US2363531A

US2363531A - Electric discharge device and electrode therefor

Info

Publication number: US2363531A
Application number: US420638A
Authority: US
Inventors: Lyman B Johnson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1941-11-27
Filing date: 1941-11-27
Publication date: 1944-11-28
Anticipated expiration: 1961-11-28

Description

NOV. 28, 1944. JOHNSON 2,363,531

ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Filed NOV. 2'7, 194].

Fig. 1.

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3% d. /5 2 E J B 1 Z I p- 2 4 Discharge Cur r-en't 'Amperes Inventor: L man B. Johnson, by

His Af't'or'neg Patented Nov. 28, 1944 ELECTRIC DISCHARGE DEVICE AND ELECTRODE THEREFOR Lyman B. Johnson, Cleveland Heights, Ohio, as-

signor to General Ele poration of New York Application November 27, 1941, Serial No. 420,638 8 Claims. (01. 176122) This invention relates to electric discharge devices, and is useful in devices producing radiation tor various purposes, such as germicidal and therapeutic ultra-violet lamps and tubes, and lamps or tubes used industrially for irradiating or treating various substances and products, or for blueprinting and other photographic purposes, as well as lamps or tubes for more ordinary iiluminatlon. Speaking in a general way, the invention aims at improvement of electric discharge devices as regards blackening oi envelope walls and wastage of electrodes or cathodes, as well as at obviating the necessity oi activating such electrodes with special electron-emissive materials. This application is a continuation-in-part of my application Serial No. 381,633, filed March 4, 1941, now U. 8. Patent No. 2,313,646, granted applicable to both the cooperating electrodes, since they function as cathode and anode alter nately.

Discharge devices in which a. discharge is passed between electrodes in an ionizable atmosphere have come into wide use in recent years. Generally, these devices have a metal vapor (such as that of mercury) as the operating atmosphere, and also include an inert auxiliary or starting gas (such as one of the rare gases) at pressures up to several millimeters. Lamps 01' this character are classified in two groups or types, according to the pressure of the mercury vapor. In the low-pressure group, represented for example by the well-known Cooper Hewitt lamps, the devices operate at a relatively low temperature, and consequently at ver low pressure. The discharge at such low pressures characteristically spreads out into a uniform glow which fills the entire tube. In the high-pressure field, of which U. S. Patent No. 2,202,199 to Germer is representative. the lamp is allowed to heat considerably, and the pressures may build up to very high values, ranging anywhere from a fraction of an atmosphere to many atmospheres As such pressures, the arc is characteristically constricted into a narrow cord. The dividing line between the two types may be taken as the pressure at which the discharge begins to contract: and this is in practice determined by the size of the envelope and its consequent temperature during operation. While the division of types is not well defined, it does occur somewhere between several hundred millimeters and about one atmosphere.

ctric Company, a cor- One of the chief problems in the design of discharge devices 01' either type for a reasonably long useful life has been the construction of electrodes which do not disintegrate under bombardment to which they are necessarily subjected by the current carriers in the arc. This bombardment is caused principally by positive gas ions which are accelerated to high velocities by the so-called cathode fall. Unless preventive measures are taken, such ions attain suflicient velocity to knock minute particles oi metal or 01' activating coating from a cathode. This phenomenon results in a gradual disintegration and final destruction oi the cathode, as well as a blackening oi' the tube wall upon which the particles collect,

The destructive eflect. upon the cathode is particularly prevalent during the starting period of the lamp: i. e., the period of time before the cathode has attained proper emission temperature, including, in the case or the high pressure lamp, the time before the heating of the lamp has built up an atmospheric pressure of full operating value. The destructive effect of bombardment during starting on an activated cathode is enhanced by a tendency of the cathode spot or the arc to jump around on the surface of the cathode, as if in an attempt to find highiy activated areas. During the starting period, a relatively high potential-fall exists in the neighborhood oi the cold cathode, so that the positive ions that strike the cathode have much more energy. and produce more sputtering; and as the pressure (even in the high-pressure type of lamp) is low at this time, there are relatively few gas particles to be in collision with the sputtered material from the cathode, and thereby reduce the actual net loss of material by sputtering. On the other hand, when the cathode is at proper emission temperature, the cathode fall is greatly reduced; and when the gas pressure (in the high pressure case) is at normal operating value, the cathode is blanketed by gas particles which repel sputtered particles 01' metal back to the cathode. After starting is over, therefore, positive ions do not so greatly damage the cathode when they strike it.

