US3377492A

US3377492A - Flood gun for storage tubes having a dome-shaped cathode and dome-shaped grid electrodes

Info

Publication number: US3377492A
Application number: US476925A
Authority: US
Inventors: Frederick G Oess
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1965-08-03
Filing date: 1965-08-03
Publication date: 1968-04-09
Anticipated expiration: 1985-04-09

Description

F. G. OESS A ril 9, 1968 FLOOD GUN FOR STORAGE TUBES HAVING A DOME-SHAPED CATHODE AND DOME-SHAPED GRID ELECTRODES Filed Aug. 5, 1965 [iii-ilk all ilawd Z. K M: I a w 4 /J M, M m a f w 5 N NMN 3,377,492 FLOOD GUN FOR STORAGE TUBES HAVING A DOME-SHAPED CATHODE AND DOME-SHAPED GRID ELECTRODES Frederick G. Oess, Oceanside, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Aug. 3, 1965, Ser. No. 476,925 9 Claims. (Cl. 313-71) ABSTRACT 6F THE DISCLOSURE A flood electron gun for storage cathode ray tubes wherein the flood gun cathode has a spheroidal electron emissive surface with two electrode members nested over and around the electron emissive surface and having spheroidal mesh members (grids) covering the apertures in the electrode members with the, spacing between the spheroidal mesh member and the electron emissive surface adjacent thereto being substantially constant.

This invention relates to cathode ray tubes and especially to the direct-viewing storage display type. More particularly, the invention relates to means for forming a flood beam of electrons especially but not necessarily exclusively for use in direct-viewing storage dis play tubes. Certain types of stored display tubes utilize a relatively high velocity writing electron beam to scan a storage tar-get in accordance with intelligence signals to establish a charge pattern on the storage target corresponding to the information to be stored and displayed. In such tubes, the storage target is sprayed or flooded with a relatively broad beam of electrons which pass through the storage target according to the potentials of the discrete areas thereof defining the stored charge pattern. These flood electrons are then accelerated to impinge upon the viewing screen to create a visual replica of the stored charge pattern. The operation of such tubes is well known and ample description thereof may be found in US. Patent No. 2,790,929 issued to E. E. Herman and G. F. Smith, entitled Direct-Viewing Half- Tone Storage Device and in US. Patent No. 3,086,139 issued to N. H. Lehrer, entitled Cathode Ray Storage Tube.

Heretofore it has been common practice to generate such flood electrons by means of a flood electron gun comprising a tubular support member (hereinafter called the cathode cylinder) having cathode material on the outside of the closed end thereof and mounting this cathode assembly within a cylindrical electrode member (hereinafter called the G1 g id) having a small, centrallydisposed opening in the closed end thereof so that the cathode material was adjacent this small opening. A second electrode member (hereinafter called the G2 grid) is commonly mounted external of an adjacent the G1 grid and may take the form of a plate or cylinder having an opening therein either large-r or the same as the opening in the G1 grid and coaxially aligned with the opening in the G1 grid and the cathode material, A typical flood gun arrangement is shown in the aforementioned patent to Lehrer as Well as in the co-pending application of R. G. Spangler, Ser. No. 276,835 filed States Patent Apr. 30, 1963, now abandoned and assigned to the instant assignee. It is understood that difierent electric potentials are applied to the G1 and G2 grid electrodes in order to form a broad beam of flood electrons with which to blanket the surface of the storage target in such storage display tubes as those described in the aforementioned patents. While this flood gun structure has proven eminently satisfactory for use in most direct-viewing storage tubes of the past, the fabrication of extremely large storage tubes having a faceplate diameter of about 21" or tubes having a very short length have required a flood gun structure capable of producing a flood beam of much wider divergence but which also is a high-density beam of uniform low energy electrons. The flood guns of the prior art have proven to be incapable of providing a flood beam of controlled and accurate beam divergence which can, at the same time, operate at high beam current values.

In the co-pending application of R. L. Sjoberg and N. J. Koda entitled Flood Gun for Storage Tubes, Ser. No. 450,335 filed Apr. 23, 1965, and assigned to the instant assignee, an improved flood gun structure is described and claimed for producing a high density flood beam having an accurately controlled divergence. Sjoberg and Koda achieved improvement over the prior art flood guns by utilizing a dome-shaped cathode emissive member and a domed screen or mesh over the aperture in the G2 grid electrode so that the surface of the domed mesh is substantially parallel to the domed surface of the cathode emissive member.

