US 2267821 A
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Description (OCR text may contain errors)
Dec. 30, 1941 W. ELENBAAS HIGH-PRESSURE METAL VAPOR DISCHARGE TUBE Filed May 6, 1940 Inven'tor: 'Mem Lenbaas,
Patented Dec. 30, 1941 HIGH-PRESSURE METAL VAPOR DISCHARGE TUBE I Willem Elenbaas, Eindhoven, Netherlands,
assignor to General Electric Company Application May 6, 1940, Serial No. 333,653 In the Netherlands April 1, 1939 Claims.
It is known that in high-pressure metal vapor discharge tubes the pressure of the metal vapor greatly increases upon ignition of the tube and this increase is attended by an increase in the operating voltage. A high-pressure metal vapor discharge tube therefore exhibits the property that when the tube is in normal use the operating voltage is substantially higher than (at least twice as high as) the operating voltage which occurs directly upon initiation of the arc discharge. The increase in the vapor pressure is also attended by the phenomenon that the discharge contracts, that is to say no longer fills the whole section of the discharge tube.
It is known to limit the quantity of vaporizable metal in these discharge tubes to such an extent that when the tube is in normal use this quantity of metal is entirely vaporized and the metal vapor is unsaturated. In the presence of a plentiful supply of vaporizable metal the vapor pressure and thus the operating voltage of the discharge will always increase when the heating of the discharge tube increases. By limiting the quantity of metal in the manner indicated it is ensured that from the moment when all the metal has passed into the vapor state the vapor pressure can only rise slowly when the heating of the discharge tube is continued so that the operating voltage also increases very slowly. This is very advantageous because a, more stable operation is thus obtained, as becomes manifest, for example, in that upon variation in the condition of the medium to which the heat is transmitted, that is to say in the case of variations in the heat dissipation and also in the case of variations of the voltage of the source of supply current, the operating voltage of the discharge remains substantially constant.
When using such a limited quantity of vaporizable metal there is also the possibility of choosing the electric load of the discharge tube so that the metal vapor is superheated to a great extent in the normal use of the tube, that is to say the lowest temperature of the discharge space is substantially higher than the temperature at which the saturation vapor pressure is equal to the pressure of the unsaturated metal vapor in the discharge tube. This offers the advantage of shortening the heating-up period of the discharge tube, i. e. the time necessary to bring the discharge tube from the cold condition into the condition of normal operation.
The invention relates to high-pressure metal vapor discharge tubes having such a limited is in normal use all the metal is vaporized and it has for its object to improve the said tubes. In the use of these discharge tubes the unpleasant phenomenon occurs that when the tube is in normal use and the discharge is interrupted (either deliberately by interruption of the circuit or unintentionally due to the fact that, for example, the voltage of the source of current momentarily falls below the voltage necessary for the maintenance of the discharge) the voltage necessary for reinitiation of the discharge is substantially higher than the starting-up voltage when the discharge tube is in the cold condition. In normal cases the voltage available is therefore too low to bring about immediate reignition. This is due to the high-vapor pressure which prevails in the tube on the discharge being interrupted. Re-ignition is not possible in this case until the discharge tube has cooled to such an extent that the vapor pressure has greatly fallen. This requires a certain time period; the more the metal vapor is unsaturated the longer is this cooling period since the discharge tube has first to be subjected to an intenser cooling before the metal vapor starts to condense.
It has been suggested to shorten the cooling period of high-pressure met-a1 vapor discharge tubes which are not artificially cooled during normal use by cooling a part of the tube wall by means of a. flow of liquid directly upon the interruption of the discharge. The contrivances required for this purpose, however, involve a great complication particularly if the arrangement is such that upon interruption of the discharge artificial cooling starts automatically. If use is not made of an automatically operating device the discharge tube must be watched continually.
