US2073599A - Electric discharge device - Google Patents

Description

I 4 Maich 9, 1937. L, MAL ER 2,073,599

ELECTRIC DISCHARGE DEVICE Filed Jan. so, 1955 4 Sheefs-Sheet 1 'INVE/VTOE! v v .56 TTUE/VEW March 9; 1937. L. MALTER 2,073,599

ELECTRIC DISCHARGE DEVICE Filed Jan. 30, 19:55 1 4 Sheets-Sheet 2' l LouiaMa/Lter;

- -Divan/70B.-

March 9, 1937. L. MALTER I 2,073,599

EL ECTRIC DISCHARGE DEVICE I Filed Jan. 30, 1935 4 Sheets-Sheet 5 n IL zaoq u eooy g L. MALTER ELECTRIC Di SCHARGE DEVICE Filed Jan. 30, 1955 4 Sheets-Sheet 4 Patented Mar. 9, 1937 ELECTRIC DIS Louis Malter, Camden,

CHARGE DEVICE N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1935, Serial No. 4,049

6 Claims.

invention relates to electric discharge devices and more particularly to devices of the type wherein amplification of a primary electron stream, such, for example, as is emitted from a thermionic cathode or from a photo-sensitive surface exposed to light, is accomplished through utilization of the phenomenon of secondary emission.

If an electrode is subjected to electron bom- 10 bardment', it will emit secondary electrons. The ratio of the number of secondary electrons to the number of primary electrons depends, in'part, upon the character of the surface and upon the potential difference between the surface and the source of the electrons. This ratio can be made considerably greater than unity. For example, a ratio of three or more secondary electrons to one impinging electron is readily obtainable with metallic surfaces treated in known ways and subjected to discharges at potentials of 300 to 400 volts. Since the emitted electrons exceed the impinging electrons in number, the electrodes emitting them, hereinafter, will occasionally be referred to as multiplying electrodes.

If the secondary electron current, in turn, is caused to impinge with sufiicient velocity upon a further electrode with a suitably treated surface, the ratio of secondary emission from the second multiplying electrode may also be greater than unity. Hence, one is able to obtain with n multiplying electrodes in cascade, for example, an amplification of the original or primary electron current equivalent to the amplification per electrode raised to the nth power. A million-fold amplification has been obtained in a single device.

Another disadvantage of the electrostatic type of multipliers heretofore described and exemplified by French Patent 582,428 is the fact that the field in the neighborhood of the multiplying electrodes, which serves to draw ofi the secondary electrons, is necessarily weak in magnitude. As a consequence, due to the small magnitude of this field, it becomes impossible to draw large currents 5 from these electrodes. An attempt to overcome this defect by increasing the potential of the succeeding electrode with the idea of increasing the field at the neighborhood of the preceding multiplying electrode generally results in a decreased gain due to the fact that some of the primary electrons which would normally strike the multiplying electrode are drawn past it to one of the following electrodes. As a consequence, due to this space-charge limitation of current,

it is impossible to maintain linearity between the input current or current at the source and the output current, except for extremely weak currents.

In my device the accelerating electrode and the multiplying electrode are entirely distinct and, consequently, it is possible to set the value of the accelerating field-independent of the-potential with which electrons strike a multiplying electrode. The magnitude of the current which may be drawn from any multiplying electrode is determined only by the potential of the electrode directly above itself and increasing this potential exercises practically no effect upon the amplification obtained in the succeeding stage.

It is, accordingly, an object of my invention to provide an electric discharge device, utilizing secondary electron emission, wherein maximum gain per stage is obtained and linearity exists between input and output.

Another object of my invention is to provide an amplifier or electron multiplier of the secondary electron emission type in which the secondary electron stream from each emitter is concentrated and is directed accurately to the desired target, and interference between the various secondary electron streams is minimized. Another object is to provide an amplifier or electron multiplier of this type in which there is practicallyno loss of secondary electrons and in which the best conditions for amplification or other desired results can easily be obtained by external potential adjustments.

Another object of my invention is to provide a secondary electron amplifier or.multiplier'which is eflicient and reliable in operation and in which v the amplification obtainable is very great as compared with the amplification obtainable with a thermionic amplifier of the usual type.