The destructive and blackening eflect of cathode bombardment has been so great that it has hither proved impossible to construct a commercially successful discharge lamp with bare or unactivated electrodes; for about the only practicable wa to avoid or reduce the deleterious efl'ects or bombardment has been to provide the is, material which when heated emits electrons much more copiously than does the cathode metal itself-thus reducing the cathode tall and the energy oi. the bombarding positive ions. Examples oi activators are certain metallic oxides, or materials which break down to form the oxides when heated, such as barium or strontium carbonates or hydroxides. In many cases, also, there is considerable dimculty in getting the discharge to start between electrodes without activation.

The activation method of obviating destructive cathode bombardment has various disadvantages, however. In the first place, it is necessary to provide a tough binder to insure that activating oxides shall adhere properly to the cathode. It is also generally necessary to subject the activating materials to some sort 01' an activation process before they are in proper condition for emission. All this adds to the technical difficulties and cost 01' manufacture Activating coatings on a cathode tend to flake off during the life of the cathode, which in itself adds to the blackening of the discharge envelope. Furthermore, activating material from an electrode deposits on the envelope wall over a great distance from its ends; and after initially depositing on the walls, it tends to migrate or spread toward the center of the envelope, thus increasing the obscuration. Moreover, some of the usual activating materials, such as compounds of the alkaline earth metals, have a tendency to initiate devitriflcation oi a quartz or glass envelope.

I have discovered a way of keeping down or reducing the deterioration or loss of material of discharge electrodes, and the blackening or obscuration of the associated discharge envelopes, without necessity for activating material on or in the electrodes, or for the employment of auxiliary gas of specially high pressure, as referred to in my pending application above identified, and without any real difllculty in starting the discharge. The invention is not only applicable to discharge devices of the type disclosed in my aforementioned application, but also to devices employing working substances and pressures other than there referred to.

In accordance with my invention, I have found that wastage of a bare refractory metal electrode and blackening of the associated envelope wall can be reduced or minimized, without necessity for activation, by a suitable temperature of the electrode or its portion from which the discharge or are takes oil. This temperature must be high enough to assure the requisite emission of electrons to reduce the cathode fall and obviate serious sputtering-just what has heretofore been achieved by activation-yet low enough to prevent undue evaporation of the refractory metal, which would produce blackening as serious as that from sputtering. Evaporation of the refractory electrode metal has not been a factor in the operation of activated electrodes, because the profuse electron emission from activating materials rendered it quite needless to approach operating temperatures of the metal at which evaporation would be appreciable. The optimum temperature for my purpose is that at which the combined sputtering and vaporization losses are substantially minimized, which for tungsten is substantially oi the order of some 2700-3200 K., and even a little lower and higher than this range, at the electrode area or portion from which the discharge actually takes off.

In the case oi! a "self-heating electrode, the

cathode with so-called activating material-that heat to maintain it or its arcing portion at the desired operating temperature must, of course, come from the action of the discharge or arc. This is practicable, I find, ii the electrode body is a part oi limited mass which can be heated rather uniformly, at least in starting, so that enough of it contributes to the emission oi electrons without any of it ever being seriously overheated. Accordingly, the electrode body should be fairly compact and stubby, so that the heat generated where the discharge takes oil may be rather uniformly distributed over a blunt arcing end of said electrode body. To assure that the arc shall properly heat the electrode, taking oi! oi the discharge from the connector, rather than from the electrode body, is prevented. Of course the energy supplied the electrodes through the lamp leads has to provide all the heat that is lost from an electrode arcing end or area whence the discharge takes 01!, by conduction and radiation to and from the rest of the electrode and its current lead and support-as well as the rest of the energy represented by the arc or discharge phenomena-so that the heating and the heat losses balance at the desired operating temperature of said area.

