US2249672A - Discharge device - Google Patents

Description

. July 15, 1941. H. J. SPANNER DISCHARGE DEVICE Filed March 15, 1937 3 Sheets-Sheet 1 ZOQ ' INVENTOR HANS JSPA/WVH? ATTORNEYS July 15, 1941.

H. J. SPANNER DISCHARGE DEVI CE I Filed March 15, 1937 3 Sheets-Sheet 2 INVENTOR HA/vs I SPANA/Ek W Q WW 1% ATTORNEYS July 15, 1941. H. J. SPANNER DISCHARGE DEYICE Filed March 15. 1937 3 Sheets-Sheet 3 INVENTOR HA/vs J SPAN/V5,?

. n? I!" ATTORNEYS Patented July 15, 1,941

Hans J. Spanner, Berlin, Germany, assignor to General Electric Company, a corporation of.

New York Application March 15, 1937, Serial No. 130,872 In Germany December 10, 1936 1 Claim.

This invention relates to electrical discharge devices and to electrodes adapted for use therein and to methods of making and treating the same.

More particularly the invention relates to selfstarting electrical discharge devices of the arcing type and to arcing electrodes adapted to start at relatively low voltage and to be heated to or maintained at arcing temperature by action of the discharge itself. I

Electrical discharge devices as known prior to my invention may be divided into three distinct types, namely, cold cathode, glow discharge (Geisler) tubes in which the electron current depends upon very high potential gradients, hot cathode thermionic (dependent) discharge tubes in which the electron current depends upon thermionic emission from the cathode resulting from use of extraneous heaters, and are discharge devices in which the arcing point on the cathode is maintained at an incandescent temperature but the discharge current is much greater than the thermionic current as determined by Richardsons law, and presumably results from ionic bombardment of the cathode surface. several familiar examples of arc discharge devices. In the carbon arc, canbon rod electrodes are contacted and separated, and by this contact a resistance heating and/or localized ionization at the contact point permit the arc discharge to occur. Similarly with the familiar liquid electrode mercury are a thread of mercury is broken with an effect similar to that of the contacting carbon rods, and the arc burns with a hot arcing spot on the surface of the mercury pool. In the so-called tungsten arc, a discharge is started either by'contacting, by heating the electrodes or by high tension, and the electrodes are made of a size approximating the arcing spot. Thus the entire electrode is heated to high incandescense and serves as the source of illumination. This extremely high temperature, perhaps augmented by some sputtering efiect results in relatively rapid evaporation of the electrode material and consequent rapid blackening of the walls of the tube.

In contrast to these arcing devices are the hot cathode thermionic devices using activated cathodes of the Wehnelt type heated to thermionic temperature by a resistance current. Prior to my invention it had been demozmtrated and accepted that electrodes of this type could not be subjected to ionic bombardment without rapid disintegration and activation such as to preclude their practical usefulness.

There are The electrodes of the present invention, which I have called activated arcing electrodes, and especially the self-heating electrodes, differ fundamentally from all of these earlier types, although they have points of similarity with each of the earlier types and combine advantages of each.

Like the cold cathodes, electrodes of my invention may be designed to start without preheating; but unlike them, with my electrodes the current loading of the discharge is increased rapidly to several amperes with consequent heating of the electrode, and unlike these earlier cold electrodes those of the present invention are "sputtered" in operation comparatively little or not at all and are not appreciably consumed by the art.

Like the thermionic cathodes, my electrodes start and/ or operate at low potentials; but unlike them, no extraneous apparatus is required for starting. Like them, my electrodes operate hot; but unlike them, no special heater connections or heating elements are required.

Like the earlier arcing electrodes, my electrodes operate with current loading greater than the emission, current determined by Richardsons law; but unlike them, the temperatures may be kept relatively low with avoidance of both evaporation of the electrode and disintegration by ion bombardment.

My electrodes are fundamentally new in their principle of operation in that they depend upon a combination of ionic bombardment, thermionic and probably other emission phenomena not previously combined in any electrode. In the full realization of this principle, it has been neces-,

sary to utilize new and fundamentally diflerent structural combinations, as will be more fully apparent from the following specification.

It is an object of my present invention to provide arcing electrodes which may be fixedly positioned within an envelope and adapted to start and/or to maintain a discharge without auxiliary equipment for contacting the electrodes or for heating the cathode.

Another object of my invention is to provide practicable with fixed solid electrodes prior to my invention, and with desirably long life.

With these and other objects in view my invention, in its preferred embodiment, contemplates an electrode which comprises a conductive base and an activating material carried on the base. the activating material being designed to provide cold emission, whereby the discharge may be started with cold electrodes at low potential, and thermionic emission, whereby to improve the operation of the arc and to reduce the evaporation and sputtering of the electrode material onto the walls of the device; the electrode is further designed so that the initial glow discharge which is established by reason of the cold emission tends to concentrate at relatively small portions 'on the surface of the electrodes and thereby to produce a localized heating by which the initial cold cathode discharge is quickly converted into an arc. The electrode moreover is dimensioned and shaped so that, outside of the arcing point, it is not heated above a red heat and in general not above the temperature at which the cold emissive metal in the activation would be evaporated; but the heat dissipating capacity of the electrode is advantageously sufficiently limited so that during operation the electrodes or at least that part of the electrode on which the activation material is carried reaches a thermionic temperature.

The protection of the activation from the di-' rect action of the arc is advantageously accomplished according to my invention by providing a conductive portion of the electrode near a reserve supply of activation material but projecting therebeyond in the direction of the discharge so that the arc tends to strike upon this conductive portion, and ions which otherwise would bombard the activation material are captured by such projection, or at least so far slowed down as to avoid disintegration of the activation material. The activation material may be further protected by including it in a mass of refractory material, such as calcium oxide, magnesium oxide, beryllium oxide, zirconium oxide, aluminum oxide, nickel oxide, etc.

Both of these expedients also contribute to the concentration of the discharge and the concentration of heating from the initial discharge so as to produce a rapid conversion of the initial glow discharge into an arc. If the conductive material projecting beyond the electrode is made in the form of narrow ridges or edges or otherwise of small exposed area, and especially as isolated fine wires or projecting points as described and claimed in the copending application No. 397,427, filed October 4, 1929, the heating resulting from the initial discharge will tend to concentrate upon these small areas and thereby to produce greater increase in temperature. Furthermore, the refractory compounds such as the oxides already mentioned or the metallates, as referred to in my prior patents Nos. 1,817,636, and 1,925,701, tend both by their electrical insulating effect to concentrate the discharge with resulting concentration of heating and by their thermal insulating eilect to reduce the heat dissipation from the areas on which the discharge is concentrated.

The initial glow discharge must also be provided with a current loading sufficient to effect such heating of the electrode. In each lamp and with each electrode design, rent value below which such conversion will not occur; since the glow discharge is desttuctive to the electrode activation, the current loading there is a critical curshould be well above this critical'value to hasten so far as possible the formation of the arc.

Advantageously the electrode is made with a surface as rough as possible and preferably porous, e. g., by winding 2. wire upon another wire or wires so that a reserve supply of activation material may be engaged in the interstices of the porous structure or the cavities of the rough surface.

It is an important feature of the preferred embodiment of my invention that a portion on which the are normally tends to burn is substantially closer to the other electrode so that the area of this arcing spot is more or less constricted depending primarily upon the gaseous pressure; and the activation is carried on a portion of the electrode beyond the arcing spot. With this arrangement, so long as there is sufficient activa- -on around the arcing spot, the arc will remain in its normal position; but if this spot should become deactivated, and before it is de-activated to such an extent that sputtering or evaporation of the metal would occur, the arc migrates due to the fact that the longer are path is more than compensated for by the lower electrode drop where the better activation exists. According to the present invention the activation material around the normal arcing spot is adapted to renew the activation on the arcing spot when such migrations occur. This is particularly important with alternating current arcs because, with the restarting of the are at each half cycle when the electrode acts. as a cathode, there is a tendency for the arc to strike to the point where suflicient activation exists. This action also provides an automatic use of the reserve supply of activation substance exactly as it is needed, since the migration of the arc serves to reduce additional activating substance and/or to cause its migration' along the electrode to the arcing spot. This is especially important with high pressure devices because the concentration of the arc tends to burn off any excess activating substance from the arcing spot, and to vaporize the base metal if insufficient activation is available at the arcing spot.

