US2319912A - Electrical discharge device - Google Patents

Description

May 25, 1943 w. T. ANDERSON, JR., r-iT AL 2,319,912

ELECTRICAL DI SCHARGE DEVICE Filed Jan. 18, 1940 Patented May 25, 1943 ELECTRICAL DISCHARGE DEVICE William T. Anderson, Jr., Newark, and Hans Laub, Miilburn Township, N. E., assignors to Eanovia Chemical and Manuiacturing- Company, Newark, N. J., a corporation oi' New Jersey Application anuary 18, 1940, Serial No. 314,440

1 Claim.

This invention relates to electrical discharge devices and is concerned in particular with electrical discharge devices in which the discharge is carried by metal vapor.

Electrical discharge devices of the type contemplated herein may comprise for instance an envelope of vitreous material such as quartz and the like and having a illing of inert gas and metal vapor in which an electrical discharge is carried between activated electrodes. The inert gas may be for instance argon and the metal vapor may be that of mercury or other easily vaporizable metal. The activated electrodes may consist of tungsten, molybdenum or other high fusing metal activated in known manner with an electro-positive metal such as barium or other alkali or alkali earth metal or suitable com` pound thereof.

When such discharge device is non-operating the electrodes are cold and the vaporizable metal, such as mercury, is present in non-vapor form. On application of an electrical potential to the electrodes an arc is struck between the electrodes and is carried by the inert gas, the discharge vaporizing the vaporizable metal and heating the electrodes. whereupon the arc is carried by the metal vapor. Usually such discharge devices are so constructed that in operation all of the vaporizable metal is evaporated and the vapor may be at a high pressure. When the discharge is discontinued by withdrawal of the electrical potential thetemperature will sink and the metal vapor will condense and reassume its initial non-vaporous form, such condensation taking place in that portion of the envelope where cooling takes place rst.

Condensation of the metal vapor, for instance mercury, on the activated electrodes is accompanied by various disadvantages such as slow starting of the discharge, short life of the discharge device and rapid destruction of the electrodes. In the ordinary electrical discharge device condensation of the metal vapor takes place primarily on the envelope of the device, away from the electrodes, and condensation on the electrodes is substantially avoided. It has been found, however, that condensation on the electrodes takes place easily in discharge devices having an extended elongated envelope in tubular form when such devices are subjected to passing air currents.

It is, therefore, one object of our invention to provide an electrical discharge device comprising an elongated tubular envelope provided with activated electrodes and having 'a gaseous filling consisting at least in part of condensible metal vapor, in which the electrodes shall be protected against condensation thereon of such metal vapor.

It is another object of our invention to provide such electrical discharge devices with extended elongated envelopes which shall be capable of being easily started and restarted and 4which shall have a long life.

These and further objects of our invention which will become apparent hereafter, and the construction of our electrical discharge device, are further illustrated in the accompanying drawing, forming part hereof, and in which:

The gure represents an elevational sectional view of the electrical discharge device embodying our invention.

The device illustrated in the drawing comprises an envelope i of vitreous material such as quartz or fused silica and is provided with activated electrodes 2 and 3 placed at opposite ends of the envelope. The envelope contains a filling of lnert gas such as argon and a small quantity of mercury. On application of an electric potential of relatively low voltage to the electrodes 2 and 3 through the 1ead-in conductors sealed into the envelope i at 5, a discharge will be struck between the electrode 2 and the electrode 3, which discharge is initially carried by the inert gas and subsequently by the mercury vapor maintained at high pressure. Heat losses at the ends of the tube, which would be detrimental to the maintenance of the desired high pressure of the mercury vapor, are prevented by heat insulating means such as infra-red reflectors 6 for instance of nickel.

If the tube is operated in a vertical position with electrode 2 substantially below electrode 3 the upper end of the tube will be at a higher temperature than the lower end due to convection currents outside the tube. On cooling the vaporized metal will, therefore, deposit at the lower end of the tube and on the electrode 2. If the tube is operated horizontally and cooled by a draft of air flowing for instance in the direction from the end containing electrode 2 to the end containing electrode 3 the former end will be cooled more than the latter particularly when the draft is continued after the discontinuation of the discharge and the vaporizable metal will, therefore, condense at the former end and on electrode 2.

Experiments have shown that the problem of mercury condensation on electrodes does not seriously occur in the case of small tubes limited in wise properly constructed in accordance with well-known practice. Where the length of the elongated tube is such that the distance between the electrodes exceeds several inches and approximates inches or more serious condensation is experienced in the cases described unless a double jacket is resorted to, which however increases are used for instance in photo-chemical processes such as photo-printing and in the irradiation of milk, provitamins, etc.

