaust tube, is drawn into the exhaust tube the desired distance from the end thereof and then allowed to flow out of the exhaust tube so as to deposit a coating of the slurry on the inside wall of the exhaust tube. The vehicle employed for suspending the powdered coating material may be of any suitable character. For example, where the coating material consists of magnesium oxide, the suspending vehicle may consist of a suitable denatured alcohol such as that commercially known as Synosol. The proportion of magnesium oxide to alcohol is not critical. Thus, 100 to 250 grams of magnesium oxide per 1000 ml. of alcohol will give satisfactory coatings. To reduce and delay the thickening of this slurry, a small amount of acetic acid may be added to the suspension in the range of, for example, a few drops to one or two ml. per 100 ml. of suspension. Further improvement in this respect is obtained by the addition of a small amount of a suitable sequestering agent such as ethylene diamine tetraacetic acid (commonly known as EDTA) for sequestering the free ions present in the suspension and rendering them inactive. Thus, an addition of from /2 to 1 ml. of a 5% solution of ammonium salt of EDTA to 100 ml. of the suspension has been found satisfactory for this purpose. To improve the adherence of the powdered magnesium oxide coating to the wall of the exhaust tube, and at the same time improve .the uniformity of the coatings on the exhaust tubes, at small amount of a suitable binding agent such as ethyl borate may also be added to the suspension. Thus, from 1 to 5 ml. of ethyl borate per 100 ml. of the magnesium oxide suspension will alford the above mentioned improvement.

Where the coating material. 10 consists of zirconium oxide, the suspending vehicle used to form the zirconium oxide suspension preferably comprises water and a small addition of a suitable binder such as polymerized (i.e., a. copolymer of) vinyl methyl ether and maleic anhydride, commonly known as PVM/MA. While other vehicles may be used for suspending powdered zirconium oxide, water is advantageous for such purpose because of its non-explosive and non-toxic character and its relative inexpensiveness. The polymerized vinyl methyl ether and maleic anhydride is used to increase the viscosity of the zirconium oxide slurry, to improve the suspension of the zirconium oxide particles (it acts partially as a dispersing agent), and to give good and consistent coatings on the exhaust tubes. Preferably, in addition, small amounts of ammonium hydroxide and boric oxide are also incorporated in the zirconium oxide slurry. The ammonium hydroxide addition is employed to control the pH of the slurry and thus control its viscosity. The boric oxide addition to the suspension is employed as a fusing or sintering agent to obtain improved adherence of the zirconium oxide powder particles to the wall of the exhaust tube after the binder is burned off. Without boric oxide in the zirconium oxide coating slurry, the zirconium oxide tends to flake off the wall of the exhaust passageway 7 in the press 2 of the finished lamp. While loose zirconium oxide powder in the lamp envelope 1 is not detrimental to lamp quality, nevertheless the presence of such loose powder within the lamp envelope is more or less undesirable from an appearance standpoint.

The application of the coating of slurry to the wall of the exhaust tube 9 is preferably accomplished by positioningthe end of the exhaust tube to be coated so as to touch the surface of asupply of the coating slurry. Capil lary action then draws the slurry up into the exhaust tube.

The height to which the column of slurry rises in the exhaust tube ordinarily is quite variable. However, by adding a small amount of ethyl borate to the slurry, for example, 10 ml. of ethyl borate per 300 grams of slurry, the height of the slurry rise in the exhaust tubes becomes much more uniform.

As a specific example, a suitable zirconium oxide slurry for application to the walls of exhaust tubes 9 may be prepared by milling for an extended period, preferably from 24 to 48 hours or so, a mixture of the following general composition;

Zirconium oxide grams '1200 Water ml 800 Polymerized vinyl methyl ether and maleic anhydride (10%) ml 40 Ammonium hydroxide ml 40 Boric oxide ml 18 After milling of the above ingredients, the milled suspension is washed out of the milling container with 400 ml. of water containing 4 ml. of a suitable wetting agent such as that commercially known as Igepal 530, which is a polyoxyethylated nonylphenol of such ethylene oxide nonylphenol balance as to give medium hydro! phobic qualities. It is a surface-modifying agent used to give coatings free of pin holes and other coating defects. The resulting milled zirconium oxide slurry has excellent keeping properties. For example, even after an extended period of three months or so it does not exhibit any deterioration or gelling. Shortly before use of this zirconium oxide slurry to coat exhaust tubes (preferably not more than 112 hours or so before use), 10 ml. of ethyl borate is added to each 300 grams of the slurry. This ethyl borate addition improves the coating characteristics of the slurry when applied to the exhaust tubes. Inasmuch as the keeping quality of the coating slurry after the addition of ethyl borate is relatively poor, the slurry with the ethyl borate addition should be used within a relatively short period of not more than 24 hours or so.

