US3562571A

US3562571A - Mercury-vapor discharge lamp with amalgam-type vapor-pressure regualtor and integral fail-safe and fast warmup compone

Info

Publication number: US3562571A
Application number: US832665A
Authority: US
Inventors: George S Evans; Chalmers Morehead
Original assignee: Westinghouse Electric Corp
Current assignee: Philips North America LLC
Priority date: 1969-06-12
Filing date: 1969-06-12
Publication date: 1971-02-09
Anticipated expiration: 1988-02-09
Also published as: FR2046718A1; GB1307417A; BE751637A; DE2027893A1; FR2046718B1; JPS508578B1; NL7008659A

Abstract

The time required for an amalgam-regulated fluorescent lamp to stabilize and reach its rated light output under ''''cold-start'''' conditions is reduced by providing a small auxiliary body of amalgam on a selected portion of a metal conductor that is attached to one of the electrode structures within the lamp and is so arranged that it serves the dual function of a ''''failsafe'''' component. The auxiliary body of amalgam is so located with respect to the adjacent cathode that it is rapidly heated and releases mercury vapor as soon as the lamp is energized. The ''''fail-safe'''' conductor is preferably made of wire mesh or sheet metal and provides a conductive path from one of the leads to the stem which causes an arc to impinge upon and puncture the stem when the cathodes are devoid of emission material and the lamp has thus reached the end of its useful life.

Description

Hnited States Patent Inventors George S. Evans Caldwell: Chalmers Morehead. Upper Montclalr. NJ.

Appl. No. 832,665

Filed .Iune I2. 1969 Patented Feb. 9. i971 Assignee Westinghouse Electric Corporation Pittsburgh. Pa. a corporation of Pennsylvania MERCURY-VAPOR DISCHARGE LAMP WITH AMALGAM-TYPE VAPOR-PRESSURE REGULATOR AND INTEGRAL FAIL-SAFE AND FAST WARM-UP COMPONENT 10 Claims. 5 Drawing Figs.

U.S. Cl 313/109. 313/l78Jl3/l8t) Int. Cl .t H0lj (/24 Field otSeareh 3 l 3H0), I74. I78 lill) I56! References Cited UNITED STATES PATENTS 3.227 007 l/lioo Bernier ct al. 3 l 3/178 3,265.)l7 8/1966 Ray 3 l 3/109 Prinmry la'xaminer- Raymond F. Hossl'eld Allornays-A. T. Stratton. W. l). Palmer and l). S. Buleza ABSTRACT: The time required for an amalgam-regulated fluorescent lamp to stabilize and reach its rated light output under cold-start conditions is reduced by providing a small auxiliary body of amalgam on a selected portion of a metal conductor that is attached to one of the electrode structures within the lamp and is so arranged that it serves the dual function of a fail-safe" component. The auxiliary body of amalgam is so located with respect to the adjacent cathode that it is rapidly heated and releases mercury vapor as soon as the lamp is energized. The fail-safe" conductor is preferably made of wire mesh or sheet metal and provides a conductive path from one of the leads to the stem which causes an arc to impinge upon and puncture the stem when the cathodes are devoid of emission material and the lamp has thus reached the end of its useful life.

MERCURY-VAPOR DISCHARGE LAMP WITH AMALGAM-TYPE VAPOR-PRESSURE REGULATOR AND INTEGRAL FAIL-SAFE AND FAST WARM-UP COMPONENT (ROSS-REFERENCES TO RELATED APPLICATIONS The invention disclosed in the present application is related to and is an improvement upon the subject matter disclosed in application Ser. No 381,503, filed July 9. 1964 by George S. Evans. application Ser No. 524,898. filed Feb. 3, 1966 by George S. Evans; and application Ser. No. 678,702, filed Oct. 27, 1967 by George S. Evans and Chalmers Morehead, now US. Pat. No. 3.526,804.All of the aforementioned pending applications are assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to electric discharge lamps and has particular reference to an improved fluorescent lamp in which the mercury-vapor pressure is regulated by means of an amalgam.

