US2849637A

US2849637A - Electrode for fluorescent lamp

Info

Publication number: US2849637A
Application number: US563018A
Authority: US
Inventors: Weiss Harry
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-02-02
Filing date: 1956-02-02
Publication date: 1958-08-26
Anticipated expiration: 1975-08-26

Description

z- 26, 1958 H. WEISS 2,849,637

ELECTRODE FOR FLUORESCENT LAMP Filed Feb. 2, 1956 IN V EN TOR. HARRY WEISS AT TORNEYS United States Patent ELECTRODE FOR FLUORESCENT LAMP Harry Weiss, Larchmont, N. Y. Application February 2, 1956, Serial No. 563,018

8 Claims. (Cl. 313-109) This invention relates to fluorescent lamps, and more particularly to the electrodes thereof.

Some fluorescent lamps of the hot cathode type require preheating, and therefore require a starter. More recent fluorescent lamps are made without a starter, these being hot cathode instant start lamps, frequently referred to as slimline lamps. The electrode is a wire filament, usually tungsten, coated with an electron emissive material, usually alkaline earth oxides. The filament may be a coiledcoil, or a coiled-coil-coil.

During operation of a lamp some emission from the electrode may sputter on to the glass envelope at each end, causing a darkening which is unsightly and reduces illumination. Finally the lamp may fail. The belief (probably erroneous) up to now was that use of a great thickness or filling of the alkaline earth oxide would increase the life of the lamp.

The primary object of the present invention is to prolong the life of fluorescent lamps. Lamp life is dependent largely on the number of starts, rather than on the total number of operating hours, and accordingly another object of the present invention is to increase the number of starts of a fluorescent lamp before darkening appears. Further objects are to generally improve fluorescent lamps and their electrodes.

To accomplish the foregoing general objects, and other more specific objects which will hereinafter appear, my invention resides in the fluorescent lamp and electrode elements, and their relation one to another, as are hereinafter more particularly described in the following specification. The specification is accompanied by a drawing, in which:

Fig. 1 is a front elevation of a slimline fluorescent lamp.

Fig. 2 shows an electrode prior to coating the same with electron emissive material;

Fig. 3 is a section through a fragment of the electrode while still in filamentary form, with the upper half in section and the lower half in elevation;

Fig. 4 is a similar fragmentary section after a first coating with colloidal graphite;

Fig. 5 is a similar fragmentary section after a second coating of electron emissive material;

Fig. 6 is a fragmentary view of one end of the electrode similar to the righthand end of Fig. 2, but showing the electrode after being coated with electron emissive material;

Fig. 7 is an elevation of a glass end with an electrode; and

Fig. 8 is a fragmentary elevation of one end of a slimline fluorescent tube.

Referring to the drawing, and more particularly to Fig. l, the fluorescent lamp there shown is of the slimline type, and comprises a glass tube 12 coated on the inside with conventional phosphors or fluorescent coating. The ends have

caps

14 and 16, with a

single terminal pin

18 and 20 on each end.

Referring now to Fig. 7, the electrode 22 is in helical form, and is supported by spaced, relatively

rigid support wires

24 and 26 which pass through a glass seal or reentrant stem 28 formed integrally with a frusto-conical glass end 30. The support 24 is extended as a wire 32, and the support 26 is extended as a wire 34.

The glass end shown in Fig.7 ishoused in one cap (14 or 16) of the fluorescent tube 12 shown in Fig. l, and is sealed to the glass tube around the periphery 36. The electrode at the other end of the lamp is nearly similar, and is similarly mounted in a conical glass end, except that the other glass end is provided with a glass tube (not shown) projecting outwardly from its center. This tube may be connected to a suitable vacuum apparatus to exhaust the lamp.

Referring now to Fig. 2, the helix 22 supported between

wires

24 and 26 may be a coiled-coil, and is preferably a coiled-coil-coil. More specifically, a filament wire is initially coiled to form a long slender minor coil. The individual turns of this minor coil are shown at the right end of Fig. 2, and for the rest of the helix are indicated by the double line 40. The minor coil is itself coiled to form what is heretermed a major coil, and the turns or coils of the major coil are shown at 42. These are disposed on a relatively straight axis at each end where they are held by

wires

24 and 26, and are disposed on a helical axis between the

wires

24 and 26, in order to form the main or largest coils or electrode helix 22.