In order to prevent or control electrode wastage and envelope blackening, I not only maintain the electrode (or at least its arcing portion or area where the discharge takes off) at suitable temperature during the running of the discharge device, but I provide for heating up the electrode body or its arcing portion to a suitable operating temperature, during starting, before appreciable or serious sputtering can occur. I have discovered that both rapid heating up and subsequent temperature maintenance can be assured by artiflcially reducing the thermal conduction of the electric current connector, as by increasing its length beyond what is otherwise needful. When the connector is made of such limited thermal conduction that in running the heat loss from the arcing portion balances the heating thereof from the discharge at an operating temperature 0 where sputtering losses are kept down without incurring serious losses by evaporation of metalor, at the optimum, combined sputtering and vaporization losses are substantially minimizedit is found that the electrode (or its arcing portion) will heat up before important sputtering occurs if the electrode is made of low heat capacity or thermal inertia; in other words, sufilciently small, Conversely, if the electrode is made suillciently small to heat up rapidly, it will also be small enough to be heated sufiiciently uniformly by the discharge during running.

In a word, then, the mass and thermal capacity of the electrode and the thermal conduction of the connector are correlated, for both startin and running, to control the temperature of the electrode and its arcing portion, and the value of the cathode fall. It follows that the electrode should be designed and proportioned according to the current in the discharge which it is intended to transmit, and by which it is heated; and any serious deviation from the rated discharge current, in service, will result in either sputtering or evaporation of electrode metal, with consequent blackening of the discharge envelope, because of the consequent underheating or overheating of the electrode to an operating temperature outside the above-indicated range.

In order to secure easy starting of the discharge without activation of the electrodes, I combine with their other features above set forth a roughened arcing surface. This, I find, greatly reduces the voltage necessary for starting. Such roughness of the electrode surface may consist of grooving formed in a variety of ways, or may reside in the texture of the refractory metal of the electrode. The rough character of the electrode surface is also helpi'ul for assuring that the discharge shall operate from the electrode body to the virtual exclusion of its connector.

During normal running of my device, after a stable condition has been reached, the pressure of the operating atmosphere of mercury or the like influences electrode wastage and envelope blackening, because it streets the temperature of the electrode and its arcing portion or area from which the discharge takes oil. The higher the operating pressure in the lamp, the more constricted is the arc and the smaller the effective arcing portion or area, and the greater is the heat loss by conduction from the electrode to the discharge atmosphere. Moreover, the temperature of the arcing portion is affected by conduction to the rest of the electrode body, which does not attain so high a temperature as when the are spreads out over it and so acts to heat it all over directly. Hence the electrode temperature is in some measure a function of the working pressure in the discharge envelope. While small differences in pressure may not be of any great importance, it may be desirable to take account of very large pressure differences in designing the electrode.

Pressure of the starting gas is favorable to the suppression of electrode wastage and envelope blackening during the starting period of a discharge device embodying my invention; but it is by no means a major factor, as in the lamp set forth in my aforementioned application.

Various features and advantages of the invention besides those already referred to will appear from the following description of species and forms of embodiment, and from the drawing.

In the drawing, Fig. 1 is a tilted or perspective view of a discharge device or lamp embodying my invention, with a wiring diagram of suitable electric circuit connections; and Figs. 2 and 3 show axial sections through the ends of this lamp. on a larger scale than Fig. 1.

Figs. 4 and 5 are views generally similar to Figs. 2 and 3 illustrating a somewhat different conformation of the ends of the discharge envelope.

Fig. 6 shows characteristic curves of currentvoltage and arcing tip temperature for a discharge device embodying my invention.

As shown in Fig. 1 of the drawing, the discharge device a lamp having a vitreous elongated or tubular envelope M, of quartz or glass, for example. provided with solid operating electrodes H, H in its opposite ends, and permeable to ultraviolet and visible radiation. In the present instance, there is also a solid auxiliary starting electrode I: in one end of the envelope l8, closely adjacent the corresponding operating or main electrode H. A charge of vaporizable and ionizable working substance, such as mercury, is indicated by a droplet l3 inside the envelope Hi. The charge l3 may either be greater in amount than will vaporize under the heat of the lamp. so that an unvaporized surplus will always remain, assuring operation of the device with an atmosphere of saturated vapor. or the charge l3 may be less than required to afford such an unvaporized surplus, so that the lamp will operate with an unsaturated atmosphere. The envelope iii also contains an atmosphere of starting gas such as one or more or the rare gases like argon, krypton. xenon, etc. For the device hereinafter described, having the proportions stated. argon at a pressure or 20 mm. 01' mercury is satisfactory. The proportions chosen may preferably represent a constriction or the envelope in such as results in a discharge-constricting pressure therein durlng normal operation.

bodies H, H are attached. Each inlead II and wtire 2| are joined together by a telescopic joint a 22.