It is also important that the conductive base of the electrode provides several paths from the arcing spot to the supply circuit. This may be accomplished by use of a closed ring (cups, disks, cylinders, cones, etc.) or by a wire or Wires with their ends short circuited either outside the tube or inside (as claimed in the copending application Serial No. 397,427, filed October 4,1927).

Finally best results are attained when the normal arcing spot occurs at a bend rather than at an end of the conductive base.

Although one cannot at the present time state with assurance exactly what action occurs on these activated arcing electrodes, nevertheless it is thoroughly clear that there is a complex action quite different from anything which exists on the older type electrodes. In the arcing spot itself there is an action which is apparently quite similar to that of the are produced in the same atmosphere by the contacting of unactivated solid electrodes. That it is not identical, however, is indicated by the fact that the evaporation of metal from the electrodes and consequent blackening of the envelope does not occur or occurs only to a much lesser extent if suitable activation exists at and beyond the arcing spot and V the electrode is suitably dimensioned. It is indi- 'cated furthermore by the fact, already referred to, that the arc will migrate to a point electrically more remote if the activation at such a point is sufliciently better than at the normal arcing point.

In addition to the action ithin the arcing spot itself there is evidence of-a supplemental action, thermionic in its nature, occurring over the activated surface outside of the arcing spot, and it is probable that in addition to-this there is also a photo-electric effect due to the presenceof the cold emissive electro-positive constituent of the activation material.

That the action of the electrode is not merely thermionic is clearly established by the fact that, if we calculate by known formulae the thermionic emission from the electrode, it is so low as to account for only a very few percent of the total current of a normal discharge.

On the other hand, it is apparent that the thermionic emission from the area surrounding the arcing point is important. If the rest of the electrode is too cool the electrode disintegrates, the activation is lost, and the envelope around the are is rapidly blackened. This is explained by assuming that the positive ions which otherwise bombard the electrode around the arcing point are repelled to some extent by thermionic emission from those parts of the electrode.

The photo-electric eflect is probably particularly important in the starting of the electrode. Thus the first feeble discharge, by irradiating the electro-positive metal on the electrode, increases the flow of electrons and thereby results in a stronger current loading, a more rapid ionization and consequently a more rapid heating of the electrode and conversion of the discharge into an arc. Nevertheless, although the reduced electro-positive metal is most important during the starting and heating-up period, it seems also to have importance during the normal operation of the arc.

It is also important that the surface area, and especially the heat-dissipating capacity of the electrode should be designed with regard to the design and the energy loading of the tube in which it is to be used so that its temperature, outside of the arcing spot, remains below such temperatures as would evaporate the reduced electro-positive metalor other readily vaporized activation material, and at the same maintains during normal operation a suflieient thermionic emission from the activated surfaces.

The area of the electrode to attain these desired results cannot be specified in general terms, since it depends on the current loading for which the electrode is to be used, the degree of activation, the irregularity or roughness of the surface, the heat dissipating capacity of the electrode, which includes the design of the pole vessel as well as the conductivity of the electrode material and the presence of connected radiating bodies, as for example the short-circuiting connections described in the examples given below, and also upon the design and positioning of the lamp by which heat from the discharge itself may be carried by convection or radiation to the electrodes. It will be found, however, that this areais not highly critical, and in general variations as much as 30% or more either way from the optimum size may be tolerated without excessive disintegration or vaporization of the electrodes if there is proper activation. Little difiiculty will be experienced in determining the proper area for any particular tube if these considerations are borne in mind, as well as the desired temperature relations mentioned in the preceding paragraph.

The temperature in the arcing spot itself depends more upon its activation and upon the pressure of the discharge atmosphere than upon theheat dissipating capacity.

The character of the activation can be substantially varied; and in fact the materials used for activation may be in many cases identical with those which have heretofore been used on activated thermionic electrodes. In general, however, a heavier coating of activation material must be provided giving a greater reserve than in the case of the heretofore known thermionic electrode; this reserve or at least apart of it is protected to some extent at least by the projecting conductive material of the electrode: and the treatment of the activation material must be' designed to reduce the compounds of the electropositive metal used for activation, at least to a substantial extent.

The amount of activation used is to some extent dependent upon the treatment used for activation. In general it is desirable to use as much activation material as possible but it is found that with any given treatment for reducing the activation material there is a limit to the amount of activation material which can be properly reduced by the treatment. Beyond this amount, an excessive amount of oxygen or other combined or occluded gas would remain in the activation mass to be released during operation, and this might make trouble with starting or reduce the useful life of the device. Furthermore, such an excessive activation mass, being less intimately combined with the electrode structure, has more tendency to flake off from the electrode leaving unactivated portions.

If too little activation material is used, c. g., an approximately monomolecular layer, formed by dipping in a solution, as has been common with so-called Wehnelt cathodes, the coating of activation material may be quickly burned off by the action of the arc.

The materials useful for activation include barium, strontium, calcium, thorium, cerium and other rare earth metals, lithium and magnesium,

for a satisfactory self-starting electrode except in combination with the more emissive metals. In addition to the reduced activation material, and

I advantageously admixed therewith in the coating on the electrode, is a refractory oxide which serves to concentrate the initial discharge and the heating therefrom and also in the final arc, especially with high gaseous pressure, to disperse the arcing spot somewhat with consequent reduction of the maximum temperature in the arcing spot. These oxides also may serve as a separating layer between the reduced electro-positive activating material and the conductive base material such as tungsten, especially if the base is an electro-negative material. A similar effect can be attained with advantage by providing the surface of the tungsten with a neutral metal which is oxidized when the activating substance is reduced, and thereby both assists in the reduction of the electro-positive metal and provides a barium oxide and at least oxide such, for example, as calcium oxide, magrefractory oxide separating layer. Nickel, for example, has been found usefuras a coating,'probably'because it acts in this way. However, where the electrode is to be used for a highpressure mercury arc-discharge with a relatively heavy current loading free nickel on the electrodes may be evaporated with blackening of the wall of the envelope, and in such cases should, of course, be avoided.

The refractory oxide used with the electropositive metal may be one which serves also to increase emission from the hot electrode, e. g., CaO, SrO, MgO, T1102, etc.

I find that the best electrodes are made with activation material having at least 50% of of a refractory nesium oxide, beryllium oxide, zirconium oxide, aluminum oxide, etc., and these may be combined in refractory compounds, as for example, metallates, i. e., with amphoteric oxides such as the oxides of nickel, zirconium, zinc, aluminum, or with refractory oxides such as silica and phosphorus oxides.

In the application of the coating to the electrodes these materials are preferably finely ground and suspended in a volatile liquid either without a binder or with one which leaves no objectionable residue on the electrode. This mixture should preferably have the consistency of a cream or paste and may be applied to the electrode structure, e. g., by spraying or brushing or dipping. The coating thus applied may be from several thousandths to a few hundredths of an inch in thickness and if a thin suspension is used several applications may be necessary to give the desired thickness.

A coating of oxide formed on an electrode in this way ordinarily has but little strength, unless a binder is used; and the electrode must be treated carefully in inserting it into the tube in which it is to be used. The strength of the coating can be increased by the use of silicate or phosphate material in the coating and preheating of the coated electrodes sufllciently to sinter the coating. Also, a fiuxing material as for example, bariumfiuoride may be used to give the coating a porcelain-like nature.