Fig. 3 corresponds to a fragment of the major coil taken, for example, at the straight part 48 in Fig. 2. Fig. 3 does not show the main electrode helix 22 at all. Fig. 3 is drawn to larger scale, and shows the individual turns of the minor coil. The wire itself is shown at 44. The minor coil is shown at 40, and the winding of the minor coil into the coils of the major coil is shown at 42.

The dipping or coating of the filament preferably takes place after the filament has been secured across the

support wires

24, 26. In accordance with my invention, the supported filament is first dipped into a colloidal suspension of graphite. Instead of dipping, it may be coated with the aid of a fine brush. At this time the interstices between the successive coils of the minor coil are filled with the graphite, and this change is shown by comparison of Fig. 3 with Fig. 4 of the drawing. The showing at the left of Fig. 2 also corresponds to Fig. 4, following the graphite coating, for the individual turns of the minor coil are not shown in most of Fig. 2.

The electrode is next either dipped in a suspension of electron emissive material, or it may be coated by means of a brush, and at this time the interstices between the successive coils of the major coil are filled, as shown in Fig. 5, which may be compared withFig. 4. The appearance of the electrode at this time is also shown in Fig. 6, which corresponds to the right end of Fig. 2. It is important to note that even after two coatings, the spaces between the coils of the electrode helix are not filled, and therefore the electrode remains a helix, as shown at 22 in Fig. 7.

The electrodes are air dried after the graphite coating, and prior to the second coating. After the second coating the electrodes are again air dried, and the glass ends, shown in Fig. 7, are then fused to a previously coated fluorescent tube of appropriate dimension.

To drive off volatile material, and to convert the elec-' tron emissive coating to oxides (usually initially applied as carbonates) the lamp is heated as it is evacuated. For this purpose the entire lamp is heated in a furnace while the lamp is connected to a vacuum pump, and the

conductors

32 and 34 shown in Fig. 7 are connected to a heating circuit which passes an electrical current through the electrode filament in order to additionally heat the same. There is a heating circuit of this type at each end, in addition to the overall furnace heating. When the heating and evacuation of the lamp have been completed, the glass tube leading to the pump is sealed and cut off. Because the lamp is an instant-start lamp, the

conductors

32 and 34 are twisted together to form a single conductor.

Caps

14 and 16 are cemented to the ends of the lamp, with the

twisted conductor

32, 34 passing through a single tubular metal terminal pin, as shown less 20 in Fig. 8. The excess length is cut otf and the twisted wire is soldered to the terminal pin, all in accordance with conventional practice.

The filament wire may be made of any of the metals or alloys commonly employed for the present purpose, most typically tungsten. The carbon applied to the electrode is preferably a suspensionof colloidal graphite in distilled water. A paste of colloidal graphite known commercially as Aqu-adag may be suspended in distilled water, using a ratio of one pound of Aquadag to anywhere from one quart to one gallon of distilled water. The density of the suspension depends on the dimensions of the coil structure of the electrode and on the method of applying the graphite, etc.

After air drying the electrode it is next coated with the electron emissive material, which preferably comprises a mixture of barium and strontium salts, preferably carbonates suspended in a suitable vehicle. Butyl acetate with a slight amount of lacquer added is a satisfactory vehicle. The carbonates are changed to oxides during subsequent heating and evacuation.

The coating of the electrodes is preferably performed after mounting the tungsten wire coils on the glass ends. Laterthe glass ends are secured to the glass fluorescent tube, and then heating and evacuation proceeds. The coated tube may be of conventional manufacture. The lamp is heated in a furnace to a temperature of, say, 700 to 800 F. The filaments are preferably simultaneously heated more directly by passing a suitable current through the electrodes to bring them up to a yellow to white heat, say 900 to 1200 C., while evacuating the glass tube through a glass stem provided in one of the two glass ends. This process may go on for, say, fifteen or twenty minutes, until a desired vacuum is reached, say less than one-half micron of pressure. The temperature is kept relatively low, as indicated, because I do not desire to form any metal carbides by combination of the filament with the graphite. I am not certain of the action, and the invention may be considered largely empirical. I believe the graphite is left unchanged, and that the alkaline earth carbonates are changed to oxides by driving elf carbon dioxide gas, which is evacuated by thevacuum pump.

