US2660692A - High-pressure discharge lamp - Google Patents

Description

Nov. 24, 1953 J. A. s'r. LOUIS ETAL 2,660,692

HIGH-PRESSURE DISCHARGE LAMP Filed March so, 1950 2 Sheets-Sheet 1 17 E3 /8 4O 43 .4' 4Z[.% EZN O Time A 7 ,3) 40 a Ki 3 I I 80 Invewtors:

James A. St. Louis, John M. PoTrrF'reH;

b8 M C" K W Their" A kkomweg.

Nov. 24, 1953 J. A. ST. LOUIS EIAL HIGH-PRESSURE DISCHARGE LAMP 2 Sheets-Sheet 2 Filed March 50 1950 3 T V i@ Wfm 1 A w o m, w m m HE JQ/ lrwven tors: James A. S l: Louis, John M. PQm FTe Ft; b W a Their A=t+ovneg Patented Nov. 24, 1953 UNITED STATES PATENT OFFICE Milton Pomfrett, Willoughby, Ohio, assignors to General Electric Company, a corporation of New York Application March 30, 1950, Serial No. 152,904

7 Claims.

This invention relates to electric discharge devices of the gaseous conduction type, and more particularly to means for reducing the elec trolysis of the glass or envelope material in the vicinity of the electrode Seals in high pressure mercury vapor lamps. 7

An illuminating device which has come into wide use within recent years for general industrial and street lighting is the high pressure mercury vapor discharge tube. The modern lamps of this type commonly have a cylindrical discharge tube containing a small quantity of mercury and are provided with thermionic electrodes containing a core of material of high electron emissivity. A quantity of rare gas is enclosed within the arc tube for starting purposes; and, in addition, an auxiliary starting electrode is generally located at one end of the bulb adjacent one of the electrodes in order to facilitate ignition. The actual discharge tube is generally surrounded by a glass bulb or cover fitted with a suitable base, and the space between the arc tube and the outer bulb is either evacuated or filled with a gas such as nitrogen or air.

The discharge within the arc tube has the character of a high pressure discharge, that is, the arc stream is constricted as a result of the high vapor pressure of mercury that is developed when the lamp has attained its normal operating temperature. The discharge voltage drop generally increases from the initial value as the lamp attains its normal operating temperature, at which the vapor pressure exerted by the mercury within the arc tube may be in the neighborhood of a few atmospheres. In starting such lamps when the electrodes are at ambient temperature, a very high voltage would have to be applied across the main electrodes in order to initiate an arc. In order to obviate the necessity for such a high voltage, the auxiliary electrode is connected, by means of a current-limiting impedance such as a resistor, to the opposite main electrode from that to which it is adjacent. in practice, the resistor connecting the auxiliary starting electrode to the opposite main electrode is permanently connected and sealed within the space between the arc tube and the outer envelope.

It has been found that the permanent resistive connection, between the auxiliary starting electrode and the opposite main electrode, causes a potential to exist between the starting electrode and the adjacent main electrode, which potential is always present during operation. In addition, we have found that with the type of starting electrode heretofore commonly employed, there is a tendency for a direct current component of voltage to develop between the starting electrode and the adjacent main electrode. This direct current component of voltage, and to a certain extent also the alternating component, frequently causes electrolysis to occur 2 in the hard glass or quartz between these two electrodes. The efi'ect of the electrolysis is to alter the compositicn of the glass and to destroy the glass-to metal seal of one of the adjacent lead-in wires; this is generally followed by cracking of the glass around the affected wire and destruction of the lamp.

Accordingly, it is an object of our invention to provide a new and improved high pressure arc discharge lamp containing a pair of main electrodes and an auxiliary starting electrode, wherein the starting electrode is so designed that the voltage gradient, between it and the adjacent main electrode during operating, is sub.- stantially reduced.

Another object of our invention is to provide a high pressure discharge lamp containing a, pair of main electrodes and a starting electrode so proportioned with respect to the adjacent main electrode that electrolysis of the glass therebetween is substantially eliminated.

In accordance with our invention, we have found that the deleterious eiiects previously mentioned may be greatly reduced or practically eliminated by increasing the electron emission of the starting electrode to the point where its conductivity is substantially symmetrical on both half cycles of an applied alternating voltage. The starting electrode is made of such size that it can safely withstand the current necessary to produce a voltage drop, across the resistance or impedance connecting it to the opposite main electrode, suificient to hold the potential of the starting electrode close to that of the adjacent main electrode. Thus practically no potential, either unidirectional or alternating, exists between the starting electrode and the adjacent main electrode, so that the dielectric between the seals is not affected.

