US2733371A - Internally conducttvely coated - Google Patents

Description

Jan. 31, 1956 J. H. CAMPBELL 2,733,371

INTERNALLY CONDUCTIVELY COATED DISCHARGE LAMP Filed May 12, 1950 lnven lrors John H. CampbeLL,

His A tforneg.

2,733,311 Patented Jan. 31, 1956 INTERNALLY CONDUCTIVELY COATED DISCHARGE LAMP John H. Campbell, Painesville, Ohio, assignor to General Electric Company, a corporation of New York Application May 12, 1950, Serial No. 161,621 6 Claims. (Cl. 313-185) The present invention relates to gaseous electric discharge devices generally,

relatively low resistance.

It has become relatively common to provide metallic stripes or conductive coatings on the outside glass walls of electric discharge devices. For instance, this is done with respect to cathode ray tubes and some types of dis charge lamps. In general, however, the purpose of the coating is simply to dispel static charges or accumulations of electrons on the walls, so that high ambient humidity will not prevent the lamp from starting at normal voltage.

It is known that if a transparent conductive coating is provided on the inside of the glass walls of a low pressure positive column and the blast made considerably smaller. Although some improvements in that direction have been made in the past, there is as yet no commercially available lamp wherein the starting voltage is as low as the voltage required for stabilizing the discharge.

I have found that there appears to be an optimum range of resistance for the internal coating of a gaseous discharge device which is neither as high as the resistance merely sufiicient to dispel electronic accumulations nor as low as a metal stripe, the equivalent of which might A be a wire internally disposed within the lamp. In general, the optimum resistance appears to be that which, when traversed by the current suflicient to produce a glow discharge in the lamp, is just sufficient to produce a potential gradient along the glass envelope which is slightly in excess of the gradient required within the lamp to ionize its inert gas constituent.

I have also found that a lamp provided with an internal conductive coating lends itself advantageously to combinations of multiple electrodes wherein a main electrode is provided is the provision of For further objects and advantages and for a better I the coating resistance during the understanding of my invention, attention is now directed to the following description and to the novel will be more clearly pointed out in the appended claims.

In the drawing:

Fig. l is a side view of an improved electric discharge lamp structure embodying my invention and including a schematic diagram of an operating circuit therefor.

Figs. 2, 3, and 4 are pictorial representations of consecutive stages in the transition from a glow to an arc discharge within a lamp embodying my invention.

Fig. 5 is a side view of an electric discharge lamp having an internal conductive coating and provided with an improved cathode structure in accordance with my invention, and including a schematic diagram of an operating circuit therefor.

Referring to Fig. l, the device 1 comprises an elongated tubular glass envelope 2 having sealed into the ends thereof a pair of electrodes 3 and 4. These electrodes may consist of coils of tungsten wire coated with activated electron-emitting materials such as barium and strontium oxides. The ends of the coils are brought out externally to the envelope through suitable seals in the glass in order to permit the application of a heating current to raise the activating materials to an electron-emitting temperature. The glass envelope contains an inert gas such as argon, krypton, neon, or mixtures thereof at a pressure of a few millimeters of mercury and also a small quantity of mercury which, during normal operation of the lamp, exerts a pressure of a few microns.

The inner wall of the glass envelope 2 is coated with a fine layer of conductive materials such as, for instance, one of the metallic halides. I found that stan' nous chloride, which may be applied by vaporization while the glass is at a temperature of approximately 550 0., provides a suitable transparent coating which hardly affects the light-transmitting properties of the glass. In practice, after the transparent conductive coating has been deposited on the glass, it is further coated with a suitable phosphor as in the usual fluorescent lamps.

Although internal transparent conductive coatings in electric discharge devices have been suggested before, prior investigators'have apparently not made use of the fact that heated electrodes in combination with a relatively low resistance inside coating will provide a maximum lowering of the starting voltage. Past practice has indicated that use of a conductive coating outside or inside the bulb will merely dispel static charges and thus prevent hard starting due to high ambient humidity. I have found that, in general, the optimum resistance value is that which, when traversed by a current just sufiicient to produce a glow discharge within the lamp, produces a voltage drop along the longitudinal axis of the lamp giving a voltage gradient slightly in excess of that required for ionizing the inert gas within the lamp. Naturally, the

In manufacture it is desirable to keep the resistance range to closer limits in order to allow for changes in various heat treatments.

3 The 40-watt lamp, may, therefore, require a range of 50,090 to 150,000 ohms.

Referring to Figs. 2, 3, and 4, the progress of the glow discharge within such a lamp has been illustrated in order to. facilitate comprehension of the different manner of establishing an arc in a lamp provided with an internal conductive coating such as I have described. The glow discharges illustrated refer actually to those occurring when the lamp is operated from a direct current source, since the phenomena may be more easily understood thereby. The only difference when the lamp is operated from an alternating current source is that the character of the discharge alternates from one end of the lamp to the other at the frequency of the voltage supply, so that the character of the glow at both electrodes appears to be the same. In fact, however, the type of glow established at each electrode during each half cycle of the alternating c rrent wave is identical with that established continuously with a direct current supply, so that, with these reservations in mind, the phenomena occurring with a direct current supply only need be considered.

It will be assumed that an operating circuit has been provided for a lamp such as illustrated in Fig. 1, which comprises a discharge current limiting reactor 5 inserted in series between the lamp and a pair of input terminals 6, 6 adapted to be connected to a source of voltage. A filament heating transformer 7 comprises a primary winding 8 and a pair of secondary windings 9 and 10 which are connected to electrodes 3 and 4, respectively. This circuit obviously is intended for use with an alternating current supply. However, those skilled in the art will have no difficulty in conceiving a similar circuit for operating the lamp on direct current, as will be assumed in the explanation of the character of the glow in Figs. 2, 3, and 4 now to follow.

Referring to Fig. 2, after the electrodes have been heated to an electron-emitting temperature and a voltage is applied across them to establish a gradient within the discharge space of the lamp, there occurs initially a weak glow in the immediate vicinity of both the positive and negative electrode. If the magnitude of the applied voltage is increased slightly, the glow at the positive electrode assumes a conical shape, the apex of which is pointed in the direction of the cathode. The lamp at this stage has a positive resistance characteristic. I surmise that this is due to the resistance of the internal coating and is not actually inherent in the discharge. It would appear that at this stage of the glow, in the immediate vicinity of the electrodes the current is carried by electrons and ions, but in the space intermediate the electrodes, where no glow has developed as yet, the current is flowing through the conductive coating. The reason for which there is an optimum potential gradient along the resistance path will now become apparent in that it is necessary that the glow discharge fill the whole space between the electrodes before the transition into an arc discharge can occur. For the glow to fill the. intermediate space, it is necessary that the gas molecules therein be ionized and this will naturally be assisted if the voltage gradient produced along the walls of the envelope by the current flowing through the resistive coating is sufficient to cause such an ionization. If the resistance is too high, the flow of current through the coating will be unnecessarily restricted and ionization in the vicinity of the electrodes will be hindered. If the resistance of the coating is too low, the current from the glow discharge flowing therethrough will produce a voltage gradient too low to be of any substantial help in the ionization of the gaseous column.

When the voltage applied across the electrodes is increased slightly, the apex of the conical glow emanating from the anode stretches out in the direction of the cathode. In Fig. 3 the apex of the glow has reached approximately the mid-point of the lamp, and in Fig. 4 the, apex has reached the glow which surrounds the cathode. Fig. 4 illustrates a condition which is generally unstable, as, the'glow discharge will shortly thereafter tain activating processes in. manufacture only,

spread out to fill the whole tube, and the transition to the arc. discharge will then occur with the formation of a hot spot on the cathode or negative electrode.