Another object of my invention is to provide a device of the type described that may be used for substantially any purpose for which thermionic tubes of present types are, used, such, for example, as an amplifier, a demodulator, an oscillator, a combined oscillator and modulator, etc;

Another object of my invention is to provide a combined phototube and amplifier that shall be responsive to the very highest frequencies encountered in television transmitting apparatus.

A still further and more specific object of my invention is to provide a device of the type de-- scribed that lends itself readily to mass production methods.

In accordance with my invention primary electrons from a photo-sensitive cathode or a thermionic cathode are caused, by. means of a magnetic field, to impinge upon an electrode surface,

capable of secondary emission, at a velocity sufficiently high to produce secondary emission having a ratio greater than unity to the primary electron stream. Substantially all of the secondary electron stream thus produced is directed against another similar surface, at a still higher positive potential, whereat further secondary electrons are produced. This process is repeated a number of times within the same container and the final, greatly amplified, stream of secondary electrons is collected by an output electrode.

Further, in accordance with my invention, I utilize a' single magnetic field for concentrating and directing all of the electron streams to the proper target electrodes, whereby the major portion of each secondary electron stream is utilized and the primary electrons are prevented from being drawn past the targets to impinge directly upon the output electrode.

More specifically stated, my improved electric discharge device is constituted by an evacuated container, preferably, though not necessarily cylindrical, wherein are disposed a plurality of sets of discrete electrodes, the electrodes lying in spaced apart planes parallel to each other and to the long axis of the container. Although, strictly speaking, the container in which the electrodes are mounted does not have to be oriented in any particular mannerduring operation, I find it convenient to style the multiplying electrodes as lower and the electrodes opposite to them, either as upper or accelerating electrodes. It has been found that good results are obtained if the multiplying electrodes are provided with a photo-sensitive surface.

Means are provided for impressing suitable potentials on the electrodes and for establishing a magnetic field which threads the container, the lines of force being parallel to the electrode faces and transverse to the long axis of the device. By reason of the magnetic field, when suitable potentials are applied to the electrodes, elec- 45 trons, which otherwise would be drawn to the accelerating electrodes, are caused to travel in trochoidal or cycloidal paths and to fall upon the next adjacent multiplying electrode.

The novel features which I believe to be char- 60 acteristic of my invention are set forth with particularity in the appended claims. Myinvention itself, however, both as to its organization and method of operation will best be understood by reference to thefollowing description taken in 55 connection with the accompanying drawings, in

which:

Figure l is a view in perspective of an electric discharge device constructed according to my invention, portions of the container wall and the 60 associated magnetic field structure being broken away to more clearly illustrate the disposition of the electrodes, R I

Fig. 2 is a view of an analogous device constructed according --to my invention, wherein the 65 source of primary electrons is thermionic and wherein a grid is provided to control the electron flow therefrom,

Fig. 3 is a diagrammatic sectional view of the cathode and control element in the device shown 70 in Fig. 2,

Fig. 4 is an end view of the device shown in Fig. 2 looking toward the left, illustrating my preferred means for establishing a magnetic field parallel to theelectrode surfaces,

75' Fig. 5 is a'diagrammatic view exemplifying the ing the electrode firmly in position.

manner in which the device shown in Fig. 1 is operated,

Fig. 6 is a diagrammatic view illustrating the manner in which the device shown in Fig. 2 is operated,

Fig. 7 is a diagrammatic view of an alternative deviceconstructed according to my invention,

Fig. 8 is an end view of the device shown in Fig. 7, and r Figs. 9 and 10 are graphs exemplifying certain characteristics of my improved-device.

In all figures of the drawings, equivalent elements are similarly designated.

Referring now to Figs. 1 and 5 of the drawings, a preferred embodiment of my invention is constituted by a cylindrical, evacuated container I of glass or Pyrex wherein is disposed a plurality of photo-sensitive lower electrodes 3, lying in the same plane and spaced apart along the long axis of the container and a plurality of upper or accelerating electrodes 5 disposed in a plane parallel to and spaced from the plane in which lie the first mentioned electrodes. The lower electrodes are preferably made of silver and the upper electrodes of molybdenum, tantalum, nickel, or any other metal which is easily de-gassed and not easily oxidized.

The upper and lower electrodes are grouped, vertically, in pairs. All of the electrodes have substantially the same dimensions and they are spaced apart substantially the same distance along the long axis of the container. Such configuration is desirable in that it permits accurate electron-path-control by means of a single magnetic field.

