US1893304A - Negative glow lamp - Google Patents

Description

. Jan. 3, 1933 H c. RENTSCHLYER ET AL 9 3 NEGATIVE GLOW LAMP Original Filed Marh 20. 1929 INVENTOR H. C. PENTSCHLE'E .D. 5 475MB) U AT EY Patented Jan. 3, 1933 UNITED STATES PATENT OFFICE HARVEY O. RENTSGHLER, OF EAST ORANGE, AND DONALD E. HENRY, OF BLOOMFIELD, NEW JERSEY, ASSIGNORS 'IO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA NEGATIVE GLOW LAMP Original application filed Harch20, 1929, Serial No. 848,409. Divided and this application filed Kay 81,

1980. Serial This application is a divisional application of copending application Serial No. 348,409 filed March 20, 1929 and relates to gaseous r conduction devices in which rarefied gases are rendered luminous by the sustained passage of electric current between electrodes, and more particularly to lamps of this type in which the luminosity is due to a negative glow discharge.

One of the objects of the invention is to produce a negative glow lamp which may be operated from an ordinary 110 volt alternating current lighting circuit, with or without the inter-positioning of a ballast resistance or other current limiting device in series therewith.

Another object is to produce a glow discharge device which will have a stable operating characteristic.

Another object is to produce a glow discharge television lamp of uniform glow and high intrinsic brilliancy.

A still further object is to produce an electrical discharge device, employing a rare gas in which gas purifying conditions are provided continuously during the operation of the device.

Other objects and advantages will hereinafter appear.

It has been necessary, in the operation, on

commercial line voltages, of negative glowv lamps or similar gaseous discharge devices, as heretofore constructed, to employ a our- 0 rent limiting means in series therewith in order to prevent arcing between the electrodes and the establishment of an unstable electrical discharge. Usually such current limiting device consists of a high resistance located in the base of the lamp and connected in series with one of the electrodes.

In the absence of 'such current limiting means the glow discharge cannot be maintained, but immediately forms a destructive are which causes destruction of the device.

We have discovered, however, that if one of the rare gases such as neon, is employed at a suitable pressure, as the gaseous filling within the device and the electrodes are properly spaced apart and are composed of thorium, the lamp may be operated with a stable electrical discharge without the use of current limiting means in series therewith, on an ordinary 110 volt alternating current hne. The thorium electrodes have a low cathode fall and the voltage required to start the d1scharge is near the peak voltage of the alternating current source. Since the vapor pressure of the thorium is so low that it does not assist in carrying current through the device, the arcing voltage of the lamp is relatively high, that is, above the peak voltage of the 110 volt alternating current line and consequently when the lamp is operated directly from the 110 volt line, the discharge cannot break down into an arc.

It has been proposed heretofore to employ an alkali metal on the electrodes to reduce the cathode drop but when so employed, the arcing potential is reduced below the peak voltage of the 110 volt alternating current source and such devices require the use of a current limiting resistance in series therewith to render the discharge stable.

Negative glow discharge lamps of this I type are useful for many purposes where only a small amount of light is desired at a low current consumption, such as indicator and signal lamps and in household applications.

The current consumption of a small neon lamp, such as used on electric light switches is so low that they may be operated con tinuously to indicate the location of the switch at night. They are also useful for indicating when current is turned on in electrical appliances, such as electric irons, etc. It is highly desirable that such lamps operate without the necessity of a current limiting resistance in series therewith, in order to economize in space. The elimination of 90 the ballast resistance also efiects a considerable saving in the cost of the lam In addition to render the disciiarge stable, the use of thorium electrodes, enables a more uniform operating characteristic to be obtained for the lamps and rmits the use of a hi h current density on t e electrode without ob ectionable sputtering.

One important application of the negative glow lamp is in a light source for-television apparatus. In such lamps the electrodes are composed of two parallel flat sheets of metal, placed close together so that the discharge occurs only on the outside of the electrode serving as the cathode. The plates are of the same size or slightly larger than the picture to be reproduced and the cathode is observed through a scanning disc, as is well understood in television reception. Difiiculty has been experienced in such lamps, as heretofore constructed, due to the fact that the degree of illumination of the cathode is small. The materials ordinarily used for the cathode are iron or nickel and when employing such metals, the current density of the electrode must be maintained low to prevent sputtering of the electrodes. According to Claude Patent 1,125,476 the current density must be maintained below two thirds amperes per square decimeter area of the cathode.