It must be emphasized that our invention proposes raising the electron emission of the starting electrode during normal operation of the lamp and is not directly concerned with the emissivity at starting. This is due to the fact that electrolysis of the dielectric between the seals occurs during normal operation, when a relatively high temperature has been attained, and not at starting.

There are, of course, various ways in which the electron emission of the starting electrode may be raised. The electrode may be made of a material having a very low work function, or again, an electrode coated with activating materials such as barium and strontium oxides may be used. Electrodes of these types are comparatively expensive due to the fact that special heat treat.- mentsare required for the activating materials. Accordingly, it is desirable to provide the starting electrode in the form of a relatively common refractory metal, such as tungsten or tantalum and, instead of raising the electron emission by means of activating materials, to do so by increasing the temperature of the electrode durin normal operation of the lamp.

We have found that by providing the starting electrode with a relatively small thermal mass and, by bringing a portion of it within or close to the main arc stream, it is possible to substantially reduce the voltage between it and the adjacent main electrode, even though the starting electrode is resistively connected to the opposite main electrode during operation. As a result of this construction, the starting electrode can emit electrons quite readily during normal operation, and its potential follows that of the adjacent main electrode near which it is situated rather than that of the opposite main electrode to which it is resistively connected.

In accordance with one species of our invention, the auxiliary starting electrode may consist of a tungsten wire with a small slip-over coil, of which the last turns project substantially forward of the adjacent main electrode. According to another species of our invention, the starting electrode is in the form of a fine tungsten wire loop which is situated a short distance in front of the adjacent main electrode and located to surround the arc stream. This species is particularly adapted to a lamp designed for operation in any position, as contradistinguished from a lamp restricted to operation in the vertical plane. In yet another species of our invention, the starting electrode is a fine L-shaped tungsten wire of which the crosswise portion of the L is located in front of the adjacent main electrode,

substantially within the arc stream.

For further objects and advantages and for a better understanding of our invention attention is now directed to the fOllOWlllg description and accompanying drawings. The features of our invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

Fig. 1 is a vertical elevation of a high pressure mercury vapor discharge tube mounted within an outer envelope to constitute a practical lamp in accordance with the prior art and showing typical circuit connections for same.

Fig. 2 contains a number of curves showing the voltages existing between the electrodes of the lamp of Fig. l, and illustrating certain conditions which it is the principal object of this invention to correct.

Fig. 3 is a vertical elevation of an arc tube containing an improved starting electrode and embodying our invention.

Fig. 4 contains a series of curves showing the voltages existing between the electrodes of the lamp of Fig. 3 and illustrating the correction of the conditions with which this invention is concerned.

Fig. 5 is a perspective View of one end of an arc tube illustrating another species of our invention in which the starting electrode is provided as a ring, and which is particularly designed for a lamp intended for operation in any position.

Fig. 6 is a perspective view of one end of a lamp embodying yet another species of our invention in which the starting electrode has a portion in the path of the arc stream.

Referring to Fig. 1, a gaseous discharge lamp of the high pressure mercury type is shown which comprises an envelope l of quartz or hard glass with a high melting point, so as to be able to withstand the operating temperature of the lamp. A pair of thermionic electrodes 2 and 3 are disposed therein at opposite ends of the envelope, and are supported by lead-in conductors or exterior terminals 4 and 5 preferably made of tungsten or like refractory metal.

After exhaustion of the envelope I, it is filled with an ionizable medium such as mercury, of sufficient quantity to be completely vaporized with a pressure of the order of A2 to several atmospheres during operation of the lamp; and, in addition, a small quantity of a rare gas is preferably introduced to facilitate starting. Such a rare gas may, for instance, be argon at 30 millimeters pressure.