While the flood gun structure described in the Sjiiberg and Koda application does achieve the production of a broad beam of electrons having a wide angle of divergence as well as high flood beam current, density or beam current control appears to require simultaneous adjustments of other electrodes such as the collimating lens structures, for example, in order to maintain beam uniformity. Furthermore, none of the prior art flood electron gun structures permits adjustments of the flood beam density distribution in order to compensate for geometric variations such as cosine law effects if the beam is projected onto a planar target and especially if the flood gun is offset with respect to the principal axis of the tube. Neither do these prior art flood guns permit compensation of second and third order collimating lens oberrations such as distortion of the collimating electric fields which tend to alter the uniformity of the density distribution of flood electrons at the target.

it is, therefore, an object of the present invention to prgvide an improved flood gun structure for cathode ray tu es.

Another object of the invention is to provide an improved flood gun structure especially for direct-viewing storage display cathode ray tubes.

Another object of the invention is to provide an improved flood gun structure capable of producing a fixed wide angle divergent flood electron beam of controllable uniform density and velocity.

Yet another object of the invention is to provide an improved flood gun structure for cathode ray tubes which gun structure itself masks off excess flood electrons thereby preventing or limiting collection, reflection and secondary electron emission by other electrodes in the tube such as those used for collimating the flood beam.

Still another object of the invention is to provide an im proved flood electron gun structure capable of permitting beam without requiring simuitaneous adjustments of other in the flood beam density which may be introduced by the geometry of the tube.

Another object of the invention is to provide a improved flood electron gun structure in a cathode ray tube capable of permitting density control of a flood electron beam without requiring simultaneous adjustments of other electrodes such as the collimating electrodes.

Yet another object of the invention is to provide an improved flood electron gun structure for electron tubes which permits adjustment of the flood beam density distribution to compensate for (l) geometric variations in particular tube designs for cosine law effects where the flood beam is projected onto a planar target or (2) where the flood beam gun is offset from the tube axis, or (3) density distribution alterations due to aberrations in collimating lens fields.

These and other objects and advantages of the invention are realized by providing a flood electron gun structure having a dome-shaped cathode emissive member, a dome-shaped mesh covering the aperture in the G2 or intensity grid electrode, and a spheroidal mesh cove-ring the opening in the G1 or control grid electrode. By this struc ture and preferably but not necessarily exclusively with a variable negative potential on the G1 or control grid, the over-all beam density may be varied from zero (cutoff) to maximumwithout affecting either the relative design density distribution or the cone angle of the beam with a fixed accelerating voltage on the anode or G2 grid. The variable potential on the control grid or G1 electrode permits varying the transmission across the G1 control grid which in turn makes it possible to obtain a variable but uniform beam density at the target. Other techniques are also available to permit adjustment of the design density distribution within the flood beam with the flood gun structure of the present invention as will be described hereinafter.

The invention will be explained in greater detail with reference to the drawings in which:

FIGURE 1 is an elevational view in section of a visual display storage cathode ray tube in which a flood gun structure according to the present invention is incorporated;

FIGURE 2 is an elevational view in section of one embodiment of a flood electron gun for use in a cathode ray storage tube; and

FIGURE 3 is an elevational view in section of another embodiment of a flood electron gunfor use in a cathode ray storage tube.

Referring now tothe drawings, a half-tone visual display cathode ray tube 2 of the type described in the aforementioned patent to Lehrer is shown according to the present invention. The tube 2 comprises an evacuated envelope formed by a comparatively large cylindrical section 4 and a narrower neck portion 6 communicating therewith at one side thereof (and hereinafter referred to as the neck or gun side). The side of the large cylindrical section 4 opposite the neck side comprises a faceplate 8 over the inner surface of which may be disposed a phosphor target or viewing screen 10 covered with a thin film 12 of metal, such as aluminum, for example. Adjacent and coextensive with the faceplate 8 is a storage target 14 as described in the aforementioned patent to Lehrer. The storage target 14 may comprise a nickel mesh support, which may be electroformed, having disposed on one side thereof a thin layer or film of cubic zinc sulfide, for example, which has both secondary electron emission and bombardment induced conductivity properties.

