US2477506A

US2477506A - Internal shield for vapor electric devices

Info

Publication number: US2477506A
Application number: US55204A
Authority: US
Inventors: Winograd Harold
Original assignee: Allis Chalmers Corp
Current assignee: Allis Chalmers Corp
Priority date: 1948-10-18
Filing date: 1948-10-18
Publication date: 1949-07-26
Anticipated expiration: 1966-07-26

Description

July 26, 1949. H. wlNoGRAD INTERNAL SHIELD FOR VAPOR ELECTRIC DEVICES Filed 00's. 18, 1948 .mwmwvw ma/wm Patented July 26, 1949 INTERNAL SHIELD FOR VAPOR ELECTRIC DEVICES Harold Winograd, Milwaukee, Wis., assigner to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application @ctober 18, 1948, Serial N0. 55,204

(Cl. Z50-27.5)

il Claimsn 1 This invention relates in general to improvements in electric valves, and in particular to means for controlling faults in anelectric valve using a vaporizable conducting medium, and is especially useful in connection with electric valves of the single anode type.

Electric valves are rated according to the current which they will deliverto a load circuit. A limiting factor in the current rating is the frequency of occurrence of faults such as `arc-backs and stray arcs. An arc-back may be defined as as flow of current in a reverse direction through the valve, due to the -formation of a cathode spot on the anode, and results in -a failure of the rectifying actionof the valve. A stray arc may be defined as a flow of current in the forward direction through the valve from the anode to the casing, and results in erosion o-f the casing.

In electric valves of the type in which the cathode is not insul-ated from the casing, stray arcs are created when the cathode spot is transferred from the cathode to the casing. Arcbacks occur in the same type of valve for reasons not clearly known, and may result in a current flow from the casing to the anode. Since these operating faults are more likely to occur at elevated temperatures and high vapor pressures, it has been customary to provide cooling means `for maintaining the valve temperature and vapor pressure within specified limits.

One form of cooling means heretofore used in the non-insulated cathode type of valve, comprises a cooling coil of small uniform pitch insulated from the casing and disposed within the casing surrounding the arc region. When so placed adjacent the Iarc region, the cooling coil carries olf heat transmitted to it from the anode and the arc by radiation, `as well as heat transferred to it by condensation of the vaporizable cathode medium on the surface of the coil. The cooling coils used heretofore have been so wound that adjacent turns define `a uniformly narrow gap which is relatively pervious to the flow of the vaporizable medium, but Which is substantially impervious to the formation of Iarcs between the anode and the casing wall.

The condensing action of the cooling coil is a function of the surface presented to the vapor, the rate of flow of the vapor over that surface, and the quantity and temperature of cooling fluid passed through the coil. However, with afixed surface area, a fixed rate of vapor flow, and a fixed temperature of cooling fluid, a saturation point is `reached yat which an increase in the flow of cooling fluid through thee-oil produces no appreciable increase in condensate, It is therefore desirable in the design of a cooling coil to pres-ent a condensing surface of `maximum area in the region of the highest vapor pressure and rate of flow. i

In a coil having la pitch defining a uniformly narrow gap between adjacent turns, the rate of flow of the vaporizable medium between adjacent turns is limited by the gap width. The `portion of the coil surface which faces inward toward the arc region lies in a zone of relatively high vapor pressure and unrestricted vapor iiow, and is fully effective in condensing the vapor present and returning it to the cathode. The balance of the coil surface which is not directly exposed to the arc region is much less effective in condensing vapor from the region of relatively high vapor pressure, because of the restricted gap through which the vaporizable medium must iiow to reach the balance of the coil surface. If the coil is rewound with a greater pitch defining a larger gap through which the vaporizable medium may iiow unrestricted, then the coil is no lon-ger impervious to stray arcs, or to flowiof arc-back currents from the casing. i t

Since the gap between adjacent turns is critical in an arc-impervious coil it has heretofore been necessary to construct the` coi1 with a uniform gap width, not exceeding specified maximum and minimum tolerances. This `construction necessitates the introduction fof spacers between turns and calls for a tolerance of manufacture far beyond that required in the present invention, in which the cooling coil is Wound With any width of gap, above a predetermined minimum value.

The present invention proposes to shield the casing from the arc region to prevent the formation of strayarcs and at the same time provide for an optimum rate of -heat transfer from the vaporizable medium to the cooling means, whereby the cooling means may effectively act as a surface Condenser to maintain a suitable vapor pressure in the valve. The present invention also proposes to shield the casing of the valve to pren vent the casing from participating as an electrode in the flow of fault current to the anode during an arc-back.