It is, of course, possible to individually mount each electrode in the container, but the labor involved in securing parallelism and alignment of them is excessive. Accordingly, I find it expedient to first construct the upper and lower sets" of electrodes 'as sub-assemblies and, subsequently, to introduce each sub-assembly as an unit into the device. These,sub-assemblies are physically identical, insofar as their construction is concerned, and a description of the set of upper electrodes is applicable to the lower set.

upper surface thereof. During the operation of welding the straps to an electrode, the rod II is forcibly biased againstthe. mica sheet, thus lock- Each cross rod has a lead l5 welded thereto, which lead extends through the wall of the container to the exterior thereof. Each lower cross-rod (not shown) is also provided with an output lead H.

An output electrode I9 is mounted in one end of the tube, preferably fairly closeto the sets of electrodes and in a plane transverse to the tube axis. The output electrode is provided with a terminal connection 2| which extends through the wall of the container to the outside thereof.

In manufacturing my improved device, the several sets of electrode sub-assemblies and the output electrode are first suitably mounted in the container which is then heated and evacuated.

After evacuation, oxygen is introduced into the container at a pressure in the neighborhood of 1 mm. of mercury. One of the lower accelerating electrodes is then made substantially 500 volts positive with respect to its corresponding lower electrode to cause a glow discharge which oxidizes the surfaces of the lower electrode. The oxidation is carried on until the lower surface acquires a bluish-green tinge. Each pair of electrodes is treated in the same manner and the process is continued until all of the lower electrodes have acquired an oxide surface.

After the lower electrodes are oxidized the residual oxygen is pumped out of the container and an alkali metal is distilled into it. For this purpose sodium, caesium, rubidium, or potassium may be utilized, although I prefer caesium. The container is next baked for about ten minutes at a temperature of 210 C. which causes the alkali metal to combine with the silver oxide, thus giving rise to a highly photosensitive surface. During the baking step the excess caesium is pumped out of the device.

During the heating process just mentioned any caesium or other alkali metal which is deposited upon the upper electrodes or upon the container forms a relatively stable compound with the excess caesium and prevents it from being redeposited on the inner walls of the container where it would provide leakage paths between the electrodes.

Any convenient means may be utilized for estab-' lishing a magnetic field parallel to the electrode surfaces, such, for example, as the device partially shown in perspective in Fig. 1 and in end elevation in Fig. 4. Preferably this device is constituted by an U-shape element 23 of magnetically permeable material on which is mounted an energizing coil 25 and to each upstanding portion of which is afilxed a plate 21, also of permeable material. The tube is disposed between these plates in such position that a substantially uniform magnetic field is set up parallel to the opposed surfaces of the sets of .electrodes. Obviously, a permanent magnet may be substituted for the electromagnet shown, or the tube may be disposed within a coil of wire carryingan electric current.

As heretofore mentioned, it is necessary to maintain each of the lower multiplying electrodes positive with respect to the primary electron source and with respect to any electrode between it and the said source. For this purpose, referring now to Fig. of the drawings, the first lower electrode 3 at the left, farthest from the output electrode l9,'may be connected to the negative terminal of a source of unidirectional potential, exemplified in the drawings by a resistor, and. the output electrode may be connected to the positive terminal of the source through an output device such as a resistor 3|. Starting from the electrode connected to the negative terminal, the next adjacent electrode 3, or first multiplying elec-' trode, may be connected to apoint 33 on the resistor somewhat more positive and each of the remaining multiplying electrodes 3 connected to successively more positive points 35 and 31 on the resistor.

The first upper accelerating electrode 5, paired with the first lower photo-sensitive electrode, may

be connected to a point 39 on the resistor more positive than the point 31 to which the last multiplying electrode is connected and the successive accelerating electrodes connected, respectively, to points 4|, 43 and 45 on the resistor still more positive, as clearly shown in the drawings.

It is not to be inferred from the foregoing description of Fig. 5 that I am limited to the exact in fact, it has been made to operate when the upper electrode is at the potential of the electrode directly beneath it. The amplification in a multi-stage tube under the last set of conditions is not the best obtainable.

