US2062538A - Cathode ray tube - Google Patents

Description

Dec. 1, 1936. F. J. G. VAN DEN BOSCH 2,062,538

CATHODE RAY TUBE Filed Jan. 23, 1936 INVENTOR HTTORNEY.

Patented Dec. 1, 1 936 UNITED STATES CATHODE BAY TUBE Francois Joseph Gerard Van den Bosch, London,

Application January 23, 1936, Serial No. 60,371 In Great Britain February 13, 1935 4Claims.

A known form of cathode ray tube comprises a glass envelope in the form of a cylinder flaring out into a frusto-conical bulb, the inner face of the end wall of the bulb being coated with a substance, such as willemite or calcium tungstate, which becomes fluorescent under bombardment by the cathode ray. Within the envelope are arranged, in the order mentioned, a hot emitting cathode surrounded by an open-ended metal cylinder, one or more anodes each in the form of a centrally apertured disc,- a pair of horizontal and a pair of vertical deflecting plates.

Focussing of the ray on the fluorescent screen is achieved by suitably adjusting the relative positions and potentials of these electrodes, and hitherto great difliculty has been experienced in reducing the size of the fluorescent spot on the screen to that required for such purposes as'high definition television.

It is an object of the present invention to provide means whereby the cathode ray may be more accurately and finely focussed upon the screen, giving at the same time an increased light intensity.

With this end in view, and in accordance with the invention, the plates of each of the horizontal and vertical deflecting pairs of plates are coated on their adjacent faces with a substance such as an alkali metal oxide or iodide for example, strontium-oxide, barium-oxide, lime, iodide of calcium, calcium fluoride or caesium, which are known to emit electrons easily when i even slightly heated. Preferably the plates of each of said pairs are parallel over a minor portion of their axial length nearer the cathode and diverge outwardly from the cathode over a major portion farther from the cathode.

By axis of the tube" is meant a line coincident with the centre-line of the beam of rays passing through the tube.

- A preferred form of the invention will now be described with reference to the accompanying drawing, which is a perspective view of the electrodes of a cathode ray tube constructed in accordance with the invention.

A cathode ray tube comprises a cathode l, a Wehnelt cylinder 2 surrounding the cathode, a flrst anode 3 in the form of a centrally apertured disc, a second and similar anode 4, a pair of. vertical" deflecting plates 5 and 6 and a pair of horizontal deflecting plates 1 and 8. The two pairs of plates 5, 6 and I, 8 are termed vertical" and horizontal deflecting plates respectively, since they operate to deflect the ray in the vertical and horizontal planes respectively.

The vertical plate I is earthed and the vertical deflecting potentials are applied between the plates 8 and I. The horizontal plate 5 is maintained at 200 volts negative with respect to plate 1, and the horizontal deflecting potentials are applied between the plates 5 and 6. The second anode 4, the first anode 3, the Wehnelt cylinder 2 and the cathode I are maintained respectively at the following negative potentials with respect to the plate I: 600 volts, 1200 volts, 1435 volts and 1400 volts.

Adjacent faces of the plates 1, 8, and 5, 8, are coated with a substance, such as potassium or strontium oxide or caesium which emits electrons freely when a current is passed through the tube and these plates are slightly heated. Preferably a current of 5 to 20 milli-amperes is passed to these plates which then become slightly heated. Each pair may consist of parallel rectangular plates or, as shown, each plate such as 5 may be formed from a rectangular plate bent along a line 9 which is perpendicular to the tube axis Ill. The distance II, I2 is about A; of the axial length of the plate 5. The plates of each pair are bent outwardly and disposed symmetrically about the axis Hi.

It is found that by coating the plates as described, a sharply focussed and very small fluorescent spot can be produced on the screen, the focussing efi'ect being increased by inclining the plates as shown in the drawing.

The same eifect is obtained with soft tubes containing a rare gas such as argon, neon, helium or a gas of low molecular weight such as hydrogen.

The relative potentials of the various electrodes may be varied considerably from those quoted above.

The adjacent faces of the plates of each pair I, 8 and 5, 6 may be coated either all over or, as shown, only upon thedivergent portions.

I claim:-

1. A cathode ray tube comprising a cathode, an anode, a pair of vertical deflecting plates and a pair of horizontal deflecting plates, the plates of each of said pairs being coated, on their adjacent sides, with an electron emitting substance.

2. A cathode ray tube comprising a cathode, an anode, a pair of vertical deflecting plates and a pair of horizontal deflecting plates, the plates of each of said pairs being coated, on their adjac'ent sides, with an electron emitting substance selected from strontium oxide, barium oxide, lime, calcium iodide, calcium fluoride and caesium.

3. A cathode ray tube comprising a cathode, an anode, a pair or vertical deflecting plates and a pair of horizontal deflecting plates, the plates of each pair being parallel over a minor portion of their axial length nearer the cathode, diverging outwardly from the cathode 'over the remaining major portion further from the cathode and being coated on their adjacent taces with an electron emitting substance.

4. A cathode ray tube comprising a cathode. an anode, a pair of vertical deflecting plates and a pair oi horizontal deflecting plates, the plates 01' each pair being parallel over a minor portion 4 01' their axial length nearer the cathode diverging outwardly from the cathode over the remaining major portion further from the cathode and being coated on the outwardly divergent pottions or theiradjacent faces with an electron emitting substance.

FRANQOiS JQ SEPH GERARD VAN pm 30868.

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