A collector grid 16 is disposed adjacent and coextensive with the storage target 14 and comprises a conductive screen supported about its periphery by an annular ring 18. The function of the grid '16 is to collect secondary electrons emitted from the storage target 14.

A flood gun according to the invention is disposed on the longitudinal axis of the tube 2. The flood gun 30 will be described in greater detail hereinafter. At this point, and with particular reference to FIGURES 2 and 3, it is sufficient to indicate that the flood gun comprises a cathode support cylinder 32 with cathode material 33 disposed on the end thereof, and an intensity electrode 34 or control G1 grid which encloses the cathode 32 except for an aperture 36 disposed over the cathode material 33 on the end portion of the cathode cylinder 32. An accelerating electrode or G2 anode 38 is disposed adjacent the G1 electrode 34 and may be coaxially disposed with respect to the longitudinal axis of the tube 2 which also passes through the center of the aperture 36 in the G1 control electrode 34. A filament wire is disposed withing the cathode cylinder 32 for heating the cathode material 33.

The neck portion 6 of the tube 2 houses a first electron gun 40 which may be of conventional construction for providing an electron beam of elemental cross-sectional area. The gun 40 comprises a cathode 42, an intensity electrode grid 44, and a cylindrical beam-forming section 46. Horizontal deflecting plates 48 and vertical deflecting plates 48' for controlling the deflection of the electron beam generated by the electron gun 40 are also provided. Also shown as housed in the neck portion 6 of the tube 2 is a second electron gun of conventional construction for providing a second electron beam of elemental crosssectional area. The gun 50 comprises a cathode 52, an intensity electrode grid 54 and a cylindrical beam-forming section 56. Horizontal and

vertical deflecting plates

58 and 58 respectively are likewise provided. While electrostatic deflection means are shown, the electron beam from the two guns may be magnetically focused and deflected, in which case separate neck portions may be desirable.

A collimating lens system for flood electrons may be established within the neck portion 6 and the large eylin drical section 4 of the tube 2 by means of

conductive layers

49 and 49, which may be coated over the interior surfaces of the tube as shown, and by a conductive cylinder 49".

The electron beams produced by the two electron guns 40 and 50 are caused to scan the storage target 14 and/or viewing screen 10 by means of horizontal and vertical deflection voltages applied to their respective horizontal and vertical deflection plates. The electron beam from the gun 50 may have an energy level at which the rate of charge of the storage surface by secondary electron emission is counter-balanced by the rate of discharge by bombardment induced conductivity resulting in a zero net charge on the storage surface when the potential of the storage target is substantially zero. The intensity of the electron beam from the live* gun 50 may be modulated by potentials applied to the intensity grid 54 in accordance with information representative signals.

The electron beam from the storage gun 40 may have an energy level at which the rate of charge on the storage surface by secondary emission out-balances the rate of discharge by bombardment induced conductivity whereby a pattern of charges may be formed and stored on the storage target in accordance with information representative signals by which the intensity of the 'electron beam from gun 40 is modulated.

In prior art flood guns, the cathode cylinder of the flood gun 30 may be referenced to ground by means of a connection therefrom to ground. The intensity or control grid or G1 electrode may be maintained at about the same potential as the cathode cylinder while the G2 anode electrode may be maintained at a tial.

It will be appreciated that the storage target utilizes two phenomena to produce charging effects in opposite electrical senses which effects may be balanced so as to result in no net charging effect in either electrical .sense. This is possible because there is a continuous range of different poten electron beam energy levels where both secondary electron emission and bombardment induced conductivity occur and because at different portions of this range either of these phenomena can be made dominant or the two phenomena can be balanced. The capability of balancing these two phenomena is of significance where it is desired to provide a storage target which can be writtenthrough to present direct or live information without altering the potentials on the storage target. A complete and detailed description of the operation of such a storage tube may be found in the aforementioned patent to Lehrer.

Areas of the storage target 14 impinged by the beam from the electron gun 40 in accordance with the information to be displayed are charged positively due to the emission of secondary electrons therefrom which are collected by the collector grid 16. Viewing or flood electrons from the flood gun 30 may then pass through the storage target 14 at these areas of positive potential and are then accelerated to impinge upon the phosphor layer of the viewing screen by means of a potential of about 6,000 volts positive with respect to ground which may be maintained on the aluminum film 12 of the viewing screen. In this manner, the information is displayed as white or black and the display may be maintained and viewed as long as desired.