If a fluctuating beam of light, such, for ex ample, as that obtained by passing light through a moving photographic sound record thereof or obtained by scanning'a motion picture film, is caused tofall upon the first lower electrode 3, photo-electrons will be emitted from this surface proportionally thereto. These photo-elem trons will be accelerated toward the upper electrode 5 directly above the first lower electrode because of the electrostatic field and, if no magnetic field were present, they would impinge thereon. However, the magnetic field (not exemplified in Figs. 5, 6, and 7), which is parallel to the planes of the electrodes, subjects the electrons, moving toward the accelerating electrode, to a force component at a right angle to their instantaneous direction of travel. The direction of the component depends upon the polarity of the field. If the electric anrl magnetic fields are adjusted to proper values, the electrons will describe trochoidal paths and will strike the first multitribution, the maximum velocity being determined by the color and the nature of the light and upon the work function of the emitting surface.

The photo-electrons are emitted in all directions. Nevertheless, by reason of the presence of the magnetic field, the electrons emitted from a single point, even with varyingvelocities which differ by considerable amounts from zero, will be focussed to substantially a single corresponding point on the next succeeding electrode.

will be diverted and be focussed upon the third' lower electrode. Here again, a multiplication, by reason of secondary emission, is secured and this process is repeated in any number of stages until the amplified stream of secondary electrons is collected by the output electrode and caused to drawings by the resistor-included between the out- -flow in a utilization circuit exemplified in the put electrode and the positive terminal of the potential source.

.To simplify the drawings, only three multiplying stages are shown in Fig. 1. It is to be understood, however, that as many stages as desired may be incorporated into the device.

As hereinbefore pointed out, I may utilize a thermionic primary electron source, if desired, instead of a photo-electric source, for the purpose of rendering my improved device capable of uses to which well-known thermionic tubes are put. For example, referring now to Figs. 2, 3, and 6, an alternative embodiment of my invention is illustrated wherein a controllable electron source is substituted for the first lower photo-sensitive electrode shown in Fig. 1. Specifically, I prefer to provide the container with a depending neck portion l! in which is sealed a press of the usual type which supports a virtual source of electrons in the plane of the multiplying electrodes. The electron source may be constituted by a metallic thimble 49,,the upper end of which has a layer of electron emissive oxides 50. The thimble is completely surrounded and shielded by a cylindrical metallic grid structure 5| which terminates in a perforated cap 53. The upper face of the cap lies in the plane of the multiplying electrodes 3 and, preferably, the perforation therein is coaxial with the emitting portion of the cathode, and is covered by a fine screen 55 to which it is electrically connected. This grid structure, when supplied with proper potentials, either direct or fluctuating, serves to control the emission from the thermionic cathode in the same way as emission from the first photo-sensitive lower electrode 'in the device shown in Fig. 1 is caused to be con-.

trolled by variations in the light impinging thereon.

As will be clearly apparent from a close inspectionof Fig. 6 of the drawings, the potential distribution on the various electrodes and the cathode of the tube shown in Fig. 2 may be the same as that exemplified by Fig. 5. Obviously, any desired input circuit elements may be connected between the grid and the cathode of the tube, exemplified in the drawings by an input resistor 51, a grid biasing potential source 59 and potential divider 6|.

A still further modification of my invention is shown in Figs. 7 and 8. The device'illustrated corresponds in function to the device shown in Fig. 1.' It differs from that device, however, in that the upper, set of accelerating electrodes is replaced by a single film 63 of resistive. material and the lower set of photo-sensitive electrodes is replaced by a similar film of resistive material which has been photo-sensitized. These resistive films may be formed by sputtering a metal film onto a strip of a non-conductor, such as mica, or in any other desired manner.

By means of an external source 61 the two' ends of the lower film 65 are maintained at different potentials. In the same manner a potential difference is maintained between the two ends of the upper film 63, by a suitable potential source 69 connected thereto. The negative terminal of each source is connected to the left end of the corresponding film as it appears in the drawings, thus making the said left ends at lower potentials than the right ends. By means of a further external source H the left end of the upper film electrode is maintained at a higher potential than the left end of the lower electrode. This automatically insures'that each point on the upper electrode is at some potential above the point on the lower electrode immediately beneath it.