As an example of the advantages ensuing from this type of lamp construction, the starting voltage required for 40-watt fluorescent lamps provided with argon at 3 mm. pressure has been reduced from 450 volts to 270 volts in the instant start type and from 208 volts to 150 volts in the switch type preheat ballast. Although this voltage is in excess of the operating voltage drop across such a lamp, which is in the neighborhood of volts, it is well within the voltage which is usually provided for regulating the discharge through the lamp in order to compensate for its negative resistance characteristics. Thus, commercial 40-watt fluorescent lamps are usually operated with a transformer and series choke coil or with a high reactance transformer which has an open circuit voltage of approximately 208 volts, which voltage is decreased during operation, as a result of the internal reactance drop, to approximately 100 volts. A lamp having an internal resistive coating such as I have described will start instantly, in such a circuit, without any preheat switching apparatus whatsoever provided a preheat current is applied to the. electrodes.

Although I have described the. advantages of an internal transparent coating with respect to electrodes which are preheated at starting, my invention lends itself equally well tothe lowering of the starting voltage with lamps wherein the electrodes are not preheated at starting, such lamps being commonly known as the instant start type. Without any modification whatsoever, except the provision of an internal coating having a resistance of the same order as that which has been described, a substantial lowering, of the starting voltage occurs.

I have found that a discharge lamp containing a low resistance internal transparent coating lends itself particularly Well to an improved type of electrode, as illustrated in Fig. 5. The lamp 20 therein is provided with main electrodes 21 and 22 which may consist of coiled coils of tungsten wire thickly overlaid with activated electronemitting material. Both ends of the filaments of electrodes 21 and 22 are, brought out external to the glass envelope of lamp 20. However, this is for the carrying out of cerand the upper terminal alone is utilized during operation. No preheat is required. so that a high reactance transformer 23 without any; filamentary heating windings may be utilized for operating the lamp. In association with electrodes 21 and 22, I provide small auxiliary electrodes 25 and 26, respectively. These electrodes may consist of a simple coil of finer tungsten wire than utilized in the main electrodes 21 and 22, these auxiliary electrodes being also coated with. activated.electron-emitting material and con nected to the free ends of the main electrodes. They are physically displaced from the main electrodes and preferably located closer to the envelope wall than the main electrodes. As with the free ends of electrodes 21 and 22, the free ends of electrodes 25 and 26 are brought out external to the glass envelope in order to facilitate the manufacturing process.

In operation, when voltage is applied across the electrodes, ionization occurs and surrounds the electrodes at both endsof the lamp. At this moment, the current flows between the electrodes and the conductive coating in their immediate vicinity. Auxiliary electrodes 25, and 26, being constructed of finer wire, have a lower thermal capacity and also a higher resistance than the main electrodes 21 and 22. As a result of the current, they heat up more quickly and arrive more readily at a temperature at which sufficient electrons are emitted. to permit the transition to an arc discharge. milliamperes, in the case of a 40-watt, 48-inch lamp. When the arc discharge occurs, the current becomes substantially greater; and since, it must flow through the main electrodes, connected in series with the auxiliary electrodes, it causes these main electrodes to reach electron-emitting The current above may be as low as 70:

temperature. Thereupon, the arc shifts to these main electrodes, that is 21 and 22, which electrodes are large enough to support the discharge without overheating, and normal operation ensues. It might be mentioned that the hot spots forming on the electrodes always tend to shift toward the end of the electrode closest to the point of application of the external potential causing the discharge. This is due to the fact that the voltage drop produced by the resistance of the electrodes between the point where the hot spot occurs and the connection to the lead-in wire produces a voltage diiferential which tends to shift the hot spot toward the lead-in wire. During the life of the lamp, as the activated material is slowly destroyed in the immediate vicinity of the lead-in wire, the hot spot gradually works away from the lead-in wire toward the other end of the cathode until finally the lamp has reached the end of its life.

While certain specific improvements have been shown and described, it will, of course, be understood that certain modifications may be made without departing from the invention. The appended claims are, therefore, intended to cover any such modifications coming within the true spirit and scope of the invention.