According to the invention the discharge tube is provided with an auxiliary receptacle which is heated by a heating element connected in series with the discharge path, the thermal capacity of the said receptacle being made so low and its heat dissipating capacity being made so high that when the discharge is interrupted while the tube is in normal use the temperature in the auxiliary receptacle decreases more rapidly than the lowest temperature in the discharge space proper, the heating element being in addition so proportioned that onthe discharge tube being put into service the auxiliary receptacle is caused to assume a higher temperature so rapidly that ,the heating-up period of the discharge tube is not longer than in the absence of the auxiliary quantity of vaporizable metal that when the tube receptacle.
Due to the fact that the temperature in the auxiliary receptacle falls more rapidly than in the discharge space proper a more rapid condensation of the metal vapor ensues in this auxiliary receptacle after interruption of the discharge and the cooling period necessary for reducing the vapor pressure to such extent that the discharge can re-ignite becomes shorter than in the absence of the auxiliary receptacle. It is known that, a short time after initiation of the discharge, the current strength is higher than when the tube is in normal use. Consequently, this large current, which also flows through the heating element brings about a rapid heatingup of the auxiliary receptacle with the result that the metal condensed therein is vaporized again. By making the capacity of the heating element suificiently high it may be ensured that the heating-up period of the discharge tube, that is to say the time required for bringing the tube from the cold condition into the normal operating condition, is not increased by the provision of the auxiliary receptacle. Obviously, the heating element must also be proportioned in such manner that when the discharge tube is in normal use the temperature in the auxiliary receptacle is not lower than the temperature at which the desired vapor pressure is obtained.
'The thermal capacity of the auxiliary receptacle can be reduced by decreasing the wall thickness and the width thereof. Simultaneous decrease of the wall thickness and of the width permits of high pressures being nevertheless admitted in the discharge tube. The external diameter of the auxiliary receptacle is preferably less than half the external diameter of the discharge tube.
When the auxiliary receptacle is enclosed in a gaseous atmosphere a small diameter is also favorable as regards dissipation of heat since the smaller the diameter the higher is the dissipation of heat per unit of wall surface. The dissipation of heat is also influenced by the pressure and the nature of the surrounding gaseous atmosphere and increases as the pressure and the heat conduction of the gas increase. In order to intensify the dissipation of heat by radiation it may be advisable to blacken the outside of the wall of the auxiliary receptacle.
The wall of the auxiliary receptacle has preferably one or more metal bodies, for example metal wires, led through it. Due to the heat conduction by these bodies the heat dissipating capacity of the auxiliary receptacle is increased, it being preferable for the said bodies to have a large surface area outside the auxiliary receptacle and in some circumstances, to be black- .ened. For this purpose, for example, a small metal plate may be fixed to a wire led through the wall.
As an alternative, a heat-radiating body of low thermal capacity, for example a thin metal plate, may be housed in the auxiliary receptacle. In the cooling period this body dissipates its heat rapidly by radiation so that it very rapidly assumes a lower temperature than the wall of the auxiliary receptacle and the metal vapor is thus condensed on the said body.
The heating element is preferably so constructed as to leave free a large part, for example more than 75%, of the wall of the auxiliary recept'acle so that this part can dissipate its heat directly to the surroundings.
In order that the invention may be clearly understood and readily carried into effect it will now be described more fully, with reference to the accompanying drawing in which the single figure illustrates a high-pressure vapor tube embodying my invention.
Referring to the single figure of the drawing, l designates a cylindrical discharge tube made of quartz and having an internal and an external diameter of 7.5 and 10 mms. respectively. This discharge tube comprises two incandescent electrodes 2 and 3 which are provided with a material capable of copiously emitting electrons and are heated to the required 'high temperature solely by the discharge. The spacing between the electrodes is 22 mms. The gas-filling of the tube is constituted by argon at a pressure of about 2 cms. to which a small quantity of mercury is added.
