US2214973A - Cathode ray tube - Google Patents

Description

Sept. 17, 1940. ROSE 2,214,973

CATHODE RAY TUBE Filed Feb. 25, 1938 2 Sheets-Sheet l INVENTOR.

ALBERT ROSE 2 Sheets-Sheet 2 INVENTOR. ALBERT ROSE ATTORNEY.

Patented Sept. 17, 1940 vireo STATES ATENT trio carnonn nay TUBE Albert Rose, East Orange, N. J assignor, by mesne assignments, to Radio Corporation of America, New York, Delaware N. Y.', a corporation of Application February 25, 1938, Serial No. 192,451 Claims. (01. 250-153) My invention relates to electron discharge aD-' composed of low velocity electrons focused on the mosaic electrode by a uniform magnetic field,

the mosaic electrode being wholly immersed in the field. I have found that notwithstanding the presence of the magnetic field, the electrons tend to spread from the desired path or paths as they approach the mosaic electrode, probably because of the building up of negative electrostatic charges on the surface of the electrode which repel the electrons causing some of them to be so directed to those portions of the mosaic electrode which are less negatively charged. Since all the electrons do not reach those portions of the mosaic electrode at which they are directed, the final result is to reduce the resolution and sensitivity that should be obtained.

One object of my invention is to provide a television transmitting tube of the magnetically focused type of greater sensitivity capable of producing television signals which are of greater intensity than those obtained in the usual way and which are capable of producing television signals which will recreate an optical replica of the transmitted image with good fidelity and high resolution. Another object is to provide for use in such tubes mosaic electrode structures having active elemental areas which receive practically all of the electrons directed toward said areas. A further object is to provide a method of projecting electrons on a mosaic electrode made in accordance with my invention.

40 In accordance with my invention I provide a mosaic electrode, for use in magnetically focused televisiontransmitting tube, which is so designed and made that the elemental active particles of the mosaic have a higher magnetic permeability 45 than the support on which the particles are carried. With a mosaic electrode made in accordnace with my invention the magnetic lines of force normal to the mosaic electrode are concentrated at the elemental active areas.

A better understanding of my invention will be obtained and other-objects, features, and ad-- vantages will appear from the following descrip tion taken in connection with the accompanying 55 drawings in which:

Figure 1 is a longitudinal view partially in section illustrating one form of television transmitting tube incorporating my invention;

Figure 2 is a longitudinal view partially in section illustrating a modification of the tube shown 5 in Figure 1; r

Figures 3 and 4 are views of a portion of theelectrode structure shown in Figures 1 and 2;

Figure 5 is longitudinal view partially in section illustrating a television transmitting tube in- 10 corporating one modification of my invention; and, Figure 6 is a view of 'a portion of the mosaic electrode structure shown in Figure 5.

Referring to Figure 1 a television transmitting 15 tube, such as is more fully described in my copending application Serial No. 165,250, filed September 23, 1937, and incorporating my invention, includes in a highly evacuated envelope or bulb I a semi-transparent photosensitive cathode 2 which liberates electrons under the influence of an optical image of an object represented by the arrow 3. These electrons are focused on the front of a double sided mosaic electrode 4 by a magnetic focusing field produced by the field coil 5. The magnetic field also focuses on the back-of the mosaic'electrodea low velocity scanning beam of photoelectrons liberated from the semi-transparent photocathode B by a flying spot of light produced on the fluorescent screen i when it is scanned by a high velocity electron beam from the cathode ray gun 8. The photoelectrons from the photocathode 2 impinge on the front of the mosaic electrode at a sufflcient velocity to cause a greater number of secondary electrons to be liberated than there are photoelectrons, thus forming on the mosaic electrode a positive electrostatic image of the optical image. "A positive electrostatic image may also be formed by photoelectrons liberated directly from the active elements of the mosaic electrode in a modifled tube structure such as shown in Figure 2 where the optical image is formed directly on the mosaic electrode 4.

In accordance 'with my invention the mosaic electrode 6 which is wholly immersed in the magnetic focusing field generated by the magnetic coil 5 is so made and the field is so adjusted that the magnetic lines of force passing through the mosaic electrodes are concentrated in and around the elemental areas of the mosaic electrode to such an extent that the electrostatic charges on the elemental areas cause negligible deviation of the electrons from their desired paths toward the elemental areas of the electrode.

One form of my new and improved mosaic electrode shown in Figure 2 comprises a foundation member I of nonmagnetic metal or nonmagnetic alloy, that is a metal or alloy of low magnetic permeability, preferably woven in the form of a wire mesh screen which is coated with electrically insulating material such as the vitreous enamel H leaving apertures or interstices which are filled with electrically conducting material which is magnetic, that is, a material having relatively high magnetic permeability to form the plugs l2.