It will thus be understoodthat flood electrons for creating visible stored displays penetrate the storage target in accordance with the potentials thereon and that the storage potentials control the intensity of the flood beam so as to create half-tone or color displays ranging in brightness from 0-100 percent. The necessity of flooding the storage target with a broad beam of high-intensity uniformly low energy level electrons will thus be readily understood and appreciated in order that an entire display may be presented. In the case of large diameter storage tubes or tubes having a very short length, or in tubes requiring a high light output as for display projection, wide flood beam divergence angles of the order of 65 or more are required. In addition, accurately controlled divergence is desirable for good beam collimation. Hence, with these objects in mind, the flood gun structure of the present invention has been conceived and designed.

Referring now to the drawings and to FIGURE 2 in in particular, the flood gun structure 30 for use in an onaxis disposition in the cathode ray tube as shown in FIG- URE 1 includes a hollow metallic cylindrical member 32 in which is disposed a filamentary heating wire 35. Cathode emitting material 33 in the form of a dome is disposed on the closed end of the support cylinder 32. Disposed around the cathode cylinder 32 is a cathode surface continuation shield member 37 in the form of a metallic cylinder slightly separated from the cathode 33 so as to prevent heat loss at the cathods surface. The cathode shield member 37 may be electrically tied to the cathode so as to eliminate undesirable fringe field effects which could adversely affect the desired electron density distribution. The exterior surface of the partially closed end 37 of this shield member 37 is coextensive with and has substantially the same curvature as the surface of the cathode material 33. Disposed around the cathode shield member 37 is the G1 electrode 34 in the form of a metallic cylinder having an aperture 36 in what would otherwise be the closed end thereof. The G1 cylindrical electrode 34 may be coaxially disposed and electrically isolated by means of an insulating ring 41 with respect to the cathode cylinder 32 and the cathode shield member 37. The anode or G2 electrode 38 may be in the form of an apertured metallic cylinder whose aperture may be coaxial with the aperture in the G1 electrode 34. The G2 electrode 38 may be spaced and isolated electrically from the G1 electrode 34 by means of an insulating ring 43.

Both the G1 and

G2 electrode members

34 and 38 are provided with dome-shaped, high transmission mesh or screen members 39 and 38', respectively, which may be welded to the cylindrical portions of these electrodes and over the apertures therein. The radius of curvature of the screen portion 39 of the G1 electrode 34 is such that the domed mesh surface 39 is substantially uniformly spaced from or parallel to the surface of the dome-shaped cathode 33. Likewise, the radius of curvature of the screen portion 38' of the G2 electrode 38 is such that the domed mesh surface 38 is substantially uniformly spaced from or parallel tothe screen portion 39 of the G1 electrode 34 as well as to the surface of the dome-shaped cathode 37. By this flood gun structure, a high-density flood beam having the desired divergence angle of uniform low energy electrons may be provided for blanketing the storage target surface in a cathode ray tube as in FIGURE 1 where the flood electron gun is mounted on the main axis of the tube. Unlike conventional flood gun structures of the prior art in which an increase in the G2 electrode voltage changes the beam shape and/or divergence angle while increasing the beam current, the flood gun structure of FIGURE 2 permits the attainment of a desired divergence angle and a uniform beam current density by adjusting or varying the transmission across the G1 control grid electrode 34 with a fixed accelerating voltage on the G2 anode grid 38. The adjustment of the transmission across the G1 control grid 34 may be accomplished by applying a predetermined potential, which may be negative with respect to the cathode 33, to the G1 control grid electrode 34. Thus, high-density electron beams having beam currents on the order of 5 milliamperes have been obtained with the flood gun structure of the present invention while maintaining a fixed divergence angle of about It is also possible to compensate or correct for minor density variations at the target by utilizing a flood gun structure in which the spacing between the G1 mesh 39 and the G2 mesh 38 is not constant. In such an arrangement, transmission through the G1 mesh 39 can be held at a constant value white still attaining the desired density distribution within the flood beam so as to obtain a uniform density thereof at the target structure 14. Various other alternatives can be practiced to obtain this objective. Thus, for example, the spacing between the surface of the cathode 33 and the mesh portion 39 of the G1 electrode 34 may be varied or made non-constant. However, this alternative flood gun structure requires smaller parts thus increasing the accuracy requirements of the parts. Another alternative is to vary or adjust the transmission across the G2 anode grid 38 which in turn would require an increase in the mesh-web thickness of the screen 38 in some areas. The resultant higher degree of localized masking could result in local non-uniformities in the electron density distribution across the target 14.