2. Description of the Prior Art Low-pressure mercury-vapor discharge lamps that contain a strategically located metal such as indium, cadmium or the like which forms an amalgam with mercury and regulates the mercury vapor pressure within the lamp during operation are well known in the art. A fluorescent lamp of this type is described in US. Pat. No. 3,007,071, issued Oct. 31, 1961 to A. Lompe et al.

While such amalgam-regulated fluorescent lamps achieve the desired objective of providing satisfactory lamp operation over a wide range of ambient temperatures, the amalgam is located in such a position within the lamp envelope that the amalgam heats up slowly. As a result, the lamp operates in a mercury starved" unstable condition for a considerable period of time after it is started and the lamp is at ambient temperature that is, under cold-start conditions. This is undesirable since the lamp does not reach its full light output until the amalgam has heated up and released a sufficient amount of mercury vapor to permit the lamp to operate in a stabilized manner.

It has been found that amalgam-regulated lamps may take 5 minutes or more at room temperature to stabilize and reach the lamp output for which they are rated. The time required for the amalgam to warm up and the released mercury vapor to diffuse throughout the lamp increases, of course, as the ambient temperature decreases. Thus, the problem of slow warmup under cold-start conditions constitutes a serious drawback in outdoor lighting applications where the lamps may be subjected to extremely low ambient temperatures. Comparative tests in a typical four-lamp enclosed outdoor fixture have shown that conventional amalgam-regulated lamps take about 36 minutes to reach stabilized light output when cold started at F. and as long as 60 minutes when cold-started at 20 F.

Various modified amalgam-type fluorescent lamps have been disclosed in the prior art to overcome this slow warmup" problem under cold-start conditions. According to one such prior art modification an auxiliary quantity of amalgamforming metal is placed on one of the anodes or on the glass stem press near the cathode. A lamp having this construction is disclosed in US. Pat. No. 3,227,907, issued Jan. 4, 1966 to C. J. Bernier et al. A somewhat similar arrangement wherein an auxiliary body of amalgam is placed on a metal ring or cap that surrounds but is electrically insulated from the cathode is disclosed in German patent 1,274,228, issued Aug. 1, 1968.

Tests have shown that highly-loaded fluorescent lamps (1500 ma, T12, approximately 23 arc watts per foot of arc length) with an auxiliary quantity of amalgam attached to the anodes exhibited severe end discoloration of the envelope, thus indicating that the anodes operate too hot and cause the amalgam-forming metal to vaporize and migrate to the envelope wall. While placement of the amalgam-forming metal directly on the stem press would reduce the temperature of the amalgam and perhaps avoid the vaporization and bulbdarkening problem, the amalgam would still be located immediately adjacent the cathode and thus operate at a temperature near or above its melting point, particularly in a highlyloaded lamp. Retention of a fluid amalgam on the glass stem press without interfering with the passage of mercury vapor from and to the amalgam constitutes a rather formidable problem Highly-loaded fluorescent lamps operate at current densities such that the arc may anchor on the bulb wall under certain end-of-life conditions and cause the bulbs to crack. As a safeguard against this potential hazard the lamps are provided with an internal fail-safe" component consisting of a conductor that is connected to one of the lead wires and extends to a portion of the glass stem that is adjacent an exterior portion of the lead wire. When the emission material on the cathodes is exhausted, the arc is directed by the fail-safe conductor to the stern thereby puncturing it and rendering the lamp inoperative. A fluorescent lamp wherein the fail-safe" component comprises a wire or a strip of conductive material applied to the stem press is disclosed in US Pat. No. 3,265,917, issued Aug. 9, 1966 to J. G. Ray.

SUMMARY OF THE INVENTION It is accordingly the general object of the present invention to provide a mercury-vapor discharge lamp that rapidly achieves stabilized operation and output under cold-start conditions and will fail safely at the end of its useful life.

Another and more specific object is the provision of a highly-loaded fluorescent lamp that is designed to operate at power loadings in excess if 20 are watts per foot of arc length and overcomes the aforementioned fast warm-up and safety problems by means which can be readily and inexpensively incorporated within the lamp during manufacture and does not produce discoloring deposits on the envelope or otherwise affect the quality or performance of the lamp during its useful life.