The two electrodes 2 and 3 may be of identical construction and each is shown to comprise a core 1, 8 made of a suitable refractory metal such as tungsten and surrounded by wire helixes 9, l0 made of like refractory metal, as tungsten. A small elongated piece or sliver l l, !2 of thorium metal is inserted between the core and the helix in each electrode. In operation, the outer tungsten helix protects the thorium sliver from ion bombardment which would cause sputtering thereof; and, at the same time, the high temperature of the electrodes appears to cause a molecular diffusion of the thorium metal over the surface of the tungsten wire, whose work function of electron emission is thereby lowered practically to that of pure thorium. An auxiliary starting electrode 13, which may consist of a piece of tungsten wire projecting a short distance within the arc tube I, is located adjacent electrode 3.

The are tube l is held in place within an outer glass envelope 5% by means of suitable uprights l1 and 58 provided with spring support members l9 and 2d at their upper ends bearing against the glass envelope. Uprights l1 and it are fixed at their lower end by spot welding to a conductor 2!, which leads to the customary screw base 22 through a press 23. Conductor 2| serves at the same time to supply current through the uprights ll and 8 to the upper electrode 2 which is electrlcally connected thereto. The other conductor 24 is connected to the lower disc terminal 25 of the screw base at one end, and to electrode 3 at the other end. Starting electrode i3 is internally connected within envelope [6 to electrode 2 by means of a resistor 26 which is shown welded to the upright l8 at one end. This resistor may be of the order to 10,000 to 40,000 ohms. The operating circuit for the lamp consists of input terminals 21, 23 adapted to be connected to a suitable source of voltage, for instance, 220 volts, 60 cycles, and across which the lamp is connected in series with a current-limiting reactor 29.

The lamp as described is of conventional construction, and its operation may be summarized briefly as follows: When voltage is first applied to terminals 21, 28, a glow discharge is initiated between starting electrode l3 and main electrode 3. The ionization spreads throughout the arc tube l and very quickly thereafter conduction starts between the main electrodes 2 and 3. The main electrodes gradually become heated to thermionic emission and the glow discharge thereupon changes over into an arc discharge which, as the temperature of the arc tube increases and the vapor pressure of mercury within it builds up, is gradually constricted to a narrow discharge of high intensity.

Our invention resulted from attempts to operate such a lamp with a greater power input and consequent higher operating temperature. We found that the current which may be allowed to flow between the electrodes is limited mainly by the advent? of electrolysis occurring between starting electrode I3 and main electrode This electrolysis causesa deterioration of the glass seal around these electrodes and a cracking of the glass.

Theoccurre'nce or electrolysismay be explained with" reference to; 2, denoting the" voltage or potential existing at the arcing surfaces of the main electrodes 3 'an'd as A and B, respectively, and the potential at the auxiliary starting electrode- Basra-. These curves representexperimental values taken with an actual: construction-of a lamp,- sis-illustrated in Fig. l, and operated with thebase up at 40'0" watts input. The curves M, M, and '42 represent, respectively, the potential- VAB existing across the main electrodes 3; 2, potential VAa existing between the adjacent main electrode 3-- and starting electrode 3, and the potential vaB existingbetween starting electrode 13 and the opposite main electrode 2'. It will-be observed that the potential of the auxiliary starting electrode during the positive haltcycle is very close to that of the opposite main electrode, that is, VAa is almost equal to Vina Thisis due to the fact that the starting electrode must operate as acathode whereas the adi'ac'erit rnainelectrode operates as an anode. However the starting electrode is a poor electron emitter;- and, accordingly, a large potential difference exists between it and the adjacent main electrode Oh the other hand, during the negative-half cycle curve vAa has a much lesser amplitude; and there is a lesser difference in voltage between the starting electrode is and adjacentma'inel'ectrode 3, as shown by curve vAa.

appearsto be due to the fact that the adjacent: main electrode, being activated and at a higli temperature, is a good electron emitter, so that the conductivity between it and the starting electrode operatin as an anode is high.

From the above; two conclusions readily appear: Inthe first place, a large alternating com-- ponent of voltage, practically equal to that existing between the main electrodes, is always present in the glass or'dielectric between the starting electrode and the adjacent main electrode; in the s'econd place, due to-the' unequal conductivity of the starting electrode on positive and negative half cycles, conduction through the starting electrode is asymmetrical and results in an average value or direct component of voltage which may bare-presented by the line as in Fig. 2'. This direct component of voltage produces a unidirectional potential gradient in the glass between the auxiliary and the adjacent main electrodes,

t3 and 3 ,1 respectively.

