US 3366825 A
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VACUUM GAP DISCHARGE DEVICE HAVING GROOVED ELEGTRODES FOR THERMAL INSULATION Filed July ll, 1966 Jan. 30, 1968 J. M. I Ar-'FERTY 3,366,825
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United States Patent O ABSTRACT F vTHE DISCLOSURE Vacuum arc devicesincluding a pair of primary arc electrodes adapted to sustain an arc therebetween. Improved current carrying capacity results from arc dispersal means at the periphery of one arc electrode, including an annular slot at the electrode periphery defining a peripheral ridge which isuniformly heated to vapor emission temperature to avoid formation of anode spots.
The present invention is a continuation-impart of my copending applicationyser. No. 422,373, filed Dec. 30, 1964 and assigned to the present assignee, now abandoned.
This invention relates to improved vacuum gap devices and particularly those of the triggered vacuum gapand triggered vacuum switch type and to improved electrode configuration therefor.
Vacuum switches and vacuum gaps, particularly triggered vacuum gaps have recently undergone great im provements. A number of such 'devices generally referred to as vacuum gap devices are presently of important commercial considerations. vIn most of these vacuum gap devices a limiting factor as to the current carrying capacity lies in the ability of the anode to withstand destructive melting caused by the formation of intense anode spots. Similar destructive melting also occurs lat the cathode eletcrode, although the vanode destruction is the most extensive. f Accordingly, it is an object of the present invention to provide improved vacuum gap devices having electrode configurations which facilitate the carrying `of eX- tremely high currents without destructive melting ofthe anodes thereof.
A further object of the invention is to 'provide triggerf able vacuum switch and fixed vacuum gap devices in whichl the electrodes are adapted to carry high values of current between arc initiation and arc interruption' without destructive melting thereof. v
A further object ofthe present invention is to provide improved vacuum gap devices which are' suitable for utilization over a greater number'of circuit interruptions or arc estabilshments than devices of the prior art.
In accord'with one feature of the presentinvention I provide a vacuum 'gap device including a pair of primary electrodes defining therebetween a vacurn gap. These electrodes are located within an evacuable envelope which envelope is composed partially of metal and at least in part of `a high voltage insulating lsubstanfce so as to provide electricalv isolation between the primary electrodes to prevent sh'ort circuliting of the gap defined tlhereby. InV one embodiment of my invention the man gap electrodes are provided with an annular groove closely adjacent to the external periphery thereof, which groove provides a portion thereof which is, as far as thermal conduction characteristics are concerned, partially isolated from the remainder of the main electrode in order that the isolated portions thereof may achieve a highertemperature caus- 3,366,825- K Patented Jan. 30, 1968 ICC 2 y ing a substantially `uniform emission'ofvapor. therefrom and thepreventionl of -destructive`spots.7In accord with a further feature ofthe present invention the electrodes ofthe wacuum'devices thereof lare so shaped'afs to puoi/ide a central portion at which the gap'between'theprima'ry electrodes is larger than the gap lat the peripheryJ thereof so as to facilitateA the location of the arc discharge' between the peripheral portions of the 'electrodes at which the thermally isolated portions are located.Inaccord with a further feat-ure of the present invention a trigger electrode is located within the central portion of at least one of'the electrodes and is-connected so as to cause the `injection of a highly ionized gaseous plasmaintoy the gap between the main electrodes tofcause thel breakdown thereof-upon the initiation of a voltage pulse thereto. rThe novel features believe-d characteristic'of thepr'esent invetnion are set forthin the appened claims. IThe invention itself, together with further objects andv advantages thereof may'lbe more readily understoodliby a reference to the appended drawing in which:
FIGURE 1 is a vertical cross-sectional view of a fixed gap triggerable vacuum gap device constructed in accord with the present'invention, and i FIGURE 2 is a vertical cross-sectional view of the trig`` ger electrode utilized inthe device 0f FIGURE 1'.- 'i The vacuum gap device of FIGURE f1 includesv an envelope represented generally as 1` containing ttherein a pair'of primary arc electrodes 2 andS'deni'ng therebetween a primarygap 4. A trigger'electrode 6'havin'g a trigger gap 7 is concentrically located withina central aperture 8 of electrode 2 and extends to substantially to the center of the primary gap 4. Envelope 1 is comprised of a substantiallycylindrical metallic ymember 9 having a central bulbous portion 10 which. is symmetrical `along the longitudinal axis thereof,
