US2682007A - Compact type electrical discharge device - Google Patents

Description

INVENTORS- David J, Hayley y fPez/erefi faerfsozz June 22, 1954 D. L. HILDER ETAL COMPACT TYPE ELECTRICAL DISCHARGE DEVICE Filed Jan. 11, 1951 Patented June 22,1954

COMPACT TYPE ELECTRICAL DISCHARGE DEVICE David L. Hilder, Allentown, Pa., and Revere H.

Robertson, Westfield, N. J., assignors to Hanovia Chemical and Manufacturing Company, Newark, N. J., a corporation of New Jersey Application January 11,1951, Serial No. 205,549

Claims.

The present invention deals with a compact type electrical discharge device and more particularly with an electrode therefor.

The compact type electrical discharge lamp is distinguished from the tubular and capillary type discharge lamps in that the lamp envelope is of a substantially spherical configuration and the electrodes contained therein are spaced to provide a discharge path of from a few millimeters to about two centimeters in length. The electrodes in lamps of this type are subjected to very high current densities and, especially during the starting of the discharge when the electrodes are cold, to a high evaporation rate of the metal forming the electrodes, which blaclrens the walls of the lamp envelopes, e. g. light transmissive envelopes of fused silica, fused quartz, or other glasses of high melting point, and thereby decreases the light output of the lamps. The rate at which the electrodes evaporate is the determining factor of the useful life of the lamp.

Tungsten is generally used as the electrode material. The electron emissivity of tungsten increases enormously with temperature, being of the order of one hundred billion times greater at near the melting point than at about 25 Centigrade, and the temperatures developed in the discharge chamber of compact type lamps employing such electrodes are exceedingly high, e. g. between about 5,000 and 11,000 Kelvin. However, tungsten melts at 3,643 Kelvin and the electrodes will melt unless employed in very specificforms to dissipate by radiation and conduction the heat received from the discharge.

Usually, the electrodes used in compact type lamps are massive in structure-in comparison with the electrodes of other type lamps having envelopes of comparable size-in order to provide ample surface for radiation because radiation is a surface phenomenon. Massive electrodes have certain undesirable features in that the initial emissivity is low and sputtering of the tungsten metal is likely to occur, the extent of sputtering being dependent in part on the rate at which the metal warms. For the same current, a massive electrode warms more slowly than a substantially smaller-sized electrode and, therefore, sputters to a greater extent and more rapidly blackens the envelope than the smaller electrode during the heating up period.

Electrodes of substantially less mass than the aforesaid massive electrode will heat faster and sputter less but will evaporate more rapidly after they have been heated and blacken the envelope more rapidly after the heating up period. Moreover, such smaller-sized electrodes, due to the rate of evaporation of the tungsten after the heating up period, causes the lamp employing such electrodes to have a shorter useful life than lamps employing the more massive electrodes.

Therefore, specific forms of electrodes for compact type lamps are a compromise between rate of initial sputtering action and subsequent evaporation.

It is an object of the present invention to provide an improved electrode for compact type lamps. It is another object of the present invention to provide an electrode which combines the advantages of massive and substantially smal1ersized electrodes. It is a further object of the present invention to provide a compact type electrical discharge lamp which employs the improved electrodes for a longer useful life of such lamps. It is a still further object of the present invention to provide a compact type lamp and electrodes therefor having a combination of embodiments which cooperate to provide a superior compact type lamp. Other objects and advantages of the present invention will become apparent from the description hereinafter following and drawings forming part hereof, in which:

Figure 1 illustrate partly in elevation and partly in section a major portion of a compact type lamp according to the present invention, and

Figure 2 illustrates an enlarged view of the salient features according to the present invention.

The present invention is concerned with high pressure and super high pressure rare gas or rare gas and metal vapor compact type discharge lamps having light transmissive substantially spherical envelopes of fused quartz and the like glasses, and an improved electrode structure which avoids in a large measure the inadequacies and disadvantages of electrodes heretofore em ployed in compact type lamps.

Referring to Figure l, the compact type lamp of this invention comprises a substantially spherical vitreous e. g. fused quartz, envelope i defining a discharge chamber and having tubular fused quartzextensions, e. g. tubes 2 and 3 projecting outwardly therefrom preferably oppositely of each other, which support a pair of spaced electrodes 4 and 5 hereinafter referred to as main electrodes. The main electrodes are spaced from each other in the discharge chamber to provide a discharge path of from a few millimeters, e. g. 5 millimeters, to about two centimeters in length. The discharge chamber contains an ionizable atmosphere of a rare gas at at least one atmosphere pressure at about 25 C., or a rare gas and a vaporizable metal, e. g. mercury, in an amount sufiicient when vaporized to provide a gas-vapor filling of at least one atmosphere total pressure at about 25 C.