The activation treatment required for these electrodes is much more than the ordinary treatment to which electrodes have been subjected prior to my invention. After the preliminary degassing of the electrode it should be heated to a temperature well above that to which it will be subjected in operation, e. g., to about 1000 to 1400 C. in a vacuum maintained by a vacuum pump which removes substantially as soon as they are released any gases released from the electrode or from its coating. This heating may be effected by galvanic glowing if the electrode is designed so that a resistance current can be passed therethrough, or advantageously by high frequency heating as described and claimed in the copending application, Serial No. 397,427, filed October 4, 1929, or otherwise.

The final activation treatment requires the burning of a discharge from the electrodes acting as cathodes, whereby it is subjected to ionic bombardment. During this final treatment the envelope in which the electrodes are mounted should be pumped continuously or intermittently to remove gases released by the treatment, and

advantageously the wall of the envelope should be kept sufficiently hot to prevent deposition thereon of any material which may be evaporated from the electrode by the activation treatment. This treatment should'be continued or repeated, with a discharge current substantially beyond that to which the electrodes will be subjected during normal operation, until the electrode starts readllywithout preheating at the voltage for which it is designed.

The discharge for this final activation may be carried on in an atmosphere of rare gas or of a suitable vapor, e. g., mercury. There is advantage in the mercury pumping in that the ions of the mercury vapor are heavier and apparently more effective in reducing the activation material and also in the fact that an excess of the mercury can be supplied in the envelope and thus the discharge maintained at any given pressure according to the temperature of the mercury pool while the pump is operating to withdraw the mercury vapor and with it any gases given oil by the electrodes. Thus the final activation step can be made continuous. If desired a smaller amount of mercury can be used and returned by a suitable reflux condenser while the gases given of! from the electrode are withdrawn by the pump. If mercury is used in this way, however, great care must be taken to assure purity of the mercury, since otherwise this distillation of mercury from the tube impurities in the tube.

Alternatively, a discharge may be started in a suitable gas, e. g., argon or neon, and. after a short period of burning the envelope may be opened to the pump and the gas pumped out. The initial pumping can be done while the discharge continues, but the pressure should not fall below about 2 or 3 mm. before stopping the discharge. It is better to stop the discharge before opening the pump connection. This burning and evacuation may be repeated several times. The gaseous pressure during this ionic bombardment treatment is preferably kept relatively low, e. g., below about 15, or better 10, mm. pressure' in order to avoid unnecessary concentration of the arcing spot upon the electrode and thereby to produce activation as complete as possible over the surface of the electrode. When this treatment is complete the activation coating which before the treatment was white will have been reduced to a metallic gray, at least in patches, and the electrode will emit electrons when cold so that a discharge may be started at relatively low voltage without preliminary heating. When this has been accomplished the envelope is pumped clean of all gases and advantageously flushed with the gas which is to be filled therein, after which the filling gas and/or vapor is filled into the envelope and the latter sealed oil from the pump connection.

In some devices, and especially where relatively high voltage is available for operation, auxiliary electrodes are not necessary; but where devices using the self-heated electrodes are to start and operate on or 220 volts there is very important advantage in using auxiliary electrodes either inside or outside for assisting in the starting of the discharge. Such use of auxiliary electrodes should not be confused with the use of auxiliary electrodes in cold cathode Geisler tubes or in the thermionic cathode tubes as known to the prior art. The auxiliary electrodes in this case'are no mere. reduction of the distance over which the discharge should occur nor a mere introduction of capacity between the electrodes and an out-- side cuff. On the contrary they have a special action when combined with my electrodes which may leave undesirable cloud or space charge which may so far repel the further emission of electrons as to throttle the initial discharge and prevent it from carrying sumcient current at the available voltage to effect the conversion into an arcing electrode. Under an inside electrode 'close to the cathode (e. g.,

slightly more than the mean free path of electrons) the electron cloud may be dissipated by the formation of ions in an auxiliary discharge between the cathodes and the auxiliary electrode. In the case of a capacitative electrode, and to a lesser extent in the case of a resistance or inductance connected inside electrode, the effect may be obtained by binding the space charge; and for this reason such auxiliary should have a substantial area near the electrode and along the discharge path toward the opposite electrode.

This space charge effect may be avoided by preheating of the electrode. Presumably, the effect is due to the fact that cold emission occurs at a low energy level, and, therefore, the electrons have a low velocity which is insufficient to get them away from the vicinity of the electrode; whereas, when the activated electrode is heated its electrons are emitted at relatively high velocity which carries them away from the electrode.

In the accompanying drawings, I have shown several embodiments of my invention. These are given by way of examples which are not intended to be either exhaustive or limiting of the invention. On the contrary it is my intention by means of these illustrations so fully to explain the invention and the principles thereof that others skilled in the art may readily apply the- Fig. 4 is a view similar to Fig. 3. but showing another type of electrode;

Fig. 5 is a. view in side elevationv of still another type of electrode;

Fig. 6 is aview in side elevation of another modified form of an electrode similar to that of 7 Fig.

Fig. '7 is a view partly in elevation and partly in axial section of another lamp embodying my invention;

Fig. 8 is a view in longitudinal section Qi another lamp embodying my invention; 4 A

Figs. 9 and 10 are perspective views of the electrodes'made according to my invention such as are shown in theiamps of Figs. 7 and 8;

Fig. 11 is a fragmentary view in longitudinal section of a lamp similar to that of Fig. 8 but using a resistance heater within the electrode-for preheating the activated electrode surface to facilitate starting;

Fig. 12 is a fragmentary view in longitudinal section of a lamp similar to that of Fig. 8, but using an auxiliary electrode;

Fig. 13 is a view partly in side elevation and partly in section of an apparatus for activating the electrodes in alamp embodying my invention;

Fig. 14 is a view in longitudinal section of an-. other lamp embodying my invention;

Fig. 15 is a diagrammatic view showing an alternative electrode structure which may be used in place of the coiled wire electrode shown in Fig. 13;

- Fig. 16 is a view partly in longitudinal section and partly in side elevation and partly diagrame matic of another lamp embodying my invention;

Fig. 1'7 is a view in longitudinalsectlon with the electrical connection shown diagrammatically of still another embodiment of my invention; and

Fig. 18 is a view in longitudinal section with the circuit connections shown diagrammatically of another lamp embodying my invention.

into commercial use. This consists broadly of an inner sealed envelope 20 resiliently mounted within an outer jacket 2|. The electrodes 22 are -melt or vaporize or disintegrate. v

rule as to the determination of the size can be mounted on the electrode lead-in wires 23, sealed into the envelope 20 by suitable sealing means 24 and 24'. The electrode lead-in wires are suitably connected to the respective contacts of the base 25. The envelope 20 is filled with an will not heat sufllciently to properly maintain the arc and thus the lamp may continue to give only a glow discharge or the electrodes may even sputter like the Geisler tube electrodes. If on the other hand the electrode is too smallit will No absolute stated, but in general the size should be such that an operating temperature of about 700 C. will be maintained on the electrodes outside of the arcing spot.

An electrode which is tapered toward the discharge, e. g., as shown in Figs. 5 and 6, is of a special advantage particularly with tubes burnedin horizontal position because this tends to main-.

5 tain the are at a substantially constant point on the electrode and reduces the tendency of the arc tomigrate from side to side of the electrode.

With an electrode as shown for example in Fig. 3 in a 300 watt lamp using a filling of 14 0 mm. of argon and mercury sufllcient to bring the pressure during operation to approximately 1 atmosphere, an envelope 14.5 centimeters in length (not inclusive of the sealing bead) and 4 centimeters in diameter in the arc tube and having pole vessels 3.9 centimeters in diameter established to one of these points.

and 2.2- centimeters between the bottom of the constriction 26 and the end of the tube at the base of the sealing bead, theelectrode may be 1.2 centimeters at the outer rim and 4' mm. in depth. The outer cup 21 may be stamped from .010 inch thick sheet refractor metal, e. g., nickel, and the wire mesh inside may be 40 strands per inch in both directions of .007 inch refractory metal, e. 8., nickel.

