US2530990A - Electric discharge device - Google Patents

Description

Nm 2, Egg@ Lm R, PETERS 2953999 ELECTRIC DISCHARGE DEVICE Filed April 21, 1945 z Il W 9 5V MMM/,M

H/S AWD/wn Patented Nov. 21, 1950 ELECTRIC DISCHARGE DEVICE Leo R. Peters, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New York Application April 21, 1945, Serial No. 589.489

11 Claims. l

This invention relates to electric discharge lamps and their electrodes. and aims at improving the operation and light-output maintenance of such lamps. Advantages which may be obtained include preventing or minimizing what is known as spiralling of the discharge; prevention of so-called anode oscillations; and prevention or minimization of end-blackening of the lamp envelopes around and near the electrodes, which renders the lamp unsightly and is strongly objected to by users. The invention is especially advantageous for lamps intended to be started cold, or without adequate preliminary preheat of the cathode(s); for lamps in which the discharge current is relatively low, around some 100 milliamperes or less; for long, slender tubular lamps whose envelopes are of comparatively small diameter, including the recently evolved slender fluorescent tubes operating on high voltages with low currents; and for germicidal lamps.

The invention is also useful in conjunction with starting arrangements for discharge lamps. For the purposes of my invention, I screen oi one or both electrodes of a lamp by means of an openwork enclosure or screen, which has a suitable potential relative to the associated electrode. Perforated sheet metal, wire mesh screening, or even a helical coil of fine wire Wound to a close pitch are illustrative types of conductive grillage or foraminous structure suitable in such a screen. While the high conductivity of metal is not necessary for the screen, nevertheless metal is generally the most convenient material. The screen may be kept at suitable potential by allowing it to float in the discharge while insulating it from the conductive lamp parts and circuits. Preferred means which I have devised for insulatively holding the screen in position away from the surrounding envelope wall are also described and illustrated hereinafter.

Various other features and advantages of the invention will become apparent from the description of species and forms of embodiment, and from the drawings.

In the drawings, Fig. 1 is a side view of a tubular fluorescent lamp conveniently embodying the invention, a mid-portion of the lamp tube being broken out and omitted; Fig. 2 is a fragmentary view of one end of the lamp on a larger scale, the

lamp envelope and a screen around the electrode being shown in longitudinal mid-section: and Fig. 3 is a tilted side view of the lamp mount before sealing into the envelope, a somewhat modifled screen being shown in longitudinal midsection'.

Fig. 4 is `a diagrammatic view illustrating the application of my invention in conjunction with lamp starting arrangements.

Broadly speaking, 'the lamp illustrated in Fig. 1 is similar in construction and operation to low pressure positive column iluorescent or germicidal tubes now in general use, whose envelopes are substantially filled by a diiuse discharge, and are permeable to the desired radiation of the lamp; although some of the details shown are particularly adapted for long, slender tubes operating on low current at high voltage, as mentioned above. As in ordinary low pressure positive column lamps, Fig. 2 shows the vitreous envelope l as comprising (for each end) a reentrant tubular glass stem 2 with an inner end seal 3 of flat pressed type. Wires 4, 5 (one or both serving as current leads) extend through the stem 2 and seal 3, and at one end of the lamp (at least) an inner exhaust tube 6 opens through the seal 3 and extends out through the stem 2 to its own seal l. Electrodes 8, 8 in the ends of the envelope l adjacent the stems 2, 2 may be supported from the latter, in whole or in part through connection or attachment to the wires 4, 5. The operating atmosphere in the envelope I may consist of gas or of some vaporizable and ionizable working substance, such as metal, or of both together, as, for example, inert rare starting gas like argon at a pressure of some 2 to '6 mm., and mercury to an amount exceeding what will vaporize during operation, indicated in Fig. 1 by a droplet 9. A suitable pressure of the mercury vapor during operation is of the order of 10 microns. In the case of a fluorescent lamp, the envelope wall l includesor carries a layer of luminescent material or phosphor excitable by the radiation from the discharge, this wall layer being indicated in Fig. 1 as an internal coating Ill extending substantially throughout the length of the envelope.