The bottom end of the discharge tube I has sealed to it an auxiliary receptacle 4 constituted by a small cylindrical tube of quartz having an internal and an external diameter of 2 and 2.5 mms. respectively. The length of this auxiliary receptacle is about 10 mms. The closed end of this auxiliary receptacle has a tungsten wire 5 of 400 microns thickness sealed in it. The auxiliary receptacle contains a small molybdenum plate 6 which is fixed to the said tungsten wire and is about 4 mms. in length and almost 2 mms.
in width. In order to reduce the thermal capacity of this plate the thickness is made very low; it is 18 microns. In order to increase heat radiation the plate 6 is blackened. Outside the auxiliary receptacle the wire 5 has fixed to it a blackened nickel plate 1 which has a thickness of microns and is 20 mms. in length and width. The auxiliary receptacle 4 is surrounded by a heating element 8 constituted by 10 turns of a nickel-chromium wire (diameter microns). One end of this heating element is connected through a wire 9 to the electrode 3, whereas the other end is connected through wire H] to the leading-in wire II which is lead through the pinch i2 of the bulb l3 and is connected to the contact member [4 of the cap I 5. This pinch has also two supporting wires l6 and" I1 secured in it to carry the discharge tube 1, the wire I! also serving as a current-supply member for the electrode 2 and being connected to the contact member IS. The space between the discharge tube and the bulb I3 is filled with nitrogen at a pressure of 50 cms.
The lamp is used for the emission of rays, for example for illumination or radiation by ultraviolet rays, 'and is supplied from an alternating current supply circuit of 220 volts with the interposition of a choke coil. When the tube is in use it exhibits a high-pressure mercury vapor discharge at which the absorption of energy in the discharge tube is 75 watts and the strength of the discharge current is 0.75 ampere. The quantity of mercury introduced into the discharge tube is so low that the operating voltage of the discharge when the tube is in normal use is .120 volts. In this case the mercury vapor is superheated.
When the above discharge tube was tested and was cut out of circuit while in normal operation the time in which the vapor pressure fell to such an extent that a discharge was re-initiated amounted to 50 seconds. When the auxiliary receptacle was not provided and the construction remained otherwise unaltered this time period was 95 seconds. The invention consequently ensured a considerable shortening of this time period.
by way of example,
What I claim is:
1. A high-pressure metal vapor discharge tube comprising a sealed envelope containing a pair of electrodes and a gaseous atmosphere comprising a vaporizable metal in an amount so small that it is completely vaporized during operation or the tube, an auxiliary receptacle extending from said envelope and communicating with the interior thereof, a heating element connected in series with the discharge path between said electrodes and located adjacent to said receptacle so as to supply appreciably more heat thereto than to the envelope proper, the thermal capacity of said receptacle being so lowand its heat dissipation being so high that upon interruption of the discharge the receptacle cools more rapidly than any portion of said envelope, the said heating element serving to heat the said receptacle, upon restarting of the discharge in the tube, so that the restarting time is not longer that it would be in the absence of the said receptacle. 2. A discharge tube as set forth in claim 1 wherein the auxiliary receptacle is provided with a metallic conductor member extending through the end thereof and serving to increase the heat dissipating capacity of the receptacle.
3. A discharge tube as set forth in claim 1 wherein the auxiliary receptacle contains a thin metallic conductor member which serves, during the cooling period of the tube, to rapidly dissipate heat by radiation so that the vaporizable material is condensed thereon.
4. A discharge tube as set forth in claim 1 wherein the auxiliary receptacle is provided with a metallic conductor member extending through the end thereof and having a large surface area outside said receptacle thereby serving to increase the heat dissipating capacity of the said receptacle.
5. A discharge tube as set forth in claim 1 wherein the auxiliary receptacle contains a thin metallic conductor member which serves, during the cooling period or the tube, to rapidly dissipate heat by radiation so that the vaporizable metal is condensed thereon and a metallic conductor member connected to said thin conductor member and extending through the end of the receptacle and having a large surface area outside the receptacle thereby increasing the heat dissipating capacity of said receptacle.
' WILLEM ELENBAAS