With special reference to FIGURES 3, another embodiment of a flood gun structure according to the present invention is shown for use in cathode ray tubes where the flood gun is in an off-axis arrangement and the axis of the flood gun is not orthogonal with respect to the target 14. The structure shown permits the achievement of a uniform density electron distribution with a relatively wide angle of divergence by non-constant spacing between the G1 electrode screen 39 and the G2 electron screen 40 while maintaining the spacing between the G1 electrode screen 39 and the surface of the cathode material 37.

It will thus be understood that by providing a flood gun structure in which the G1 control grid utilizes a meshcovered aperture in addition to a mesh-covered aperture in the G2 anode grid as suggested in the aforementioned application of Sjtiberg and Koda is the ability to control the flood beam by means of the G1 grid electrode so as to vary the over-all flood beam density in electron tubes requiring a uniform flood electron beam and resultant uniform light output in direct-view storage tubes. By the present invention, this advantage is realized without requiring readjustment of other electrodes such as the collimating electrodes. By the flood gun structure of the present invention, one may apply continuously variable control potentials to the flood gun to obtain a uniform flood electron beam without changing the velocity of the flood electrons as they leave the flood gun. The structure of the present invention also provides the ability to cut off the flood beam uniformly by the use of either the G1 control grid or the G2 anode grid without requiring additional compensation to be applied to the collimating system if the G1 control electrode is used for this purpose. The structure of the present invention also results in an improvement in the uniformity of electron emission from the flood gun cathode even when there may be local nonuniformities in the cathode. This latter advantage results from the space charge redistribution in the cathode-to- Gl-electrode region, especially at relatively higher values of control grid bias.

A flood gun according to the present invention also permits one to essentially prevent the flood electrons from reaching any portion of the internal tube structure other than the desired target and thus reducing or preventing reflection and secondary emission of electrons from the structures as well as insulator charging thereof by flood electrons landing thereon. One of the general considerations in designing a flood gun utilizing mesh-covered G1 and G2 electrodes is to mask as little of the active cathode surface as possible in order to obtain maximum efficiency.

This design advantage is achievable by making the meshmasked G1 electrode exposed as much as possible the cathode surface and by making the G2 electrode mask as large as possible or at least larger than the cone angle or configuration of the flood beam which is determined by the G1 masking.

Although the G1 mesh electrode portion and the surface of the cathode are shown and described herein as concave segments of spheroidal surfaces, such concavity of these surfaces may not always be necessary. Thus the G1 mesh electrode portion and the cathode surface may be flat or even somewhat convex, especially in instances where the spacing between the G1 and G2 electrodes is not constant. On the other hand, the G2 mesh surface will normally always be concave in tubes requiring a divergent flood beam.

The flood gun structure of the present invention is particularly useful in all direct-view and projection storage tubes because the light output can be varied at will with a single electrical adjustment of the G1 electrode bias voltage. In addition, the light output efficiency compared to conventional flood gun structures is much greater due to the elimination of losses in the emission current to tube elements other than the direct and/or viewing or projection screens. Likewise, the uniformity of light output will be much better with a flood gun structure according to the present invention than with conventional flood guns. The present flood gun structure also permits pulsing of the flood gun without the normally associated undesirable and transient decollimating eflects characteristic with prior flood gun designs. The present flood gun structure also permits the use of off-axis flood guns without the loss of uniform electron density at the target.

The flood gun structure of the present invention may also be utilized in ring-type flood electron guns such as shown and described in US. Patent No. 2,967,971 issued to C. D. Beintema and assigned to the instant assignee. This latter method of construction becomes apparent if the elevational cross section of the flood gun shown in FIGURE 3 is viewed as a cross section through a continuous toroidal structure concentric With the axis of the two with the writing gun or guns disposed at the center thereof. In such an embodiment, the cathode would be a flattened toroid with the heater element being located coaxially therewith. The shape of the G1 and G2 electrode meshes in this construction would thus be segments of toroidal surfaces. Such a structure would possess the advantage of having a higher cathode current at lower heater power.

