US2673304A - Crater lamp - Google Patents

Description

March 23, 1954 N. c. BEESE CRATER LAMP Filed Oct. 25, 1951 W5 m IC. AVA d Patented Mar. 23,1954

CRATER LAMP Norman Q. Beese,oVerona, N. J assignor'to. West.-

inghouse- Electric Corporation, East, Pittsburgh. Pa., a corporation of Pennsylvania Application October 26, 1951, Serial No. 253,406

2 Claims.

This invention relates to lamps and, more particularly, to improved electrodes for crater lamps.

The crater lamp. is a. low wattage gas discharge device designed to produce a point light source that can be readily modulated. It is used primarily by wire photo news services for receiving pictures transmitted on telephone lines. At the transmitting station the picture to be transmitted is mounted on a slowly revolving, drum and scanned circumferentially by a minute pencil of light directed from a. filamentary light source by an optical system. The. light reflected from the picture is converted into electrical energy by means of a photocell. This electrical energy is in turn sent. over the wire to the receiver. At the receiving station an amplifier builds up the signal strength of the received electrical energy before it is superimposed on the carrier current of the crater lamp to modulate its light output. The light emitted by the crater lamp is directed by a suitable optical system on to a film mounted on a rotating drum whose speed is synchronized with that, of the transmitting station drum. As the scanning light on the film varies due to fluctuations in. the reflected light at the transmitting station, the result produced on the film is a negative of the picture transmitted.

The crater lamp consists of two electrodes, one of which is of the cold cathode type. The cathode electrode is machined or gouged out to form a crater within. The other electrode. which acts as an anode is a metal cap surrounding the crater and having an aperture axial to the crater. The light produced by the inert gas discharge between the electrodes has a high actinic value.

The conventional cathode electrode is, a metal rod with a hole or crater drilled into one end. This hole has straight sides and an essentially fiat surface at. the bottom. When Viewed along the axis of the lamp, the light produced forms a ring-shaped luminous area having av highlight intensity donut-like sheath or ring near the sides of the hole where the electric field is intense and a relative lesser light intensity at the center. This non-uniformity of brightness across the cathode crater prevents clear definition of detail on the receiving station film.

The conventional cathode electrode is manufactured by vacuum baking pulverulent thorium to sinter or frit theparticles into a coherent rod and then machining the-porous sintered rod to the desired crater-like configuration. Due to the'porous structure of the cathode electrode, water vapor is absorbed therein during fabricae tion. During lamp manufacturing and normal use this vapor dissociates due to cathode heating by the discharge, liberates hydrogen and raises. the lamp breakdown voltage. On cooling, the thorium cathode, having a getter-like afiinity for the hydrogen, re-absorbs the gas. Lamps made with sintered thorium electrodes often have unstable operating characteristics, have variable light output during life, and the light output fadesv during the first few minutes. of operation at any given current setting.

Hence it has been found advantageous, accordingto my invention, to provide an improved cathode electrode for a. crater lamp. having a uniform light distribution across. the. crater, stable operating characteristics, constant light output with long life, and freedom from cyclic fading. To. produce the desired characteristics. of stable operation, constant output. and freedom from fading in such a lamp, I vacuum melt or cast thorium powder for the cathode into a. fused rod to eliminate any intersticial voids into. which water vapor can be absorbed and from which hydrogen can be liberated during operation of the lamp.

To produce uniform light intensity across the crater, I increase the. light intensity at the center of the crater by introducing a zone of high electric field intensity thereat in any of the following ways An axial hole provided in the bottom of the conventionally-shapedcrater produces light which augments. that of the main crater hole, to give essentially uniform luminosity across the entire area. In lieu of the axe ial hole, I may provide a small wire on the axis of the. conventional Crater where the field is normally a minimum, to create a zone. of high field intensity thereat, thereby improving the uniformity of light emitted therefrom. Again the straight. cylindrical sides. of the conventional crater may be replaced by inverted conically ta r d s des to ain mp o d l ht di t ibu ion- Use of the axial hole or the axial wire with the comically tapered crater further improves light distribution across the crater.

In its general aspect, the present invention has as its object the elimination of the disadvantages and defects of the prior art crater lamps.

A spe ific. obj t of. the i v ti n is, an. m-. proved cathode electrode for a crater lamp having a uniform light distribution across the crater.

Another object is an improved cathode elec, trode for a crater lamp having stable operating characteristics, constant light output. with long 3 life, and freedom from fading during the first few minutes of operation at any given operating current.

Another object is an improved cathode electrode for a crater lamp which Will not absorb water vapor during fabrication and liberate dissociated hydrogen during lamp operation.

A further object is an improved'cathode electrode for a crater lamp in which any intersticial voids are eliminated.

Another object is an improved cathode electrode for a crater lamp which has a crater and a surface for creating a zone of high electric field intensity along the crater axis.

An additional object is an improved electrode for a crater lamp which has a conventional straight-sided cylindrical crater hole and an axial hole in the bottom thereof for producing light which augments that of the main crater hole.