A barrier extending from the anode to the cathode coextensive with the cooling coil is clisposed between the cooling coil and the casing to provide this shielding without interfering with the coolingand condensing function of the coil. Such a barrier, yby confining an` arc-back to the space between the anode and the cathode, is instrumental Vin restricting thearc path. As a com sequence, the arc-drop voltage is increased with a resulting lower value of fault current through the valve.

It is therefore an object of the present invention to provide an electric valve of the vaporizable cathode type in which the aforementioned disadvantages' of"th`eprior art: are obviated, and in which the" aforementioned advantages are achieved.

Another object of the present invention is to provide an improved electric-valve of the noninsulated cathode type in which it is unnecessart7 to wind the cooling coil'with auniform pitch.

Another object of the invent-ion" i's4 to: provide an improved electric valve of'l the Vaporizable cathode type in which the are region' between the anode and the cathode is shieldedfagainst stray arcs and at the same time effectively cooled through the unrestricted transfer of heat to the cooling means.

Another Objectl (if 1 th'liv'erltii 1S' to pDVde anelectric'v'alv of the vaporizable'cathode type in' which the cooling'-y mearis' provides for unrestricted iiowftherethroiigh loithefvaporizable medium of the Valve for the condition of optimum thamediurn andthe coolan'k improved ele6tric"va1ve of the vaporizable .ii

cathode type; in* which the fca'sing of the valve is prevented'Afrom-parti patin'g'jaslananode in the flow vQfcurrentj during attivare-back;

Another object o ffthe" invention is to provide an improved electric'A valvel of the vaporizable cathode type inv which',v theca'sing-y ofthe valve is shieldedagainst theformationofstray arcs between the casingcand theancde.

Another objectv of theinvention is to provide an electric valve of the vaporizable cathode type in-which-arcbaclisare'confined-to the arc region between the' anode-'andthe cathode.`V Y

Another objectfof-theinvention is to provide an electric Valve'in which restricting'the-path of the arc-back current'is'instrumental in reducing the fault current owing through the' valve.

Another' object-ofthe invention is to provide an electric valve` in' Whiohthe cooling means is simpler` to-construct, cheaper to manufacture, and is more Yefcient'in operation.

4'I he novelV features of the invention and how the objects arerobtain'ed'will be apparent from this specification and the accompanying drawings showing several embodiments of the invention and forming partof` this specification and all the novel features are" intended tobe pointed out in theappended'claim's; V v

Fig. 1 ishavertical section" through a sealed electric Valve of` th'efsinglefjanode typeV in which a shield embodyingjtheinvention is shown surrounding the arc region;y

Fig. 2 is a fragmentary vertical section through a similar Valvein whichk the shield is mechanically supported-by the coljir'igcoil; Y

Fig. 3 is a fragmentaryview, partly in elevation andp'artlyV in-sfelction; of an alternate embodiment-of the shield shown' in Fig.v 1;

Fig; iis e; fragmentargfiferizentm"section shewing theelectrical headerofI the anode `baffle disposed between adjacent'pelrtsofthe shield;

Fig. 5 is a vertical section through one of the risers of the cooling coil showing the fluid-tight seal between the riser and the casing of the valve.

The invention lends itself to embodiment in any Well known type of valve with single or plural anodes, but for purposes of exemplification the invention is shown in Fig. lf-.einbodied in an electric valve II of the single anode type'. The electric valve here shown comprises a substantially cylindrical cup-shaped casing I2 which is preferably made inwhole' or in part of a conducting material, such as steel. The casing is closed at its open end by a disk-shaped anode plate or cover I3; theplatei-Ibeing dished upward at its outer rim to`f`ori'v a lip following the inner peripheral contourVV of the casing I2. The anode plate I3 is preferably made of steel and is sealed in fluid-tight relation to the casing I2 by any suitable means, as by welding the lip of the plate i3 to the casing I2. l