With a 100 watt tube using an electrode as shown for example in Fig. 4 and filled with 18 mm. argon pressure and mercury sufllcient to provide approximately 1 atmosphere pressure during operation and with an envelope 2.5 centimeters in diameter, centimeters in length and pole vessels 1.5 centimeters long and 2.5 centimeters in diameter the electrode disc may be 9 mm. in diameter and approximately 3 mm. thick, including the face disc of .010 inch sheet nickel and the 2 flares of wire mesh 40 strands per inch .007 inch refractory metal, e. g., nickel.

The activating material is pressed into and over the interstices of the wire mesh so as to 'be anchored thereby in intimate contact with the refractory metal structure. This activating material, in the examples for which dimensions are given above, consist of barium oxide reduced by treatment as more fully specified hereinafter. I have found this type of electrode most satisfactory for general purposes. The activation when properly prepared so far reduces the resistance of the electrodes to the passage of the discharge as to permit operation at temperatures well below the temperature at which the metal of the electrodes would be volatilized, e. g., around 700 C. and furthermore permits cold starting of the lamp either directly from the supply line through auxiliary electrodes (either the strip 20 as shown in Fig. 1 or an internal auxiliary electrode as shown at 29 in Fig, 2) or with very simple inductive starting devices as hereinafter described, which maybe the ballasting device as well.

The use of the wire mesh for their construction, which leaves irregular points or thin edges projecting from the electrodes, I have found to be of particular advantage, apparently because it facilitates the heating of the electrodes and the conversion of the discharge to an arc type discharge. These small points of electrode material are readily surrounded with the activation material and consequently the first discharge which occurs through the auxiliary electrode is readily material of such a point is more or less isolated, it is readily heated by the discharge to a temperature .at which it is highly emissive and at which the discharge is readily converted into an arc. Thus, although the initial discharge might be inadequate to heat the entire electrode, if it can occur first to such an isolated point of the electrode such heating may readily occur and the arc-type discharge then will be initiated and will serve to heat the entire electrode.

In Fig. 12, I have shown a modified construction intended to accomplish substantially the same purpose. In this case instead of using wire mesh a series of sheet nickel cups are nested one in the other. These are preferably roughly stamped out so that the edges are more or less frayed in the process. These frayed edges may,

therefore, serve the same purpose as just disv Since the cussed above for the ends or points of the wires of the mesh used as shown in Fig. 3 or the mesh disc shown in Fig. 4. With this ty e of electrode 7g 'to. In the examples illustrated in Fig. 3 lips 30 are turneddown from the material of the cups and are welded to the lead-in wire 23.

In Figs. 9 and 10 I have shown another type of electrode comprising a twisted roll 3| of fine refractory metal wires secured to one end of a horseshoe or disc 12 of nickel, tungsten or other refractory metal and the latter in its turn being connected to the lead-inwire 23. The roll 3| of fine wires may be made of short lengths of wire twisted together so that there will be numerous isolated points for the purpose as already described. Into this fabric of wires the activation material is worked as already described on conne'ction with the wire mesh of Figs. 3 and 4; and its operation will be much the same as with those electrodes.

In all of these electrodes it will be observed that there is a closed path for passage of eddy currents so that after the tube is formed with the electrode in place activation material may be reduced and the electrode cleaned of occluded and combined gases by an induction heating with high frequency current.

In these examples I have referred to nickel as 'the material used in the electrodes. My invention, however, is not to be limited to nickel, in fact I have found that many other metals can be used, but nickel, I have found seems to hold the activation material best and does not inter fere with the effectiveness of the activation material. If, however, special requirements of design or operation necessitate overheating of elec-- trodes, a more refractory material may be preferred. Thus I have used tantalium, molybdenum, tungsten,; fetc. When'these metals are used, they may be covered with a thin surfacing of nickel, but this is not essential. On the contrary I have obtained good results with tungsten alone as the carrier metal of the electrode, but I prefer to use nickel or equivalent metal to support the activation material where operating conditions permit.

In- Fig. 5 I have shown a massive type of electrode Md. The electrode shown in Fig. 5 is made by winding into a return spiral, preferably in substantially conical form, a heavy wire of tungsten, molybdenum. etc., e. g., wire of about 0.50 mm. diameter, or better yet a stranded wire, e. g., 8 wires of .1 to .3 mm. diameter, twisted or parallel and bound together.

This spiral is returned to the center at the bottom and welded to the lead wire in order to form a closed. circuit for high frequency heating and to close in the space within the cone. This space or the interstices of the stranded wire may be filled with. activation material.

A similar electrode is shown in Fig. 6. In this and the activation material held within the interstices of the stranded wire the spiral need not be close together andin this case a substantial distance is left betweemeach of the turns of the sp ra Another feature of the electrodes shown in Figs. and 6 which is of special advantage is the shape, tapering toward the axis in the direction of the discharge, as in the conical form shown in Fig. 5. This tapering form tends to hold the discharge at all times centered within the tube and to prevent its wandering over the surface of the electrode. This is particularly important in lamps which are designed or intended for horizontal burning, since in the horizontal position the wandering of the arc to the upper edge of a flat electrode may bring it between the electrodes too close to the wall of the envelope.

It will be observed that with this form of electrode and especially in a high pressure atmosphere the are normally strikes the tip of the electrode and thus this portion of the electrode serves to protect the activation on the remainder of the electrode. If, however, due to constant burning on this point or for other reasons the tip should become so far de-activated that the electrode drop is substantially increased more than the difference in resistance through the gas path and thereby the arcing point may shift along the electrode and strike at a point better activated beyond the tip. When this occurs additional activation material will migrate along the wire to the tip until it again becomes sufliciently activated so that the arc can strike at that point. The activation material is used automatically as required without excessive evaporation. In low pressure rare gas atmosphere the discharge is more difiuse, but even in this case there will ordinarily be a spot, e. g., 3-5 mm. along the wire which is at a higher temperature than the rest of the wire.

I have found that the electro-positive metals, such as barium if properly combined in the electrodes will render them emissive even when cold so that a lamp in which they are included may be started without resistance heaters or other heat-- ing devices and will be self-heating, i. e., the electrodes after starting the discharge will be heated directly by the action of the discharge itself. Thus with the lamp as shown in Fig. 2

for example, a small glow or corona discharge is first formed between the electrode 22 and the auxiliary electrode 29 or starting strip 28. This discharge quickly ionizes the gas and dispels or neutralizes any space charge to such extent that the path between the electrodes offer less impedance than the path through the wall of the envelope 20 (acting as the dielectric of a condenser) or through the resistance 33 in the auxiliary electrode circuit and the principal discharge is transferred therefore to the path between the electrodes. As the discharge conconnection. Through this connection the tube is evacuated by pumping in the usual way. The electrodes are then subjected to intense heati e. g., by means of eddy currents induced therein by a high frequency apparatus commonly called a. bombarder."v During this or some other equivalent preliminary heating the pumping is continued to exhaust any gases which are released by the heat treatment, and this pumping should preferably be continued throughout the electrode treatment until the lamp is finally filled with the gas and vapors which are to serve in the use of the lamp.

This preliminary heating is carried to preferably just under 1000' C. or to a temperature just below that at which the nickel or other electrode metal would begin to vaporize. A higher temperature may be used and the activation of the electrode would be entirely satisfactory, but the tendency of the tubes to darken by vaporization of the electrode metal would be increased. If this preliminary heating is not intense enough the electrodes will not be sufficiently activated for the subsequent treatment by the discharge,

and unless special precautions are taken they may be fused so badly as to be spoiled in the subsequent treatment.