The electrodes Ii, tel-parts of one another) are of relatively small the electrode H. the electrode H is stubby, so that the Though small in total mass. shown relatively compact, or

portions behind reaching any very high temperature. A helical construction for the electrode II is here illustrated, and will presently be described.

electrode H itself. though smaller. This thickness of the connector assures that the arc will not take 01! from it to the exclusion of the electrode ii. But while the connector is thus decidedly stout or thick-set as a mere matter of cross-section, this is compensated by extra eifectlve length, so that the actual, eflective heat conduction is relatively low. To provide this extra length, a loosely coiled helical form for the connective part II is here illustrated, and will presently be described. However, the important point is not any particular means of securing low heat conduction, but rather the thing for which low heat conduction is the means. Indeed, a fine wire connector without coiling may be used, if surrounded with suitable insulation to prevent the discharge from taking oil therefrom.

The low thermal conduction of the connector formed by the connective part II and the lead it is determined by two considerations: control of the conduction of heat away from the electrode II during running on the one hand, and during starting on the other. During running.

the loss of heat from the arcing portion 20 must balance the heating thereof by the discharge at an operating temperature for the portion 20 at which combined sputtering and vaporization losses are reasonably kept down or minimized: otherwise, the lamp would blacken badly during running. (In devices with activated electrodes, this condition has not been reached, because the ,activation supplied electrons to prevent sputtering at temperatures well below that at which vaporization could become serious.) Durin starting, the loss 01' heat from the electrode ll must be so far overbalanced by the heating from the discharge that the electrode actually heats up to operating temperature before appreciable sputtering can occur.

that engages around the current lead 18 to make atelescopic joint. Actually, the arc may sometimes take on initially from the part 22, especially when a new lamp is started i'or'the very first time. In addition to end turns the coiled wire, the electrode Ii may include a heat-distributor in the form of a short wire insert plug 26 filling these turns. Preferably this piece 25 is soundly welded to the helical turns that it occupies, especially the outer end turn, and this turn and the piece 25 may be ground oi! flush to provide an even end face for the tip 20. The rear socket coils 12 may also be attached by welding to the lead end it around which they fit.

Making the coils forming the electrode II and the connective part II separate from the lead it greatly facilitates construction, since it allows of winding wire in a continuous coil whose pitch is suitably varied, and simply cutting this coll into sections each consisting of the desired turns to form an electrode and connector device. Functionally, however, it will be understood that the coil turns 20, 2! and the straight lead wire it might equally well be one continuous length of assassi' tially to that in Figs. 1-3, and its corresponding parts and ieaturcs aromafked with the same reference characters.

Illustrative circuit connections for the startin and running or the discharge device are shown in Fig. i as including a high-leakage-reactance transformer T of semi-auto type with its primary connected across an A. C. power supply circuit P and with its secondaries ti, it connected in series across the main discharge electrodes II, it. Through a high current-limiting -resistance R and a thermal (bimetallic) switch 5, one of the main electrodes ii and the associated auxiliary starting electrode I! are connected across the transformer secondaries ti, ti in parallel with the electrodes H, II. The heating resistor r of the thermal switch 8 is shown connected in one side or the secondary circuit to the main electrodes H, H, so as to be heated whenever the are operates. For a discharge device intended to run on a voltage of 225 volts, the'transiormer '1 may be so chosen as to produce this voltage across its serially connected secondaries tl, it on open circuit, and to give a secondary current of about 3.8 amperes on short-circuit.

Under these conditions, energization of the circuit P will automatically start the auxiliary discharge across the short electrode gap II, II and then the main discharge across the electrode gap II, II. Thereafter the thermal switch 8 will disconnect the auxiliary electrode I! from the sec ondary circuit, so that the seal around the leads II, I! will not be injured by the D. C. voltage subsisting between them when both are in circuit: and this switch 8 will remain open at all times when current is on.

