US2351616A - Electric discharge device - Google Patents

Description

June ,1 w. J. KARASH ETAL ,3

ELECTRIC DISCHARGE DEVICE Filed April 21, 1942 H igh Leakage reac'tance/ hr'ans+ormer'.

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Wakter J. Karash, Eugene Lemmevs, by N, flaw/- 1 His At'tofney.

Patented June 2b, 1944 UNITED STATES PATENT OFFICE ELECTRIC DISCHARGE DEVICE Walter J. Karash, Annapolis, Md., and Eugene Lemmers, Cleveland Heights, Ohio, assignorsto General Electric Company, a corporation of New York Application'April 21, 1942, Serial No. 439,896

9 Claims.

This invention relates to electric discharge devices, and is useful in devices producing radiation for various purposes. The invention facilitates the operation of discharge devices, makes it possible to dispense with certain usual accessories, and also affords means of increasing the radiant or luminous output. It is particularly adapted for positive column discharge devices characterized by low operating pressure and a diffuse discharge, and is hereinafter explained with reference to fluorescent lamps or tubes of this character. It has proved very advantageous with respect to the starting of such lamps.

The early fluorescent lamp were of cold-startin type, having activated electrodes or cathodes at their ends which were heated only by the discharge between them, as exemplified by U. S.

Patent 2,114,842 to Inman. Such lamps were somewhat subject to blackening by deposits of material thrown off (whether vaporized or sputtered) from their cathodes during the starting The electron emission from the hot electrode (s) when functio'ningas cathode (s) lowers the voltage drop and reduces the severity of ion bombardment during starting, and likewise reduces the tendency toward sputtering of cathode material. However, while they are satisfactory as regards freedom from blackening and candlepower maintenance, these hot-starting fluorescent lamps have the disadvantage of requiring extra wiring and starting devices to enable them to be preheated and then started as described. Automatic starter of this character are themselves liable to derangement, which results in putting out of service the lamps with which they are associated, and requires troublesome replacements, besides tending to discredit the lamps themselves.

Positive column discharge devices (including fluorescent lamps. of that type) exhibit what is known as "the Faraday dark space, a dark region between the negative glow" adjacent the cathode and the "positive column glow which extends toward the anode. In alternating current devices, whose electrodes function as cathode and anode alternately, this sequence of cathodic glow, dark space, and positive column glow likewise'takes place from opposite ends of the device alternately; but by persistence of vision,

dark spaces are seen near both electrodes simultaneously. The Faraday dark space phenomenon is due to an excess of positive ions in the region, which reduce the negative charge and field strength there, so that the voltage gradient across this space does not maintain the electron velocity at a level suiflcient to produce luminous or other radiation,-such as the 2537 A. line of mercury vapor that is mainly relied 'on to excite theluminescent materials or phosphors in ordinary fluorescent lamps. Hence the region of the Faraday dark space is externally more or less deficient in luminosity in the cas of fluorescent positive column lamps coated with phosphors, as well as in the case of non-fluorescent discharge lamps. In all discharge tubes, therefore, the Faradaydark' space represents an essentially ineffective fraction of the tube length. I

We have found that by means hereinafter explained the drawbacks of cold-starting discharge devices as regards envelope blackening and maintenance of radiant output can be obviated very simply and inexpensively, so that such lamps generally give a long life at high efficiency, even with thousands of cold starts. This allows of dispensing with the usual starting circuit and starter, thus reducing the cost of installation as well as the possibilities of derangement and the trouble and expense of operating maintenance.

The invention also permits of virtually abolishing or greatly ameliorating the Faraday dark space phenomenon, thus increasing the useful radiation from the device. The flicker noticeable 'at the ends, of discharge lamps operating of species and forms of embodiment, and from the drawing.

In the drawing; Fig. 1 is a tilted side view of a positive column electric discharge tube embodying the invention 7 in a form suitable for a fluorescent lamp,a midportion of the device being broken out and omitted to permit the drawing to be made on a larger b I scale, and suitable circuit connections for the device being diagrammatically shown.

Fig. 2 is a tilted side view of the end portion of a similar discharge tube, showing a modification; and Fig. 3 is a circuit diagram for the modification illustrated in Fig. 2.

As shown in Fig. 1, the discharge device comprises an envelope tube III with electrodes II at its opposite ends, each of which acts as cath-- ode and anode alternately, during successive half cycles, whenthe device is operated on A. C., and is connected by current leads l2, l2 to the contact terminals |2, |2 of the usual bipost base I. The electrodes II are spaced apart lengthwise of theenvelopetube I. sufliciently phosphor 5 on the cylindrical tube wall. For

an ionizable discharge atmosphere, a surplus supply of mercury is indicated by a droplet i6,

and the envelope It may also contain starting gas, such as argon at a pressure of some 2 to 4 mm. of mercury. As shown in Fig. 1, the cathodes II are connected (through a lead I2 and terminal ll of each, and through an external circuit l1, l1) across the secondary of a step-up leakage-reactance autotransformer T, whose primary is connected across the A. C. power-supply circuit P. Such a transformer T connected in this manner gives a sufllciently high open-circuit voltage for starting the discharge, and a suitably lower operating voltage when discharge current is drawn from it after starting. Instead of this arrangement, any other suitable current supply and discharge-starting circuit means may be employed to produce discharge-initiating voltage between the electrodes I while they are cold,

followed by lower discharge-maintaining voltage during ordinary operation.

-As shown in Fig. 1, each cathode comprises a thermionic cathode member 2| in the form of a wire coil or coiled coil of such fine gauge that it is self-heating," and is very quickl heated by the discharge to effective electron emission, during starting, before substantial sputtering from it can occur. This member 2| may be ac: tivated with refractory oxides for more abundant emission of electrons. Across the envelope III in circular cross-section of the tube ll, yet discs oi polygonal or other non-circular outlines might also be used, and might-be rather more eillcient than a circular disc if the tube "were of correspondingnon-circular cross-section. Nor need the disc 22 be imperforate as shown, nor even of sheet metal: on the contrary, a disc pierced with a multitude of small holes (e. g., about 1 inch in diameter), or of wire mesh (e. g., wires of 10 mil diameter 1 inch apart both ways) has been found just as eifective, as regards both cold starting and amelioration of the Faraday dark space. However, such wire mesh or other perforate constructions have not shown any advantages beyond the obvious structural ones, such as lightness and economy of metal.

With the external power connection made to the terminal II of the lead |2 to which the disc 22 is directly connected, current flow via the disc 22 would be independent of the thermionic member 2|; but with the external connection made to the terminal I3 of the other lead l2, as shown at I! in Fig. 1, current flow via the disc 22 traverses the thermionic member 2|, which may be found more advantageous. The advantage of thus having the current flow via the disc 22 traverse the thermionic member 2| is that this current flow assists in heating said member to adequate emissive temperature. In detail, with a current supply connection only to one suitable point of the thermionic member 2|, and with an electrode disc 22 and other parts proportioned and arranged as hereinafter indicated, at least as close to the other electrode of the device as front of the self-heating thermionic member 2| is an associated transverse electrode member 22 of thin sheet metal (e. g., nickel), electrically connected to the thermionic member 2| ofits current supply connection, and interposed directly between said member 2| and the cathode II at the other end of the tube III. This part 22 is illustrated and generally referred to as a flat disc, since that is the most convenient form for it commercially, though by no means the only practicable form. The disc 22 is shown of suflicient size to obstruct the direct discharge path and largely occlude the tube Ill, and is fairly close to the thermionic member 2|. While it is not necessary for this disc 22 to be activated, activation does somewhat enhance the benefits due to its presence as regards cold starting. The disc 22 may be mounted across the envelope III by means of a support wire 22 welded to it and to one of the leads l2, l2.

Though the disc 22 is shown as of circular outline in the drawing, in correspondence with the is said member 2|, the 'discharge goes to the disc 22 when the electrode II to which it belongs functions as anode, and the discharge current then .flows through the member 2| of this electrode and heats it by resistance heating; while when this electrode functions as cathode, the discharge goes to its member 2| and the discharge current also flows in and heats at least part of member 2| by resistance heating,in addition to such heating of member 2| as may be produced more directly by the discharge itself at this time. In alternating current operation, therefore, each member 2| of a device with cathodes ll of this character at both ends is heated by resistance heating on both the positive and the negative A. C. half-cycles, while on the positive or anode half-cycles there is substantially no discharge to either member 2|. In sum, each member 2| is heated to efiective emission by the energy that it receives from the discharge current under these conditions.

For 3, 15-watt discharge tube It of 1 inch internal diameter 18 inches long, each cathode may have a coil 2| consisting of 377 turns of 2.55 mil tungsten wire wound 215 turns per inch on a 6 mil mandrel, which may be again wound 50 turns per inch on a 22.6 mil mandrel. After removal of both'mandrels, the coil 2| may be activated in the usual way with a mixture of refractory oxides such as commonly used for cathode coils of fluorescent lamps of similar wattage. Each electrode disc 22 may be a flat disc of sheet nickel 20 mils thick and 1 inch in diameter, and may be mounted coaxial with the tube l0 about inch in front of the coil 2|, on a nickel supporting wire 23. If activated, each disc 22 may be activated with the same oxide mixture, coated on its front side.

It is to be understood that these specific particulars are illustrative, for the convenience of those wishing to practice the invention, and are flcial effects.

not intended to limit or define it, since they can be widely varied or changed. Moreover, the subject of disc location and size is explained in one of the leads l2, l2. *This has the advantage of always giving a large electrode disc area connected to the external circuit I1, I! through the thermionic member 2|, regardless of which terminal l3, l3 happens to be connected to this circult, thus assuring that said member 2| shall always be heated by current flow therethrough during starting. Also, there is always a large disc area connected to the .circuit H, H independently of the member-2|, to serve as anode during A. C. half-cycles when the member 2| is positive. Of course the disc segments 22a, 22a

, should not be in contact; but they should be as close together as possible short of contact, for reasons that will appear hereinafter.

When suitably placed and proportioned, the electrode disc 22 (whether integral 'as in Fig. 1, or divided as in Fig. 2) produces several bene- Not only is its position such that it would shield the tube wall III in front of it from any particles of material that "might be thrown off from the thermionic member 2|- whether by evaporation or by sputtering-but its very presence and connection as described actually prevent appreciable sputtering from taking place, so that the envelope wall I0 is not black- 1 to substantially the sam intrinsic brilliance as the rest of its lengthor, in other words, it extends the positive column glow right up to or into the cathodic glow at the end of the tube. It also -minimizes flicker at the end of discharge tubes operating on A. C., as already mentioned. In addition, the disc 22 affords an ample area to act as anode during the half-cycles when the associated thermionic member 2| is positive.

As regards the prevention of sputtering, the action of the electrode disc 22 appears to be that it takes discharge current at starting without being materially affected by the concomitant positive ion bombardment, owing to its very large surface area as compared with the fine thermionic cathode member 2|, and that it also large- 13 shields the member 2| from the ions. By taking a large part of the discharge current during starting, it lowers the voltage in the tube and the velocity of such ions as do reach the member 2| behind it, thus reducing the force of their impact and their sputtering power. As soon as the discharge to the disc 22 has produced abundance of ions in the tube, however, the discharge shifts over from the disc to the thermionic member 2| because of its lower work function. This is due not only to its activation, but also to the freer electron emission from it as it is heated up. The disc 22, on the other hand, does not heat up sufficiently to become emissive, owing to its relatively large size and mass. It is difficult to assess the benefit amongst these several factors. or to be sure that there is no other action of the" disc 22 which may be more or less responsible; but therexis no doubt that the disc'does prevent sputtering of the thermionic member .2| and blackening of the tube wall l0 so long as any substantial amount of activating material remains on said member 2 I. 1

As regards eliminating or ameliorating the Faraday dark space phenomenon, the disc 22 is effective by trapping out and eliminating the causative surplus of positive ions (wholly or partially) from. the region(s) that would otherwise be dark, without shading or blanketing apprecia-.

ble areas or annular zones of the tube wall from the desired photon radiation. Being at a negative potential relative to the discharge at the point where it is situated, the disc 22 can be highly effective in collecting positive ions. without impairing the desired even distribution of the discharge in the tube III, the disc 22 must be of relatively small thickness, in order that the discharge may spread out around the barrier offered by the disc 22, rather than canalize itself in one single path of least resistance, or jump around spasmodically over a variety of paths. In

general, any thickness of the disc 22 consistent with desirable lightness will answer; Discs about 30 mils thick are satisfactory inl'a tube'of the dimensions above indicated operating on current up to 0.5 ampere, though for higher currents. the disc should be thinner; 1

It is to be remarked that the effect ,of eliminating or reducing a Faraday dark space, which has almost a zero voltage gradient, is to extend correspondingly the positive column portion of the discharge, which does have a substantial, uniform voltage gradient throughout its length, and hence such amelioration of the dark space necessarily increases, the voltage required to maintain the discharge. However, this increase in discharge voltage is too small to weight against the amelioration of the dark space.

There is a certain latitude in proportioning and locating the cathode disc 22 relative to the discharge envelope I0 and the thermionic member 2|. A disc 22 of any relative size at all comparable to those shown in the drawing will produce some beneficial effects, as regards both cold starting and the Faraday dark space, if it is located in front 'of the thermionic member 2| anywhere within the normal sphere of the Faraday dark space and the negativecathode glow, and is suitably connectedthough if the disc were rather small, or were unfavorably placed, its effects might be negligible or so small as to escape observation. But a disc of suitable size properly placed is effective to reduce or virtually eliminate the Fara- .day dark space for current densities over a considerable range; e. g., from about 0.2 ampere to about 0.5 ampere for the 1% inch tube hereinbefore referred to.

Considering the question of disc location more particularly, it is found that a disc 22 does not affect the Faraday dark space if it is locatedon the outer boundary of this dark space, remote from the thermionic member 2|; it reduces the extent of the dark space more and more as its location is shifted toward the member 2| and the inner dark space boundary at the fringe of O do this 7 still further; and the minimized darkspace n correspondingly shifted toward the thermionic cathode member 2|, though-the extent or ,widtn of the dark space'remains, unchanged: So far glow in producing the 2537 A. radiation by which the phosphors generally used in fluorescent tubes or lamps are specially excited. For a tube such as mentionedabove, it is found advantageous to place the disc 22 about /4 inch from the cathode member 22, or even closer. Commercially, of course, the limit ofproximity of the electrode disc 22 to the thermionic cathode member 2| is where the flat disc face and the member 2| might 2,351,010 a coacting discharge eiectrodes in said envelope touch under severe vibration or ither unfavorable conditions of. operation-unless, of course, it' should be deemed worthwhile to dish" or recess the part 22 adjacent the member 2i to allow the general plane of the part 22 to extend to or beyond the transverse plane of said member.

While the range of beneficial disc location as regards the Faraday dark space is definitely limited to the outer boundary of that space, remote from the member 2|, the, range for improving cold starting performance is more extendedthough here to there is a limit, beyond which the beneficial effect on cold starting diminishes. As a working rule for practical Purposes, it sufnces to say that any position of the disc 22 within the above-stated range up to the outer boundary of the Faraday dark space will also give maximum benefit as regards cold starting. As to the size or area of the disc 22, there is closer parallelism as regards Faraday dark space and cold starting. A disc 22 that is inch in diameter will reduce the dark space in a tube ll of 1% inches internal diameter to a minor fraction of its normal extent lengthwise oi' the tube, if it is well placed; a disc 1 inch in diamter will reduce the dark space to look like a mere dark line across the tube, only somewhat thicker to the eye than the disc itself; a disc still larger than this produces no better eflect. but does tend, on the other hand, to reduce the discharge current in the device for a given discharge voltage. From this it is to be concluded that the maximum benefit as regards reduction of the dark space is obtained with a disc whose projected area is about half the internal crosssectlonal area of the discharge envelope II, more or less, or rather less.

It has also been found that a disc 22 of such size as this, which produces maximum reduction of the Faraday dark space, likewise produces the maximum of improvement as regards cold starting. Merely to prevent sputtering, indeed, the area of either disc segment 22a in Fig. 2 alone sumces,though both of them are required for the effectual amelioration of the Faraday dark space as explained above.

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

1. A cold-starting positive column electric discharge device characterized by low pressure and a difluse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and

ed to eflective emission by 'the pacedapart to provide for a positive column discharge between them: oneof said electrodes, which functions as cathode, comprising a selfheating thermionic member so flne as to be heatharge, and also comprising an electrode disc electrically connected to said thermionic member and mounted across the envelope in front of said member,

within the outer boundary of where the Faraday" dark space would otherwise be, and of such ample surface area, as compared with the crosssection of the envelope where it is located and with the thermionic member, that no substantial sputtering from said member occurs during starting,'while the Faraday dark space is reduced at least to a minor fraction of the extent it would otherwise have. I W Y 2. A cold-starting positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes in said envelope spaced apart to provide for a positive column discharge between them; one of said electrodes, which functions as cathode, comprising a self-heating thermionic member so fine as to be heated to eflective emission by the discharge, and also comprising an electrode disc electrically connected to said thermionic member and mounted across the envelope in front of said member, within substantially the inner boundary of where the Faraday dark space would otherwise be, and occupying as much as approximately half the cross-sectional area of the envelope where it is located.

3. A cold-starting positive columnelectric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes in said envelope spaced apart to provide for a positive column discharge between them; one of said electrodes, which functions as cathode, comprising a self-heating thermionic member so fine as to be heated to effective emission by the discharge, and having in operation only a single current supply connection, and also comprising an electrode disc electrically connected to said current supply connection only through said thermionic member and mounted across the envelope in front of said member, and of such ample surface area, as compared with the cross-section of the envelope where it is located and with the thermionic member, that no substantial sputtering from said member occurs during starting.