The electrodes 8, 8 may be of any suitable or preferred type (s) for either A. C. or D. C. operation, and either for hot starting after preheat or for cold starting without preheat, although the benefits of the invention are greater in the case of a cold-starting lamp, as already indicated. In a cold-starting lamp, the wires 5, l5 may be useful as supports, but are usually not required to carry current. One form of electrode that is adapted for either hot or cold starting is disclosed in U. S. Patent No. 2l306,925 to Aicher. To adapt the lamp for operation on either A. C. or

I5. C., both the electrodes 8. 8 may be constructed as thermionic cathodes, although in D. C. operation only one of them need be emissive. As shown in Fig. 2, each electrode 8 comprises a tungsten wire filament wound in a coil or a coiled-coil and connected at its ends to the wires 4, 5 sealed into the stem 2, and preferably coated and activated with a charge of alkaline earth metal oxidefs), such as a mixture including barium and strontium oxides. To make the electrode more compact, the coil 8 is shown as bent into the form of a narrow arch rising from the clamps at the inner ends of the. wires 4, 5.

In accordance with my invention as here illustrated, there is associated with each of vthe electrodes 8, 8 a screen II setting apart or closing off a space around the electrode that is substantially constricted as compared with the general cross-section of the discharge path afforded by the envelope I. 'I'his space may conveniently be referred to as the electrode space. The screen II shown comprises an openwork or foraminous grillage forming a sort of enclosure around the electrode 8. Such an enclosure II may preferably be constructed as a tube of woven-wire mesh or screening, with an end w'all I2 of the same character in front of the electrode, and with the rear screen end closed in behind the electrode, as by being blocked off by the stem 2. As shown, the screen II is cylindrical and fits around'the cylindrical body of the stem 2 with a snug, tight fit. The grip of the screen II on the stern 2 vis further enhanced by the elastic properties of the wire mesh and by the fact that as shown in Fig. 2 the stem press 3 is wider than the diameter ofthestem and the screen, so as to distortvthe 1attere1astically to an'elliptic form at I3, above its lower end, Fig. 2. In other Words, the screenII locks over the wide stem press 8 elastically.- If desired, the screen II may be more positively keyedon the stem by providing a, smal1 lug I4 on the edge of the .press 3 to project through the wire'mesh, or may even be sealed to the glass by fusion of the latter. In a lamp tube I of 1% inch diameter operating on a current of some 50 to 200 milliamperes, good results have been obtained with a screen II of about 1%; inch diameter and about 1 inch to 1115 inch long extending about 1/,2 to inch beyond the inner end of the stem 2 with its seal 3, and about Re to ya inch beyond the electrode 8. and formed of nickel wire of 0.007 inch diameter woven to provide aibout 20 meshes per inch.

I have found that when the screen II is kept at a suitable potential, positive to that of the electrode 8 when the latter is functioning as cathode, the screen prevents .the .usual end blackening" of the envelope I around and near the electrode 8, which is produced during the cathode cycles of the. electrode, while allowing the luminous or other radiation from the discharge to reach and pass through said wall, or to excite its luminescent layer I0. Thus the envelope wall I around the screen II appears substantially as bright as though the screen were not there. The simplest way of thus maintaining the screen II at. a. suitable potential is to insulate it from all electrical parts and allow it to float in the discharge, without any electrical connection whatever. Under this condition, the screen I I acquires or assumes a potential which is negative to that of the discharge around it, but is positive to that of the electrode 8 when functioning as cathode. In fluorescent lamps containing argon and mercury that I have tested on A. C., this screen potential is some 10 volts R. M. S., more or less, above the cathode potential. A proper positive potential of the screen II relative to the electrode 8 while functioning as cathode is essential; for if the screen is connected to the electrode so as to have the electrode potential, the screen is altogether ineffective against envelope darkening. Within reason, the size of the screen openings is unimportant: I have found screens as coarse as 16 meshes per inch just as effective as those so fine as mesh. The presence of the screen II does no t materially aiect the voltage taken by the lamp or its energy consumption. y

While the nature and causes of the usual end blackening" or darkening of a discharge envelope wall near an electrode are not fully understood or agreed on by those skilled in the art, it is concededV that it is somehow due to matter reaching the Walls 'from the electrode, or from its lead(s).. Whether this matter is activating material, electrode vmaterial, or lead-wire material need not be decided; nor do I express any opinon whether this matter itself forms the darkening wall deposit, or combines with the wall material .or phosphor coatingl to darken it, or somehow draws a darkening deposit of the vaporized working substance, such as mercury. But itis generally agreed that the loss of material from the electrode occurs very largely during starting ofthe discharge and while the elec-'- trode is functioning as cathode.

The prevention of end-blackening by the screen II is not a simple blocking of matter vaporized or sputtered from the electrode` 8; for if the screen is electrically connected to the electrode so as to have the same potential, it ceases to be eiective; and besides this, theopenings of a 16-mesh screen are so large that-it would offer little real obstruction to such particles. Neither does it appear to be a mere case of electrostatic attraction of particles from the cathode 8 or leads 4 to the screen I2; because the screen appears perfectly clean throughout the life of the lamp, and no increase in its weight has been detected by even the most careful weighing. It seems, therefore, that the screenA II must somehow actually prevent departure from the electrode of matter such as ordinarily produces blackening.