The anode plate: I3i cairres a; rha'inanode`l Ill which depen-ds'ther'efronilinto th" sin'g`I2C The anodev I l-is electrically? coiiee cal conducting shaft? If In'ounte A within an in'- sulating sleeve Iext'en'ding upv? "through the anode plate I3. Mea'risfo'r' sealing# the shaft I5 against the anode'p'late' I3 comprisesa laminated vitreous seal l1, Wlriih; iajddition t0' providing a huid-tight seal, alsof eieetrie'anyfinsularesthe Shaft I5 from-theead'plat; la'rdprov'des the Shaft i5A Wthth'A-'Iebes'sary mchaiicalspl port. As shown, the* shaftl I5- threads into a tapped axial bore inthe a'ri'ol Ill` A portion of the shaft I5 extendingabovetheanode plate I3 is tted With a colirl'g'ra'ditor"Igfordisslpating heat fromv the' anode I4Lk Vduring operation of the valve II.- lindrical in shape-aintlpeferably madeio'f'acnductive materials'uch' as graphite;` The e1ectrie x'falv'eV lvl ispovlied'witn aneth: ode I9 comprising afpdolofmer'curycr other suitable vapori'z'able'material in electrical Contact with the casing I 2. An upwardeextending'annu-L lar insulator 2-I,Vknown asiac'athode'rin'g and preferably made'` of a-materialisuch' as' quartz', bounds the activesurface-ofthe-".nreicuryg' The cathode ring ZI is" coagnally'mdiiiitedin any siite able manner oiifaradiallyrnotchdidisk 22 which in turn is carriedby'fthe bttomgko'fltliecasing`I2. A control electrodegpr giidlZSiis' ajrarig'edili spaced proximity tothe .anode llt-"for" controlling the new Vof current-j' betweerr thexar'iode I4 and the cathode'iS. Thez'ggliZBiis pref'eiably'nade of a conductive-3material*such as graphite, and is desirably ba'sket;sh'aped` t0 `f1loWthe`c0ritoir of the sinodale;v Numerous perforationsarefprovided in' thesi'de" ai'l'dbotto'i Walls'O'fLtlfieV grid-'.23 to permit the p'assageof't ejarcdis'charge'b'e'- tween anode; and icathodel Qand to" permit "the radiation therethrough 0f Heat from the surface ofY the anode4V Illduringfoifi'tibY of the" valve II The grid 231s prefere :f supp'orted" frmtne anodeV plate I3 bye; plurality;` ofdepehdingse'g mental insulators"`2fIjr only' brief 'ofwliichfisv shown. A control potential' maybeeiipress'd uponfthe grid 2s` through aaireieet @any insulated' fluidtight con'nection",l notf shown;- penetratiriglthe anode plate I3.'` Y

frnepeths-effthe are disiehargepetween anode e4 and cathode" la? and or the; new ormatenal vaporized kfrom the f cathode'J I 911 during] operation er the valve aifejce'rayrpueg vby meae'ef a dislieheped parte 2f' exi'aiiyidspesea' below-nie end 2s, andY by ef; eee-luie;meansV` aucuneid:

The baiiie 25 is preferably made of a conductive material such as graphite. The cooling means comprises a cooling coil 26 which encircles the arc region and extends below the disk-shaped baille 25. The cooling coil 261 is preferably made of stainless steel which prevents free hydrogen from passing through the pipe wall from the cooling fluid to the interior of the casing.

The coil 26 is preferably constructed of pipe of circular cross section, forming a passage for the flow of water or other cooling fluid. It may be wound as a single helix, or it may be wound as two helixes interleaved with one another and joined by a common connection. Assuming that the coil 26 is wound as a single helix, the pitch is so chosen that the gap defined by adjacent turns has any width above a predetermined minimum value so as to permit an unimpeded flow of mercury vapor across the turns.` The selection of a predetermined minimum gap width is an empiric one since it is mathematicallyinfeasible to make such a determination. Theoretically a straight pipe axially disposed within the casing would offer the least resistance to vapor flow but such a pipe would lack the condensing surface ren quired. A balance therefore is struck between the amount of condensing surface required and a gap which will permit substantially unimpeded iiow of the vapor. Since the maximum clearance is not critical, no spacers are required between turns to hold the gap to a maximum width.

The free ends of the coil 26 terminate in upward-extending risers 21 forleading the coil 26 through the anode plate I3. The risers 21 penetrate the anode plate I3 diametrically and are respectively sealed thereagainstby means of a nexible composite vitreous seal 23, as shown in Fig. 5. If desired, the risers may be provided with insulating means, such as insulating sleeves, not shown. In addition to sealing the risers 21 in fluid-tight relation against the anode plate I3, the seals 23 electrically insulate the coil 26 from the anode plate I3 and the casing I2. Protection against breaking the seals 28 is afforded by upward-extending sleeves 29 mounted in any suitable manner on the anode plate I3. An annular insulator 3I closes each sleeve and electrically insulates the risers 21 therefrom. An annular gland 32 cooperates with the insulator 3l to hold the insulator 3I against the riser 21. `Attached to the upper end of each riser is an elbow fitting 33 having a ange 34 for connection to a source of cooling uid, not shown.