The foregoing objects and other advantages are achieved in accordance with the present invention by constructing the fail-safe component in such a manner that it also serves as a holder for an auxiliary quantity of amalgam. in accordance with a preferred embodiment, the fail-safe component is made from wire mesh or sheet metal that is attached to one of the lead wires and has a depending segment that snugly fits around the stem press at its outermost edge and extends transversely from the stem into the lamp interior. The auxiliary quantity of amalgam is placed on the protruding segment of the planar fail-safe conductor with at least a portion of the amalgam on the side of the conductor which faces the adjacent cathode and the arc stream. The amalgam is thus rapidly heated by the cathode and is so located that the released mercury-vapor diffuses directly into the arc stream without circulating through the cool end of the lamp. The spacing between the auxiliary quantity of amalgam and the adjacent cathode is such that the amalgam heats up and releases mercury vapor quickly but operates at a temperature which prevents the amalgam-forming metal from vaporizing and depositing on the envelope walls. The desired rapid stabilization of light output and the elimination of the potential bulb-breakage hazard at the end of lamp life is thus achieved by a single inexpensive component that can be readily attached to the lamp mount during the regular sequence of manufacturing operations.

A fail-safe component is attached to each of the electrode structures. According to the preferred embodiment, each of these fail-safe components is provided with a small quantity of strategically-located amalgamative material in order to provide an auxiliary body of amalgam at both ends of the lamp and thus reduce the warm-up time to a minimum. The main body of amalgam which controls the mercury-vapor pressure during stabilized operating conditions is located remote from both electrode structures and, in the preferred embodiment, is held in place on a tubular part of one of the lamp stems by a wire mesh collar.

BRIEF DESCRIPTION OF THE DRAWING A better understanding of the invention will be obtained by referring to the accompanying drawing, wherein:

FIG. 1 is a side elevational view of a highly-loaded fluorescent lamp that embodies the invention, a portion of the envelope being removed to show the structural features and relationship of the various components;

FIG. 2 is an enlarged perspective view of the mount assembly which is provided with the main and auxiliary bodies of amalgam-forming material in accordance with the present invention and is ready to be sealed into the lamp envelope;

FIG. 3 is an enlarged cross-sectional view through the press seal portion of the stem assembly along the line III-Ill of FIG. 2;

FIG. 4 is an enlarged fragmentary perspective view of an alternative embodiment wherein the dual purpose amalgam holder and fail-safe component is fabricated from sheet metal; and

FIG. 5 is a graph illustrating the warm-up characteristics of the improved lamp at various ambient temperatures compared to those of a standard lamp that contains only the main body of amalgam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS (FIGS. l-3) In FIG. I there is shown a highly-loaded fluorescent lamp which embodies the present invention and includes the usual tubular envelope 12 of light-transmitting vitreous material and glass stems 14 that are fused to the ends of the envelope and extend longitudinally into its interior. Suitable lead-in conductors such as a pair of lead wires l6, 17 are provided at each end of the envelope 12 and are embedded in press seals 18 formed on the inner ends of the glass stems 14 in accordance with standard lamp-making practice. Each of the tubular hollow stems 14 are, accordingly, hermetically sealed at their inner ends and constitute reentrant wall portions of the envelope 12 that define elongated cavities at each end of the lamp 10 that are open to the atmosphere. The outer portions of the

lead wires

16, 17 extend through these cavities and are anchored in the bosses of suitable base members 24 that are composed of insulating material and attached to the sealed ends of the envelope 12, thus providing a set of recessed contacts at each end of the lamp 10.

The inner ends of each set of

lead wires

16, 17 are fastened to a thermionic electrode such as a tungsten wire coil 20 that is coated with a suitable electron-emissive material, for example, the well-known alkaline earth carbonates. The coated coils 20 function as cathodes which sustain the electric discharge within the energized lamp 10. A pair of plate anodes 22 (see FIG. 2) are fastened to the

respective lead wires

16 and 17 and disposed on opposite sides of the associated cathode 20.

The inner surface of the envelope 12 is coated with a layer 25 of a suitable ultraviolet-responsive phosphor, as shown in FIG. I, and the envelope is evacuated and charged with a predetermined amount of mercury and a suitable inert fill gas through an exhaust tube 26 which is subsequently tipped off in the usual manner. The fill gas consists of argon, neon, or a mixture of argon and neon at a pressure of from 0.5 to 3 millimeters of mercury.

The mercury-vapor pressure within the lamp 10 during normal or stabilized operating conditions is controlled by a main body of suitable mercury-amalgamative material 28, such as indium, indium-tin alloy, etc., which is retained on a tubular portion of one of the stems 14 by a wire mesh collar 27 of the type described in the aforementioned pending application Ser. No. 524,898. The main body of amalgam-forming metal is thus divided into segments 28 (as shown in FIG. 2) that are disposed within the mesh collar. As will also be noted in FIG. 2, the collar-amalgam assembly 27 is locked in place on the stem 14 by a wire ring 29.

As illustrated in FIG. 1, the lamp 10 is provided at each end with a small auxiliary quantity of mercury-amalgamative material 30 that is carried by a wire mesh holder 32 which is fastened to one of the lead wires and extends to the press seal 18 of the associated stem 14. The holders 32 serve as fail-safe conductors and are so constructed that small auxiliary quantities of amalgam 30 are spaced a predetermined axial distance x from the adjacent cathode 20.

As shown more particularly in FIG. 2, the auxiliary amalgam holder 32 comprises a rectangular panel of wire mesh that extends transversely from the stem press 18 and has an upstanding tongue T coined from one end thereof, which tongue is fastened as by spot welding to one of the lead wires 17. The end of the mesh holder 32 fromwhich the tongue T is coined accordingly has a slot 33 and is of U-shaped configuration. As will be noted, tlie slotted end snugly straddles the outennost edge of the press seal 18 and is thus located proximate the exterior portion of the lead wire 17 which emerges from the press seal and extends into the stem cavity. The press seal 188 can, if desired, be constricted in this region to form a transverse groove 19 which accommodates the U-shaped end portion of the mesh holder 32.

The rectangular slot 33 formed by cutting the tongue T and bending it upwardly is so dimensioned that the edges of the mesh holder 32 which define the slot are immediately adjacent the embedded portion of the lead wire 17, as shown in FIG. 3. and are thus insulated by the stem 14 from the exterior portion of the lead wire 17 that emerges from the press seal 18 into the stem cavity. Hence, the mesh holder 32 defines an electrically conductive path from an interior part of the lead wire 17 to an exterior part of the same lead and thus serves as a fail-safe component that causes the arc to impinge on the insulating portion of the stem at the end of the life of the lamp and punctures the stem thus rendering the lamp l0 inoperative.

As will be noted in FIGS. 2 and 3, the auxiliary quantity of amalgamative material 30 is preferably a narrow strip of soft metal (such as indium) that is pressed into and embeddably anchored in the planar depending surface of the holder 32 that faces toward the cathode 20. The amalgam metal strip 30 is also preferably located inwardly from the ends of the mesh holder 30 to insure that it does not migrate to the outermost edge and drop off the holder when the metal is in a fluid condition.

As a specific example, satisfactory results with regard to both the warm-up and fail-safe features have been obtained in a 72 inch T12 lamp which operated at 1500 ma (a loading of approximately 23 arc watts per foot of arc length) by using a one-sixteenth inch wide strip 30 of indium that contained l0 milligrams and was centrally embedded in a wire mesh holder 32 that was woven from stainless steel and had a length of about 0.5 inch, 21 width ofabout 0.4 inch and a thickness of approximately 0.005 inch. The length of the tongue T and the point at which it was secured to the lead wire 17 were such that the axial distance (dimension x) in FIG. 1) between the holder 32 and the cathode 20 was approximately 0.5 inch. The actual amalgam-cathode spacing (dimension y) was greater since the amalgam 30 was located to one side of the stem 14. In the 72 inch T12 lamp referred to, this dimension was approximately 0.62 inch. T,he wire mesh collar 27 contained 490 milligrams of indium and the leading edge of the collar was spaced an axial distance of about Hfinches from the cathode 20. The lamp 10 was dosed with approximately I50 milligrams of mercury which was absorbed by the indium and formed the desired amalgam within the lamp after the latter was completed and operated.