4. A positive column electric discharge device characterized by low pressure and a diil'use discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes in said envelope spaced apart to provide for a positive column discharge between them, and functioning alternately as anode and as cathode during A. C. operation of said device; at least one of said electrodes comprising both a thermionic member having in operation only a current supply connection to one point thereing of such ample surface area, as compared with the thermionic member and with the envelope cross-section where the disc is located, that the discharge goes to the thermionic member and to the disc, respectively, on the cathode and anode half-cyclesof the discharge, and there is substantially no discharge ,to the thermionic member during the anode half-cycles, while the discharge current flows in the thermionic memher during both of these half-cycles; and said thermionic member being proportioned to be beated to eflective emission by the energy which it receives from the discharge current.

5. A cold-starting positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and co acting discharge electrodes in said envelope spaced apart to provide for a positive column discharge between them; one of said electrodes, which functions as cathode, comprising a. self-heating thermionic member so fine as to be heated to effective emission by the discharge, and having current leads to both ends thereof only one of which is a current supply connection in operation, and also comprising an electrode disc mounted across the envelope in front of said thermionic member and divided into segments each of which is connected to one of said leads independently of said thermionic member, said electrode disc segments having such ample surface area, as compared with the cross-section of the envelope where they are located and with the thermionic member, that no substantial sputtering from said member occurs during starting.

6. The combination with a positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes spaced apart in said envelope to provide for a positive column discharge between them, one of said electrodes comprising a self-heating thermionic cathode member so fine as to be heated to effective emission by the discharge: of current supply and discharge-starting circuit means connected to said electrodes for producing discharge-initiating voltage between them while they are cold, followed by lower discharge-maintaining voltage; and an electrode disc electrically connected to said thermionic member and mounted across the envelope in front of said member substantially within the outer boundary of where the Faraday dark space would otherwise be, and of such ample surface area, as compared with the cross-section of the envelope where it is located and with the thermionic member, that no substantial sputtering from the latter occurs during cold starting as aforesaid, while the Faraday dark space during subsequent operation is substantially abolished.

'7. The combination with a positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes spaced apart in said envelope to provide for a positive column discharge between them, one of said electrodes member so fine as to be heated to eflective emission by the discharge; of current supply and discharge-starting circuit means connected to said electrodes for producing dischargeeinitiating voltage between them while they are cold, followed by lower discharge-maintaining voltage, through a single current supply connection only; and an electrode disc mounted across the envelope in front of said thermionic member substantially within the outer boundary of where the Faraday dark space would otherwise be and electrically connected to said current supply connection only through said member, and of such ample surface area, as compared with the cross-section of the envelope where it is located and with the thermionic member, that no substantial sputtering from the latter occurs during cold starting as aforesaid, while the Faraday dark space during subsequent operation is substantially abolished.

8. The combination with a positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes spaced apart in said envelope to provide for a positive column discharge between them, one of said electrodes comprising a self-heating thermionic cathode member so fine as to be heated to efiective emission by the discharge; of current supply and discharge-starting circuit means connected to said electrodes for producing discharge-initiating voltage between them while they are cold, followed by lower discharge-maintaining voltage, through a single current supply connection only; and an electrode disc mounted across the envelope in front 0! said thermionic member substantially within the outer boundary of where the Faraday dark space would otherwise be and divided into segments one of which only is elec- H trically connected to said current supply connection through said member, said electrod disc segments having such ample surface area, as compared with the cross-section of the envelope where they are located and with the thermionic member, that no substantial sputtering from the latter occurs during cold starting as aforesaid, while the Faraday dark space during subsequent operation is substantially abolished.

9. A cold-starting positive column electric discharge device characterized by low pressure and a diffuse discharge therein during operation, said device comprising a discharge envelope with an ionizable operating atmosphere therein, and coacting discharge electrodes in said envelope spaced apart to provide for a positive column discharge between them; one of said electrodes, which functions as cathode, comprising a selfheating activated thermionic member so fine as to'be heated to efiective emission by the discharge, and also comprising an electrode electrically connected to said thermionic'member and mounted across the envelope in front of said member, substantially within the outer boundary of where the Faraday dark space would otherwise be, and cocupying as much as approximately half the crosssectional area of the envelope where it is located.

' WALTER J. KARASH.

' EUGENE LEMMERS.

' Patent No. 2 ,5'51,61'6.

CERTIFIGA'I 'EOF CORRECTION Y June 20, 19141;. WALTER J. KARASH, ET AL.

' It is hereby certified that'error appears in the printed specification of the above numbered patent requiring correction es follows: Page 5, sec

0nd column, line 62, claim 9, after the word "electrode" ineert --d1ecand 'that the said Letters Patent shouldbe read witn'thia correction there in that the Same may conform to the record of the case in the 'Patent Office.

Signed and sealed this 16th day of January, A. D. 1914.5.

Leslie Frazer (Seal) "Act ing Commissioner of- Patents,

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