` Referring to Fig. l it will be observed that the lower end of the cooling coil 26 supports the diskshapedbafiie 25. In turn the baille supports an excitation anode 35 insulated therefrom. A suitable potential may be impressed on the anode 35 through an insulated lead-in 36. Below the cathode I9, a starting device 31, preferably of the electro-magnetic type, momentarily connects the cathode pool I9 with the excitation anode 35 when it is desired to ignite an are at the cathode I3 to place the valve I I in service.

The shielding means comprises a cylindrical shield 38,` preferably made of metal such as steel, but which may be made of an insulating material, such as porcelain. The shield 38 encloses the coil 26 and extends substantially coaxially therewith from the region of the anode I4 to the cathode ring 2i. In this manner the shield 38 surrounds the arc region between the anode I4 and thecathodel. I f The shield; 38 is suspendedfrom the anode plate `I 3 by a plurality of insulators 39 `depending therefrom. An upper closure for the shield is provided by a disk-shaped cover 40 having a depending rim 4 I, the cover 40 being supported from the anode plate I3 by the segmental insulators 39. The shield 38 may be fastened to the cover 43 in any suitable manner, as by welding, as shown in Fig. 1, or as by screws 43 penetrating the rim 4I of the cover 40, as shown in Fig. 2. In the connection shown in Fig. 2, the shield 38 is separable from the cover 40 for ready access to those elements of the valve I I concealed within the shield `At its lower end, the shield 33 converges toward the cathode I9 to form a funnel 44 integral with the shield 33. Although the funnel 44 is here shown integral with the shield 36, the funnel 44 may be constructed as a separate element and fastened tothe bottom of the shield 38 in any suitable manner. It will be observed that the funnel 44 opens into the space above the cathode I9 enclosed by the cathode ring 2l. Thus positioned, the funnel 44 acts as a collector for `catching mercury condensed by the coil 26 and returning it to the cathode I9.

The shield 33 is axially notched as shown in Fig. 4 to incorporate the insulated lead-in 36 of the excitation anode 35 within the curved surface of the shield 38.

Referring `to Fig. l, it will be noted that the cooling coil 26 rests on a series of annularly-disposed metal pads 45 supported by the funnel 44. An electrical connection is thus made between the shield 38 and the cooling coil 26 at the lower end thereof. In this way, the coil 26 and shield 38 are maintained at the same electrical potential with respect to one another. "Since both the coil 26 and the shield 38 are electrically insulated from the anode plate I3, they are also insulated from the casing I2. In operation of the valve II, the coil 26 and shield 33 have a floating potential with respect to the cathode I9 which, as hereinbefore disclosed, is directly connected to the casing I2. Although the` shield 33 is electrically connected to one end of the coil 23 through the pads 45, it is spaced sufficiently from the coil 26 throughout its axial length to permit the free flow of mercury vapor around the turns of the coil 26 during operation of the valve II.

As shown in Fig. 2, an alternative method of supporting the shield 38 comprises hangingiit on the risers 21 of the coil 26 by means of clamps 46 which bolt to the shield 33 and securely grip the outer surface of the risers 21. When supported in this` manner, the shield 33 does not require the support furnished by the insulators 39 of Fig. '1. If the cooling coil 33 is to be utilized as a resistance heater during forming of the valve II, as described in Patent No. 2,459,057, to Harold Winograd, then the construction of Fig. l is preferred, because the heating current may be passed through the coil 26 without being shunted by the shield 33. The clearances between the shield 33 and each of the following elements, the

anode shaft I5, risers 21, lead-in 36, and cathode ring 2l, while suflicient to provide a gap therebetween for insulating andA degassing' purposes, are substantially impervious to the passage of stray arcs therethrough. It will thus be noted that the shield 38 together with the cover 46 provides an arc-impervious barrier surrounding the arc region of the valve II. How the shield 38 operates to prevent stray arcs is described as follows. u

Prior to placing thevalve II in operation, the valve is formed or desassed by evacuating the casing .l2 .and applyingA heat '.to :drive out vthe occlud'ed gasses. One method of forming itne "valve l l. is `described in Patent No. ...2.459.057 :to

'Harold Winograd. `Following dega'ssing :of ithe Vvvalve H, the casing l2 is sealed-oil andthe valve il cpr-:rated as a hermetically sealed unit.