The initial heating treatment may not and advantageously does not completely reduce the electrode; and it is, therefore, subjected to a further treatment by passing a discharge through the lamp while the pum ing is continued. At this stage a gas, preferably one of the rare gases, is allowed to enter the tube to carry the discharge therein, and a stream of this gas preferably is continually exhausted from the tube by the pumping.

I have found that in a filling of such a gas the discharge can be made to spread upon the entire electrode and thereby to give more uniform activation. Mercury vapor, which is superior in many respects for this treatment, tends to hold the discharge more to one spot on the electrode and thereby to interfere with uniform activation. but mercury can be used to advantage at the end of this stage of the treatment,

I prefer mercury vapor for this final discharge treatment because its ions are heavier and therefore produce a more intense bombardment. Furthermore, the mercury can be gradually'vaporized either by the heat of the discharge or by externally applied heat, and thus a stream of its vapor may serve to sweep impurities .out from tinues it rapidly heats the electrodes until the point is reached at which the initial glow discharge is automatically converted into an arc discharge. Thereafter the temperature of the electrodes is sustained by the heat of the discharge.

The electrodes used for this purpose are not oxide coated electrodes of the Wehnelt type although in their manufacture they are coated with oxide. The action which makes possible the operation as just described is dependent upon an intense reduction of the electrodes after the tube in which they are mounted is sealed from the atmosphere. For example in the preferred embodiment of my invention the electrodes are coated with barium oxide, preferably an oxide having a high content of water of crystallizathe tube, and gradually take over the discharge from the gas.

During this final treatment the electrodes are overloaded, i. e., are subjected to a current greater than that of normal operation. so that the surface of the electrode is heated to a yellow or yellowish white heat.

The preliminary heating may be done in other ways, and in fact can be combined with the treatment by the discharge; but'it is difficult to get satisfactory initial heating of the electrodes by the discharge until they are at least partially activated. For this reason the electrodes are preferably brought to a glowing temperature by other means, e. g., by high frequency or resistance heating, unless they have been substantially activated by a pre-treatment.

The action of the discharge effects'a further reduction of the barium oxide at the surfaceof the electrode and leaves it with a metallic grey appearance which apparently is caused by the particles of reduced free metallic barium. I believe that these particles of free metallic barium are isolated from the carrier metal, e. g., nickel by a certain amount of unreduced barium oxide or sub-oxide or nickel oxide or other compounds. This seems to be important, since I have found that an alloy of nickel and barium cannot serve as the full equivalent of the reduced electrode as just described. The oxide, sub-oxide or other barium compounds apparently serves as a reservoir of activation material, thus prolonging the useful life of the electrode. If at any time the activation should be impaired the electrode begins to overheat and the arc to migrate and a new layer of barium is thereby reduced. Thus fresh activation material is automatically produced as required, yet at no time is there an excess of barium which would be vaporized from the electrode.

Other reducible barium compounds may be used. and other electro-positive materials, especially reducible alkaline earth compounds. Furthermore the reduction may be effected in other ways. As for example, solely by a high frequency heating, in which case it would have to be continued for a very long period of time at a very high temperature. and even then the reduced electrode would not be quite as satisfactory as the electrode produced by the method as just described. Also the entire reduction may be effected by direct action of the discharge upon the electrode, e. g., starting the discharge by means of high tension or high frequency ionization and continuing it under conditions to effect the desired heating of the electrode.

In any case, I have found that the most active electrode results if the reduction treatment is Just short of eliminating all the separable oxygen.

I have mentioned above and illustrated in Fig. 2 the use of auxiliary electrodes to provide a shorter and consequently a lower voltage path for starting the discharge. The auxiliary electrode shown in Fig. 2 is of a very simple form, but one which has proven entirely satisfactory to give low voltage starting.

It is not essential that this auxiliary electrode be given any panticular form or location and as shown in Fig. 2 both have been chosen primarily for ease and economy of manufacture. I have found, however, that certain advantages may be attained with special forms and/or positioning. I have found, for example, that it is better to space the auxiliary electrode from the main electrode a distance at least equal to the mean free path of electrons in the filling at starting.

With the auxiliary electrode placed as shown in Fig. 2, its circuit may be opened after starting or it may be connected through a resistance such that during operation there is only an insignificant current passing to the auxiliary.

The impedance connected to the auxiliary electrode may be for example a resistance of from 500 to 50,000 (e. g. 2000) ohms for lamps of the size specifically mentioned above. In gen eral, the impedance should be as high as is consistent with a good starting discharge and reliable conversion of the initial glow into an arc, so that the "leakage discharge to the auxiliary electrodes during operation will be minimized. If the resistance is too high starting may be diflicult or impossible and the glow discharge may continue too long without conversion to an arc and with consequent disintegration of the electrodes. If the impedance is too low the efficiency of the lamp may be impaired.

In Fig. 7, I have shown another example of the invention. Here a lighting tube of a type similar to that shown in Fig. 1, but without the external jacket 2!, is shown with an electrode 220 of the type shown in Figs. 9 and 10 mounted in the envelope "0. In this case. as in the case illustrated in Fig- 1, the return lead connection 280 from the upper lead-in wire 23c forms an outside capacitative auxiliary electrode. As shown, this is in the form of a nickel strip, beneath which on the outer surface of the tube is a conductive coating 84, e. g., graphite (aquadag).

The electrodes in this case are adapted to become incandescent without any additional heating means. When used with alternating current, preferably at least two such electrodes are" fitted in the tube, and they alternately become cathode and anode. When direct current is used, only one electrode 01' the self-heating type is necessary, and the anode can be made of some non-disintegrating material. It is advantageous with direct current to make the anode of larger dimensions than the cathode, according to the different cathode and anode drops.

The electrodes consist of a metal, such as nickel, which reacts favorably 'to the activating materials,especially barium and compounds of barium and caesium or compounds of caesium. In order to reduce the destructive effect of the arc on the electrodes, these are made of layers of nickel sheeting or nested cups, or of twisted nickel wires. By the use of this porous structure, it is possible to insert the activating materials to a suilicient depth in extremely narrow gaps and holes.

To assure reliable working of the arc lamp and reliable starting of the are after a glow discharge has initially set in, in the gas filling, the cathode consists in practice of a fabric of thin wire or any other porous structure made of some metal which, by virtue of its chemical properties, will not prevent the emission of electrons. This fabric is coated with an activating material, as, for instance, a compound of barium,

or, in the case of smaller loads, of caesium or rubidium, or with mixtures of these and/or other emissive substances.

The activating process of an electrode of this kind is the most diflicult problem. In manufacture, care must be taken to see not only that the pure metal is formed at the surface, but also that for instance, some of the barium oxide or other activating material is reduced to a suboxide, e. g., partially reduced by a high temperature treatment and preferably that an intimate combination of the barium, etc., with the base body, e. g., nickel, is formed. To this end the core may be covered with a mixture of substances, including compounds of an emissive substance, the whole chemical structure of which is unstable. Now experiments by the applicant have shown that unstable sub-compounds of this kind are actually extremely suitable for the emission of electrons.

Such electrode will be suitable for the production of a glow discharge which afterwards turns into an arc, if the electrodes are formed in such a manner that the final result is as described above.

It is also an advantage to have as large a proportion as possible of the electrode surface activated and to have any unactivated connections to the electrode within the tube of relatively small surface and positioned beyond the activated portion relatively remote from the discharge. I

With a lamp as shown in Fig. 7, designed to operate on a 110 volt circuit with an initial arc current of 6 amperes at about 16 volts and a final operating current, after evaporation of mercury, of about 3.4 amperes at 55 volts the electrodes shown in Figs. '7 and 9 may consist of about 40 wires of nickel or tungsten or other refractory metal of about 0.25 mm. diameter twisted or interwoven and wound into three coils spaced apart suiflciently so that the coils themselves do not short-circuit between one another. The ends of this stranded wire are welded to a bow 32 of horse-shoe shape which may be made of a wire of nickel or tungsten or other refractory metal of about 3 mm. diameter, which advantageously may be flattened down to about 1 mm. thickness.