One material suitable for the foundation member is an alloy of 80% nickel and 20% chromium. The non-magnetic screen or foundation I0 is rolled and treated with acid to increase the area of the apertures or interstices as described by.

W. H. Hickok in U. S. Patent 2,047,369, July 14, 1936. Prior to enameling the non-magnetic foundation member, I find it particularly advantageous to clean the metal thoroughly then de-oxidize it by heating in hydrogen to remove any film of oxide which may be present. I then spray the foundation with the enamel ground to a particle size under 2 microns and held in suspension in either water or alcohol. .The sprayed screen or foundation is then fired in a furnace at about 900 C. in air for a period of one to two minutes to fuse the enamel into a smooth glassy coating which completely covers all of the metal surface and adheres firmly to it. I prefer to build the enamel coating up to a thickness of approximately 3 mils on the surface of the electrode and to a thickness of one-half to one mil on the walls of the holes or interstices in the electrode by applying the enamel in several thin coats and firing the foundation after each coat is applied. I find in this way that well insulated screen electrodes highly suitable for the production of mosaic electrodes for television transmitting tubes can be made with such a non-magnetic wire mesh screen.

Following the formation of the insulating coating on the non-magnetic foundation member and further in accordance with my invention, I fill the interstices thereof with a powder of finely divided metal such as nickel or other metal having magnetic properities. Nickel, iron, cobalt or alloys of iron, nickel and cobalt are suitable for this purpose, the main requirement being that the material used should have good magnetic properties. These metals and alloys are magnetic in a high degree and are usually referred to as ferromagnetic materials. Other metals or alloys which have only slight magnetic properties and are commonly referred to as paramagnetic materials. are not suificiently magnetic to warrant their use to obtain the desired objects of my invention, hence I use only the ferromagnetic materials for my purposes. This metal powder may be mixed with a binder such as paraffin and applied to the insulated screen while the screen is heated to approximately the melting point of the binder to fill the interstices with the metal powder, as more fully described in my co-pending application Serial No. 166,540 filed September 20, 1937.

The prepared electrode is fired or baked by placing it in a hydrogen atmosphere furnace which is preferably at about 400 to 600 C. to volatilize the binder below the sintering point of the metal powder. The baking is continued for one-half to one hour at 400 to 600 C. to sinter the metal powder and thereby form-the metal plugs l2. The sintering of'the metal powder proaaiaera vides a mosaic electrode wherein the plugs ii are electrically conducting and firmly held in the foundation so that the assembly will'withstand normal mechanical shock during use without the metal plugs falling out of the interstices. By performing the above baking operations in a hydrogen atmosphere the oxide of the metal such as nickel is prevented from forming which is desirable inasmuch as nickel oxide has relatively poor magnetic permeability.

While an electrode made as above described is suitable for use in television tubes such as shown in Figure 1 I find it advantageous, when the electrode is to be used in tubes such as shown in Figure 2 and where high photosensitivity of the metal plugs i2 is desired, to deposit on the exposed surfaces of the plugs l2 another metal which forms a better base for photoemissivity than the metal of the plugs. The use of a thin metal layer or cap l3, of metal such as silver, on the plugs is advantageous for this reason and also for obtaining uniform secondary emission when the electrode is used as shown in Figure l. The metal layer or cap l3 may be conveniently formed on the individual plugs H by an eletcrolytic deposition process and for this purpose I use a solution which may be'of the following constituents:

Sodium cyanide "ounces-.. 5 Silver cyanide .do 3% Sodium carbonate do 5 Water gall0n 1 This solution is used as an electrolyte in which the mosaic electrode is suspended between a silver anode and a nickel cathode. An electric current of 600 ma. is passed between the cathode and anode for a period of minutes to coat the metal plugs in a. screen having an area of approximately 20 square inches. Such treatment produces a coating ofsilver sufiiciently thick for optimum photosensitivity when oxidized and photosensitized with caesium or other alkali metal.

Figure 5 shows a television transmitting tube incorporating a modification of my invention where the mosaic electrode is of the photosensitive single sided type. An optical image of an object of which a picture is to be transmitted is focused on the photocathode 20 to liberate electrons which are focused on the mosaic electrodt 2! by the focusing coil 22 in much the same manner as in the tube shown in Figure 1 except that these electrons are accelerated to only a very low velocity so that a negative electrostatic image of the optical image is formed on the mosaic electrode.