For the convenience of those desiring to practice my invention: I will now give specific conwire, without anything corresponding to the socket turns 22, 21. l

The construction shown in Figs. 4 and d1!- i'ers from that of Figs. 1, 2 and 3 in having the molded tube ends Ila, Ila around the electrodes ii, iii smaller than the tube ends, ll, II in Figs. l-3. Otherwise, this device corresponds essenstruction data for lamps such as illustrated in Figs. 1-5 suitable for operation with a discharge current 01' about 3.0 amperes, the Figs. 1-3 device being suitable ior an operating voltage of about volts and the Figs. 4 and 5 device for about 135 volts. These lamps may be built with envelope tubes Ill oi clear fused quartz about to V4 inch in internal diameter, and with an electrode gap 01' 3 /4 inches for the shorter tube of Figs. 1-3, or about 8% inches for the longer tube oi Figs. 4 and 5. The reduced end chambers i4, ll may each extend about inch axially of the envelope It, with an internal diameter oi about inch for Figs. 1-3, or inch for Figs. 4 and 5. The inleads l0, l8 and i! may be of 30 mil molybdenum wire, extending into the necks i5, it to about the positions shown,--i e., a distance of about /4 inch i'or Figs. l-3, or 1 inch for Figs. 4 and 5.

The coil forming each electrode ii and its conductor 2| may be 0! 30 mil tungsten wire, and may be wound in a lathe on a mandrel wire of the same size, 1. e., of 0.030 inch diameter. Each coil II, it, 22 may comprise three close turns Ii 01' per cent pitch (33.3 T. P. 1.), three loosecoiled, open turns 2| oi 200 per cent pitch (16.7 T. P. 1.), and three close turns 22 of 100 per cent pitch. In practice, it is advantageous to wind 8 close turns, 3 open turns, 6 close turns, I open turns, 8 close turns, etc., in alteration. and to cut through the middle of each group of I close turns to form the individual coils or electrode and connector devices. The heat distributor insert or plug 20 for each electrode Ii may consist oi a piece of 30 mil tungsten wire 0.090 inch long. inserted into the three end turns 20 asoassr and electrically welded in place in a protective inert or reducing atmosphere. The three end turns 22 may be similarly welded in place on the inleads IS, IS, before sealing the latter into the glass of the end chambers l5, l5.

For lamps of higher wattages, appropriate enlargement of essential parts may be found desirable; e. g., a lamp for a discharge current of 6 amperes operating at 525 volts may have an envelope ill of 1% inch internal diameter, an electrode gap of 48 inches, and electrodes ll, ll similar in essentials to those above described, except that the coils of 30-mil wire forming them may be wound on a 45-mil mandrel, their inserts 25 may be of this same size, and their inleads l6, l6 may also be of this same size.

It will be understood, of course, that the foregoing particulars are illustrative of suitable designs. and are not intended as defining or limiting my invention in its broader aspects; on the contrary, the proportions and forms of construction can be widely varied.

In operation, the discharge or arc prefers the compact electrode body II, with its grooves formed by the close wire convolutions and the plug 25 in them, to the connective part 2! where the turns are opened up, or the socket 22; i. e., even when it starts on the part 22 or on the connective part 2|, the discharge straightway shifts to the electrode l i. As regards the parts ll, 22, this is at once intelligible in view of their having about the same mass, since the electrode ii aflords a shorter arc path. It becomes intelligible as regards the parts H, II when it is considered that the openness of the turns :4 makes them a mere length of plain, smooth wire. rather than a grooved or otherwise rough-surfaced body like the electrode I I. As already mentioned, the discharge prefers a rough surface, and starts more easily from such a surface than from a plain, smooth one. Such definite preference of the discharge for the electrode ii and its tip 26 is important, since otherwise the heating effect of the discharge might be so dissipated over the parts H, 2|, 22, as the are shifted from one part to another, that the electrode ll would not be heated to adequate emissive temperature, and objectionable sputtering would occur.