A still further object is an improved cathode electrode for a crater lamp which has a small Wire on the axis of a conventionally-shaped crater hole for introducing a zone of high electric field intensity thereat and improving the uniformity of light therefrom.

A further object is an improved cathode electrode for a crater lamp which has an inverted conically-shaped crater with tapered sides for achieving improved light distribution.

Another object is an improved cathode electrode for a crater lamp which has an inverted conically-shaped crater with tapered sides and an axial hole in the bottom thereof for augmenting the light from the main crater hole.

Another object is an improved cathode electrode for a crater lamp which has an inverted conically-shaped crater with tapered sides and a small wire on the axis of the crater for introducing a zone of high electric field intensity thereat and improving the uniformity of light therefrom.

Other objects of the invention will appear to those skilled in the art to which it appertains as the description thereof proceeds both by direct recitation thereof and by implication from the context.

Referring to the accompanying drawing, in which like numerals of reference indicate similar parts throughout the several views:

Fig. 1 is an elevational view of a crater lamp embodying my invention;

Fig. 2 is an enlarged sectional view of an electrode mount for the lamp of Fig. 1, and showing an embodiment of my improved cathode electrode;

Fig. 3 is a detailed sectional view of the electrode mount of Fig. 2, showing a conventionallyshaped crater with an axial hole in the bottom thereof;

Fig. 4 is a plan view of the electrode mount of Fig. 2, showing especially the uniform light distribution across the crater, applicable to all embodiments of my invention;

Fig. 5 is a view similar to Fig. 3 of an alternative embodiment of my invention, wherein a conventional crater is shown with an axial wire therein;

Fig. 6 is a view similar to Fig. 3 of a preferred embodiment of my invention, wherein is shown an inverted conically-shaped crater with an axial wire therein;

Fig. '7 is a view similar to Fig. 3, wherein is shown an improved cathode electrode having an inverted conically-shaped crater with an axial hole therein;

Fig. 8 is a view also similar to Fig. 3, showing the conventional crater of the prior art cathode electrodes.

Fig. 9 is a plan view of the mount of Fig. showing the donut-like ring of high light intensity along the sides of the conventional crater and the center portion of relatively lesser light intensity. v

Referring to the drawing, and more particularly to Fig. l, a gas discharge device, for producing a point light source that can be readily modulated, such as a crater lamp, is designated by the reference numeral 10. This lamp l0 comprises a tubular vitreous envelope [2, an electrode mount l4 sealed to the open end thereof, a pre-focused base It secured about the seal by a suitable cement and a mixture of inert gases within the envelope, such as neon, argon and nitrogen, for starting and sustaining the discharge.

This electrode mount I4 comprises a reentrant type vitreous stem l8 (Fig. 1), a cathode electrode 20 (Fig. 2) on said stem, a ceramic insulator 22 about said cathode electrode 20, and an inverted cup-like anode electrode 24 on said insulator 22. The stem l8 has a press 26, an exhaust tubulation 28 depending therefrom and three leading-in conductors projecting therefrom, namely, an anode conductor 30, a cathode conductor 3| and a dummy conductor 32. A threaded cathode support stud 34 is secured, as by brazing, to the inner end of cathode conductor 31 for axially mounting the cathode 20 thereon by screwing said stud into a suitable hole 35 in the lower end of the cathode.

This cathode 20 is a rod-like electrode, suitably vaouum melted thorium metal. Whereas melted or cast thorium is preferred because of its superior machining properties, it will be understood that my improved cathode may also be either fabricated from pure zirconium with a small amount of oxygen such as 0.5% in solid solution therein, or comprise a nickel or iron shell with a misch metal alloy core, such as 24% iron, 1% magnesium and 75% misch metal. Misch metal is an alloy containing approximately 50% cerium with the remainder other rare earth metals, such as lanthanum, neodymium, etc. In the upper end of cathode 20, shown in Figs. 2 and 3, there is provided an axial crater or hole 36 and a surface 38 for creating a zone of high electric field intensity along the crater axis. In the showing of Figs. 2 and 3 crater 36 has the conventional cylindrical shape and the surface 38 is provided by the side walls of an axial hole 39 in the bottom of crater 36.

The ceramic insulator 22, suitably cylindrical in shape, is axially hollowed to slide over cathode 20 with a small amount of clearance and is provided with an inner lip or shoulder 42 on the upper end which rests on the top surface of cathode electrode 20. The outside diameter of said insulator is such that it slides between'anode conductor 3!], and dummy conductor 32 with sufficient clearance to permit the insertion therebetween of the anode 24. This anode 24, suitably nickel, fits over the upper end of insulator 22 and is provided with an aperture 44 therein axial to crater hole 36. The anode is secured, as by welding, to the upper ends of anode conductor 30 and dummy conductor 32, thereby locking the insulator 22 and hence the cathode 2B in a prede termined spaced relationship.