The mosaic electrode 2| as best shown in Figure 6 comprises a thin sheet of transparent insulating material such as the sheet of mica 25 coated on one side with a semi-transparent film of metal 26, such as platinum which is substantially nonmagnetic, which in turn supports the fluorescent screen 23. Although platinum does, have slight magnetic properties, it is here referred to as being non-magnetic, because its magnetic susceptibility is negligible, being only one one-millionth that of nickel, and it, like other metals exhibiting negligible magnetic properties, is usually referred to as paramagnetic. Diamagnetic metals such as bismuth and antimony, are also substantially nonmagnetic in comparison with the ferromagnetic metals, so that both the paramagnetic and diamagnetic metals may be broadly classified as non- ,magnetic. On the opposite side of the sheet of mica 25Iprovide mutually separated metallic particles 21 of a metal having good magnetic properties such as nickel or the metals or alloys above referred to. These metal particles may be deposited on the mica sheet by laying a wire mesh screen on the mica sheet, vaporizing the metal which is to form the particles such as nickel and allowing it to condense as a deposit on the sheet of mica 25 through the interstices of the wire mesh screen. This deposition may be done very conveniently by enclosing the sheet of mica with the superposed wire mesh screen in an envelope or bell jar provided witha source of metal to be evaporated, vaporizing the metal by heat and allowing it to deposit on the mica through the wire mesh screen. Such a process for forming metal particles on a sheet of insulation is described by Tedham et al in U. S. Patent 2,077,442, April 20, 1937. The metal particles 21 should be of a size determined by the re -so1ution desired and of a thickness sufiicient to give the necessary mass of magnetic material in the particles to cause concentration of the magnetic field through the particles. Thus the thickness of the particles fiiould be several times the diameter of the paroles.

The negative electrostatic image is neutralized by photoelectrons liberated from'the surface of the mosaic by light from the fluorescent screen 23 when the screen is bombarded by electrons from the electron gun 24.

While a single sided mosaic electrode made in accordance with the above is satisfactory and the metal particles may be photosensitized, I have found it desirable to provide a thin film of silver or cap 28 on the metal particles 21 which allows even greater photosensitivity to be obtained. This film of 'silver may be deposited on the particles through the wire mesh screen subsequent to the formation of the particles and prior to removal of the wire mesh screen from the mica sheet and from the evacuated bell jar. The thickness of the film of silver is not critical, the only requirement being that the silver should cover the magnetic particles 21 with a thin coating.

In operation of a television tube incorporating my new and improved mosaic electrode the magnetic field in which the electrode is immersed is so adjusted that the field is sufiiciently concentrated through the magnetic particles to overcome the electrostatic forces created by the charges on the mosaic which would otherwise tend to deflect the electrons from the desired paths. Any increase in intensity of the field beyond the point at which the particles are magnetically saturated will not be eflective in overcoming the electrostatic forces created by the charges on the mosaic.

While I do not wish to be limited to any particular theory of operation, it seems probable that when my new and improved mosaic electrode is used in a television tube using magnetic focusing arrangements and where the mosaic electrode is wholly immersed in the magnetic field the magnetic lines of force are concentrated in and around the magnetic mosaic particles and that electrons which would be deviated by electrostatic charges on the mosaic electrode are controlled by the concentrated magnetic field so that the electrons impinge on those areas of the mosaic electrode to which they are directed.

While I have indicated the preferred embodiment of my invention of which I am now aware and have indicated the specific applications as directed to cathode ray tubes of the television transmitting type, it will be apparent that my invention is by no means limited to the exact dation, and a plurality of mutually separated metallic particles of ferromagnetic material supported by said foundation.

2. In combination with a cathode ray tube, means for generating a magnetic field, a mosaic electrode wholly immersed in said magnetic field comprising a base member of substantially nonfield comprising an electrically insulating founmagnetic material, and a plurality of mutually separated metallic particles of a ferromagnetic material supported by said base member and selected from the group of metals consisting of iron, nickel, cobalt, and alloys of iron, nickel and cobalt.

3. A method of scanning a mosaic electrode having mutually separated metallic particles with a stream of electrons which comprises pro: jecting a stream of electrons toward said mosaic electrode, producing a magnetic field substantially perpendicular to the plane of said mosaic electrode, tie-concentrating the magnetic field between the mutually separated particles, and concentrating the magnetic lines of force through said mutually separated particles to converge said electrons toward said metallic particles.

4. In combination with a cathode ray tube having an envelope enclosing a mosaic electrode comprising an electrically insulating foundation, a plurality of mutually separated metallic particles of ferro-magnetic material supported by said foundation and an electrical conductor of non-magnetic material adjacent said metallic particles and in capacitive relationship with said metallic particles, and means surrounding the portion of said envelope enclosing said electrode to generate a magnetic field having lines of force passing through said electrode.

5. In combination with a cathode ray tube having an envelope, means for generating a magnetic field having substantially parallel lines of force extending over a portion of the length of said envelope, and an electrode wholly immersed in said field and supported within said envelope in a plane substantially normal to said lines of force, said electrode comprising an electrically insulating foundation, a multiplicity of mutually separated nickel particles supported by said foundation and an electrically conductive coating coextensive with said nickel particles and in capacitive relation with said particles.

ALBERT ROSE.

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