The lead-in wire 230 is preferably a tungsten wire welded to the back of this metal bow. Before the electrode is sealed in, its emitting part 3I, is covered with a layer of finely ground barium oxide which has been worked up into a viscous mass in distilled water. Ordinarily it should not thereafter be allowed to remain long in contact with the air, but should be sealed into the envelope as quickly as possible.

The envelope with the electrodes sealed in is connected to the usual pumping stand and heated to as high temperature as possible and subjected to a vacuum to clear the glass of any free or adsorbed gases. Then the metal bow electrode is brought up to temperature of about 1200 C. by means of high frequency induced currents. During this process the oxide mass, which up until now has been of a pure white, runs and melts, giving off oxygen and foreign gases; and after cooling down it remains as a darker coating on the electrode wires, presumably as a sub-oxide on the surface.

While the electrode is still in this condition it is not ready for useand it would not start reliably if charged with the ordinary supply voltage.

To proceed with the process, the tube is now filled with a suitable discharge gas, e. g., argon, at a pressure of from 4 to mm. mercury column, and a tension somewhat higher than the intended operating voltage, e. g., 220 volts, is connected to the tube with an impedance in series. charge is now started and is worked up in the tube. It is to be recommended that in the first moments of operation the discharge should not be loaded over 4 amperes.

Mercury can be used for the final treatment of the electrodes with advantage, a already suggested above, by filling in a large quantity of mercury before the tube is connected onto the pump stand, preferably before the discharge is started. After the discharge has continued for some time the mercury evaporates as the result of the heat developed by the discharge and the mercury takes over the carrying of the current. For some time after starting the electrode continues to give off foreign gases and With the aid of high frequency a disby distillation into the pump connection. When thetube is burning well with this strong current the connection to the high vacuum pump is opened once more and the gases are exhaust ed. The mercury, however, continues to evapcrate so that the mercury arc continues to burn that the quantity in the tube is not exhausted quietly.

The entire tube is now heated up, e. g., by means of a burner flame or an electric furnace, the latter being better as it allows a continuous observation of the discharge and also provides more uniform and more perfectly controllable conditions. During this procedure care must be taken to prevent the mercury vapor pressure from becoming so high that the discharge will be extinguished. As the supply of mercury diminishes the pressure of the vapor drops more and more. This can be seen from the swelling of the luminous discharge. Shortly before the last of the mercury has been evaporated from the tube the supply of current must be cut oil, and the tube may be heated for a while longer, the pumping being carried on at the same time.

The surface of the activated parts of the electrode structure after this treatment should have assumed a dull metallic appearance and free barium will have been formed in the suboxide layer and possibly also compounds and/or alloys of the barium with the metal, e. g., nickel, of which the surface of the electrode structure is formed.

The tube is now filled a second time with mercury and rare gas and treated as described above, except for the fact that one can now operate on a supply tension of volts; otherwise the figures given above for current are adhered to.

After cooling'of the lamp a starting strip, made for example of a colloidal suspension of graphite in water (the so-called aquadag), or

rounding it. If the tube thus treated is now filled with argon at a pressure of approximately 10 mm. and a suitable amount of mercury to provide the desired. operating pressure and is sealed off from the pumping stand it' will be found capable of starting itself when connection is made with the supply current of, e. g., 110 volts, provided that the current available for discharge is not too low. If this current should be too low a glow discharge sets in which is very damaging to the electrodes and should, therefore, be avoided under all circumstances.

After the tube is first filled in this manner and sealed 03, the first few starts may be difli-' cult and the tubes should, therefore, be started several times with the aid of high frequency or an increased voltage until it starts reliably on the supply voltage.

Before sealing off the tube it is advantageous to raise the pressure of the gas somewhat above the figure specified to take care of the clean up, i. e., the taking up of gas by the metal parts in the tube during the first part of the operation of the tube.

Instead of the-particular form of electrodes shown in this case other forms .of electrodes as described above, e. g., several nickel cups nested one inside the other may be used and subjected to the same treatment for activation as described fitted to the lead wires 23c and sealed attheir outer extremities, sufilciently iar irom the inside oi the tube to avoid injury, by the sealing compound 36, e. g., sealing wax, lead or a suitable heat resistant resin. These and other ieatures in the design oi the tube as shown are not part oi the invention claimed in this application, but are covered in my other co-pending applications.

The electrodes 22c shown in this case are of the form shown in Figs. 9 and 10. In these, as already described above, the lead wire 23a is welded to the metal bow 32, and the ends of the metal how are in turn welded to the ends of the electrode structure proper Ii. This assembly is placed in the tube as shown with the bow 32 transverse to the axis of the tube, so that a high frequency coil it placed around the tube will induce a heating current in the short-clrcuited path through the electrode body proper and the metal bow. By this arrangement it is possible to heat to a very high degree the cathode body filled with activating material, even if oi large size and thus a quick degassing and preliminary activating process may be efiected. The eddy current produced in the bow 32 or the area surrounded by it are proportionately higher than they would be ii induced in the cathode body ll For the same purpose, if a disc 32a is used instead of the metal bow 32, it is slotted as indicated at 31 so as to force the induced current to flow through the cathode body proper. Instead of a ring or disc, any other suitable auxiliary construction based on the same principle may be used to make possible quick and efficient inductance heating of the electrodes.

The cathode body 3| itself is a compact metallic conductor of porous and absorbent quality so as to be able to receive the activating material. It may preierably be made oi twisted thin wires, closely arranged laminae, telescoped cylinders, ribbons and the like. It is important, iurthermore, that its surface has an uneven and incoherent poorly heat conducting quality which will result from the above-mentioned structures.

Furthermore, the activation material, besides its electron emissive quality, shows a definite heat and electrical insulating property. 'This result is obtained in the present example by combining suitable compounds of strongly electropositive metals such as barium oxide, barium azide, strontium boride, etc., with refractory insulating materials such as zirconium oxide, beryllium oxide, aluminum oxide, etc., which are refractory at electrode operating temperatures. These refractory compounds should, moreover, be of such material that they do not spoil the emissive quality of the electro-positive metals, e. g., by too high a degree of acidity.

In the third place, an addition may advantageously be made of reducing means, as ior example azides, cyanides, hydrides, and other easily dissociating compounds of magnesium, calcium,

assaera nickel. Also pure metals capable oi reducing the electro-positive compounds; e'. g., magnesium, calcium, nickel, etc., may be used as the reducing material.

The final mixture may consist oi 40% to 70% oi the first group (the electro-positive material) 20% to 50% of the second group (the refractory insulating material); and about 10- to 20% oi the last mentioned group (the reducing material).

,Other powdered metals, such as gold, silver, copper, etc., may also be included in this mixture. The electrode structure, or at least its suriace, consists of a reiractory metal capable oi withstanding substantially without evaporation the temperatures to which the electrode is subjected and one which will not destroy the emissivity 'oi the activating mixture by forming non-emissive compounds with the electro-positive material. Nickel, .cobalt, chromium, copper, or mixtures and alloys oi these metals have been iound most suitable in this latter respect, and pure tungsten, molybdenum, and tantalum less suitable although they may be chosen because oi their wires and projecting points is more especially I described and claimed in the copending application, Serial No. 397,427, filed October 4, 1929. By combining these various ieatures, these electrodes even when spaced a considerable distance apart, and on application of low voitages.'

e. g., the ordinary supply voltages of 220, 150 or even volts, give an initial glow discharge which by the overheating of distinct points oi the electrode surface transforms itseli rapidly into an arc.

The tube shown in the example of Fig. 8, is designed primarily for operation with mercury vapor at high pressure. Since under high pressure operating conditions, the heating of the electrodes is very great, partly because oi the high average temperature prevailing in the tube to maintain the vapor pressure, partly because oi the constriction of the arcing spot to a veryv narrow area, and, in the case of A. C. operation, partly because of the increased anodic heat generation, the electrodes should be dimensioned and designed to dissipate heat from the arcing point with sufiicient rapidity to avoidoverheating of the electrodes. At the same time it is im-' portant that the electrode be maintained at a I temperature sufilciently high to give full eiiect to its activation. In this example, and particularly with an envelope of quartz or glass of about 1 mm. thickness, a length of about 12 centimeters, a diameter of about 2 centimeters and an operating pressure of 1 to 3 atmospheres mercury vapor and a current loading of about 6.5 amperes at the maximum in the gas filling and about 3 amperes operating load aiter evaporation oi the mercury, I have found that the electrodes should have a surface area between about 1 and V square centimeters per ampere oi current loading. With the electrode at the bottom of the tube and especially it its support e; g'.. the how 32, is close to the wall oi the pole vessel and the latter uninsulated, the loading ratio may be increased to about 8 amperes per square cenfor this auxiliary electrode may be similar to timeter, or if for other reason heat dissipation from the electrode is more or less increased the loading' ratio likewise may be more or less increased. In the example illustrated a ratio of dissipating heat from the electrodes during operation. Tins is attended by special advantages, as'more particularly set forth and claimed in my Patent No. 2,047,390, dated July 14, 1936.

Although one of the important advantages of the electrode according to my invention is that it is capable of starting without auxiliary devices and converting an initial glow discharge into an arc discharge, nevertheless, it should be understood that my invention is not inconsistent with the use of pre-heating devices; and in some cases, as for example where it is not practicable to use auxiliary electrodes, there is advantage in such an arrangement. The pro-heating of the electrode produces thermionic emission of sufilcient energy to ionize the gas and to overcome a space charge effect which would prevent the formation of an arc from a cold electrode. In such a case, after the first discharge is established, the operation is substantially identical with that already described for cold-starting self-heating electrodes. In order to obtain the non-disintegrating activated arcing electrode, however, whether or not pre-heating is used, the special activation and the structural characteristics of the electrode as described above are important.

In Fig. 11, I have shown such an electrode. In this case a spiral tungsten resistance wire 38 is embedded in a refractory insulating material, such as pure beryllium oxide or zirconium oxide, within the metal thimble 39. To the outer surface of this thimble is secured a porous absorbent structure, e. g., of twisted wires, mesh, etc., similar to that already described in connection with Figs. 9 and 10; and this is filled with the activating materials, e. g., as already described above. The heater filament 38 is connected to a lead wire d0, which as shown is insulated to prevent any discharge short-circuiting the filament; and

the thimble 39 is connected to the main lead wire 23 Thus a heating current can be passed through the electrode by means of these two lead wires preliminary tothe establishing of the discharge. After the electrodeis heated the heating circuit can be disconnected or, if for any reason the heat dissipation of the electrode is excessive, the heating may be continued to a greater or less extent. In the latter case, the lead wire 40 may serve as the principal lead and the heater resistance may in such case be in series with the discharge. Ordinarily, however, such heating is not to be recommended.

In Fig. 12, I have shown another modification in which an activated electrode 29y is used near the principal electrode 220. With this arrangement the initial discharge may take place as an alternating current discharge and with a more rapid ionization of the gas and, therefore, a more immediate conversion to an arc and transfer of the discharge to the main electrodes. The circuit that shown in Fig. 2.

Inorder to obtain the fullest advantage from the novel electrode according to my invention it is important that they be properly activated by a suitable treatment. In Fig. 13, I have shown one apparatus designed for the activation treatment of the electrode. In this case the tube is shown diagrammatically at 28 and is connected by means of a tube through the reflux condenser 42 to a high vacuum exhaust pump, not shown. A cooling fluid is circulated through the condenser 43 by meansof a connection 45. Means are provided for heating the tube 28 while thus connected to the pump, and this is indicated diagrammatically in this figure as a multiple jet gas heater 46.

The tube with its electrode mounted therein and coated with a mixture such as is described above, is connected to this apparatus and an ample amount of mercury is provided in the tube. The tube is then baked in a suitable heating chest or heated up by any other suitable means. During this heating the exhaust pump is kept constantly working and at the same time the electrodes are advantageously heated up to at least 200 C. above their subsequent operating temperatures, e. g., by means of a high frequency inductance coil. In this way gases occluded in the walls or the electrodes are driven out and at the same time a strong evaporation from the mercury serves to wash away these gases from the lamp and towards the pump.

This vapor fills the lamp at a substantial pressure, e. g., of several millimeters and is continually carried off together with the impurities into the condenser 43 from which the impurities go on to the pump while the mercury condenses and flows back intothe tube.

The final activation of the electrodes is effected by a discharge between the electrodes; and this, as already stated above in connection with Fig. 7, may be carried out while the tube is still connected to this apparatus.

In Fig. 14, I have shown another example of my invention which embodies also special features more particularly described and claimed in the co-pending application, Serial No. 397,427, of myself and Oskar Gadamer. The tube shown in this figure is designed particularly for operation with fixed gases or low pressure vapors and, therefore, uses the conventional steam seal. The electrode in this case, as inthe other embodiments described above, is mounted on a single.

a coiled wire with activation engaged in the interstices between adjacent turns of the wire and the ends of the wire welded or otherwise securely connected to the ends of a short-circuiting bow 32L Instead of this, a loose structure of fine wires, as shown for example in Fig. 15, may be used. This may be made of a pl urality of fine wires loosely twisted or interlaced together and some of which may be cut at spaced points and pulled outwardly to provide fine points on which the initial discharge may concentrate with high current density to give rapid heating and conversion to an arc, as already described in connection with'the other figures. This loose barium oxide or rangement, giving the general appearance of a crownof thorns, not only provides interstices in which the activation material such .as barium may be. held inplace mechanically, but also gives these wires'thermal isolation which assists in the rapid heating and conversion to an arc, while at the same time guarding against complete destruction of the electrodes if one of the fine wires should be .burned through bya temporary overload. This construction is more particularly described and claimed in the said co-pending application, Serial No. 397,427.

The ends of the ring shown in Fig. 15 will, of course, be welded either directly to the lead-in wire or advantageously to a connecting strip to which the lead-in wire also is welded.

As another example of asuitable activating, I treatment which may be used with an electrode of this type the tube after preliminary evacuation and degassing may be filled with a rare gas, such as argon, to a pressure of about 2 mm. and a suiilcient amount of vided on or adjacent the electrodes to take up the gases released by final activation of the electrodes. The tube may then be sealed oil and connected to a 220 volt alternating current with suitable ballast and a discharge started and burned thereon at a current loading of from 3 to amperes. With such loading the cathode loops are heated to incandescence and the other activation materials suplied to the electrodes is at least partially reduced. Also the getter-material, e. g., magnesium, is to some extent, at least, evaporated and serves to take up the gases resulting from the reduction and thereby to purify the rare gas used as a discharge atmosphere.

In the example shown in Fig. 16the electrodes are mounted upon two lead-in wires 23 which are of suillcient size to carry the current required for activation of the electrode by resistance heating. These two lead-in wires are shortcircuited outside of the tube after the activation treatment so that in this case as in the other examples shown the arc may burn symmetrically upon the electrodes and the arc current may be led away from the arcing point in both directions.

The electrode in this case comprises a carrier wire or core of a refractory metal, such as nickel, platinum or tungsten,-upon which is placed an activating substance, comprising, for example, a mixture of 50% barium oxide-hydrate, 25% calcium oxide, 13% zirconium oxide and 9% zinc oxide, to which may be added a substance or substances which will give it a porcelain-like characterto enable it better to adhere to the electrodes after a preliminary heating or other treatment. Barium fluoride to the amount of .compound or a less emissive metal will prove satisfactory.

Where tungsten or other more or less electronegative materials are used for the carrier wire there is advantage in separating the reduced emissive metal from the surface of the carrier. This may be done by plating with nickel or by winding the nickel wire on the tungsten carrier wire; or the refractory. oxide may serve as the separating layer between the electropositive' J emissive metaland the carrier.

getter-material pro- The carrier wire of the electrode may be of braided or oppositely wound wires as shown in the drawings, or may be a core wire onto which is wound another wire, e. g., a core of tungsten with a winding of nickel wire; and in general the surface of the electrode should be made as rough as possible. The activating substance is applied to this carrier wire structure and is engaged in the interstices between the adjacent wires or ad-- Jacent turns of the wire winding.

Auxiliary anodes I are mounted on separate 'lead-in wires, as shown in the drawings, and

serve first for carrying a direct current discharge during the activation process and later during the normal operation of the tube one or both may serve as an auxiliary electrode for facilitating the starting of the discharge and thedispelling of any space charge over the cathode.

After the electrodes have been assembled the tube is exhausted and subjected to galvanic glowing by means of a resistance current passing through the electrode carrier wire and to bombardment in rear gases, etc. Activation of the cathodes is performed by bringing them to a bright red incandescence, e. g., approximately 1000 C. or higher and permitting a direct cur-; rent of 2 amperes to pass for about 5 minutes between the electrode being treated-and an auxiliary electrode acting as an anode. Preferably the auxiliary electrode at the other end of the tube is used during this treatment. After each of the electrodes has been treated in this way, inert gas is introduced into the tube at low pressure and as pure as possible and at the same time a getter substance, such as magnesium, may be introduced into the tube to combine with impurities in the gas. About 3 mm. of mercury may be placed at three different points in the tube, e. g., at the ends and at the bend, to later produce the desired mercury pressure.

Advantageously, the getter-material in the form of magnesium powder is mixed with the electrode activating substance. In this way the getter-material serves to effect the desired reduction to produce free electron-emitting metal about 3% may be usedfor this purpose. These -materials, of course, are specified only as exhighly emissive metal and a more refractory on the electrode without release of gaseous products of reduction to act as impurities in the gaseous atmosphere of the lamp.

This reduced free metal on the electrode is observable after glowing ,and bombardment by the appearance of dark grey metal patches or points on the surface of the cathodes.

Further activation may be effected in the presence of a rear gas, such as argon, at about 1 to 2 mm. pressure. After both cathodes have been activated by direct current bombardment the discharge, which may be started by means of a high frequency, may be allowed to take place between the two main electrodes. Instead of using the high frequency, the discharge may ordinarily be started also by applying the potential of one of the main electrodes to the auxiliary electrode associated with the other main electrode. This step produces, as already de- -metal strip or other attachment may be'-.con-.

nected inside or outside of the tube and with high ohmic resistance between the main electrodes.

Once the alternating discharge is established between the main electrodes, the high impedance auxiliary path carries negligible current and it is, therefore, not necessary that it be disconnected. After final activation treatment by operatic of the discharge the tube is filled with an inert or rare gas at low pressure, e. g., argon at 2 mm. pressure (in which case the spacing betweenthe auxiliary anode and its associated cathode will be about 3 mm.)

The device may be operated, for example, on

' carries a current of about l-2 amperes more or less depending upon the final gaseous pressure.

I have found that any impurity in the rare gas may inactivate the cathode by combining with the emissive metal to form non-emissive or less emissive compounds, so that it is important to take every reasonable precaution to establish and maintain the highest purity in the rare gas or other filling material used within the tube.

In Fig. 17, I have shown an example in which each of the electrodes is provided with an auxiliary anode, and the main electrodes are of a simpler but less advantageous form than those shown in the other examples. In this case the electrode comprises a metallic core or support on which is provided a layer of highly emissive material whose electrons work function is less than three volts and which does not attack the is in the form of an oxide, the amphoteric oxide may be formed by reaction between these; and the material ofthe shell will difiuse slowly through the coating. Instead of this and more advantageously, however, the coating may be made of a mixture of an emissive substance, such as barium oxide with an amphoteric compound such as aluminum oxide, zirconium oxide or nickel oxide. It is possible that the amphoteric oxide may unite to some extent with the emissive oxides to form compounds such as barium aluminate, barium zirconate, barium nickelate, etc. However this may he, a cathode constructed in this way is particularly suitable for use in tubes of this kind because it is resistant to ion bombardment and. is not subject to mechanical disintegration by crumbling away. The refractory oxide so far protects the emissive substance that, with this combination, even such highly emissive substances as caesium and rubidium have been satisfactorily used for the cathode material.

The materials used for their emissive property may be supplied to the coating in the form of oxide or alsobecause oi. improved ignition and reliability they may be'supplied in the form of.

compounds such as hydrides, nitrides, cyanides,

azides, etc., which decompose by heating and free the metal. The metal is protected against evaporation and sputtering from the electrodes by the refractory oxides.

Instead of mixing the refractory amphoterlc oxide with the highly emissive material, and es- .pecially in cases where the core or support is made oi! tungsten, platinum or molybdenum an intermediate layer of the refractory oxide may be coated onto the core and it, in turn,covered with a layer or highly emissive material; or similarly, instead of the'amphoteric oxide a coating of the corresponding metal may be used to sep arate the highly emissive material from the tungsten, platinum or molybdenum support and this coating will presumably be oxidized as already suggested above. This feature is particularly describedand claimed in the copending application Ser. No. 351,368, filed March 30, 1929.

The electrodes 22k, in this case as in the other examples illustrated, have no internal means for heating but are dependent entirely upon the heating received from the gaseous discharge, either from the main discharge or from the auxiliary electrodes 29k.

As clearly shown in the drawings, each of the main electrodes 22k have positioned adjacent thereto an auxiliary electrode 29k: which is connected to one side of a supply circuit. The leadin wire 23k from the main electrodes are also connected to the supply circuit, one of them having in series therewith an inductance coil 3110 serving as ballast for the discharge in the lamp. A high resistance 33k is connected in series with each of the auxiliary electrodes so that after starting by the auxiliary discharge and ionization of the gas within the tube the discharge will pass by preference between the main electrodes.

When this tube is first put into operation by closing the switch which connects the supply line the electrical potential is only sufficient to start an auxiliary discharge between the main electrodes 20k and the auxiliary electrode 297:: as indicated at 48. These auxiliary discharges gradually heat the electrodes 2210 until the main discharge takes place as indicated at 49.

In Fig. 18, is shown an example somewhat similar to that of Fig. 16. In this case. as in Fig. 16. the electrode is formed of a carrier wire capable of heating by a resistance current and the opposite ends are connected to two lead-in wires. After the electrode has been subjected to a suitable preliminary activation treatment by resistance heating, its lead-in wires are short-circuited by a connection 50 outside of the tube. The activating coating materials and the activation treatments in this case may be substantially similar to those already described in connec tion with the other examples.

In this case as in the case illustrated in Fig. 17 both electrodes are heated by auxiliary discharges but in this case a relay 5| is provided to open the auxiliary discharge circuit after the main discharge has started. With this arrangement the resistance 33m may be much lower including an oxide of an activating metal, plac- This application is a continuatlon in part of my prior applications: Serial No; 744,206, filed September 15, 1934; Serial No. 643,502, filed November 19, 1932; Serial No. 558,148, filed August 19, 1931'; Serial No. 397,427, filed October 4, 1929; Serial No. 387,986, filed August 23, 1929; Serial No. 351,368, filed March 30, 1929; and Serial No. 241,035, filed December 19, 1927.

What I claim is:

The method of activating a refractory elec trode for use asan are heated cathode in a radiant electrical discharge device which comprises the steps of placing in contact with the surface of the refractory cathode an activating material trode to an intense impact of a discharge until the activating material is converted so as to exhibit a. dull metallic appearance at the surface, and, during the progress of and after this final conversion step, exhausting the envelope to remove any remaining impurities and oxygen, and aisoievaporating vaporizable metal in the enve- HANS J. SPANNER.

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