When employing such materials as nickel and iron as the cathode of television lamps, the glow on the cathode is often not uniform over the surface but has irregular spots which are either brighter or darker than the average intensity of illumination of the cathode.

The bright portions are believed to be due to traces on the electrode of materials of lower cathode resistance than the material of the electrode, such as one of the alkali metals, and the dark ortions to accumulations of materials of higher cathode resistance, such as carbon. It is extremely diflficult to clean iron or nickel electrodes of such materials, so as to eliminate this spotty condition. 7

We have found that when thorium is employed as the cathode that the current density at the electrode may be very much higher without objectionable sputtering or deterioration and as a consequence the brilliancy of the electrode is materially increased.

Thorium, moreover, does not exhibit the spotty appearance of the iron or nickel electrodes. Due to the chemical activity of thorium it is self-cleanin so that foreign materials having a hig er or lower cathode resistance than thorium do not remain on the thorium cathode.

The sputtering of the electrodes in a gaseous conduct-ion device of this nature we have found to be due to the voltage drop at the surface of the cathode, rather than to the current density at the electrode and the steeper the voltage current characteristic curve of the device, the greater the tendency of the cathode to sputter.

Comparative measurements made with thorium and iron or nickel electrodes have shown that for a given increase in currentdensity at the cathode, the increase in voltage drop at the electrode is substantially only one half as great with a' cathode composed of thorium, as with a nickel or iron cathode, so that the voltage-current characteristic curve for the device when employing a thorium cathode is only half as steep as when the cathode is composed of iron or nickel and similar materials.

We have also discovered that there is a definite relation between the steepness of the voltage-current curve and pressure of the gas employed and that by arranging the electrodesso as to permit the utilization of a high gas pressure, the slope of the voltage current curve of the device may be still further reduced so that a materially higher ourrent density may be employed on the cathode sputtering of the cathode.

In television lamps as now constructed, the gas pressure which can be used is limited by the permissible s acing of the parallel plate electrodes. As t e s pressure is increased the plates must be rought closer together in order to maintain the spacing therebetween well within the mean free path and hence to prevent discharges occurring on reverse side of the cathode. However, there is a mechanical limit to the closeness to which these electrodes maybe spaced which limits the maximum gas pressure which may be used.

In accordance with the present invention this difiiculty has been overcome and a high gas pressure employed by placing solid insulating material between the electrodes and in contact therewith so as to positively restrict the discharge to the face thereof. This insulating material may consist of an interposed sheet of mica or similar material of high dielectric strength or it may consist of an insulating coating on the back of the cathode. When high electrical efiiciency is not of particular importance as compared with the importance of a high intrinsic brilliancy brillancy of the observed side of the cathode is high.

The invention may be embodied in various forms and in order that a more complete understandlng of the invention may be had reference is made to the accompanying drawing, in which Fig. 1 is a side elevational View of a television lamp' constructed in accordance with the present invention for operation In a high gas pressure;

Fig. 2 is a side perspective view of a modified electrode structure for a television lamp;

Fig. 3 is a perspective view of a further modified electrode assembly for a television lamp in which the anode is spaced outside of the mean free path of the gas, and

Fig. 4 is a perspective view of a negative glow television lamp for producing a concentraied glow of small area.

The lamp shown in Fig. 1 comprises an envelope 11 containing a pair of spaced apart electrodes 12 and 13 supported upon rigid stems 14 and 15 sealed in a press 16. The stems 14 and 15 are joined to leading-in conductors connected to the terminals 19 and 20 of a standard type lamp base.

The electrodes 12 and 13 are composed of thorium, which has a cathode drop of such value that when the electrodes are spaced apart at the critical distance from the minimum starting potential, in neon gas at a substantlal pressure, the discharge will start below the peak voltage of an ordinary commercial 110 volt .A. C. line, but sufiiciently near to the peak voltage so that the normal arcing voltage of the lamp will be above the peak voltage.

The anodes 12 and .13 may be secured to the support wires 14 and 15 in any suitable manner. A gaseous filling of argon, neon, helium or mixtures thereof may be employed at a reduced pressure, but neon is preferred in order to bring the starting and arcing voltages respectively below and above the peak voltage of the 110 volt A. 0. line. The pressure of the gas may range from 5 mm. to 3 or 4 cm. When employing argon lower pressures are preferable and in the case of helium higher pressures may be used.