As a consequence, when light is caused to impinge upon the lower electrode at a point adjacent the left end thereof, photo-electrons emitted therefrom will be accelerated toward the upper electrode but, by reason of the magnetic field (not shown), parallel to the plane of the electrodes, the photo-electrons will be forced to travel in a curved path and to strike the lower electrode at a point the potential of which is higher than that of the point of origin. Such being the case, secondary electrons will be emitted from the lower electrode which, in turn, will traverse similar paths and, upon again returning to the lower electrode, .will give rise to a still greater number of secondary electrons. This process is repeated indefinitely until the final or output current is collected by the output electrode l9 which is maintained at a positive potential with respect to the upper electrode by means of a suitable potential source 13.

The final output current from an electric discharge device, constructed according to my invention, in addition to being proportional to the number of electrons emitted from the primary source, is also dependent upon the potential of any one of the multiplying electrodes with respect to the next adjacent electrode which is normally maintained at a lower potential. For example, referring now to Fig. 9 of the drawings wherein points along the Y axis represent final output current in milliamperes and points along the X axis represent the potential of the first multiplying anode with respect to the electron source, it will be noted that as the potential of the first multiplying electrode is increased from zero the output current increases proportionately until a certain critical potential is reached, after which it decreases. This is explained by reason of the fact that the electrode under discussion actually has negative resistance, the current thereto decreasing as the potential is varied from zero to the'critical value which, of course, is tantamount to saying that the secondary emission therefrom increases over this same range. This phenomenon is also apparent when the potential impressed on any other of the multiplying electrodes is varied, or when a varying potential is impressed upon several of the electrodes. My improved device, therefore, has negative resistance over the range between zero potential and the critical potential, the multiplying stages subsequent to the electrode, the. potential of which is varied, serving merely to amplify the current changes referred to.

' I am not entirely prepared at this time to state the exact theory upon which this phenomenon of negative resistance depends. It seems to be controlled by" the velocity with which the electrons strike the electrode under discussion and upon whether all of the electrons from the preceding surface or source do or do not strike the said electrode. Over the critical range of potentials referred to, the over-all amplification of the device will rise by reason of the fact that the number of secondaries per impinging primary electron increases or, in other Words, the current, to the electrode decreases and, also, by reason of the fact that all of the primaries will be focussed upon the electrode. It is my belief that if the potential of the electrode exceeds the critical value some of the electrons from the preceding surface or source may skip the electrode and strike the following one, thus resulting in decreased over-all amplification. Perhaps, also, when the potential is raised above the critical value which, in an actual device constructed by me according to the disclosure in Fig. 1 of the drawings, is 320 volts, some of the electrons from the preceding surface fall closer and closer to the edge of the electrode nearest the electron source and eventually miss it entirely.

It also is possible that, as the potential on the electrode is raised to correspond to the potential of the next higher potential multiplying electrode, the velocity of the electrons striking the next electrode diminishes. Obviously, if this is the case the secondary electronsdriven out from the succeeding electrode decrease in number. I, of course, do-not intend to be bound by these theories of operation but I am able to state with certainty that the actual tube under discussion operates as described to give decreasing amplification with increase of potential upon the first multiplying electrode above the critical value.

The fact that the multiplying electrodes of -my improved device exhibit negative resistance leads at once to the conclusion that the device will operate ina mannersimilar to that of the wellknown dynatron oscillator. ring to Fig. '5, if a parallel-tuned circuit '15 is inserted in the potential supply lead connected to the first multiplying electrode, or to any one ofthe succeeding multiplying electrodes, and the potential thereon is properly adjusted, the device will vigorously oscillate and the oscillatory current will be amplified by the succeeding multiplying electrodes.

The device shown in Fig. 2 of the drawings may also be made to oscillate in the manner just described or oscillations may be obtained by replacing the input and output resistors, shown in Fig. 6, by suitable inductors coupled to. each other magnetically in proper phase. means may be employed, if desired, for feeding back a portion of the output energy to the input circuit. 4

In Fig. 10 of the drawings, I have exemplified the grid. potential-output current characteristic of the tube shown in Fig. 2 of the drawings. Since this curve has a reasonably sharp bend, it will be at once apparent that the device may be utilized as a detector by properly choosing the static bias applied to the grid.

Either of the devices shown in Figs. 1 and 2 may be utilized as combined modulator-oscillators by introducing the modulatingpotentials into a lead which supplies potential to one of the multiplying electrodes while it is in the oscillatory state.

Any two or more of the phenomena heretofore described, such as detection, amplification, oscillation, etc., may be caused to take place in a single tube by proper choice of potentials, etc.

Since the above described'modifications are perfectly obvious to those skilled in the art no necessity is seen for complicating the drawings by illustrating them. V

An electric discharge device constructed and operated according to my invention has many advantages. By utilizing a single magnetic field 'to concentrate and direct the electron streams to their pre-.-allocated targets, the major portion of each stream will impinge upon the succeeding electrode under conditions favorable for stable operation. By reason of the facts that the ac-- That is to say,'refer- Other suitable- 'lized for directing the electrons along fairly definite and predetermined paths, the spacechargelimitations of prior devices are avoided. The efiiciency thus obtained is much greater than.- where dependence is placed solely upon electrostatic-fields between adjacent electrodes for the purpose of both accelerating the electrons and directing them to the secondary emitters. This, for the reason that the potential gradient which is favorable to the escape of the emitted electrons is adverse to and acts as a retarding field for the impinging electrons. However, by providing a uniform magnetic field parallel to and extending along all of the emitting surfaces, the impinging electron stream is concentrated and directed to the emitting surface and, at the same time, there is maintained at the surface an electrostatic potential gradient which is favorable for removing the emitted electrons with maximum eificiency. Substantiallyno interference or interaction between the high velocity impinging electrons and the low velocity secondary electrons has been observed and it seems highly probable that the magnetic field causes the emitting electrons to move out of the stream of impinging electrons as soon as the emitted electrons attain any substantial velocity.

It follows from the foregoing facts that, in my improved device, the outputcurrent is not limited by space-charges. As a result my improved device has no saturation point, the amount of output current which it can provide being dependent only upon the amount of heat which the electrodes can dissipate and upon their resistance to destructive electrostatic forces, the potentials applied to the electrodes and the strength of the magnetic field.

Although I have shown and described only a few specific embodiments of my invention I am fully aware of many modifications that will at once be apparent to those skilled in the art to 'by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. An electric discharge device comprising an elongated container, at strip of insulating material mounted in a plane parallel to the long axis of the container, a pair of spaced apart sets of discrete electrodes, the electrodes of one set being paired with the electrodes of the other set, the electrodes of one set being supported in substantially uniform spaced apart relation on said insulating strip and presenting their electrode surfaces entirely exposed to the electrodes of the other set.

2. An electric discharge device constituted by an elongated container in which are disposed a plurality ofsets of'spaced apart discrete electrodes, the electrodes of one set being paired with the electrodes of the other set and the electrodes of each set being substantially uniformly spaced apart along the long axis of the container, the container being provided with an angularly dis posed depending portion in which is supported a primary-electron source, and control means for said source disposed in a plane substantially parallel to the plane in which lies one of the said sets of electrodes. d

3. An electric discharge device constituted by ,an elongated container in 'which ;are disposed a plurality of setsof spacedapart discrete electrodes, the electrodes of one set being paired with the electrodes of the other set and the electrodes oi each set being substantially uniformly spaced apart along the long of the container, the container being provided. with an singularly dls posed depending portion in which is supported a thermionic electron source, and control means for said source disposed in a plane substantially parallel to the plane in which lies one of the said sets of electrodes.

4. The invention as set forth in claim 1 where- I in one of the exposed electrode surfaces of the set supported by said insulating strip is a primaryelectron emitter and the others of the exposed surfaces in the said set emit secondary-electrons when subjected to bombardment by said primary electrons.

5. An electric discharge device comprising an elongated container, a pair of insulating strips.

mounted in parallel planes on opposite sides of the long axis of said container, a pair of spaced apart sets of discrete electrodes, the electrodes of one set being paired with the electrodes of the other set, the electrodes of one set being supported in substantially uniform spaced apart relation on one of said strips and the electrodes of the other set being supported in similar, array on the other of said strips, the electrodes of one set presenting their electrode surfaces entirely exposed to the electrode surfaces of the other set.

6. The invention as set forth in claim 5 wherein the exposed surfaces of the electrodes sup ported on one of said insulating strips are electron emissive surfaces and the surfaces of the electrodes supported on the other of said electrodes are non-emissive.

LOUIS MAL'IER.

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