There thus has been described a novel and useful flood electron gun capable of producing a high density flood electron beam having a large angle of divergence which is especially useful in relatively large cathode ray storage tubes.

What is claimed is:

1. A flood electron gun comprising a hollow tube and having a closed end, an electron emissive source disposed on said closed end of said tubular member, means within said tubular member for heating said electron emissive source, a first cylindrical electrode member disposed around said tubular member and having an aperture in one end thereof in which at least the surface of said electron emissive source extends, a second cylindrical electrode member disposed over and around said first cylindrical electrode member and having an aperature in one end thereof through which said electron emissive source is exposed, a third cylindrical electrode member disposed over and around said second cylindrical electrode member and having an aperture therein adjacent the aperture in said second cylindrical electrode member, and mesh members mounted on said second and third cylindrical electrode member and over the apertures therein.

2. The invention according to claim 1 wherein said mesh members and the surface of said electron emissive source are spheroidal.

3. A flood electron gun comprising a hollow tubular member having a closed end, an electron emissive source disposed on said closed end of said tubular member, means within said tubular member for heating said electron emissive source, a first cylindrical electrode member disposed around said tubular member and having an aperture in one end thereof in which at least the surface of said electron emissive source is coextensive with and has substantially the same curvature as the surface of said one end of said first cylindrical electrode member, a second cylindrical electrode member disposed over and around said first cylindrical electrode member and having an aperture in one end thereof through which said electron emissive source is exposed, a third cylindrical electrode member disposed over and around said second cylindrical electrode member and having an aperture therein adjacent the aperture in said second cylindrical electrode member, and mesh members mounted on said second and third cylindrical electrode members and over the apertures therein.

4. The invention according to claim 3 wherein the aperture in said second cylindrical electrode member has a common axis with the aperture in said first cylindrical electrode member and with said electron emissive source, and the axis of said aperture in said third cylindrical electrode member being disposed at an angle with respect to said common axis.

5. The invention according to claim 3 wherein said mesh members and the surface of said electron emissive source are spheroidal.

6. A cathode ray tube including a target to be flooded with a flood beam of electrons, and a flood electron gun for forming said flood beam comprising a cathode member having an electron emissive portion, first and second nested electrode members having apertures therein disposed adjacent said electron emissive portion of said cathode member, and mesh members mounted on said first and second electrode members and over said apertures therein.

7. A cathode ray tube including a target to be flooded with a flood beam of electrons, and a flood electron gun for forming said flood beam comprising a cathode member having a dome-shaped electron emissive portion, first and second nested electrode members having apertures therein through which said dome-shaped portion of said cathode member is exposed, a d dome-shaped mesh members mounted on said first and second electrode members and over said apertures therein, the mesh member nearest said dome-shaped portion of said cathode member being substantially uniformly spaced from the surface of said domeshaped portion.

8. The invention according to claim 7 wherein the spacing between said mesh members is substantially constant.

9. The invention according to claim 7 where the spacing between said mesh members varies.

1 0 References Cited UNITED STATES PATENTS 2,532,339 12/1950 Schlesinger 3l368 2,782,334 2/1957 Gardner "313-82 5 2,283,041 5/1942 Broadway 313 s2 2,947,905 8/1960 Pierce 31382X JAMES W. LAWRENCE, Primary Examiner. 10 V. LAFRANCHI, Assistant Examiner.

@2 3? UNITED STATES PATENT ()FFICE CERTIFICATE OF CORRECTION Patent No. 3,377,492 Dated April 9, 1968 Inventor(s) F. G. Oe'ss It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

I Col. 3, line 3, after "beam" delete "Without requiring simultaneous adjustments of other" and insert thereat -density compensation to overcome nonuniformities-.

Col. 4, line 15, "withing" should be within.

Col. 5, line 45, delete "in" (first occurrence).

Col. 6, line 38, "white should be -whi1e-;

line 53, "PIGURES should be -PIGURE-.

Signed and sealed this 29th day of June 1971.

(SEAL) Attest:

EDWARD M-FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents