US2008022A - Electrical discharge device - Google Patents

Description

July 16, 1935. K. H. KINGDON ELECTRICAL DISCHARGE DEVICE Inventor: Kenneth H. Kingdom Filed June 1, 1928 His- Attorney.

- Patented July 16, 193s JPATVE NT OFFICE Emcrrarcan mscnaaan nsvrca Kenneth H. Kingdon, Schenectady, N. Y.. alsignor to General Electric Company, a corporation oi New York Application June 1, 1928,8erial No. 282,180

ClI-lml.

charge devices and especially to devices in which the. discharge is carried principally by negative electrons in the absence of substantial ionization 5 by collision.

Patent No. 1,648,312 granted November 8, 1927 to Irving Langmuir and me Jointly discloses an electron discharge'device containing a filamentary cathode treated with an alkali metal, specifically caesium or rubidium, hereinafter referred to as a caesiated filament and operating'under conditions of low vapor pressure. The purpose .Ofthe alkali metal is to enhance the electron emissivity of the'cathode. It is stated in the'patent that when operating a filament of this type in a substantial vacuum, the effectiveness of the aklali is increased many fold by first treating the cathodein an electro-negative medium, e. g. oxygen, which forms on the cathode a foundation tain products of ionization, all or part of which may find their way tothe cathode. These products which include variously charged gaseous particles of different material, may impact with the filament in force and number suflicient to disturb the alkali layer or oxygen binder or both, either by reforming and adding thereto oxygen and other gases derived from the material occlud ed in the glass or other parts or by sputtering and dr: im; off portions of the layer or binder from the filament core, which portionsmay combine with some of the caesium vapor present to form stable compounds and hence are lost to the ,filament. However, in both cases, i. e., film' reforming and disintegrating, and including all other effects produced bythe ionization products,

the electron emissivity of the cathode and the efliciency of the device as a translator of electrical energy may be impaired by the change in the character of the adsorbed layer. While the deteriorating effects are particularly noticeable in the case of filaments which depend for their operation on adsorbed surface films of alkali metals or other materials. the impairment ap- (Cl. 25%:7-5) I The present invention relates to electron dis-' plies in some degree to cathodes in general which are subjected to ion bombardment.

My invention therefore contemplates all types of cathodes immersed in an activating medium, gas or vapor, which forms on the cathode, layers 5 of electronically emitting material sensitive to ion impact. Whatever be the cause of this impairment, I have found that if a body be placed in the neighborhood of the electrodes and a definite potential assigned thereto, of a polarity opposed to the preponderant polarity of the bombarding ions, the eil'ectiveness and operating life of the adsorbed layer or other electronically active film formed on a binding material is thereby increased.

While I do not wish to be limited to a particular theory I believe the discovery to be based upon the fact that the products of ionization are for .the most part attracted to or intercepted by the performance of the device.

Among the objects of my invention are toenhance the electron emitting efiiciency and operating life of a heated electrode, the surface'of which is sensitive to ion bombardment and to improve the construction and operation of electrical discharge devices which employ for electron emission purposes, adsorbed films of alkali metals and other materials. Other objectsfof the invention will be apparent as the specification is perused when considered in connection with the accompanying drawing. As an example of an additional feature, I disclose hereinafter alternative means in the various modifications for supporting a filamentary cathode in a manner such that microphonic noises or disturbances caused by filament vibration may be substantially elim-, inated.

In the drawing which illustrates preferred embodiments of my invention Fig. 1 is a view partly broken away of a space discharge device embodying the features of the present invention; Fig. 2 Ban enlarged cross sectional view as seen in the direction of arrows from line 2-2 in Fig 1; Fig. 3 illustrates the adaptation of my invention to the so-called screen-grid amplifier or detector with the glass envelope removed for the of clearness; Fig. 4 is an enlarged sectional .view taken along line 4-4 in Fig. 3; Fig. 5 shows another modification of the electrode organization, also without the envelope; Pig. 6 is an enlarged cross sectional view of this modification; Figs. 7, 8 and 9 are diagrammatic views of typical circuits in which the various devices illustrated in Figs. 1, 3 and 5 respectively-may be utilized to advantage; while Fig. 10 shows in perspective, the construction of the upper portion of the electrodes illustrated in Figs. 5 and 6.