After the application of the slurry coating to the wall of the exhaust tube 9, the latter is then heated to a temperature sufficiently high, and for a period of time longenough to dry, and burn out the binder in the coating on the exhaust tube and to partially sinter or bind the powder particles of the coating onto the wall of the exhaust tube. To this end, the exhaust tube is heated to a temperature just below the point at which it will soften and distort. For example, in the case of the soft lead or lime glasses which are ordinarily used for exhaust tubes and having a melting point of around 650 C., the exhaust tube should be heated to a temperature of the order of 600 to 650 C. at which temperature the binder will be burned out of the slurry coating on the exhaust tube and the powder particles of the coating will,

be set onto the wall of the tube. A uniform and firmly adherent powder coating is thereby produced on the wall of the exhaust tube which will not flake off during the handling of the exhaust tube and during the formation of the-stem press and after the lamp has been completed.

In the manufacture of the lamp or other device according tothe invention, a glass exhaust tube 9 having a coating 10 thereon, a mount structure 12 and a glass envelope or bulb 1 are first positioned in proper sealing relation to each other (as illustrated in Fig. 1) in one of the sealing heads (not shown) of a conventional type lamp sealing-in machine such as commonly employed in the lamp-making art. As shown, the mount 12 comprises a pair of leadin conductors 3 and the filament 6 which is connected at its ends across the tips of the inner lead portions 5 of the lead-in conductors 3. The bulb'l is supported with its neck end 11 down, and the mount 12 is supported in a vertical position with the heavy end or pin portions 4 of the conductors 3 down and extending part way into the bulb neck 11 and with the filament 6 located within the bulbous portion of the bulb. The glass exhaust tube 9 is supported in a vertically extending position with its coated end 13 uppermost and located within the bulb neck 11 in a position between the two conductors 3.

With the exhaust tube 9, mount 12 and bulb 1 thus supported in place in the sealing head of the sealing machine in position for sealing, the neck 11 of the bulb is suitably heated as by gas fires 14 directed against opposite sides of the bulb neck, as shown in Fig. 1. Upon softening of the glass of the bulb neck,it gathers together and contracts against the upper coated end 13 of the exhaust tube 9 which is therefore heated and softened by radiation and conduction of heat from the hot glass of the bulb neck. The heating of the bulb neck 11 by the gas fires is so controlled as to maintain the temperature of the coated end of the exhaust tube below the temperature at which it becomes appreciably plastic, the coating lfl on the exhaust tube assisting in maintaining this temperature control. The heating is continued until the softened glass of the bulb neck 11 and exhaust tube 9 becomes thoroughly interfused, whereupon the gas fires 14 are removed and, as shown in Figs. 2 and 3, the molten glass of the bulb neck then pinched, as by means of pinching jaws 15 to press the glass completely around the inner lead portions 5 of the conductors 3 as well as around the inner or upper end portions of the pin ends 4 of the conductors 3 and into intimate contact with the glass of the exhaust tube so as to form a solid mass of glass embedding the conductors 3 and constituting a press 2.

During the fusion operation, as well as the subsequent pinching operation, the coating on the wall of the exhaust tube 9 effectively prevents collapsing of the exhaust tube within the press 2 and resultant closing of the bore 8 of the exhaust tube. However, to fully insure against closure of the bore 8 through the exhaust tube 9 by the pressure of the pinching jaws during the pinching operation, the pinching faces 16 of the said jaws are suitably relieved as by being provided with vertically extending semi-cylindrical shaped recesses 17 which, when the jaws are in their fully closed position, are more or less concentric with the exhaust tube 9, as shown in Fig. 3, and leave corresponding semi-cylindrical shaped enlargements 18 in the completed press 2 on each of its opposite sides. Y n a -After the pinching of themolten glass of the bulb neck 11 and exhaust tube 9 to form the press 2 of the lamp, the pinching jaws 15 are removed or opened and gas fires then directed once more against the pressed glass 2 first to further interfuse and work the glass thereof and then to anneal the glass so as to remove any strains developed therein during the pinching operation, the coating 10 on the wall of the exhaust passageway 7 meanwhile continuing to perform its function of maintaining the exhaust passageway open through the stem press 2. g

Upon completion of the sealing operation as above described, the lamp envelope 1 is evacuated to the desired degree of vacuum through the exhaust tube 9 and through the exhaust passageway 7 in the stern press" 2; after which the projecting exhaust tube is then tipped off as. closely as possible to, and preferably at the very junction with the press 2, as shown at 19 in Fig; 4, to thereby hermetically seal the lamp envelope. The tipping-off of the exhaust tube 9 may be accomplished in the conventional manner by directingsharp gas fires against opposite sides of the'exhaust tube at a point immediately adjacent the stem press 2,'the glass wall of the exhaust tube collapsing upon softening and forming a tip 19 closing off the exhaust passageway 7. Because of the physically and chemically stable character of the coating material 10 within the exhaust tube at'the temperature to which it is subjected during this tippingoif operation, no contaminating impurities are formed by the coating material and introduced into the lamp envelope 1 during the tipping-off operation such as to cause contamination of the lamp and adversely affect the operation thereof.