In operation, the valve lil is started'by,energicing the starting device 5i .and ystrikingr an arc Vbetween the excitation anode and the cathode i9. -A Acathode spot is thusestablishedand ythe cathodeiil remains continuouslyemissive. `If:a

load circuit is now connected to the valve Il and 'the grid voltage properly adjusted vto -make l.the 'valve 'Ii conductive, an arc will beo'rmed'between 'the vaifio'ole i4' and cathode l@ through the mercury vapor contained in the vvalvelfl. The :arc

However` thefheat radi ated by the anode `lil is absorbed 'in the cooling coil 226, the perforations in the grid '23'allowing the anode M to radiate'directly to fthe coi-i125.

in addition, the coil absorbs heat 4radiated by Y .the arc between the anode le and the cathode itl -A How off water or other cooling `fiuid through the coil 26 absorbs the heat radiated to the coil.

Concomitant 'with the radiation of heat to the coolingcoil V26, is the transfer of'heat -to the coole ing coil 2t "by the condensationoi mercury vapor on the surface ofthe coil. During operation oi; the valve il there Iis a constant flow oi mercury vapor throughout the casing l2 and over vthe `turns oi the coil 2t. The pitch of the lcoil `2"6"is selected to faciiitate thisV flow between turn-s. En the absence vof a shield, `such as the shield 3B,

the mercury arc, at high' current `load's,*m-ight sometimes transfer from the oathodeifd to the casing i2, forming aV cathode spot ori the casi-ng wall. In Vsuch a case, since the gap between acl-jan cent turns oi the cooling coil'Zt is not impervious to the vpassage oi an arc between turns, 'the coole ing" coil it alone would offer no barrier `to the fonvmation or" 'stray arcs. By placing a shield such as :the casing i2, vthe passage cisti-'ay arcs between turns is electively blocked. The lshield 33 there 'tore forms a Substantially perfect' barrier against :thel formation of anarc between the aiiodeV if-i and the casing l2, and yet permits the mercury Y vapor to iow :freely over the turns of thecooling in which the Wall lll' oi the shield'is perforated throughout its axial length. it has been found that numerous small periorations in the shield lll permit easier removal of the gases liberated during the degassing process Without rendering the shield pervious to arc discharge during oper-aE tion of the valve. For a like reason a perforated shield, such as the shield 4i?, is preferred for use in a valve which is continuously connected to an evacuating means during operation.v

Although but three embodiments 'of the present invention have been herein illustrated and de scribedrand although these embodimentshave been described particularly in connection with electric valves of the single anode type it Will be apparent to one skilled inthe art that the inven- .the shield 38 here shown, between the coii Et 'and tionunayhave desirable functional 'utility vin other types of electric valves. l And vit'should be understood .that vtheinvention of the appended claims `is not limited tothe exact details of design and Aconstruction'.d'escrib ed herein, forv obvious changes and modifications Within thescope of the claims will be apparent 'to those skilled in the art.

It is claimedand desired to secure by Letters Patent: l

fl.. .An electric valve comprising in combination an evacuated'casing, a plurality of cooperating electrodes .including an anode 'and 'a cathode disposed within said casing-cooling means Vfor cooling saidzvalve Adisposed*within.saidcasino ada'cerit to said electrodes, said cooling' reingreso lated from said'ca-sing,v andsiiielding means interu posed between ysaid cooling means and said easing 'adjacent'to said coolingnieansrand entending from said anode to said'cathode, said shieldirier means being impervious to arc `formation between said anode and said casing.