Referring to Figs 1, 2 and 7, I show a closedglass container or envelope I provided with an axially disposed filamentary cathode 2 having a rectilinear extension and composed of a refractory material such as tungsten or molybdenum, and an anode of metal which does not oxidize readily. The anode is formed in two parts as fiat plates or slats 3, symmetrically arranged on either side of the filament and preferably in the same plane, also directed toward the cathode (see Fig. 7). These plates are electrically connected together in any suitable manner, e. g., by a transversely mounted wire 4. Between the inner longitudinal edges of each anode slat and the filament, there is a grid or control electrode 5 in the form of two wires, 30 mils in diameter or thereabouts, which are connected in parallel by a suitable tie member at the top. The upper ends of the filament, grid and plate electrodes are carried by an anchor wire 6 secured in the stem, which wire has a transversely overhanging portion 8 for supporting a relatively large glass bead 9. Rigid wires connect the grid tie member and each of the anode members to the bead, the latter serving simply as a convenient means for supporting the top portion of the control and plate elements from the horizontal wire I and hence within the envelope, at the same time, electrically insulating the elements from one another. The upper end of the filament is attached di-, rectly to the wire 8 in the manner shown. The lower portions of each of the plate, grid and cathode electrodes are supported from the stem I by wires to which leading-in conductors are attached in themsual manner.

As a protection against ion bombardment of the filament and various parts of the envelope and in accordance with my invention, I provide a hollow metallic shield I having the general shape of a cylinder, open at both ends but deformed to accommodate and enclose the electrodes. The length and contour of the shield ill should preferably conform to that of the electrode structure considered as a unit. The shield conveniently may be made of refractory sheet metal, nickel, molybdenum, or the like, the ends of the strip being brought out to form flanges which are secured together in any suitable manner, e. 8., by welding or indenting. The shield may be supported within the envelope by means of stay wires, l I, I2, which also serve as electrical conductors, connecting the upper end of the flanged portion to the horizontal wire I and the lower end to the upright anchor wire 8. It will be noted that the shield is electrically joined to the filamentary cathode 2 and hence is at the same potential with respect thereto.

In order to prevent the slight movements of the filament which give rise to microphonic disturbances, I provide at positions intermediate the ends of the cathode two transversely disposed fine wires l3, M the upper being attached to the shield support; the lower wire is positioned angularly with respect to the upper wire by means of a stout wire i5 which rises from the stem 1. The ends of wires I I and Il extend slightly beyond the,=filament against which they bear lightly to produce frictional or rubbing contact at the surface oi the filament. I They also maintain the position of the filament rigidly against vibration and in conjunction with the frictional contact serve to reduce microphonic noise and other disturbances.

In order to provide a source of alkali metal from which to derive an electronically active film or layer on the oxygenous cathode surface, caesium or other desired alkali metal is introduced into the envelope in any suitable manner, e. g., by distillation from a compound contained in a side tube which is thereafter sealed off. The pressure of the vapor at operative bulb temperature corresponding to afilament temperature above about 900 K., will average in the neighborhood of .001 or .002 micron, too low to permit of substantial ionization by collision. I prefer to obtain the vaporby reducing a caesium yielding compound located within the envelope. For this purpose, there is provided a pair of capsules l6, secured to a circular plate each containing a pellet of caesium dichromate and a reducing agent such for example as silicon in powdered form. The plate conveniently may be attached to the shield It, as shown.

After the electrodes and the ion collecting'shield are mounted in position and the air, water vapor and other occluded material removed by thorough evacuation and baking in the well understood manner, the envelope is sealed and the plate and pellet members are heated by high frequency induced current to evolve caesium vapor. If it is desired to produce an adsorbed film of caesium on the filament in accordance with the subject of this invention, the latter preferably should first be treated in an atmosphere of oxygen after the preliminary evacuation and before the envelope is sealed, to form an oxygenous foundation layer on the cathode to which the caesium in metallic state will adhere tenaciously as stated in the patent referred to hereinbefore. The presence of caesium as an adsorbed film enhances the electron emissivity of the cathode which efliciency is maintained over a long period of operating life by the action of the surrounding plate member ll.