a llirst flanged metallic end piece 11 having a central aperture thereinand a second yflanged metallic apertured end'plate 12 also having a central aperture therein. A pair of substantially cylindrical, centrallyk apertured `insulator bushings 13 and v14 each of which has' a large outside diameter` portiony and a smaller outsider diameter portion connected by an annular shoulder serve "as 'insulating members for the device and `are assembled with 'the smaller outside diameter portion thereof extending in'- wardly with the annular shoulder `resting against the planar exterior surface of aperturedlend`plates`11y and 12 respectively. The lcentral apertures'within insulator bushings 13 and` 14 are closed by the insertion therewithin of annular cylindrical electrode support members `1'5and 16r respectively; The interior of cylindric-alelectrode sup port member 15 is'hollow and'is sealed to vaccurn by the insertion therein and sealing thereto 'of ltrigger electrode assembly v6. Each'of the central cylindrical aper-l tures in bushings 13 and 14 are slightly larger than the outside diameterof electrode support memb ers15 and 16 forvadded insulation properties andtoprovide an insulator surface -not 'likely to be short circuited by conf densed metal vapor. A vacuum seal is madefbetween these membersby 4the brazing thereto of `annular seal members 17 and :18 respectively each of which lhasqa larger diameter portion which fits. tightly over-the outside diameter ofthe respective insulator bushing' and a smaller diameter portionjlwhich lits tightlyy over the exteriorporltion ofthe respective electrode `support member.
The-rnaterials comprising the devices illustrated f in FIGURE 1 are substantiallyfasfollows. Main envelope member 9 may be a metal which willwithstand high temperatures and which possessesv sufficienty physical strength as to constitute lthe outside 'body of thede'vice, as for example stainless steel. `Flanged end plate 'members 11 `and 12 and flange seal members 17 and 184a1e` comprised of a material which makes good vaccum-tight seals to ceramic bushings 13 and 14 and to metallic members 9, 15 and 16 respectively and may conveniently be a Fernico or Kovar alloy generally utilized in electric discharge devices for this purpose. Bushings 13 and 14 are comprised of a high temperature g-as impermeable ceramic material, as for example high density alumina (94% or higher or A1203). Electrode support members 15 and 16 are preferably composed of a highly conductive substantially gas-free material, as for example OFHC copper Premium Grade. No special precautions need be taken to render this material gas-free, other than vacuum firing prior to assembly. Primary electrodes 2 and 3 are composed of highly purified copper or other high vapor pressure material, 4as for example any of the matrials .set forth in Lee and Cobine Patent No. 2,975,256 issued Mar. 14, 1961. This material is rendered substantially free of `all gases and gaseous-forming compounds by vacuum melting, as for example by repeated zone reining steps .so as to reduce the concentration of gas and gas-forming impurities therein to a figure of less than one part in 106.
Electrodes 2 and 3 are preferably provided with at least one arc dispersion means 19 although only the anode need be so provided. Arc dispersion means 19, in the illustrated embodiment comprises an annular slot which is cut from the arcing surface 8 of electrode 2, for example, so as to provide a vertical surface 20 and a thermally substantially isolated annulus 21 on electrode 2. The dimensions of arc dispersion slots 19 is related to the material from which the electrodes are formed and to the gap distance. In general the slot should be located far enough in from the periphery so that the peripheral isolated region so formed is raised to a vaporemittin'g temperature without destructive boiling of vapor at too rapid a rate. The depth of the slot is not critical but is related to the thickness of the electrode body. Naturally, the slot depth should be sufficient to substantially increase the surface-to-volume ratio'of the peripheral region and to s`o constrict the path of thermal conduction as to allow for the controlled heating of the peripheral region as described hereinbefore. For a given geometry, as the chosen electrode material, the width of the slot 19 varies indirectly with vapor pressure and directly with thermal conductivity of the material. As one example of dimensions, for a 21/2 diameter, 1/2" thick electrode of copper, with a gap width of Mz operated on l60 c.p.s. alternating current voltage, slot 19 was Ms" wide and 1A, deep and the width of the peripheral region 21 Was M3". Arc dispersion means may take other forms. Thus, for example, a series of slots may be drilled in a circular pattern along a radius only slightly less than the radius of the electrodes. Preferably, such slots should be interconnected to provide for arc rotation and for a uniform dispersion of the arc. Alternatively, a plurality of annular slots could be c'ut at different radial distances from the center of the arc to create several peripheral vapor emitting ridges.