ALTERNATIVE EMBODIMENT (FIG. 4)

The dual purpose amalgam holder and fail-safe component can also be made from sheet metal, if desired, as shown in FIG. 4. As will be noted, the sheet metal holder 34 is of the same construction as the mesh holder previously described in that it has an upstanding tongue T which is attached to one of the lead wires 17a of the stem 14a and provides a slot 36 which snugly engages the grooved portion 19a at the outermost edge of the press seal 18a. The amalgam forming metal 38 in this case is melted and bonded directly to the are side of the holder 34. The amalgam-forming metal can also be deposited on the depending segment of the holder 34 by plating, spraying or by any other suitable method. The holder 34 can be stamped from stainless steel or other suitable metal.

TEST RESULTS Comparative test results on 72 inch Tl2 inch highly-loaded fluorescent lamps (1500 ma) embodying the invention have shown that the improved lamps have excellent warm-up characteristics. This is apparent from the output vs. stabilization curves shown in FIG. 5 which were obtained by placing the lamps in an enclosed fixture and starting them cold at various ambient temperatures that was controlled by circulating refrigerated air past the fixtures at a velocity of 5 miles per hour. As is indicated by curve 40, the standard lamps having only the main regulating-amalgam on one of the stems required about 36 minutes to stabilize and reach 100 percent light output at an ambient temperature of F. In contrast, curve 42 shows that the improved lamps having additional small auxiliary quantities of amalgam strategically located at each end of the lamps as described above stabilized and reached their rated light output in only 8 minutes under the same cold-starting and ambient temperature conditions.

At an ambient temperature of 20 ,F, curve 44 shows that the standard lamps required about 29 minutes to achieve 90 percent light output and did not become fully stabilized until after 60 minutes had elapsed (not shown on the graph). The improved lamps (curve 46) reached 90 percent of their rated light output in 13 minutes at the same ambient temperature and reached full light output and stabilized in 28 minutes.

It will be appreciated from the foregoing that the objects of the invention have been achieved in that a very inexpensive and convenient means has been provided for reducing the time required for a fluorescent lamp to stabilize under cold starting conditions and to insure that the lamp fails safely at the end of its life. The invention is thus particularly adapted for use in highly loaded lamps designed to operate at 20 arc watts per foot of arc length and higher power loadings. The use of a single component to achieve both of the aforesaid objectives materially reduces the manufacturing costs of the lamp and enables them to be rapidly assembled on a massproduction basis.

While several embodiments have been illustrated and described, it will be appreciated that various modifications can be made without departing from the spirit and scope of the invention. For example, while indium was referred to in the specific example, other metals or metal alloys can be used with the same advantageous results. An alloy consisting of 95 percent indium and percent tin of the type disclosed in the pending Evans and Morehead application Ser. No. 678,702 (now US. Pat. No. 3,526,804) can thus be employed. ln addition, the holder need not be fabricated entirely from wire mesh or sheet metal but can be made from a combination of these or similar materials.

Moreover, all of the auxiliary amalgam need not be disposed on the arc side of the holder but may extend to the opposite side of the holder. The auxiliary amalgam can be provided on only one of the fail-safe components, although its use on both of these components is preferred. The invention is also not limited to 72 inch Tl 2 lamps but can be employed in highly-loaded lamps of various sizes, for example, 48 inch and 96 inch lamps having envelopes of different diameters (T10, T17, etc.).