End blackening is especially bad in lamps operating on high voltages, and on small currents of the order of some 50 milliamperes, more or less, and it is worse when lamps are started and extinguished very frequently. Since at starting the electrode is too cold for vaporization, while the cathode drop is very high, and remains so until the electrode heats up enough to yield thermicnic emission to lower the drop, the opinion is widely held that end blackening is not much due to vaporization. Rather, it arises from sputtering of (negatively charged) particles of electrode and leadwire material(s) due to bombardment of an electrode while functioning as cathode by positive ions of the discharge when the voltage drop in the discharge closely adjacent the cathode (called the cathode drop) exceeds what is known as the disintegration voltage, amounting to about 20 volts (more or less) for a mercury vapor discharge and an electrode activated with alkaline earth oxides. In harmony with this is the fact that cathodes which are especially diicult and slow to heat up at low starting currents give bad end blackening.

It is my view that the action of the screen II to prevent end blackening is intimately related to the formation of positive ions, which concur with the electrons in carrying the discharge current.. Situated in suitable proximity to the electrode 8, the screen II receives both electrons and ions of the discharge, which come together on the screen and neutralize one another. Thus the screen offers -a certain resistance to the passage ofthe current and gives rise to the formation of more positive ions in its vicinity, resulting in an accumulation of ions that constitutes a positive space charge; and a voltage drop arises at the screen II to provide for this ionization. Formed in or entering the inner space around the 4electrode 8, some of the positive ions meet in part the ion requirements for the passage of current there. Accordingly, fewer ions need be formed close to the electrode 8,- and so the cathode drop required to produce the smaller number of ions is lower. The net effect is that the voltage drops adjacent electrode 8 and screen II are both below disintegration or sputtering value; or, in other words, the screen I I in effect divides the cathode drop which is ordinarily high enough in starting to produce sputtering into two parts neither of which ever attains the disintegration value.

It is in harmony with this, at least, that in operation the voltage difference between the electrode 8 and the screen I I is about 10 volts R. M. S., since this means that the peak value of this voltage difference is above the required ionizing potential of 10.4 for mercury vapor, but below the above-'mentioned disintegration value of some volts.

By preventing sputtering ofl negatively charged particles in the envelope I, the screen II also prevents or minimizes the spiral swirling appearance of the discharge in the tube, and of the light from its luminescent wall layer i0, which occurs when such particles get into the positive column of a discharge in a lamp containing gas, as distinguished from mere vapor. In addition, the screen II prevents rapid fluctuations of voltage drop and current close to an electrode 8 functioning as anode (whether in A. C. operation on commercial frequencies such as 60, 50, or cycles, or in D. C. operation), which are known as anode oscillations, and may be a cause of telephone interference due to fluorescent lamps. This prevention of anode oscillations I believe to be due to the positive space charge at the screen II, which serves as a sort of equalizing reservoir of positive ions on both cathode and anode cycles, and thus obviates periodic excesses and deficiencies of ionization in the tube which give rise to such anode cycleoscillations.

Various modifications n the f orm, arrangement, and construction of the screen II may be made consistently with my invention, so long as it offers sufficient but not excessive resistance to the passage of current through the discharge atmosphere and constricts a space around the electrode 8, but does not cut oil radiation to the envelope wall I. For example, the shape of the screen might be varied considerably; portions of its walls might be imperforate, instead of foraminous (as, for example, the portion I2 in front of the electrode 8) and portions of its wall might be omitted or provided with fairly large openings. In particular, the front end wall I2 shown in Fig. 2 may be omitted as in Fig. 3 without destroying the effectiveness of the screen II, or its rear endv may be left open, or both. But

even with an end of the screen I I open, the openwork character of its wall cannot be altogether sacrificed without making it an obstacle to the discharge: e. g., if it were simply made as a tube of imperforate material instead of wire screening, this tube would absorb more positive ions than th'e openwork structure does.; while if the structure took the form of a mere transverse imperforate disc mounted close in front of the electrode 8, with an annular opening around its periphery, positive ions formed at the annular opening would be absorbed by the tube wall I, as well as by the disc. In sum, an imperforate structure would not afford so large a net output of ions available for the current to the cathode 8 as does the foraminous screen II, and either the cathode drop or the voltage drop at the structure would tend to'be higher to compensate. Moreover, the imperforate structure would tend to shut oir desired light or radiation from the tube wall I.