It is know that the useful life of the lamp, judged particularly by darkening of the lamp surface, depends predominantly on the number of starts, rather than on the number of hours of continuous operation. One test of fluorescent lamps is to operate the same for three hours on and three hours off, in repeated cycles. I have employed an accelerated test in which the lamps are turned on for five minutes and then otf for five minutes. The lamps tested were Type 96T12 (96" long and 1 /2" D) using a normal ballast current of 425 milliamperes. Ordinary lamps as previously made began to show appreciable darkening after two or three days of this accelerated test, whereas lamps made in accordance with my invention ran for twenty to thirty days before appreciable darkening appeared.

It is believed that the method and structure of my improvement, as well as the advantages thereof, will be apparent from the foregoing detailed description. It will be understood that while I have shown and described an electrode the filament of which is a coiled-coil-coil, the invention is also applicable to electrodes in which the filament is a coiled-coil. It will also be apparent that while I have shown and described my invention in a preferred form, changes may be made without departing from the scope of the invention, as sought to be defined in the following claims. In the claims the term minor coil applies to the smallest diameter coil, shown in Fig. 3, and at the right of Fig. 2. The term major coil refers to the next larger diameter coil, shown in Fig. 3, or at the left of Fig. 2. It will be understood that in Fig. 2 what appears to be a single wire at the left is in fact the minor coil, with no attempt to show the individual turns of the minor coil. The term electrode helix refers to the largest coils, which are not shown at all in Fig. 3, but which are shown in Figs. 2 and 6.

I claim:

1. A fluorescent lamp comprising a glass tube with fluorescent coating and an electrode at each end, the electrodes each comprising a filament wire coated with 'carbon and additionally coated with an electron emissive coating.

2. A fluorescent lamp comprising a glass tube with fluorescent coating and an electrode at each end, the electrodes each comprising a coiled-coil of filament wire coated with graphite sufficient to fill the interstices between successive coils of the minor coil, and additionally coated with an alkaline earth electron emissive coating, the latter coating being sufficient to fill the interstices between the successive coils of the major coil.

3. A hot cathode instant-start slimline fluorescent lamp comprising a glass tube with fluorescent coating and an electrode at each end with a single pin terminal, the electrodes each comprising a coiled-coil of filament wire coated with graphite, and additionally coated with an electron emissive coating comprising barium and strontium oxides, the said coatings being sufiicient to fill the interstices between at least some of the coils.

4. A fluorescent lamp comprising a glass tube with fluorescent coating and an electrode at each end, at least one electrode comprising a coiled-coil-coil of filament wire coated with graphite sufficient to fill the interstices between successive coils of the minor coil, but not to fill the interstices between the coils of the major coil, and additionally coated with an electron emissive coating sufficient to fill the interstices between the successive coils of the major coil, but not to fill'the spaces between the coils of the electrode helix.

5. An electrode for a fluorescent lamp, said electrode comprising a filament wire coated with carbon and additionally coated with an electron emissive coating.

6. An electrode for a fluorescent lamp, said electrode comprising afilament wire formed. into a coiled-coil, said filament wire being coated with graphite suflicient to fill the interstices between the successive coils of the minor coil, said electrode being additionally coated with an alkaline'earth electron emissive coating suflicient to fill the interstices between the coils of the major coil.

7. An electrode for a fluorescent lamp, said electrode comprising a filament wire formed into a coiled-coil, said filament wire being coated with graphite, said electrode being additionally coated with an electron emissive coating comprising barium and strontium oxides, said coatings being sufficient to fill the interstices between at least some of the coils.

8. An electrode for a fluorescent lamp, said electrode comprising a filament wire formed into a coiled-coil-coil, said filament wire being coated with graphite sufficient to fill the interstices between the successive coils of the minor coil, but not to fill the interstices between'the coils of the major coil, said electrode being additionally coated with an electron emissive coating sufficient to fill the interstices between the coils of the major coil, but not to fill the spaces between the coils of the electrode helix.

References Cited in the file of this patent UNITED STATES PATENTS 1,961,814 Charlton June 5, 1934 2,479,193 Zabel Aug. 16, 1949 2,496,065 O Hearn Jan. 31, 1950

Claims

1. A FLUORESCENT LAMP COMPRISING A GLASS TUBE WITH FLUORESCENT COATING AND AN ELECTRODE AT EACH END, THE ELECTRODES EACH COMPRISING A FILAMENT WIRE COATED WITH CARBON AND ADDITIONALLY COATED WITH AN ELECTRON EMISSIVE COATING.