We believethatthe electrolysis of the glass is caused mainly by the direct component of voltage; We have'ineasured this direct current componentorraverage value and have observed it to have'an amplitude of 30 to 40 volts in lamps of standard design asdescribed. This direct ourrentcomponent likely causes migration of alkali metal, such as freasodiurn atoms, from" the main electrode, through the glass, toward the starting electrode, thereby causing discoloration of theglass and eventual destruction of the seal and crack-ing of the-glass surrounding the lead wire. I-t-islikely also caused to some extent by the alternating component so that it appears highly desirable to. reduce both components to as low a value aspossible. Thiswe have accomplished through ourinvention as will now be described.

Referringto Fig, 3, the arctube l along-with main: electrodesl-and 3 isrsimilar in construeor "f tion to that shown with respect to Fig: 1 like reference numerals denoting corresponding el'e' ments. In accordance with our invention, we" have increased the length of the auxiliary starting electrode l3so that it is about thesarne length as the main electrode 3. Our purpose is to bring the end of the starting electrode nearer the arc stream where it willbe in a region ofhigh temperature and high ion concentration. By this means, the temperature at theend of the'start ing electrode is raised and its electron emission is increased. Lengthening the starting electrode caused a considerable reduction in the voltage VAa, that is, in the potential between the" adjacent main electrode .i and the starting electrode l3.

Merely lengthening electrode [8 as described produced a very marked improvement. How'- ever, it did not eliminate the asymmetry between successive half cycles so that the direct compo nent remained. In order to remove the asym- Inetry, a portion of the starting electrode must be brought to a yet higher electron emitting temperature, and this we have done by providing a small helix of fine refractory metal wire, for instance 5 mil diameter tungsten wire, which isattached to the end of the lengthened starting electrode or central support [3 so that several turns project beyond the forward end of the main eiectrcde 3. These projecting turns of ver fine wire are heated by the discharge to a higher temperature than the main leadwire 53 of the starting electrode and, consequently, emit enough electrons to carry the low current in the starting circuit comprising resistor 26: As a result voltage drop Vim-between assessest and it becomes quite small in over-all magnitude and, in addition, is practically symmetrical. so that no direct current component remains;-

Referring to Fig. 4, the operating conditions with the new electrode structure shownin Fig. 3' are illustrated by curvesii, and 42 which have the same significance as the curves- 46; iii, and ii. of Fig. 2. These curves likewise represent experimental results obtained with anactual construction of a lamp comprising an arc tube in accordance with Fig. 3 and operated in a vertical position with the starting electrode on top and at 400 watts loading. It will be observed that 'the voltage between the starting electrode [3 andthe' adjacent main electrode 3, denoted by VM and illustrated by curve 45-, is quite low and" prac tically symmetrical'on both sides of thezero axis so that the direct voltage component represented by line 4-3 is only a few volts. Intact, we have: measured the direct component of voltage in: lamps containin our improved starting electrode in accordance with Fig. 3, and have observed. values for the direct voltagecomponent less than 1.5 volts. The resulting low value of unidirectional gradient causes little or no electrolysis inthe glass or vitreous alkaliecontaining material between the auxiliaryand the adjacent main electrodes, even though freealkali or sodium atoms are present andthe glass is ata high temperature.

Referring to Fig. 5, there. is shown another embodiment of our invention in which the auxiliary starting electrode i3 is extended by means of a tungsten wire ring 51 which is located a few m'illimeters in front of the main electrode 3 and substantially surrounds the arc stream. This type of construction is particularly adapted for lamps which are designed to be operated in any position 1 and not necessarily restricted to vertical opera tion. When a lamp such as that illustrated by Fig. l is operated horizontally, the arc stream has a tendency to bow upwards. Were a starting electrode such as that illustrated in Fig. 3 employed under these circumstances, it would be necessary to fix the angle at which the lamp is screwed into its socket. Otherwise, if the lamp were placed in operation with the auxiliary starting electrode located below its adjacent main electrode, the arc stream might be so far deflected from it as to substantially reduce its tempera.- ture. Thus, the elimination of electrolysis which this invention has for its object, would not be achieved. However, by providing the starting electrode in the form of a ring 5i as illustrated, no matter how the lamp is situated and in which direction the arc stream is bowed, some portion of the ring will always be heated by the discharge so that a high electron emissivity will be secured and electrolysis prevented.