While the discharge current is materially highor during the starting period than during the stable running condition after the device has fully heated up-becmuse of the low voltage drop in the discharge under the low pressure existing in the discharge envelope it at starting-this (1085 not entail material overheating of the electrode II or of its tip 20, because under the low pressure during starting the discharge spreads out over all or most of the surface of the electrode ll, heating it evenly and providing sufflcient electron emission to lower the cathode fall and prevent sputtering during starting. As the mercury pressure in the envelope HI increases above a value of several hundred millimeters and the device reaches a. stable running condition, however, the discharge or are contracts to cocupy and heat directly only the end portion or tip 28, including the end turn of the electrode coil and the end of the plug 25, say. (In general. it is the temperature of this end portion that is hereinafter referred to as the operating temperature during running.) For example, observations made during normal, stabilized running on an electrode ll constructed and proportioned as illustratively stated above in connection with Figs. 1-5 showed a falling oil of temperature at the starting the lamps can be made 5 rate of about 100 K. to 200 K. per wire turn from the tip Illbacl: to the connective part 2|. corresponding to a reduction in emission per unit of area of as much as per cent in the length of the first electrode turn that forms the tip 20, so that this first turn aflol'ds by far the major proportion oi the electron emission from the electrode.

Discharge devices with unactivated solid electrodes afford a variety of important advantages over those with activated electrodes. The electrode construction is simpler and less expensive, special manufacturing steps incident to activated electrodes are materially shorten The warm-up time in shorter, and changes in the starting voltage during their useful life do not occur. There is no active alkaline material of high vapor pressure to be sputtered or vaporized on to the envelope walls, and hence no devitrltlcation or other deterioration of the walls by such material. 0n the contrary. the only material that can deposit on the walls by sputtering or vaporization is tungsten or other refractory metal of the electrodes, which is unreactive toward the vitreous material, deposits only on the very ends of the envelope tube, close to the electrodes, and does not migrate along the tube toward its middle after it deposits, as do the ordinary activating materials. Accordingly, the radiant output of the tube is better maintained during its life, both in the luminous range and in. the ultraviolet.

The characteristic of my discharge device with unactivated electrodes difiers' essentially from that of lamps having activated electrodes; viz., the lamp voltage is dependent on the discharge current, and increases as the current is reduced. as by a resistor or the like in series with the lamp. This increase in lamp voltage with reduction in current is due to a rapid reduction of the average electrode temperature and of the thermionic electron emission as the discharge current which heats the electrode is reduced. Corresponding current-voltage and arcing tip temperature curves for the lamp of Figs. 1-3 are shown in Fig. 6, from which it will be seen that as the discharge current is increased, the discharge voltage approaches an asymptotic minivated electrode, corresponding to the lower emission of electrons from electrode metal as compared with that from activating materials.

In studying the characteristics of an electrode design, curves such as shown in Fig. 6 should be plotted from a device embodying a pair of the electrodes and having an unsaturated atmosphere of mercury vapor in the range covered by the curves, in order to obviate masking of the electrode characteristics by effects arising from condensation and vaporization of mercury.

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

I. An electric discharge device of the high-pregsure type comprising a radiation-transmitting envelope containing vaporizable and its exterior, and one of which, ternately as anode and as cathode. is an unactivated stubby compact refractory metal body presentingtofliedischargcaaolidcenteranda blunt arcing end throughout which the high heat from the discharge is distributed by the compactly cmcentrated metal, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode. and with the operating pressure of the discharge atmosphere, that the heating of said arcing end by the discharge and the cooling thereof by conduction and radiation to and from the rest of the electrode and its current lead and support are in balance for a temperature of said arcing end that is of the order of 3000" K.

2. An electric discharge device of the high-pressure type comprising a radiation-transmitting envelope containing vaporizable and ionizable metal and starting gas, together with co-operating widely spaced electrodes in said envelope which are provided with current lead connections from its exterior, and at least one of which is a coldstarting, self-heating electrode functioning alternately as anode and as cathode, and is an unactivated rough, stubby, compact, refractory metal body presenting to the discharge a solid center and a. blunt arcing end throughout which the high heat from the discharge is distributed by the compactly concentrated metal, thus producing a corresponding distribution of the discharge over said end, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the operating pressure of the discharge atmosphere, that the heating of said arcing end by the discharge and the cooling thereof by conduction and radiation to and from the rest of the electrode and its current lead and support are in balance for a temperature of said arcing end that is of the order of 3000 IL, said electrode, fu th more, being of such small total mass and thermal capacity that in starting its said arcing end heats up to a temperature of the order aforesaid before substantial sputtering of the refractory metal can occur; whereby wastage of said electrode and blackening of said envelope wall are prevented or minimized during both running and starting.