The effectiveness of the creen II would be destroyed if its relation to the electrode 8 were auch that a voltage drop above sputtering value were necessary at the electrode 8 to maintain ionization to carry the current, as, for example, if the screenl were very large, or very far in front of the electrode 8. In general, the limit on the distance of thescreen II from the electrode 8 is that it must lie well within the outer boundary of the Faraday dark space as this phenomenon would exist if there were no screen. Per contra, the screen affords the advantage of minimizing or even abolishing the Faraday-dark space when the screen is close to the electrode. However. the screen II would be ineffective to prevent end blackening if it were so close to the electrode 8 and offered such great'resistance to passage of current thatv enough ions were produced in the vicinity of the screento make ion formation close tothe electrode unnecessary; for in this case the voltage drop at the screen would tend to exceed sputtering value, just as does the voltage drop adjacent the electrode 8 in the absence of the screen, and with the same effect as regards darkening of the envelope I. And, of course, the screen must not be so close to the electrode as to be overheated and fused, vaporized, or badly warped. Y

Fig. 4 illustrates my invention in conjunction with arrangements to facilitate starting the discharge, particularly in cases where the cathodes are not preheated. For this purpose, a connection I5 in parallel with the discharge path through the gas and vapor in the tube I is provided, from near one electrode 8 to near the other electrode 8. As shown, the connection I5 terminates at each end in a hollow or annular conductive structure I8 outside of the corresponding screen II, here represented as a surrounding outer tubular wire mesh screen similar to the screen II, but insulated from it. The connection I5 extending the length of the envelope I preferably outside the same, or suitably insulated if inside, may be a mere wire of such trifling resistance as to amount to a short circuit connection. For a 40 watt fluorescent tube of 11/2 inch diameter 48 inches long, such as hereinbefore referred to, this arrangement has reduced the starting voltage more than 25%, from about 400 volts to 290 volts. If desired, the circuit I5 may be interrupted after starting has been eil'ected.

It is an important advantage of using inner and outer screens II and I6 that high resistance in the circuit I5 is unnecessary to keep this Y circuit shorting the main discharge path from preventing or "dimming the direct discharge between the electrodes 8, 8; whereas if the outer screens are omitted and the circuit I connected directly to the screens Il, Il, a resistance of the order of 1A megohm is necessary in said circuit l5 to prevent suchshorting out of the main discharge. In eiect, tlie inner screen I I blocks the outer structure I6 from emitting electrons when the4 corresponding electrode 8 is lfunctioning as anode. y

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

l. In an electric discharge lamp, the combination with a tubular lradiation-transmitting discharge envelope having reentrant stems, coacting electrodes and .other Aconductive parts therein, at least one of which is thermionic, widely spaced apart lengthwise of the tube to provide for a long-gap positive column discharge through the envelope, and an ionlzable operating atmosphere in said envelope having in operation a low pressure. resulting in a diiuse' discharge substantially filling the envelope, of a foraminous electrically conductive screen in the envelope and supported exclusively by one of said stems and enclosing the sides and operating-end of the associated electrode and providing therearound a space that is constricted as compared with the general cross-section of the discharge path in the envelope, while allowing the radiation from the discharge to reach the envelope, said one stem holding said screen in position and insulating it from the conductive lamp parts.

2. In an electric discharge lamp, the combination with a tubular radiation-transmitting discharge envelope having reentrant stems, coacting thermionic electrodes and other` conductive parts therein widely spaced apart lengthwise of the tube to provide for a long-gap positive column discharge through the envelope, and starting gas and ionizable`and vaporizable metal in said envelope, of enclosures comprising conductive foraminous walls supported exclusively by said stems and enclosing the electrode sides and operating-ends thereof and screening off electrode spaces that are constricted as compared with the general cross-section of the discharge path in the envelope, while allowing the radiation from the discharge within the enclosures to reach said walls, said stems holding said foraminous walls in position away from said envelope walls and also insulating them from the conductive lamp parts.