An electrode mount i4 is shown in Fig. 5 having a cathode electrode 20*, ceramic insulator 22,

and an anode electrode 24 mounted on stem l8 (not shown) as described above. This electrode 2!] is pure zirconium metal with a small amount of oxygen in solid solution therein and is provided with a conventional crater 36 and a surface 38 such as is provided by a small axial wire #5 suitably tungsten .010 to 0.020 inch in diameter, on the crater axis. This wire extends preferably /2 to of the distance from the bottom of crater 36 to the top.

In Fig. 6 an electrode mount 14* is shown having a cathode electrode suitably cast thorium, a ceramic insulator 22 and an anode electrode 24. The cathode electrode 20* is provided with a crater 36 having inverted conically-shaped side walls and a surface 38 in this case provided by a wire 45 similar to the wire 45 as shown in Fig. 5.

An electrode mount id shown particularly in Fig. 7, differs from electrode mount [4 shown in Fig. 6, in that the cathode electrode 20" consists of a nickel or iron shell 48 with a misch metal alloy core and is provided with an inverted conically-shaped crater 36 and a surface 38, in this case provided by the side walls of an axial hole 39 in the bottom thereof similar to the hole 39, shown in Figs. 2 and 3.

The conventional electrode mount hi shown in Figs. 8 and 9, comprises a porous sintered thorium cathode electrode 29 having a conventional crater 36, a ceramic insulator 22 and a cup-like anode electrode 24. As shown particularly in Fig. 9, the light distribution produced from said crater 35 is non-uniform in intensity and consists of an outer ring or sheath along the side walls of crater 36 of high light intensity and a center portion of relatively lesser light intensity. Due to the porosity of electrode 20 and absorption of water vapor therein, lamps with such electrodes often have unstable operating characteristics, varying light output over a long life, and are subject to cyclic fading.

According to my invention, I have found that a lamp II! with a melted thorium cathode electrode 20 gives stable operating characteristics, uniformity between individual lamps of one batch and successive lots, constant light output over a long life and freedom from fading at one setting with length of burning or increase in cathode operating temperatures.

The improved cathode electrode, shown in Figs. 2 and 3, or electrode 20, shown in Fig. 6, for a crater lamp I 0, preferably melted thorium metal, is manufactured by the powder metallurgy technique. Pulverulent thorium powder is vacuum baked in a ceramic holder or in a molybdenum container with a ceramic liner at temperatures in excess of 1850 C. to melt the powder. The molten metal is then cast into a fused rod-like electrode without any intersticial void spaces in which water vapor can be absorbed during the subsequent cathode fabrication operations. This fabrication includes providing the threaded hole 35 in the lower end, as well as the crater 35 and surface 38 for creating a zone of high electric field intensity along the crater axis in the upper end of electrode 20. It will be understood that the zirconium cathode electrode 20, shown in Fig. 5

and electrode 20 having a metallic shell 48 with a misch metal alloy core 50, shown in Fig. 7, do not absorb water vapor and hence eliminate the same undesirable lamp characteristics.

The cathode 20, the insulator 22 and the anode 24 are mounted on the stem l8 to form the electrode mount l 4 which is sealed to the envelope I2. After a suitable exhaust which may consist of a bake, induction heating, and bombardment of the metal electrodes 20 and 24, and a final fill with a mixture of inert gases, the lamp I0 is tipped off and the base 18 cemented about the sealed portion of the envelope I2. It will be understood that during the bombardment operation on exhaust the central wire 45 and 45'", shown in Figs. 5 and 6 respectively, becomes coated with sputtered cathode material from the side walls of crater 36 and 36 Further uniform light distribution across the crater 36 of cathode 20 and hence from the lamp I0 is obtained by employment of a surface 38 for creating a zone of high electric field intensity along the axis of said crater, either by an axial hole in the bottom of said crater, such as hole 39 in Figs. 2 and 3 and hole 39, or by central wire 45 in Fig. 5 and 45 in Fig. 6. Further, I have found the preferred combination of either the axial wire 45* or the axial hole 39 and an inverted conically-shaped crater, such as 36 and 36, as shown in Figs. 6 and 7, produces a further improved uniform light distribution across the crater.

Whereas a preferred embodiment of my invention has been disclosed, it will be understood that modifications may be made within the spirit and scope of the appended claims.

I claim:

1. A gas discharge device for producing a point light source that can be readily modulated, comprising an envelope, an electrode mount sealed to said envelope, and an inert gas in said envelope for initiating and sustaining a discharge, said mount comprising a, stem, a cast thorium cathode electrode on said stem, an insulator about said cathode and an anode electrode on said insulator, said cathode being vacuum melted to eliminate any interstitial voids therein and provided with a. crater having inverted conically-shaped side walls and an axial wire extending to the distance from the bottom to the top of said crater.

2. An improved cathode for a crater lamp of zirconium vacuum melted to eliminate any interstitial voids therein and with a small amount of oxygen in solid solution therein provided with a. crater having inverted conically shaped side walls and an axial wire extending to the distance from the bottom of said crater to the top.

NORMAN C. BEESE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,915,934 Holst June 27, 1933 1,936,793 Knowles Nov. 28, 1933 2,190,308 Blackburn Feb. 13, 1940 2,433,809 Clapp Dec. 30, 1947

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