In Fig. 7, I show a typical amplifying circuit which employs a device containing a. caesiated filament such as described; the input transformer I! is connected across the grid andfilament members through a bias battery iii of a few volts, the output circuit comprising transformer i9 and plate battery 20 of approximately 90 volts connected in series across the anode and the filament. The shield l0 and the source of filament energy are not shown in this figure. During the operation of the tube it has been found that the ions formed within the space between electrodes due to collision between the gas molecules and the electrons moving from the cathode, which molecules may comprise alkali vapor or other gas in the envelope have a tendency to be attracted to thefilament with which they impact. However, by providing a surface in the neighborhood of the cathode and which also is negatively charged with respect to the products of ionization and preferably at cathode potential since the ions are normally preponderantly positive in character, the surface supplants to a large degree that of the cathode as a target for the bombarding ions. The ion deflecting power of the surface which comprises theshieid member is shown roughly in'l'ig. 2 wherein the curved dotted lines indicate the I path taken by an electron after leaving the filamentand colliding with a gas or vapor molecule. the full lines representing the paths assumed by the ions in traveling toward the collecting memher ll instead of towardthe cathode 2. It will or ion collector may bev variedaccording to its shape and position withrespectto the electrodes,

also in accordancewith its potential when conside ered with respect to the position of the formed ion in the electrostatic field. However, the shield isordinarily maintained at cathode, potential.

In the figures described, the position of the shield is also such as substantially to preclude any of the products of ionization from migrating toward the' envelope'of the stem where they may otherwise cause considerable harm.

In Figs. 3, 4 and 8, I show tleo""- my invention to an amplifier containing a grid or electrostatic control element screened from the capaoitative r energy feed-back eflect which ordinarily wo d be present between the grid and anode members. In these figures, the filamentary cathode 2| is axially disposed of the envelope (not shown) and there are two grid wires 22 connected together at their ends by tie members and arranged symmetrically on either side of the filament in the same manner described. with respect to Fig. l. The anodes 22, are of slat or strip form and connected together by a horizontally disposed conductor 24 but unlike Fig. 1, are arranged in parallel planes as shown more clearly in Fig. 8. Each anode may advantageously be surrounded by an'electrostatic screen 25, hereinafter termed a "screen-grid", which is illustrated in the form of a closely wound cylindrical spiral terminating in a closed top but open at the bottom. As shown,

the screen-grid is materially longer than the anode and extends an appreciable distance beyond each end of the latter so as to effectively intercept the electrostatic lines of force which radiate from the ends of the anode.

In order to lend rigidity to the spiral screen members, the turns are afilxed to a rod which I extends throughout the length of each helix. The

support rods of the helices are welded toa metallic bottom plate 21 which serves to space the convolutions at the desired distance apart and also to complete the electrostatic screening between the control grid and the anode. The bottom plate is supported from the press by two heavy wires 25, one of which is carried through the stem as a leadin conductor. The purpose of this screen which is to be sharply distinguished from that of the ion collecting member of the present invention, is to ,electrically isolate the control element 22 from electrode capacitative effects thereby to preclude the production of self-sustained oscillations, when the tube is connected in anamplifier circuit. The

the top of the envelope, the farthest removed from the-.plate or anode lead which extends through the bottom, thereby further to reduce energy feedback and inherent oscillations.

The filament is hung at the upper end from a conductor which projects transversely from the anchor wire II; the latter also serves as one of the lead-in conductors for the filament. The lower end 'of the filament passes through an enlarged opening in the bottom plate '21 and is attached to a stout conductor 34- secured to the stem and serving as the other lead. The material from which the anodes are formed should be sumciently heavy in order that these members conveniently may be supported solely from the press end by wires 35, and so that their position within the electrostatic screens 25, may be maintained relatively fixed throughout their length. As in the case of the filament, the anodes also pass through an opening (not shown) in the plate member 21 and are attached to an external contact terminal, also not shown. The lower ends of the control wires 22 fall'short of the plate 21, are tied together and supported from the stem in any suitable manner, e; g., by wires 26 and 21, fused in insulating connector ll shown in the form of a glass bead.

' In accordance with my invention and similarly to the device shown in Fig. ,1, I enclose the electrodes within a metal casing or shield 39 which casing has for a function that of providing a target of rugged character for'the ions or other charged particles generated within the tube. The shield is of elliptical shape to conform closely to the area enclosed by the electrode organization and may be constructed of two relatively thin sheet or strip metallic members secured together in any suitable manner along the surfaces of di ametrically opposed flanges formed on each strip.

The shield is maintained approximately at cathode potential by two conductors 40 attached to the upright anchor wire 3| which also furnishes a support for the shield. In order to minimize the vibration of the filament and as an alternative to the expedient described in connection with Fig. 1, I provide a fine wire 4| which is attached to the anchor member 3| but insulated therefrom by a glass bead 42, the wire extending through an opening'in the shield 39 and toward the filament 2| against which it bears lightly. friction exerted on the filament by this wire is quite effective in reducing microphonic disturbances in view of its approximate mid-way position with respect to the ends of the filament.

After thorough evacuation and baking of the envelope, caesium or other cathode activating substance, of relatively low vapor or gas pressure may be admitted in the manner set forth in connection with Fig. l, i. e., by heat reduction.

of a suitable compound contained in capsules attached to a circular plate 43.

In Fig. 8, there is illustrated one of the many circuits to which the screen-grid type of device is adapted. These circuits are exemplified by an input transformer 44 connectedthrough a biasing battery 45 to the grid and filament electrodes of the amplifier. The output circuit consists of a telephone receiver load 46 and a relatively high potential plate battery 41 connected between the anodes 23 and filament 2|. The member 25 derives a substantial positive potential conveniently The from'a terminal in the plate battery 41, and

under these conditions, eflectively shields the grid from the electrostatic lines of force which emanate from the anode. For the sake of clearness,. the ion collecting or interceptingmember ll has not been shown in this figure. However, it is present and has for a purpose that of increasing the operating life and efilciency of the filament when employed in electrostatically screened grid discharge devices exemplified in Fig. 8 as an amplifier, wihch it does most eilectively by protecting the filament from ion bombardment. Moreover, by reason of the fact that it completely surrounds the electrodes and is at a relatively fixed potential with respect to ground. the member I! precludes extraneous disturbances from adversely affecting the grid electrode and to that extent aids the helical screens 25 in reducing uncontrolled oscillations.

While I have described the position of the screen-grid as surrounding the anodes 23, it is evident that with suitable change in the external circuits I may enclose the grid structure 22 in an electrostatic screen. These screens may also be applied to each of the anodes I separately or to grid of Fig. 1, the circuits being modified accordingly. The change of position of the screengrid from that of surrounding the anode members to that of enclosing the grid electrode, ordinarily necesitates a smaller positive potential on the screen-grid. However, regardless of the position of the electrostatic screens with respect to the electrodes, the externally positioned ion collecting member cooperates with the filament and the remaining electrodes to produce a particularly efilcient device.

Figs. 5, 6, 9 and 10 illustrate still another form of my invention in which the grid structure is modified to exercise the usual electrostatic control function and in addition to collect or intercept the ions, thus preventing the latter from interferingwith the operation of the cathode. As in the preceding figures the rectilinear filament I is axially aligned of the envelope (not shown) and supported at the top from a transversely disposed rod 13 carried by a rigid upright member I! which constitutes the main support of the electrodes. The member I2 is positioned from the stem or press 14 by means of a right-angle extension 48 to which it is attached. The anode takes the form of a hollow cylinder 49! concentrically arranged with respect to the filament and has a plurality of vanes or slats 5|! equidistantly spaced about the inner periphery of the cylinder. each directed toward the filament. For convenience in manufacture, the anode may be fashioned out of relatively thin sheet metal stock and comprises two parts secured together at their inner faces to complete the plate member. Each strip is cut sufliciently wide so that when bent along predetermined parallel lines, two of the vanes 5| are formed; each strip in addition, provides one-half of the flange member 52. The latter terminates in a semicircular extension of small radius so that when the adjacent flange surfaces are afilxed, for example, by punching two holes 5| and turning over the edges as shown, the extensions form a circular housing running longitudinally of the anode with which to embrace upright rods 53. These rods are secured,

to the press in convenient manner and one is extended through the stem to serve as a leading-in conductor. At their upper ends, the rods 53 are maintained rigidly in a direction of the tube axis by means of two transversely disposed wires 54 fused in a glass bead 55 which is secured to upright rod 12 by connector 56.

The grid or control element which preferably containsasmanymembersasthereareanode vanes comprises wires ll (see Figs. 9 and 10) disposed longitudinally of the envelope and positioned in line between each anode vane BI and the filamentary cathode 60. The grid wires are equidistantly spaced about the filament by means of a hollow spider 68 upon which ears or lugs 59 are raised at suitable places around the upper portion of the spider, the wires being secured to the latter at their lower ends by welding and are threaded through apertures in the ears at their upper ends; the sliding support ofiered by these openings provides for thermal expansion of the wires. The spider 58, the function of which will be described hereinafter, is of general rectangular design with radial extending vanes or slots 62 similar to those of the anode but which are interleaved with the latter in the manner shown. The interior of the spider is hollow to accommodate the filament I.