3. In an electric discharge lamp, the combination with a tubular radiation-transmitting discharge envelope having reentrant stems provided substantially throughout its length with a luminescent wall' layer excitable by radiation, conductive parts and coacting thermionic electrodes in the envelope widely spaced apart lengthwise thereof to provide for a long-gap positive column discharge through the envelope, and starting gas and ionizable and vaporizable metal in said tube affording in operating a low pressure resulting in a diffuse discharge producing radiation for exciting said luminescent layer, of foraminous metal screens inthe envelope .supported exclusively by said stems and enclosing the sides and operating-ends of said electrodes and screening oil electrode spaces that are constricted as compared with the general cross-section of the discharge path in the envelope, while allowing the radiation to reach the envelope wall and excite their luminescent material, said stems holding electric discharge lamp comprising electrical,

partsand containing an ionizable discharge atmosphere, the combination with a vitreous reentrant stem at an end of the tubular discharge envelope, and a thermionic electrode adjacent the inner end of said stem supported exclusively from the stem and having current lead means sealed into the latter, a screen of vforaminous electrically conductive elastic material. surroundy ing the sides and operating-end andspaced from said electrode and having its en d iltted around the stem, whereby said screen is held in position as aforesaid and also insulated .from the electrical parts. l y A 5. In a mount for -a tubular positive column electric discharge lamp comprising electrical parts and containing'an ionizable discharge atmosphere, the combination with a vitreous stem having at its inner end a seal press wider than the stem, and a thermionic electrodeadjacent the inner end of said stem supported from the stem and having current lead means sealed through said stem press, of aforaminous screen of elastic wire mesh surrounding the-sides and operating-end and spaced from said electrode and having its end tted'around the stem and elastically distorted by said seal press, whereby said screen is keyed in positionfas aforesaid and also insulated from the electrical parts.

6. In an electric discharge lamp,` the combination with a tubular radiation-transmitting discharge envelope, coacting`thermionic electrodes therein widely spaced apart lengthwise of the tube to provide for a long-gap positive columnl discharge through the tube, and an ionizabl operating atmosphere in said tube, of hollow conductive structures in said tube, around said electrodes, with means for electrically interconnecting said structures; and enclosures around said electrodes, inside said hollow structures, comprising conductive foraminous walls electrically insulated from the conductive lamp parts and circuits, and screening off electrode spaces that are constricted as compared with the general cross-section of the discharge path in the envelope.

7. In an electric discharge lamp, the combination comprising a tubular radiation-transmitting discharge enevelop'e, a pair, of electrodes therein, lead-in conductors for supporting said electrodes, an ionizable atmosphere having lin operation a low pressure, and a foraminous electrically conductive screen around one of said electrodes electrically insulated therefrom and enclosing the sides and operating-end of one electrode and its associated lead-in conductors for defining a space therearound which is constricted as compared with the general crosssection of the dischargepath in the envelope.

8. In an electric discharge'lamp, the combination comprising a tubular radiation-transmitting discharge envelope, a pair of electrodes therein, means for supporting said electrodes in spaced relation comprising lead-in conductors, an ionizable atmospherehaving in operation a low pressure, and a pair of foraminous Velectrically conductive screens each enclosing the sides and operating-end of a'different electrode and being electrically insulated therefrom for defining a localized and constrictedspace about the associated electrode in order to establish betweenv the screen and the associated electrode during operation a voltage diiierence greater than the ionizing potential of said atmosphere and less than the disintegration voltage thereof.

, 9. In an electric discharge lamp, the combination comprising a tubular radiation-transmitting discharge envelope and having reentrant stems, a pair of electrodes therein, means for supporting said electrodes inspaced relation comprising leadin conductors, and ionizable atmosphere comprising mercury and a starting gas having in operation a low pressure, and a pair of foraminous electrically conductive screens each supported exclusides and operating-end of a different electrode and its associated lead-in conductors and being electrically insulated therefrom for'deilning a localized and constrictedvspace about the associated electrode in order to establish between the screen and the electrode during operation a voltage diiference greater than the ionizing potential of mercury and less than the disintegration voltage thereof.

10. Inan electric discharge lamp, the combination comprising a tubular radiation-transmitting discharge envelope and having reentrant stems, a pair of electrodes therein, means for supporting said electrodes in spaced relation comprising lead-in conductors, an ionizable atmosphere comprising mercury and a starting gas having in operation a low pressure, and a pair of foraminous electrically conductive screens each supported exclusively by the associated stem and enclosing the sides and operating-end of a diierent electrode and its associated lead-inl coqsively by the associated stem and enclosing the z' ing said cathode, and a tubular wire screen having a portion thereof surrounding said cathode and lead-in conductors and another portion thereof elastically gripping the exteriorv surface of said stem whereby to be supported exclusively anddirectly by said stem.

LEOLR. PETERS.

REFERENCES CITED The following references are of record yin the file of this patent:

UNITED STATES PATENTS Number Name Date 1,879,470 Pirani Sept. 27. 1932 1,947,417 Holst das... Feb. 13, 1934 1,971,940 Pirani (b) -1 Aug. 28, 1934 1,980,534 Kirsten Nov. 13, 1934 2,038,049 Kirsten (b) Apr. 21, 1936 2,388,277 Smith Oct. 9. 1945

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