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

1. An electric discharge device of the low pressure positive column type comprising an elongated vitreous light-transmitting envelope containing an ionizable medium including an inert gas at a pressure of a few millimeters and a small quantity of mercury, a pair of main coiled filamentary thermionic electrodes, activated with electronemissive materials, sealed into opposite ends of said envelope, lead-in wires connected to one side of said main electrodes to provide current terminals, a pair of auxiliary pressure posivitreous lightrespectively connected to the other side of said main electrodes and physically displaced therefrom, and a low resistance transparent coating of a metallic halide covertrodes and physically displaced therefrom, and a low resistance transparent coating of stannous chloride covering substantially the whole inside suiface of said envelope.

4. An electric discharge device of the low pressure positive column type comprising an elongated vitreous lighttransmitting envelope containing an ionizable medium including an inert gas at a pressure of a few millimeters and a small quantity of mercury, 21 pair of main coiled filamentary thermionic electrodes, activated with electronemissive materials, sealed into opposite ends of said envelope, lead-in wires connected to one side of said main electrodes to provide current terminals, a pair of auxiliary coiled filamentary activated electrodes of substantially lower thermal capacity than said main electrodes and respectively connected to the other side of said main electrodes and physically displaced therefrom, and a low resistance transparent coating covering substantially the to raise said auxiliary electron emitting temperature.

5. An electric discharge device of the low pressure positive column type comprising an elongated vitreous lighttransmitting envelope containing an ionizable medium including an inert gas at a pressure of a few millimeters and a small quantity of mercury, a pair of main coiled filamentary thermionic electrodes, activated with electronemissive materials, sealed into opposite ends of said envelope, lead-in wires connected to one side of said main electrodes to provide current terminals, a pair of auxiliary coiled filamentary activated electrodes of substantially lower thermal capacity than said main electrodes and respectively connected to the other side of said main electrodes, said 6. An electric discharge device of the low pressure positive column type comprising an elongated vitreous light-transmitting envelope containing an ionizable medium including an inert gas at a pressure of a few millimeters and a small quantity of mercury, a pair of main coiled filamentary thermionic electrodes, activated with electron- References Cited in the file of this patent UNITED STATES PATENTS 1,980,534 Kirsten a- Nov. 13, 1934 2,038,049 Kirsten Apr. 21, 1936 2,042,147 Fairbrother May 26, 1936 2,064,369 Biggs Dec. 15, 1936 2,291,965 Jahncke Aug. 4, 1942 2,297,454 Berger Sept. 29, 1942 2,306,925 Aicher Dec. 29, 1942 2,429,420 McMaster Oct. 21, 1947 2,441,831 Moore May 18, 1948

Claims (1)

1. AN ELECTRIC DISCHARGE DEVICE OF THE LOW PRESSURE POSITIVE COLUMN TYPE COMPRISING AN ELONGATED VITREOUS LIGHT-TRANSMITTING ENVELOPE CONTAINING AN IONIZABLE MEDIUM INCLUDING AN INERT GAS AT A PRESSURE OF A FEW MILLIMETERS AND A SMALL QUANTITY OF MERCURY, A PAIR OF MAIN COILED FILAMENTARY THERMIONIC ELECTRODES, ACTIVATED WITH ELECTRONEMMISIVE MATERIALS, SEALED INTO OPPOSITE ENDS OF SAID ENVELOPE, LEAD-IN WIRES CONNECTED TO ONE SIDE OF SAID MAIN ELECTRODES TO PROVIDE CURRENT TERMINALS, A PAIR OF AUXIALLIARY COILED FILAMENTARY ACTIVATED ELECTRODES OF SUBSTANTIALLY LOWER THERMAL CAPACITY THAN SAID MAIN ELECTRODES AND RESPECTIVELY CONNECTED TO THE OTHER SIDE OF SAID MAIN ELETRODES AND PHYSICALLY DISPLACED THEREFROM, AND A LOW RESISTANCE TRANSPARENT COATING COVERING SUBSTANTIALLY THE WHOLE INSIDE SURFACE OF SAID ENVELOPE.

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