While various constructions will at once suggest themselves to those skilled in the art, I prefer to fabricate the spider out of a number of component sheet metal parts as in the case of the anode. Two header or frame members 6| are first formed and spaced apart a distance corresponding to the length of the spider which roughly is comparable to that of the anode; the longitudinal portions 62 are then attached to both headers. Each header is conveniently made of four relatively narrow strips, the ends of which are bent outwardly and secured together to form, an approximate star shaped construction with four fiat surfaces radiating from the center. The surfaces of a pair of headers are then aligned at the proper distance from one another and the metal strip 62 laid on and secured to each of the surfac:s thus joining the header or frame members together to form a compact and rugged unit 58. The unit is suspended within the envelope between two pairs of wire supports 63, the two upper of which are supported from the bead 55 and the two lower, fused in the press, one being extended through the stem as a leading-in conductor (not shown). I

In this figure, I have also illustrated the same construction for reducing vibration of the filament as was explained in connection with Fig. 1, i. e., two fine'wires 64, angularly disposed with respect to one another, are provided at intermediate points of the filament. As in the other modifications, a source of alkali metal is provided for coating the filament from a capsule 65 containing a suitable compound.

Fig. 9 is a typical detecting circuit within which the device shown in Fig. 5 finds utility. However, it is not to be understood my invention is limited sp1cifically to detecting or amplifying circuits but is broad enough to cover all forms of energy translating systems which employ a cathode sensitive to ion bombardment. In the detecting circuit the input is represented by a receiving-antenna-ground circuit 66, the energy of which is supplied to grid wires 51 and filament 60 through a transformer 67, also through a positive biasing battery 68 and grid-leak-condenser network 69. The output circuit comprises a high voltage battery 10 and a telephone receiver li connected across the filament and anode members.

In a tube of this construction, the ions are formed almost entirely in the space between the anode and control electrodes and not between the filament and control member. The ions, therefore, are produced at points where the electric field draws them for the most part to the slat members 62 of the spider 58 upon which the grid wires 51 are hung. As in the case of the filament protecting shirelds shown in the other figures. the efiicacy with which the ion collecting or deflecting function is performed depends to some extent upon their potential with respect to that of the cathode and the preponderant potential and polarity of the ions produced in this manner which function is exercised without interfering with or detracting from the usual electrostatic control of the grid as a whole. The combined wire and vane construction is advantageous over that shown in'the modifications of Figs. 1 and 3 in that an external ion collecting member is not necessary and hence, there is involved one less element. However, it is to be understood that, if desired, the additional member may also be employed but it is found in practice that this form of grid construction affords sufiicient protection to the filament from ion bombardment.

While I have shown but three embodiments of my invention for the purpose of describing the same and more fully illustrating its operation and its construction of details, it is apparent that various changes and modifications may be made in the nature, the mode of operation and the details of construction without departing from the spirit of my invention.

No claims are made in this application on the feature of having the metallic member which receives the products of ionization form part of the control member but claims on this feature are contained in my divisional application Serial No. 703,285, filed December 20, 1933, entitled Electrical discharge devices" and assigned to the same assignee as the present application. Furthermore, there are no claims in the present application on the feature of providing a plurality of members arranged to provide a rubbing contact with the filament at a point intermediate the ends thereof but this feature is claimed in my divisional application Serial No. 724,048, filed May 5, 1934, entitled Electrical discharge devices and assigned to the same assignee as the present application.

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

1. An electron discharge device comprising a plurality of electrodes including an anode, a control member and a cathode having an electronicaliy active coating which is sensitive to positive ion bombardment, means for binding the cathode to the cathode material, said control member being positioned in line with the anode and cathode and interposed therebetween, means including a metal shield surrounding said electrodes for preventing disintegration of the binder by positive ion impact during operation of the device whereby the active coating is maintained.

2. An electron discharge device comprising a plurality of electrodes, including an anode, a control member and a cathode having an adsorbed layer of alkali metal formed on an electro-negative substance, said electrodes being in line with one another and the control member interposed between the cathode and anode, means for stabilizing the condition of the alkali layer, said means including a member of conducting material electrically connected to the cathode.