We claim:

1. An electric discharge lamp comprising;

a sealed light-transmitting envelope of vitreous material that contains a charge of mercury and has a reentrant wall portion,

a'pair of spaced electrode structures within said envelope adapted to sustain an electric discharge when the lamp is energized,

said electrode structures including lead-in conductor means that extend through the walls of said envelope and are connected to the respective electrode structures,

one of said electrode structures being located proximate the reentrant wall portion of said envelope and the associated lead-in conductor means having an exterior part that is disposed adjacent the outer surface of said'reentrant wall portion,

a fail-safe component of electrically conductive material disposed within said envelope and fastened to said one electrode structure, said component having a depending segment which extends to the part of said reentrant wall portion that is adjacent the said exterior part of the associated lead-in conductor means so that said fail-safe component is electrically insulated from the exterior part of the conductor means by the reentrant wall portion of the envelope,

means for regulating the mercury-vapor pressure withinthe lamp during stabilized operating conditions comprising mercury-amalgamative material disposed at a location that is remote from both of said electrode structures, and

means for rapidly providing mercury vapor within the lamp during the period of unstabilized operation which prevails when the lamp is energized while at ambient temperature comprising an auxiliary quantity of mercury-amalgamative material that is carried by a selected portion of said fail-safe component and is disposed in heat-receptive proximity to said one electrode structure.

2. The electric discharge lamp of claim 1 wherein;

the depending segment of said fail-safe component is of such configuration that it extends transversely from said reentrant wall portion, and

said auxiliary quantity of mercury-amalgamative material is located on the transversely-extending segment of said fail-safe component.

3. The electric discharge lamp ofclaim I wherein;

the reentrant wall portion of said envelope comprises a hollow tubular stem that is closed by a hermetic seal and defines an external cavity,

the lead-in conductor means that is connected to said one electrode structure extends through said hermetic seal and into the stem cavity, and

the depending segment of said fail-safe component extends to the part of said stem that is adjacent the portion of the lead-in conductor means which emerges from said hermetic seal.

4. The electric discharge lamp of claim 3 wherein;

each of said lead-in conductor means comprises a pair of spaced lead wires,

each of said electrode structures include a thermionic electrode that is fastened to and supported by the respective pairs of lead wires, and

said fail-safe component is fastened to an interior part of one of said lead wires.

5. The electric discharge lamp of claim 4 wherein;

said envelope is of elongated tubular configuration and contains an inert fill gas at low pressure,

the inner surface of said envelope is coated with phosphor and said lamp thus comprises a fluorescent lamp,

both ends of said envelope are closed by hollow tubular glass stems that extend longitudinally into the envelope interior,

the respective pairs of lead wires extend through press seals formed on the ends of said stems,

the depending transversely-extending segment of said failsafe component is of planar configuration and is so oriented that one of its planar surfaces faces toward the proximate thermionic electrode; and

said auxiliary quantity of mercury-amalgamative material is located on the planar segment of said fail-safe component and is so disposed that part of said material is on the surface of said planar segment that faces said proximate thermionic electrode.

6. The fluorescent lamp of claim 5 wherein;

said thermionic electrodes comprise refractory wire coils that are coated with electron-emissive material and thus function as cathodes,

an anode is disposed adjacent each of said cathodes and supported in such position by one of the associated lead wires, and

said lamp is of the highly-loaded type and adapted to be operated at a predetermined power loading in excess of 20 are watts per foot of arc length.

7. The fluorescent lamp of claim wherein;

said fail-safe component comprises a rigid panel of sheet metal that has a protruding tongue which is secured to the inner part of one of the associated lead wires, and

said auxiliary quantity of mercury-amalgamative material comprises an adherent body of amalgam-forming metal.

8. The fluorescent lamp of claim 5 wherein;

said fail-safe component comprises a rigid panel of wire mesh that has a protruding tongue,

said tongue is fastened to the interior part of one of the associated lead wires, and

the auxiliary quantity of mercury-amalgamative material comprises a metallic body that is embeddably anchored to the planar transversely-extending portion of said wire mesh panel.

9. The fluorescent lamp of claim 8 wherein;

said protruding tongue is coined from one end of the wire mesh panel and the latter is of slotted U-shaped configuration,

the press seal of the associated stern has a transversely-extending construction therein which is located proximate the outer edge of the seal from which the associated lead wires emerge,

the slotted U-shaped end of the wire mesh panel is disposed in straddled close-fitting relationship with the constricted portion of said press seal, and

said metallic body of mercury-amalgamative material comprises a narrow strip which is located on the surface of said wire mesh panel that faces the associated thermionic electrode and is spaced inwardly from both ends of said panel.