The trigger electrode assembly 6 of FIGURE 1 of the drawing is illustrated in greater detail in vertical cross section in FIGURE 2 of the'drawing. In FIGURE 2 the trigger is composed of a hollow cylindrical metallized ceramic base member 22 which has a narrow circumferential groove 23 cut therein, which may be tapered as shown, near the inward extremity thereof. Means for sealing the trigger electrode in a vacuum-tight seal to the interior of electrode support member 15 are provided in the form of an annular flange member 24 having an extended shoulder and an apertured collar member 25 which rests thereupon and is sealed thereto. Both of these members are conveniently constructed of a metal suitable for .making metal to ceramic seals, as for example one of the general class of metal alloys known as Fernicos. A first hollow cylindrical shield member 26 having a first counter-bore 27 therein and a second counterbore 28 of greater diameter also therein rests upon collar member 25. The smallest diameter of cylindrical member 26 makes electrical contact with metallized ceramic cylindrical member 22. A lirst metallic sleeve member 29, which is beveled at one end thereof to fit the bevel in the annular groove 23 in ceramic cylinder member 22, slides over member 22 and is aligned with the inward or lower bevel of the annular groove therein. A second cylindrical sleeve 30, which is likewise beveled at one of its ends to match the groove 23 in cylindrical member 22, slides over ceramic -member 22 and is aligned with the outward or upward bevel of groove 23 therein. A metallic cap and shield piece 31 having a re-entrant portion 32 .and a central aperture 33 rests upon the upper end of ceramic cylindrical member 22 and Within the interior end of second sleeve member 30. A trigger lead wire 34 centers the central aperture within ceramic trigger support member 22 and is affixed to the re-entrant end cap 31 at the uppermost portion thereof. This lead may conveniently be of nickel or any other material conventionally utilized for providing lead wires in electric discharge devices. Members 24 and 25 are conveniently selected as a Fernico or equivalent. The remaining metallic members 26, 29, 30 and 31 are conveniently selected as a refractory metal, as `for example, tungsten or molybdenum. Appropriate vacuum-tight seals are made to the mating surfaces members of all of the ceramic and metal parts by conventional, well-known techniques. When assembled, the trigger electrode assembly is lowered into the open end of electrode support member 15 and rests upon a counterbore shoulder therein where ange member 24 is sealed in a vacuum-tight seal thereto. Prior to nal assembly the portions of metallic sleeve members 29 and 30 which are to be exposed between the upper portion of cylindrical shield member 26 and cylindrical end cap and shield member 31 are coated with a material, as for example a hydride of titanium, zirconium, hafnium, yttrium, erbium or other rare earth metals which hydrides serve as a source of ionizable gas which m-ay be emitted upon the initiation of a voltage pulse between sleeve members 29 and 30.
The device of the present invention may be fabricated in accord with standard ceramic and metal tube technology in which case the device is assembled and the various members thereof are sealed into assemblies and sub-assemblies and the device is finally sealed leaving only plug 35 unsealed. The device is then placed in an outgassing furnace and raised to a temperature of, for example, 800 C., and held at this temperature for one or two hours in order to cause outgassing and the removal of sorbed gases from all of the constituent parts thereof. After suflicient outgassing, and while the device is still at an elevated temperature an atmosphere of, for example, hydrogen is introduced through plug 35 and the device is sealed. Since, in the case of a titanium hydride -coating upon met-al sleeves 29 and 30, a large fraction of the hydrogen has been evolved therefrom, hydrogen is taken up from the furnace atmosphere to recharge the trigger electrode and to cause the establishment of a vacuum within the sealed envelope. In order to enhance this operation a reservoir for an active gas, as for example hydrogen, may be provided in the form of reservoir 36 which -comprises two larger metallic, preferably .stainless steel annular washers 37 and 38 with an interposed smaller outside diameter w-asher 39 of the same material. The assembly of these three washers creates an annular cavity 40 which is filled with granular active gas absorbing material, as for example titanium 41, the opening thereto being covered with a wire mesh 42 which may conveniently be 200 mesh stainless steel screen. Since, in the case of titanium hydride used as a gas reservoir, the titanium hydride of the reservoir is substantially exhausted of hydrogen by the outgassing process, after the device has been sealed by plug 3S and the device cools to room temperature substantially all of the remaining hydrogen gas within the device is taken up by the titanium to form titanium hydride. Since the maximum concentration of hydrogen in the titanium hydride at room temperature is substantially represented by the formula TiH1 73 a suicient quantity of titanium is provided within reservoir 41 to ensure that not all of the hydrogen which could be absorbed thereby is present within the envelope prior to sealing. Thus, a hard vacuum of at least -5 millimeters of mercury is provided by the absorption of hydrogen by the trigger electrode and by the reservoir.
An alternative method for fabricating devices in accord with the present invention is substantially disclosed and claimed in my copending application Ser. No. 417,562 tiled Dec. 11, 1964, now U.S. Patent No. 3,331,961 assigned to the assignee of the present invention. In accord with the method disclosed and claimed in my copending application, the materials are assembled unsealed and the charged material utilized for the trigger electrode and the reservoir are initially supplied within the device. The device is then placed in an atmosphere of the active gas contained within the reservoir, as for example, hydrogen in the case of titanium hydride, and the temperature is elevated to a temperature to sufficiently outgas the component parts of the device and to simultaneously cause pure hydrogen to be evolved from the trigger electrode and from the reservoir to further purge the device of all impurities other than pure active gas. The temperature of the device is then further raised to cause sealing of the ceramic and metal parts together after which the sealed device is allowed to cool to room temperature to provide the requisite vacuum of less than 10*5 millimeters of mercury.
The operation of a triggered vacuum gap in accord with the present invention and as illustrated on FIGURE 1 of the drawing is quite similar to that of the triggered gaps of the prior art as is disclosed with great particularity in my Patent No. 3,087,092, issued Apr. 23, 1963. Briefly stated, this action is substantially as follows. Electrodes and 16 are connected to a source of high voltage, as for example, a primary power transmission line to be protected from overvoltages. A high electric eld is therefore established within gap 4 between electrodes 2 and 3. When, because of a high voltage transient which could damage the equipment to be protected, a preselected pulse of voltage is supplied between trigger lead 34 and electrode 2. Since electrode 2 is connected to the lower sleeve 29 abutting the triggered gap and the trigger lead is connected to the upper sleeve 30 abutting the trigger gap, a transient high voltage is applied across the trigger gap. Due to the low breakdown potential required at the interface between metal and ceramic, a spark discharge is init-iated between sleeves 29 and 30. The heat of this discharge immediately causes the evolution of an active gas, as for example hydrogen from an active gas storing substance, as for example titanium hydride, which comprises the coating upon these two sleeves. As the hydrogen is evolved the hydrogen molecules are ionized, thus causing the arc discharge across the triggered gap to be intensified, causing the release and ionization of a large quantity of hydrogen gas. A pulse of a hydrogen ion-electron plasma is thus injected into the gap between the main electrodes 2 and 3 and the main arc is broken down within a matter of microseconds or less.
Since the initial breakdown between primary electrodes 2 and 3 occurs in the immediate vicinity of the trigger electrode where the path length between the two electrodes is long and since the vacuum arc is essentially vapor starved, the natural tendency of the arc is to conserve vapor causing a propultion of the arc out into the smaller gap-length region between the two electrodes. This operation so far is conventional.
In my experimental work I have determined that the prime cause of the failure of vacuum gap devices, as for example the triggered vacuum gap illustrated in FIGURE l of the drawing is the destructive melting of the anode electrode by virtue of the formation of anode spots. As used herein an anode spot is intended to indicate the footpoint of the arc between the primary electrodes where it is anchored to or terminates at the anode electrode.
Under normal operation of a vacuum arc discharge the anode electrode is surrounded by a space charge of negative charge carriers, namely electrons. Electrons which are attracted to the positive or anode electrode must fall through the potential represented by the space charge surrounding the anode. At the anode, all of the discharge current is carried by electrons impinging thereupon. This electron bombardment of the anode is primarily responsible for anode heating. Anode heating is produced both by the kinetic energy of the incident electrons corresponding to the anode voltage drop and by the heat of condensation of the electrons. As the arc current between the primary electrodes is increased the density of the space charge sheath surrounding the anode increases, thus increasing the anode potential drop. This increases the energy of electrons impinging upon the anode, causing its temperature to further increase.