'I'he deposit of the vaporizable metal on the electrode positioned in the cooler end of the elongated tube, electrode 2l in the illustrating drawing',` is'detrimental to the performance and life of the discharge device, Condensation of mercuryv on the side of the electrode turned toward the discharge interferes with the activation material, for instance barium, and its ability to emit sumcient electrons at room temperature for starting the device; when the barium is thus interfered with the discharge can be started only by the application of a considerably higher than normal voltage potential, of high frequency or by resorting to external application of heat as by Bunsen iiame or electric heating coil. Con" densation of mercury on the side of the electrode turned away from the discharge increases the warming up period beyond that experienced when metal vapor condenses on the walls of the envelope between the electrodes. Highest currents, and therewith greatest-demand on the electrodes, occur during the starting and immediately thereafter, and an increased warming up period, therefore, decreases the life of the electrode and therewith of the discharge device: while metal condensed behind the electrodes may thus be evaporated in time the evaporation may take twice as long and the great demands put upon the electrodes during the warming up period are thus increased considerably.

In 'accordance with our invention we eliminate the disadvantage of condensation of mercury vapor on activated electrodes in long elongated tubular discharge devices by providing equalized heat insulation in proximity to the electrode affected by decreased temperature. Referring to the drawing, we may for instance provide around the lower end of the tube containing electrode 2 a wrapping 'I of asbestos, porcelain, woven glass Wool or other material having low heat conductivity. l i

'I'he beneficial result produced by our invention is surprising. A test was carried out with a quartz mercury arc discharge device having a filling of inert gas and mercury vapor and consisting of an elongated straight tube having a diameter of 24 mm., molybdenum electrodes activated with barium spaced 22 inches apart inside the tube and heat shields of nickel infrared reectors around both electrode chambers. The discharge device was operated in vertical position with a burner current of 4.7 amperes and a burner wattage of 2450 watts. After 30 minutes temperature measurements on the quartz wall showed a temperature of 855 F. near the upper electrode and a temperature of '792 F. near the length to a few inches, provided the tube is otherlower electrode. On withdrawal of the electrical potential, and subsequent cooling, the mercury began to condense'near the lower electrode and nally onv the electrode itself. After complete cooling the electrical potential was again applied and it was found that the burner started with diiiiculty, the discharge at first glowing and finally turning into an arc; the lighting was accompanied by a slight sputtering of the lower electrode, a factor which is detrimental to the life of an electrode. The same discharge device was then provided with a heat balancing wrapping of asbestos paperr placed around the lower end of the tube and it was found that the temperature on the wall near the lower electrode was increased to 855 F. When the burner was extinguished the mercury condensed in the central portion of the tube and'not at or near any of the electrodes. When the electrical potential was again applied the burner lighted with ease and without sputtering of the electrodes.

In another experiment on a similar burner having, however, activated electrodes spaced 47 inches apart, it was operated in horizontal position'with air cooling directed over the tube from one end. -Again it was found that by enclosing, as described, the end from which the air draft was coming both electrodes operated at similar temperatures and condensation on any of the electrodes was prevented, with the result that relighting took place with greater ease and Without sputtering. v

In mercury arc devices great care must also 4 be taken that the mercury vapor should not condense on the electrode support as thus, on restarting, the device might rectify, an arc being struck between parts which are not activated, thus causing rapid destruction of the discharge lamp. Our invention prevents not only condensation of the vaporizable metal on the electrode but also near the electrodes. v

Our invention, therefore, consists in equalizing the temperatures prevailing at electrodes spaced apart in elongated tubular discharge devices of the type described where there is a tendency by the electrodes to operate at substantially difierent temperatures or to cool at different speed due to air currents along the envelope of the discharge device, such'air currents being due either to external convection or special air cooling drafts.. The provision of an equalizing heat insulation, permits easier restarting, decreases the Warming up period, prolongs the life of activated electrodes and increases the life of discharge devices of the type described, experience over an extended period of time having shown an increase in life of asl much as 300% in addition to more satisfactory performance of the discharge devices.

In the appended claims we shall use the term upper end in reference to the end of the tube which except for the equalizing heat iunslation .of our construction would be the end operating A at a higher temperature and subjected to slower cooling on the discontinuation of the discharge, whereas we will refer to the opposite end as the lower end both terms to apply to elongated electrical discharge devices of the type described, irrespective oi whether operation thereof takes place in a vertical or 'horizontal position.

The invention is equally applicable to electrical discharge devices provided with only one activated electrode, as 'i'or instance in discharge devices operated with D. C. current. If, in such case, the activated electrode is at the lower end oi the envelope the heat insulating means of greater heat insulating capacity at the lower end of the envelope are imperative in order to prevent condensation of metal vapor on the electrode as well as behind the electrode;if the activated electrode is at the upper end of the envelope the heat insulating means of greater heat insulating capacity at the lower end of the envelope serve to prevent condensation of the metal vapor behind the electrode at the lower end, where it is usually lost from further participation in the operation of the discharge device, as there is not imparted sufficient heat to the condensed metal such as mercury to vaporize it.

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is: l

jAn elongated tubular vapor electric discharge device having a lower and an upper end for use in air `currents passing along the outside of the device, comprising a sealed tubular envelope of vitreous material containing a gaseous illingmetallic heat insulation at the lower end of the Y envelope, whereby the temperature prevailing at both ends is not below that prevailing at the part of the envelope through which the arc isl struck and whereby the greater heat losses at the lower end due to air currents are balanced by the greater heat insulating capacity of the said heat insulating means at such lower end.

WILLIAM T. ANDERSON, JR. HANS LAUB. Y

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