3. An electrical discharge device comprising an evacuated container, electrodes therein including an anode, a control member, and a thermionic cathode, said electrodes being positioned in the same plane, a quantity of caesium in said container, and means whereby the aflinlty of said cathode for caesium vapor at vapor pressures too low to permit substantial ionization by collision is materially increased at cathode operating temperatures above about 900 K., said means comprising an electronegative substance, and means including a metal shield for preventing diSSOC18-.

tion of the substance from the cathode material I under operating conditions.

4. An electron discharge device comprising an evacuated envelope containing an attenuated atmosphere of an ion-producing vapor, a filamentary cathode and cooperating anode and control electrodes, sa d electrodes being positioned in the same plane as the cathode, said cathode comprising an adsorbed layer of alkali material, a binder for securing the material to the filament, and means including a metal member at fixed potential for preventing substantial ion impact with each of said layer and said binder.

5. A thermionic discharge device comprising an envelope containing an activating vapor at a pressure too low to permit of substantial ioniza-- tion, and,a plurality of electrodes including a source of electrons, grid and anode members, said electrodes being positioned in the same plane whereby the electrons are constrained to travel in curved paths from the source to the anode and means including an electrode for directing the passage of ions, formed by collision between the electrons and gaseous molecules in the attenuated atmosphere, to a position within the envelope remote from said source.

6. Athermionic discharge device comprising an envelope containing an activating vapor at a pressure too low to permit of substantial ionization,

find a plurality of electrodes including a source of electrons, grid and anode members, said electrodes being positioned in the same plane whereby the electrons are constrained to travel in curved paths from the source to the anode and means for directing the passage of ions formed by collision between the electrons and gaseous molecules, in the attenuated atmosphere, to a position within the envelope remote from said source, said means comprising a member surrounding the electrodes and which is adapted to be charged to a potential negatively with respect to the position in the electrostatic field of the ionization products at the time they are formed.

7. An electrical discharge device in which the discharge is carried principally by negative electrons, a plurality of electrodes including a cathode sensitive to ion bombardment, a surrounding body of ion-producing vapor, screen members for electrostatically isolating the electrodes from one another with respect to internal capacity effects and means including an electrode cooperating with the screen members for increasing the efi'ectiveness of the screening function and in addition, for shielding the cathode from ion bombardment.

8. An electrical discharge device in which the discharge is carried principally by negative electrons, a plurality of electrodes including a cathode sensitive to ion bombardment, a surrounding body of ion-producing vapor, screen members for electrostatically isolating the electrodes from one another with respect to internal capacity effects and means cooperating with the screen members for increasing the eifectiveness of the screening function and in addition, for shielding the oathode from ion bombardment, said means comprising an electrical conductor surrounding the electrodes and adapted to be maintained at a potential less than that corresponding to the position in the electrostatic iield oi the produced ions. v 9. An electrical discharge device in which the e is carried principally by negative electrons, a plurality of electrodes'including a cathode sensitive to ion bombardment a surrounding body 01 ion-producing vapor, screen members (or electrostatically isolating the electrodes from one another with respect to internal capacity eil'ects and means cooperating with the screen members for increasing the eflectiveness of the screening function and in addition, for shielding the cathode from ion bombardmnt, said means comprising a conductor located in an ionareceiving position and 10. An electron discharge device comprising a thermionic cathode with an adsorbed film or caesium formed on a binding material, an anode and control electrode,- an ionizable medium between said electrodes at a pressure too low to permit substantial ionization, and a metallic member cooperating with said electrodes to receive the ion impact thereby to shield the caesium film and binding material from bombardment, said member surrounding the electrodes and adapted m to be charged to a potential corresponding substantially to that oi. the cathode.

mum r1. KINGDON. 15

w mime or connection Patent No. 2,008,022. 4 July 16. 1935.

KENNETH R. xmcnou.

It is hereby certified that error appears in the printed specification of the above It red patent requiring correction as follows; Page 3, first column,- line 24, for "of" read or; page 4, second column, line 15, for "slots" read slats; and page 6, first column, line l2, for "bombardmnt" read bombardment; and that the said Letters. Patent should be read with this correction therein that the same may eonform-twt he record of the case in the Patent Office.

Signed and s ealedthis'27th day of August, A. D. 1935.

Leslie Frazer (Seal) 7 Acting Commissioner of Patents.

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