The discharge lamp just described, employing neon gas, may be placed in the ordinary incandescent lamp socket and operated directly on a 110 volt A. C. commercial lighting circuit without employing a stabilizing resistance and will operate under such conditions with a stable electrical discharge. This is contrary to the behavior of negative glow discharge lamps of the same general character, as heretofore constructed.

The thorium electrodes exert a cleanup action or gas purifying action and maintain the gas in a pure condition, thus enabling the operating characteristics to be maintained over a long commercial life. In addition to these advantages the thorium electrodes are more resistant to sputtering at high current densities.

We have found that the resistance to sputtering of an electrode, in a glow discharge device depends not on the current density of the electrode, as ordinarily presumed, but

on the voltage drop at the cathode.

This voltage drop or resistance to the discharge at the cathode, increases as the current density increases but when thorium is employed as the cathode we'have discovered that the increase in voltage drop is only about one-half of that obtained when iron, nickel or similar materials are used. For this reason it is possible to increase the current density to a much higher value with a thorium cathode than with ordinary metals.

Thesputtering may be further reduced by increasing the gas pressure since we have found that at higher gas pressures the voltage current characteristic curve it flatter, than heretofore obtained.

This possible increase in current density while not important in certain types of lamps, such as indicator lamps, is of extreme importance where high luminosity is desired, as for television purposes. In Figs. 1 to 4 in elusive, are shown television lamps which are designed to be operated with a high current density on the electrodes.

In Fig. 1 we have a construction in which the gas pressure may be increased indefinitely while restricting the discharge to the outer face of the cathode, it being understood of course that the pressure must be maintained ing as by oxidation and the plates placed either in contact or at any convenient distance apart. 7

The electrode structure shown in Fig. 2 has a pair of parallel closely spaced plate electrodes 32 and 33, one of which at least is composed of thorium. The thorium electrode is operated as the cathode, such a lamp being operated on direct current. The electrodes 32 and 33 are of a size at least as great as the area covered by the scanning disc of a television receiving apparatus and are spaced apart a distance which is within the mean free path of the gaseous filling so that the discharge takes place only between the outer surfaces of the electrodes. The electrodes are supported above a press 34 by suitable support wires 35 and 36 which also serve as current conductors.

Such a lamp may be operated at a relativeof the cathode.

1y high current density and therefore with a brilliant glow because of the resistance of the thorium cathode against sputtering. However, since the minimum distance which must be maintained between the plates 33 and 34 is limited by mechanical difliculties it is not possible to increase the gas pressure. above a definite limit without causing a discharge to take place between the inner faces of the electrodes.

By increasing the gas pressure in the lamp shown in Fig. 1, the current density and hence the intrinsic brilliancy of the cathode increases very materially. The main purpose for restricting the discharge to the outside of the electrodes is for electrical efficiency since the glow on the inner surface of the cathode cannot be utilized. If electrical efiiciency is not of importance the'parallel plate anode may be replaced by an electrode of any other suitable shape and the discharge permitted to take place between the anode and both sides Such an electrode structure is shown in Fig. 3 and comprises a thorium cathode 4:2 and an anode 43 of nickel or any other suitable material.

Since there is no appreciable sputtering at the anode of such a lamp this electrode may be made of small size and of an desired material. With the construction 1n Fig. 3 any suitable high gas pressure may be employed i111l order to increase the brilliancy of the dis- 0 arge. g

In Fig. 4 we have shown an electrode structure for a lamp of the negative glow type adapted to produce a very intensive beam of light of small cross section. The lamp comprises two concentric cylindrical plates 52 and 53 serving as a cathode. and being composed of thorium and the outer cylinder 52 being spaced therefrom a distance which is wlithirii the mean free path of the gas emoye p In such a lamp all of the glow is confined to the interior surface of the cathode 53 and when such an electrode is viewed along the rectifiers, and we do not desire to be limited to the exact details shown and described except in accordance with the appended claim:

What is claimed is:

A discharge lamp comprising an enclosing envelope containing neon gas at a pressure of 5 millimeters to about 4 centimeters, a pair of electrodes therein having parallel surfaces spaced apart a distance substantially no greater than the mean free path of an electron in the gas at the pressure employed, one

of said electrodes belng composed of solid HARVEY O. RENTSCHLER.

DONALD E. HENRY.

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