10. The fluorescent lamp of claim 8 wherein;

a wire mesh fail-safe component with an anchored body of metallic mercury-amalgamative material is attached to one of each pair of lead wires and an auxiliary source of mercury vapor is thus provided at each end of the lamp, and

the mercury-amalgamative material which regulates the mercury-vapor pressure during stabilized operating conditions is located on one of said stems remote from said fail-safe component and the associated thermionic electrode.

Claims

2. The electric discharge lamp of claim 1 wherein; the depending segment of said fail-safe component is of such configuration that it extends transversely from said reentrant wall portion, and said auxiliary quantity of mercury-amalgamative material is located on the transversely-extending segment of said fail-safe component.
3. The elEctric discharge lamp of claim 1 wherein; the reentrant wall portion of said envelope comprises a hollow tubular stem that is closed by a hermetic seal and defines an external cavity, the lead-in conductor means that is connected to said one electrode structure extends through said hermetic seal and into the stem cavity, and the depending segment of said fail-safe component extends to the part of said stem that is adjacent the portion of the lead-in conductor means which emerges from said hermetic seal.
4. The electric discharge lamp of claim 3 wherein; each of said lead-in conductor means comprises a pair of spaced lead wires, each of said electrode structures include a thermionic electrode that is fastened to and supported by the respective pairs of lead wires, and said fail-safe component is fastened to an interior part of one of said lead wires.
5. The electric discharge lamp of claim 4 wherein; said envelope is of elongated tubular configuration and contains an inert fill gas at low pressure, the inner surface of said envelope is coated with phosphor and said lamp thus comprises a fluorescent lamp, both ends of said envelope are closed by hollow tubular glass stems that extend longitudinally into the envelope interior, the respective pairs of lead wires extend through press seals formed on the ends of said stems, the depending transversely-extending segment of said fail-safe component is of planar configuration and is so oriented that one of its planar surfaces faces toward the proximate thermionic electrode; and said auxiliary quantity of mercury-amalgamative material is located on the planar segment of said fail-safe component and is so disposed that part of said material is on the surface of said planar segment that faces said proximate thermionic electrode.
6. The fluorescent lamp of claim 5 wherein; said thermionic electrodes comprise refractory wire coils that are coated with electron-emissive material and thus function as cathodes, an anode is disposed adjacent each of said cathodes and supported in such position by one of the associated lead wires, and said lamp is of the highly-loaded type and adapted to be operated at a predetermined power loading in excess of 20 arc watts per foot of arc length.
7. The fluorescent lamp of claim 5 wherein; said fail-safe component comprises a rigid panel of sheet metal that has a protruding tongue which is secured to the inner part of one of the associated lead wires, and said auxiliary quantity of mercury-amalgamative material comprises an adherent body of amalgam-forming metal.
8. The fluorescent lamp of claim 5 wherein; said fail-safe component comprises a rigid panel of wire mesh that has a protruding tongue, said tongue is fastened to the interior part of one of the associated lead wires, and the auxiliary quantity of mercury-amalgamative material comprises a metallic body that is embeddably anchored to the planar transversely-extending portion of said wire mesh panel.
9. The fluorescent lamp of claim 8 wherein; said protruding tongue is coined from one end of the wire mesh panel and the latter is of slotted U-shaped configuration, the press seal of the associated stem has a transversely-extending construction therein which is located proximate the outer edge of the seal from which the associated lead wires emerge, the slotted U-shaped end of the wire mesh panel is disposed in straddled close-fitting relationship with the constricted portion of said press seal, and said metallic body of mercury-amalgamative material comprises a narrow strip which is located on the surface of said wire mesh panel that faces the associated thermionic electrode and is spaced inwardly from both ends of said panel.
10. The fluorescent lamp of claim 8 wherein; a wire mesh fail-safe component with an anchored body of metallic mercury-amalgamative material is attached to one of each pair of leAd wires and an auxiliary source of mercury vapor is thus provided at each end of the lamp, and the mercury-amalgamative material which regulates the mercury-vapor pressure during stabilized operating conditions is located on one of said stems remote from said fail-safe component and the associated thermionic electrode.