2. in an electric "valve, the combination oilan evacuated casing comprising a 'po-ol of vapo'riza-ble conductive material serving as 'a Acathode disposed in said casing in conductive relation thereto; V'an anode disposed in said casingr in Vinsulated relation thereto; cooling and condensing means 'cc'nmn pricing a coil forming a passage for the ilo'w oi cooling fluid therethrough, said coil havingspaced 'turns defining a gap therebetween perviouv's to the unrestricted flowv of material vaporized from said cathode'pool to enable said vaporiz'ed material to have access to all surfaces of lsaid member to condense thereon,Y said coil being insulated from said anode and said casingand shieldingm'eans 3. in an electric Valve employing a 'vapori'zabl'e cathode to provide an ionizable medium for the conduction o i an arc therethrough, cooling inea-ns pervious to the flow of the ionizable medium disposed adjacent s'aid arc for cooling said arc and returning said ionizable medium to said cathode, and a cylindrical arc impervious shield disposed vt'fitnin said valve jacketing said* coolingineans, said shield cooperating with said cooling means to conne said' arc iso-'said cathode.VA

e. iin electric valve comprising in combination an evacuated casing, a'plurality of 'cooperating electrodes .including an anode and a cathode disposed within said casing, cooling means for cooling said valvefdisposed within said casing adiacent to said electrodes, and shielding mean-s in terposed between said cooling means and said casing and extending from said anode to said cathode and insulated from said casing 'for prei venting are formation between said anode and casina.

5 An electric valve 'comprising incombination an 'evacuated casing, a plurality of cooperating eiectrodes including" an anode Aand a cathode dise posed within said casing`,`cooling means for cool ing said valve disposed within said casing adjacent to said eiectrodes, and shielding means cofm= prising a cylindrical envelope substantially closed at one end thereof interposed between'said coolr` ing means and said casing and'eiitending from sadanode to said cathode and insulated from said casing vfor preventingv arc format'ion'between said anode and said casing.

6. in an electric valve: the combination of an evacuated casing comprising a pool ofY vaporiz.- abieconducuve materisiserving' ssa cathode-dis;

posed in said casing in conductive relation thereto; an anode disposed in Vsaid casing in insulated relation thereto; and means for preventing arcing between said anode and said casing comprising in combination a member forming a passage for the iiow of cooling iiuid therethrough, said member being pervious to the unrestricted ilow of material vaporized from said cathode pool to enable said vaporized material to have access to all surfaces of said member to condense thereon, and a shield coextensive with said member disposed between said member and said casing to form with said member an arc impervious barrier between said anode and said casing.

7. An electric valve comprising in combination an evacuated casing, a plurality of cooperating electrodes including an anode and a pool of vaporizable material serving as a cathode disposed within said casing in conductive relation thereto, cooling means for cooling said valve disposed within said casing adjacent to said electrodes, and shielding means comprising a perforated cylindrical envelope substantially closed at one end thereof interposed between said cooling means and said casing and extending from said anode to said cathode and insulated from said casing, said envelope being pervious to the flow of said material therethrough and impervious to arc formation between said anode and said casing.

8. Means for preventing the formation of cathode spots on the casing of an electric valve having a plurality of electrodes comprising an arc impervious shield disposed within said casing in spaced relation thereto, said shield substantially enclosing said electrodes; cooling means disposed within said shield and spaced therefrom for cooling said electrodes; and insulating means disposed within said casing for insulating said shield from said casing.

9. In an electric valve of the single anode type having an evacuated casing, and a vaporizable cathode electrically connected to said casing, the combination of cooling means for cooling said valve disposed Within said casing, said cooling means comprising a coil having spaced turns extending irom said anode to said cathode and insulated therefrom, and shielding means comprising an arc-impervious element interposed between said coil and said casing and insulated from said casing, said arc-impervious element extending coextensively with said coil to prevent the passage of stray arcs through the turns of said coil.

10. An electric valve comprising an evacuated casing, an anode disposed within said casing, a vaporizable cathode within said casing providing an ionizable medium for arc conduction between said anode and said cathode, a control electrode adjacent said anode for controlling the initiation of said arc, cooling means adjacent said control electrode for cooling said arc, and arc impervious shielding means between said cooling means and said casing for coniining said arc to said cathode and said anode.

1l. In an electric valve: the combination of an evacuated casingcomprising a pool of vaporizable conducting material serving as a cathode disposed in said casing in conductive relation thereto; an anode disposed in said casing in insulated relation thereto; and means insulated from said anode and said casing for preventing arcing between said anode and said casing comprising in combination a member forming a passage for the flow of cooling fluid therethrough; said member being pervious to the unrestricted flow of material vaporized from said cathode pool to enable said vaporized material to have access to all surfaces of said member to condense thereon, and a shield coextensive with said member and connected thereto disposed between said member and said casing to form with said member an arc impervious barrier between said anode and said casing.

HAROLD WINOGRAD.

No references cited.