Referring to Fig. there is shown yet another embodiment of our invention in which the starting electrode :3 is extended by a fine tungsten wire 52 or which a vertical portion is bent over crosswise to the axis of the tube and located a few millimeters in front of the main electrode 3 so as to lie substantially within the arc stream. This embodiment of the invention is likewise adapted to lamps designed for operation at any angle.

While certain specific embodiments have been shown and described, it will be, of course, understood that various modifications be made without departing from the invention. Thus, it

will be understood that the species which have been described are non-limitative examples and that the essential feature of the invention is to provide the starting electrode in such form and so located with respect to the adjacent main electrode that it is thermionically active and capable of supporting the current necessary for maintaining it at substantially the potential of the adjacent main electrode on both half cycles of the alternating current wave. This structure is quite evidently adaptable to other types or sizes of lamps in which diiferent vapor fillings, for instance, sodium, may be used. It will also be apparent that the invention is not restricted to lamps providing visible radiation, and can be used with other types of electric discharge devices where this problem of electrolysis occurs as, for instance, with ultra-violet lamps where the envelope is quartz. The appended claims are, therefore, intended to cover any such modifications coming within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device for high teinperature operation, comprising a closed envelope including a vitreous alkali-containing material subject to electrolysis and deterioration when placed under substantial electrical stress at said high temperature, an ionizable medium enclosed within said envelope, 2. pair of main thermionic are supporting electrodes located within said envelope and provided with external connections through seals at opposite ends thereof, a starting electrode located adjacent one of said main electrodes and provided with an external connection through a seal generally side-by-side with and in close proximity to that of said adjacent main electrode at one end of said envelope, and an external resistance permanently connecting said starting electrode to the opposite main electrode for starting purposes, said starting electrode being constructed to have an electron emission, during normal operation, sufficient to develop a voltage drop across said resistance such as to reduce the voltage between said starting electrode and said adjacent electrode to a low value and substantially prevent the development of a direct voltage component therebetween, thereby substantially to eliminate electrical stress and resulting deterioration in said material in the vicinity of the seals of said starting and said adjacent main electrode.

2. An electric discharge device for high temperature operation, comprising a closed envelope including a vitreous alkali-containing material subject to electrolysis and deterioration when placed under substantial electrical stress at said high temperature, an ionizable medium enclosed within said envelope, a pair of main thermionic are supporting electrodes located within said envelope provided with external connections through seals at opposite ends thereof, a starting electrode located adjacent one of said main electrodes and provided with an external connection through a seal generally side-by-side with and in close proximity to that of said adjacent main electrode at one end of said envelope, and an external resistance permanently connecting said starting electrode to the opposite main electrode for starting purposes, said starting electrode being constructed to have an electron emission, during normal operation, suihcient to provide substantially symmetrical conduction on opposite half cycles of an applied alternating voltage and develop a voltage drop across said resistance such as to reduce the voltage between said starting electrode and said adjacent electrode to a low value, thereby substantially to prevent the development of a direct voltage component therebetween and to eliminate electrical stress and resulting deterioration in said material in the vicinity of the seals of said starting and said adjacent main electrode.

3. An electric discharge device for high temperature operation, comprising a closed envelope including a vitreous alkali-containing material subject to electrolysis and deterioration when placed under substantial electrical stress at said high temperature, an ionizable medium enclosed within said envelope, 9. pair of main thermionic arc supporting, electrodes located within said envelope and provided with external connections through seals at opposite ends thereof, a starting electrode located adjacent one of said main electrodes and provided with an external connection through a seal generally side-by-side with and in close proximity to that of said adjacent rnain electrode at one end of said envelope, and an external resistance permanently connecting said starting electrode to the opposite main electrode for starting purposes, said starting electrode comprising portion of low thermal mass extending into the vicinity of the arc stream to said adjacent main electrode and adapted to have, during normal operation, an electron emissufiicient to develop a voltage drop across said resistance such as to reduce the voltage between said starting electrode and said adjacent main electrode to a low value and to prevent substantially the development of a direct voltage component therebetween, thereby to eliminate electrical stress and resulting deterioration in said material in the vicinity of the seals of said starting and said adjacent main electrode.