The improved electrode structure of the present invention comprises the aforesaid pair of spaced main electrodes 4 and 5 upon each of which is mounted an arrangement of auxiliary-electrodes. Each main electrode consists of a rod of substantial mass, e. g. a rod having adiameter of approximately 0.004 inch per ampere of normal current rating for the electrode, said current being in the order of at least 5 amperes for a compact type lamp. The length ofleach main electrode or rod is an essential feature in that it is at least three times the length of the inside radius of the spherical envelope and a major portion of the rod is encased or sealed and supported by a vitreous or glass tube or extension, e. g. tube 2, and a minor portion of the rod extends into the discharge chamber, said major portion acting to conduct heat from said minor portion and said glass tube assisting in the conduction of heat away from the discharge and electrode and thereby cooling the electrode by conduction and convection. It is apparent that the tubes 2 and 3, since they seal-in major portions of the electrodes 4 and 5, are themselves of substantial length thereby allowing sufficient conduction and convectionto prevent at least the ends of the rods 4 and 5 outside the discharge chamber from incandescing. If the length of the electrode 4 sealed into the tube 2 is too short relative to that portion extending into the discharge chamber, the arcing ends of the electrode will overheat, possibly melt, and in such case will evaporate rapidly and blacken the walls of the envelope. Accordingly, if the length of the major portion of the electrode outside the discharge chamber is at least the length of the electrode, preferably from about to the length, suflicient conduction and convection are maintained to prevent the above mentioned detrimental conditions. However, in regard to the lower limits of lamp temperature and the diameter of the electrode, in some instances it is sufficient that a major portion of the electrode is outside the discharge chamber.

The main electrodes .are preferably composed of commercial tungsten without the addition of activators since the latter are not satisfactory because they evaporate too readily at the operating temperature of the electrode, whichat its arcing end is at about 3400" Kelvin. It is apparent that the major portion of the main electrode is not sealed vacuum tightly. Therefore, a vacuum tight seal is connected to the end of the main electrode which is outside the discharge chamber. This vacuum tight seal is sealed by said tube l as is the main electrode and is electrically conductive and of sufficient capacity to carry the normal current rating forthe electrode.

For example, the vacuum tight .seal may be in the form of a substantially thin strip .or sleevetype structure 6 as illustrated and composed of molybdenum and the like materials known to the art to constitute suitable materials forzsuch seals.

The minor portion of both main electrodes. which extend into the dischargechamber, have mounted thereon an arrangement .of first auxiliary electrodes 1 and 8 which assist the start of the discharge and second auxiliary electrodes 9 and I0 which warm rapidly to take up the discharge from electrodes 1 and 8 and sustains the discharge until the ends H and I2 are heated sufficiently to take up the discharge to establish a fixed arc. The whole starting procedure including the establishment of the fixed arc occurs in a fraction of a second when a lamp employing such an electrode structure is connected to a suitable power supply.

In order to carry out the objectives hereinbefore set forth and to enable the electrodes to operate as above described, it is essential that the main electrode and the auxiliary electrodes supported thereon form an electrode structure which comprises a combination of the said electrodes in a substantially critical relationship to each other. Since the minor portions of main electrodes 4 and 5, particularly the ends H and 12, are relatively massive they can operate and sustain a discharge are without evaporating sufficiently to render the lamp susceptible to a short useful life. However, the second auxiliary electrodes, in order to operate as described must be of less mass than the minor portions of the main electrodes in order to heat up rapidly enough to take up the discharge from the first auxiliary electrodes almost instantly, and must not be positioned too near to the fixed are sustained by the main electrodes to prevent the evaporation thereof. Therefore the second auxiliary electrodes 5: and I6 are supported on the minor portions of the main electrodes but spaced from the arcing ends I i and i2. These second auxiliary electrodes are preferably spaced from the ends I i and i2 for a distance relative to the difference in mass between the two solid electrodes. For example, as a preferred embodiment the second auxiliary electrodes may comprise tungsten coils of about 5 to 7 turns each and the diameter of the wire forming said coils being about fiveeights the diameter of the main electrodes, said coils being spaced from the ends Ii and 2 at a distance approximately twice the diameter of the main electrode not only to prevent the evaporation thereof, but to also conduct and radiate energy conducted to them from the ends i i and I2 and to thereby extend the useful life of the discharge device.