Although a preferred embodiment of our invention has been disclosed it will be understood that the invention is not to be limited to the specific construction and arrangement of parts shown, or to the specific procedures described, but that they may be widely modified within the spirit and scope of our invention as defined by the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In the manufactureof anelectrical device, the method of sealing an end portion of a glass exhaust tube into a surrounding glass member which comprises applying to the inner wall of the said end portion of the exhaust tube a coating of an inorganic material of the group consisting of magnesium oxide and the oxide and silicates of zirconium, heating the surrounding glass member and the inserted coated end portion of the exhaust tube to a temperature sufficient to soften and fuse them together and at which the said coating will react with the underlying inner wall surface of the glass tube to raise the softening point thereof to a degree effective to prevent collapsing of the glass tube and closure of the passageway. therethrough at the saidtemperature, and then pinching the softened glass of the said glass member to form it into a press portion and compress it around the exhaust tube sufiiciently to firmly unite therewith but insufficiently to close the passageway through the said exhaust tube. 5

2. In the manufacture of an electrical device, the method of sealing an end portion of a glass exhaust tube into a surrounding glass-member which comprises applying to the inner wall of thesaid end portion of the exhaust tube a coating of powdered zirconium oxide, heating the surrounding glass member and the inserted coated end portion of the exhaust tube to a temperature sufficient to soften and fuse them together and at which the said coating will react with the underlying inner wall surface of the glass tube to raise the softening point thereof to a degree effective to prevent collapsing of the. glass tube and closure of the passageway therethrough at the said temperature, and then pinching the softened glass of the said glass member to form it into a press portion and compress it around the exhaust tube sufficiently to firmly unite therewith but insufficiently to close the passageway through the said exhaust tube.

.3. In the manufacture of an electrical device, the method of sealing an end portion of a glass exhaust tube into a surrounding glass member which comprises applying to the inner wall of the said end portion of the exhaust tube a coating of a suspension comprising a powdered material of the group consisting of the oxides of zirconium and magnesium suspended in a suitable vehicle and containing a binder, heating the coated exhaust tube to the softening temperature thereof to dry the coating and burn out the binder therein and set the powdered material thereof onto the wall of the exhaust tube, positioning the coated end of the exhaust tube within the glass member, heating the said glass member and the inserted coated end portion of the exhaust tube to a temperature sufiicient to soften and fuse them together and at which the said coating will react with the underlying inner wall surface of the glass tube to raise the softening point thereof to a degree effective to prevent collapsing of the glass tube and closure of the passageway therethrough at the said temperature, and then pinching the softened glass of the said glass member to form it into a press portion and compress it around the exhaust tube sufficiently to firmly unite therewith but insufiiciently to close the passageway through the said exhaust tube.

4. In the manufacture of an electrical device, the method of sealing an end portion of a glass exhaust tube into a surrounding glass member which comprises applying to the inner wall of the said end portion of the exhaust tube a coating of a suspension comprising powdered zirconium oxide suspended in water and containing small amounts of boric oxide, ammonium hydroxide, ethyl borate and a copolymer of vinyl methyl ether and maleic anhydride, heating the coated exhaust tube to the softening temperature thereof to dry the coating and burn out the binder therein and set the powdered material thereof onto the Wall of the exhaust tube, positioning the coated end of the exhaust tube within the glass member, heating the said glass member and the inserted coated end portion of the exhaust tube to a temperature sufficient to soften and fuse them together and at which the said coating will react with the underlying inner wall surface of the glass tube to raise the softening point thereof to a degree efiective to prevent collapsing of the glass tube and closure of the passageway therethrough at the said temperature, and then pinching the softened glass of the said glass member to form it into a press portion and compress it around the exhaust tube sufiiciently to firmly unite therewith but insufficiently to close the passageway through the said exhaust tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,565,598 Sproesser Dec. 15, 1925 1,569,185 Higgins Jan. 12, 1926 2,020,729 Knoeppel Nov. 12, 1935 2,762,168 McCutchen Sept. 11, 1956