4. An electric discharge device for high temperature operation, comprising a closed envelope hiding vitreous alkalbcontaining material an it T'rable medium enclosed i envelop-e, a pair of main thermionic are supporting elec *3 located Within said enc external connections e ends thereof, a startadia -t electrode into the vicinity of the arc stre thereto, whereby said starting electrode heated suillciently, during normal operation, to attain electron emission assuring suhstanti s mmetrical conduction on opposit-e l. f cycles or" an applied alternating volt age in 03. er to prevent the development of a direct voltage component between said starting adjacent main electrodes, thereby substantially to eliminate electrical stress resulting dete rioration in material in the vicinity or" the seals of starting and said main electrodes,

5. An electric discharge device for high temperature operation, comprising a closed envelope including a vitreous alkali-containing material suhject to electrolysis and deterioration vvhen under ntial electrical stress at said temperature, an ionizaole medium enclosed thin nvelope, oi main ther. lionic supper g electrodes located Within said envelopc provided with external connections through scale at opposite ends thereof, a starting electrode located one of said main electrodes and prcvid with an external connection hrough a seal side hy-side with and in close proximity to that or adjacent electrode at one end of envelope, and eX- ternal resistance permanently connecting said electrode to the opposite electrode ting purposes, said starting electrode corocrisine" central conductive support and a helix or fine wire of a refractory metal mounted over t ereof having a number of free 'rdly or" said end and on y of the arc stream to said 1 electrode, whereby to assure sufdof said starting electrode to provide .7 symmetrical conduction on opposite hair cycles of an applied alternating voltage and prevent the development of a direct voltage component destructive of said material in the vicinity of the seals of said starting and said adjacent main electrodes.

6. An electric discharge device for high term perature operation, comprising a closed envelope including a vitreous alkali-containing material subject to electrolysis and deterioration when placed under substantial electrical stress at said high temperature, an ionizable medium enclosed within said envelope, a pair of main thermionic are supporting electrodes located Within said envelope and provided with external connections through seals at opposite ends thereof, a starting electrode located adjacent one of said main electrodes and provided with an external connection through a seal generally side-hy-side with and in close proximity to that of said adjacent main electrode at one end of said envelope, and an external resistance permanently connecting said starting electrode to the opposite main electrode for starting purposes, said starting electrode comprising a central support and a loop of fine wire of a refractory metal attached thereto, said loop being located to lie in a plane transverse to the arc stream between main electrodes and surrounding said are stream slightly forward of said adjacent main electrode, whereby to assure that some portion or said loop be heated by said are stream irrespecti sly of any bowing thereof caused by the operating position of said lamp, said heating rendering said starting electrode suitlciently emissive to develop a voltage acros said resistance such as to reduce the voltage between said start ing electrode and said adjacent electrode to a low value and substantially prevent the development of a direct voltage component thereoetween, whereby substantially to eliminate electrical stress and resulting deterioration in said material in the vicinity of the seals of said starting and said adjacent main electrode.

7. An electric discharge device for high temperature operation, comprising a closed envelope including a vitreous alkali-containing material subject to electrolysis and deterioration when. placed under substantial electrical stress at said high temperature, an ionizable medium enclosed Within said envelope, a pair of main thermionic are supporting electrodes located within said onvelope and provided with external connections through seals at opposite ends thereof, a starting electrode located adjacent one of said main electrodes and provided with an external connection through a seal generally side by-side with and in close proximity to that of said adjacent main electrode at one end of said envelope, and an external resistance permanently connecting said starting electrode to the opposite main electrode for starting purposes, said starting electrode comprising a central conductive support and a fine Wire of a refractory metal attached thereto, said wire having a transverse portion located in the arc stream between said main electrodes and slightly forward of the adjacent main electrode, the heating of said portion during normal operation assuring sunicient electron emissivity to provide a voltage drop across said resistance such as to reduce the voltage between said starting electrode and said adjacent main electrode to a lowvalue and substantially prevent the development of a direct voltage component therebetween, thereby to eliminate electrical stress and resulting deterioration in said material in the vicinity of the seals of said starting and said adjacent main electrode.

JAMES A. ST. LOUIS. JOHN MILTON PGMFRETT.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,980,534 Kirsten Nov. 13, 1934 2,020,737 Pirani Nov. 12, 1935 2,103,048 Weidhaas Dec. 21, 1937 2,208,998 Mey July 23, 1940 2,247,198 Kreirt June 24, 1941 2,315,286 Hays Mar. 30, 1943

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