As the current density is increased further, a point is eventually reached at which a given local area of the anode, randomly selected because of anode geometry and other considerations or by instabilities in the arc is suciently heated as to cause the release of metal vapor. This metal vapor is immediately ionized by the high velocity electrons striking the anode. This is because, in all but the very shortest of vacuum arcs the anode potential drop exceeds by several times the ionization potential of the metal vapor. The ions created by such ionization tend to neutralize the space charge in the localized area of the anode emitting metal vapor. This neutralization of the space charge causes the anode voltage drop at the localized area to decrease causing more electron current to tiow t0 the anode in this area. A cyclic reaction thus takes place with a continued build-up of localized anode current, 4re- Y sulting in a hot spot at the localized portion and a constriction of the arc at the anode. This is known as an anode spot. The intense local heating produced by the anode spot causes destructive localizing melting of the anode. If the vacuum discharge device is operating on unidirectional current, the damage can be great upon the formation of the spot. Even if the device is operating on alternating current and only one-half cycle of time elapses between striking and re-striking of the arc, the anode spot formed within this time can cause destructive erosion of the anode electrode.
The utilization of the electrode structure of the present invention greatly diminishes or eliminates such anode spot formation. Thus, in accord'with the present invention the anode is caused to emit vapor over an extended area in a controlled fashion. This is achieved by providing a large area of the anode electrode (or of both electrodes if there is some uncertainty as to the electrode which will be the anode upon striking of an are) which so emits to prevent localized `spot formation. In the illustrated embodiment of the invention shown in FIGURE 1 of the drawing, the entire peripheral region 21 of the electrode is caused to so emit. As is described hereinbefore, conservation of vapor principles tend to propagate the arc to the exterior portions of the electrodes. When the arc located between the peripheral regions of the mainA gap electrodes, peripheral region 21 is raised in temperature to a higher degree than the remainder of the electrode. This is due to two effects. Firstly, heating is increased because the surface-to-volume ratio of peripheral portion 21 is larger than the remaining portion of the electrodes since the impinging electrons may strike the grooved vertical surface, the top surface, and the vertical outside surface of the peripheral region of the electrode. Additionally, since the peripheral region of the anode electrode is connected to the main body of the electrode only by a restricted crosssectional area under the peripheral slot, the conduction of heat away from the peripheral portion of the electrode is less than would be in the absence of slot 19. Accordingly, peripheral region 21 becomes uniformly heated to a temperature to cause metallic vapors to be emitted at the edges of the outer ring unifor-mly around the electrode, thus preventing the formation of a single anode spot with destructive melting and destruction of the anode electrode. Further accentuation of this effect may be achieved by the location of a radial magnetic field normal to the longitudinal axis of the device of FIGURE 1 which tends to rotate the arc current conducting path circularly around the electrode, thus further reducing the probability of the formation of anode spots.
Although one embodiment, namely a peripheral groove, has been illustrated and discussed in detail it will be appreciated that other equivalent means to achieve the results contemplated by the present invention may be utilized. The necessary criterion to be met is that the outer periphery of the electrode be heated to a higher temperature than the remaining portion of the electrode. This is achieved by causing increased surface-to-volume ratio for that portion and by causing a restricted thermal conduction path between that portion of the electrode and the main portion of the electrode. As mentioned hereinbefore this may be achieved by other expedients than the cutting of a peripheral slot as is illustrated in FIGURE 1.
' Although the vacuum gap device of the present invention has been illustrated and described with reference to the specific embodiment of a fixed gap triggerable device, the advantages of the present invention may be equally achieved by the incorporation of the disclosed electrode structure in a fixed gap device which is not triggerable or in a vacuum switch of the circuit interruptor or recloser type in which one electrode is fixed and the other is movable from a circuit closing position to a circuit breaking position and in such devices whether the discharge is triggerable or whether the interrupter is of the conventional vacuum type.
While the invention has been described in detail herein in accord with certain preferred embodiments thereof, many modifications and changes therein may be effected by those skilled in the art. Accordingly, it is the intention in the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.
What I claim as new and desire to secure by Letters Patent of the United States is:
1. A vacuum gap device comprising: lan envelope evacuated to a vacuum of at least 5 mm. of mercury and having at least a portion thereof composed of a high voltage insulator; a pair of primary electrodes constituting an anode electrode and a cathode electrode disposed within said envelope and adapted to define therebetween a breakdown gap, at least said anode electrode having arc dispersion means at the periphery thereof sufficient to inhibit the formation of destructive anode spots, said peripheral region of said anode electrode being a portion ofthe arcing surface thereof, being substantially uniformly in poor thermal contact with the remainder of said electrode and having a high substantially uniform surfaceto-volnme ratio, such that said peripheral region under arcing conditions becomes uniformly heated to a temperature at which arc sustaining vapor is uniformly e-mitted; and means for connecting said electrodes in circuit with an electrical load.