The first auxiliary electrodes 1 and 8 are essentially of less mass than the second auxiliary electrodes and, although preferably in the form of a round wire coil of tantalum or molybdenum, may be of any form as above specified with regards to the second auxiliary electrode. fantalum or molybdenum is preferably used for the first auxiliary electrode because these metals have better electron emission qualities when cold than does tungsten they possess a sufficiently high melting point to exist in an unmelted form when positioned or supported on the minor portion of the main electrodes near or in contact with the inner wall of the quartz envelope 1 and between the inner envelope wall and at least some of the turns of the second auxiliary electrode. Other materials are better emitters than tantalum and molybdenum, e. g. thorium, akaline earths, lead, etc, but they all evaporate rapidly at the temperatures encountered on the main electrodes. Preferably, the first auxiliary electrodes consist of a few turns each of round wire having .a diameter less than that of the second auxiliary electrodes, e. g. about oneeighth the diameter of the main electrodes,.and are in electrical contact with the main electrodes as are the second auxiliary electrodes and the auxiliary electrodes may or may not be in contact with each other. Moreover, since the first auxiliary electrodes are positioned farther from the ends of the main electrodes than are at least some of the windings of the second auxiliary electrodes, they are shielded from the temperatures of the are between the said ends by the said second auxiliary electrodes which at least to some extent contribute to prevent the said first auxiliary electrodes from reaching excessive temperatures.

In accordance with the present invention hereinbefore set forth, a compact type discharge device employing the above described electrode structure has a greatly reduced electrode evaporation rate, consequently a reduction of blackening of the lamp envelope, providing for a longer useful life of compact type discharge devices.

What we claim is:

1. A compact type electrical discharge device comprising a substantially spherical vitreous envelope defining a discharge chamber and having a pair of tubular vitreous extensions projecting outwardly therefrom, said discharge chamber containing an ionizable atmosphere, a pair of electrically conductive rods, each of said rods having a length equal to at least three times the length of the inside radius of said envelope, a major portion of each of said rods being contained and sealed by each of said extensions, a continuous minor portion of each of said rods extending into said discharge chamber, said minor portions being spaced from each other for supporting a discharge arc, each of said minor portions comprising a main electrode, first and second discontinuous auxiliary electrodes supported on said main electrode in electrical contact therewith, said second auxiliary electrode being of less mass than said main electrode and spaced from the are supporting end of said main electrode, said first auxiliary electrode being of less mass than said second auxiliary electrode and positioned between the inner wall of said envelope and at least a part of said second auxiliary electrode.

2. A compact type electrical discharge device according to claim 1, wherein said vitreous extensions project outwardly of said envelope and diametrically of each other, and said minor portions of said rods being spaced less than two centimeters of each other.

3. A compact type electrical discharge device according to claim 1, wherein said ionizable atmosphere comprises a rare gas and sufficient mercury when vaporized to provide a total pressure of at least one atmosphere at about 25 C.

being tungsten bodies, and each of said first auxiliary electrodes being tantalum bodies.

5. A compact type electrical discharge device comprising a substantially spherical vitreous envelope defining a discharge chamber and having a pair of tubular vitreous extensions projecting outwardly therefrom, said discharge chamber containing an ionizable atmosphere, a pair of tungsten rods, each of said rods having a length equal to at least three times the length of the inside radius of said envelope, a major portion of each of said rods being contained and sealed by each of said extensions, a continuous minor portion of each of said rods extending into said discharge chamber, said minor portions being spaced from each other for supporting a discharge arc, each of said minor portions comprising a main electrode, first and second discontinuous auxiliary electrodes main electrode in electrical supported on said contact therewith,

said second auxiliary electrode being a tungsten wire coil co-axial with said main electrode and the diameter of the wire of said coil being less than the diameter of said main electrode, said second auxiliary electrode being spaced from the are supporting end of said main electrode, said first auxiliary electrode being a wire formed 01 at least one of the metals tungsten and tantalum, the wire of said first auxiliary electrode being of less diameter than that of said second auxiliary electrode, said first auxiliary electrode being positioned on and co-axial with said main electrode between the inner wall of said envelope and at least part of the windings of said second electrode.

References cited in the file of this patent UNITED STATES PATENTS Number Number

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