3. An electric discharge device of the high-pressure type comprising a radiation-transmitting envelope containing vaporizable and lonizable metal and starting gas, together with cooperating widely.

spaced electrodes in said envelope which are provided with current lead connections from its exterior, and at least one of which is a. self-heating electrode functioning alternately as anode and as cathode. and is unactivated and consists of a coil of refractory metal wire and a. refractory metal heat-distributing plug occupying and electrically and thermally interconnecting the adjacent convolutions of said coil, thus forming a rough, stubby compact body prmenting to the discharge a blunt arcing end throughout which the high heat is distributed by the compactly concentrated metal, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the operating pressure of the discharge atmosphere, that the heating of said arcing end by the discharge and the cooling thereof by conduction and radiation to and from the rest of the electrode and its current lead and support are in balance for a temperature of said arcing end that is of the order of 3000 K., said electrode, furthermore, being of such small total mass and thermal capacity that in starting its said arcing end heats up to a temperature of the order aforesaid before substantial sputtering of the refractory metal can aacasai occur; whereby wastage of said electrode and blackening of said envelope wall are prevented or minimized during both running and starting.

4. An unactivated refractory tungatenelectrode for functionmg alternately as anode and as cathode in a long-gap high-pressure discharge. said electrode being a stubby compact body presenting to the discharge a solid center and a blunt arcing end throughout which the high heat from the discharge is distributed by the compactly concentrated metal, thus producing a corresponding distribution of the discharge over said end, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the operating pressure of the discharge atmosphere, that the heating of said arcing end by the discharge and the cooling thereof by conduction and radiation to and from the rest of the electrode and its current lead and support are in balance for a temperature of said arcing end that is of the order or 3000" K., said electrode, furthermore, having a rough arcing surface and being of such small total mass and thermal capacity that in starting its said arcing end heats up to a temperature of the order aforesaid before substantial sputtering of the refractory metal can occur.

5. An unactivated refractory metal electrode for functioning in a. long-gap high-pressure discharge, said electrode being a rough, stubby compact body presenting to the discharge a solid center and a blunt arcing end throughout which the high heat from the discharge is distributed by the compactly concentrated metal, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the energy losses therefrom during operation, that the heating of said arcing end by the discharge and the cooling thereof as a result of said energy losses are in balance for a temperature of said arcing end that is of the order of 3000 K.

6. An unactivated refractory metal electrode for self-heating operation in a long-gap highpressure discharge, said electrode being provided with a current connection and consisting of a coil of refractory metal wire and a refractory metal heat-distributing plug occupyin and electrically and thermally interconnecting the adjacent convolutions of said coll, thus forming a rough, stubby compact body presenting to the discharge a blunt arcing end throughout which the high heat is distributed by the compactly concentrated metal, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the energy losses therefrom during operation, that the heating of said arcing end by the discharge andthe cooling thereof'as a result of said energy losses are in balance for a temperature of said arcing end that is of the order of 3000 K.

7 An unactivated refractory tungsten electrode for functioning in a long-gap high-pressure discharge, said electrode consisting of a coil of tungsten wire with a current connection directly thereto, and a tungsten heat-distributing plug occupying and electrically and thermally interconnecting the adjacent oonvolutions of said coil, thus forming a stubby compact body with a blunt arcing end throughout which the high heat is distributed by the compactly concentrated metal there, while the total exposed electrode surface is so correlated with the rated discharge current through the electrode, and with the operating pressure of the discharge atmosphere, that body, with a current-carrying portion of widely spaced convolutions interconnecting the socket and electrode-body convolutions, and a refractory metal heat-distributing plug occupy and electrically and thermally interconnecting adjacent electrode body convolutions of said coil, thus forming a squat, compact body with a blunt arcing end throughout which the high heat is distributed by the compactly concentrated metal LYMAN B. JOHNSON.

there.

CERTIFICATE OF CORRECII ON Batent 110.256.5531.

November 28, 191411..

LYMAN B. JOHNSON.

It is hereby certified that error appears in the above numbered patent requiring correction as follows: In the drawing, strike out Figure 6 in the lower left-hand corner thereof; and that the said Letters Patent should be read with this correction therein that the same may conform to the rec- 0rd of the case in the Patent Office.

Signed and sealed this Zhth day of April, A. D. 19MB.

(Seal) Leslie Frazer Acting Commissioner of Patents.