2. The vacuum gap device of claim 1 wherein both electrodes are fixed to define a fixed dimension gap.
3. The vacuum gap device of claim 1 wherein at least one electrode is movable between circuit making position in contact with said other electrode and a circuit breaking position out of contact with said other electrode and defining a gap therewith.
4. A vacuum gap device comprising: an envelope evacuated to a vacuum of at least 105 millimeters of mercury and having at least a portion thereof composed of a high-voltage insulator; a plurality of primary electrodes constituting an anode electrode and a cathode electrode disposed within said envelope and adapted to define therewith a breakdown gap, at least said anode electrode having arc-dispersion means at the periphery thereof sufficient to prohibit the formation of destructive anode spots, said peripheral region of said electrode being a portion of the arcing surface thereof, being in poor thermal contact with the remainder of said electrode and having a high surfaceto-volume ratio, such that said peripheral region under arcing conditions becomes uniformly heated to a temperature at which arc-sustaining vapor is uniformly emitted therefrom; trigger means associated with one of said electrodes and comprising a trigger electrode having therein a secondary gap and having located immediately adjacent said gap a quantity of a material having chemically bound therein a quantity of an active ionizable gas which is thermally evolved therefrom upon the initiation of a spark breakdown across said trigger gap; means for connecting a primary voltage between said primary electrodes and means for delivering a pulsed voltage to said trigger electro-de independent of the primary voltage to cause breakdown of the primary gap.
5. The vacuum device of claim 4 wherein both primary electrodes are fixed to define a fixed gap.
6. The vacuum device of claim 4 wherein one primary electrode is fixed and the other primary electrode is movable between a closed, contact-making position and an open contact breaking position.
7. The vacuum device of claim 4 wherein both primary electrodes are substantially circular discs with substantially flat arcing surfaces and the arc-dispersion means comprises a peripheral annular slot in the arcing surface of at least said anode electrode.
8. A vacuum gap device comprising: an envelope evacuated to a vacuum of at least 10-5 millimeters of mercury and having at least a portion thereof composed of a high voltage insulator', a pair of primary electrodes constituting an anode electrode and a cathode electrode disposed within said envelope and adapted to define therewith a breakdown gap, at least said anode electrode having an annular slot in the arcing surface thereof near the periphery thereof so as to define a peripheral surface arcing region of very small radial dimension as compared with the radial dimension of the entire electrode and having a high surface-to-volume ratio, said peripheral region under arcing conditions becoming uniformly heated to a temperature at which arc-sustaining vapor is uniformly emitted therefrom; means associated with one of said electrodes to inject a gaseous plasma into said primary gap upon receipt of a pulsed signal and comprising a trigger electrode having thereon a trigger gap, and a quantity of a hydride of an active metal immediately adjacent to such trigger gap so that hydrogen gas is thermally released therefrom upon the initiation of a spark breakdown across said trigger gap; and means for connecting said primary electrodes in circuit with an electrical load; and means for supplying a source of pulsed voltage to said trigger electrode to cause the breakdown of the primary gap.
9. The vacuum device of claim 8 wherein both primary electrodes are fixed to define a fixed gap.
10. The vacuum device of claim 8 wherein one primary electrode is fixed and the other primary electrode is movable between a closed, contact-making position and an l open contact breaking position.
11. The vacuum arc device of claim 8 wherein the hydride utilized to store hydrogen is titanium hydride.
12. The vacuum arc device of claim 8 wherein the hydride utilized to store hydrogen is yttrium hydride.
13. A vacuum gap device comprising: an envelope evacuated to a vacuum of at least 105 mm. of mercury and having at least a portion thereof composed of a high voltage insulator; a pair of primary electrodes constituting an anode electrode and a cathode electrode having substantially planar arcing surfaces disposed in closed juxta- 9 l0 position Within said envelope to define therebetween a References Cited breakdown gap, at least said anode electrode having an UNITED STATES PATENTS annular slot in the arcing surface thereof at the periphery thereof, said slot isolating a peripheral surface arcing 311631734 12/1964 Lee' region of very small radial dimension as compared with 5 the radial dimension of the entire electrode and having FOREIGN PATENTS a high substantially uniform surface to volume surface 1,306,918 9/ 1962 France.
ratio, said peripheral region under arcing conditions becoming uniformly heated to a temperature at which arc- JAMES W, LAWRENCE, Primary Exmminer. sustaining vapor is uniformly emitted therefrom to` prevent the formation of anode spots, and means for connectlo S- SCHLOSSER R JUDD, ASSSG'H Examinersing said electrodes in circuit with an electrical load.