US2132933A

US2132933A - Cathode ray tube apparatus

Info

Publication number: US2132933A
Application number: US99194A
Authority: US
Inventors: Bowman-Manifold Michael; Blumlein Alan Dower
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1934-10-24
Filing date: 1936-09-03
Publication date: 1938-10-11
Anticipated expiration: 1955-10-11
Also published as: FR796978A; NL59466C; DE763825C

Description

Oct. 11, 1938. M. BOWMAN-MANIFOLD ET AL 2,132,933

, CATHODE RAY TUBE APPARATUS Original Fil'ed Oct. 25, 1935 2 Sheets-Sheet 1 l VENTQIS MICHAEL BOWMANMANI ALAN DOV/ BLUMLEI ATTORNEY CATHODE RAY TUBE APPARATUS 2 SheetsSheet 2 OriginaLFiled Oct. 25, 1935 INVENTOR Nw Y AL E MB N mgm B w L0 5% A HN Patented Oct. 11, 1938 UNITED STATES PATENT OFFICE CATHODE RAY TUBE APPARATUS Original application October 23, 1935, Serial No.

46,272. Divided and this application September 3, 1936, Serial No. 99,194. In Great Britain October 24, 1934 3 Claims.

The present invention relates broadly to improvements in cathode ray tube apparatus and more particularly to an improved form of deflecting means for controlling the cathode ray beam developed therein. This is a division of my application, Serial No. 46,272, filed October 23, 1935.

Cathode ray tubes for current and potential analysis, and for use in television and like systems,are provided with means for deflecting the cathode ray and for causing it to trace out a desired path on a fluorescent or other screen associated with the tube. The deflecting means often comprise two deflecting coils, or two pairs of deflecting coils, arranged to produce deflection of the ray in two mutually perpendicular directions.

In television systems for example, it is generally required that the cathode ray should trace out on the screen a number of successive parallel lines, each slightly below the one preceding it, the arrangement being such that the area scanned is of rectangular shape. It is accordingly required that the deflecting coils should produce deflection of the ray in two coordinate directions, the deflection in one direction taking place at a higher frequency than the deflection in the other direction. Such a deflection of the ray is achieved in practice, in a manner well known per se, by feeding to the deflecting coils, or pairs of coils, deflecting currents of suitably chosen different frequencies. The deflecting currents are generally of saw-tooth wave form.

The method of scanning outlined above produces distortion if the area scanned is of other than the desired shape; consequently, the deflecting means must be so constructed and arranged that the area scanned is of the desired shape and dimensions. For example, if it is desired to scan a rectangular area on the screen, using deflecting currents of saw-tooth wave form, it must be arranged that for a given change of current in one deflecting coil or pair of coils, the distance moved through by the cathode ray on the screen is independent of the initial position of the scanning spot where the cathode ray strikes the screen.

If the deflection of the beam in either coordinate is not a linear function of the deflecting current, that is, if it is not independent of the initial position of the scanning spot, the following undesirable result will occur; if the beam is converging while it passes through the deflecting field, difierent portions of the beam will be dif ferently deflected, and sharp focus will be lost,

while even if the beam be assumed to be infinitely thin, distortion of the picture will arise, as already explained.

It has proved in practice very difiicult to provide deflecting coils capable of operating without introducing distortion of the kind discussed above.

A further difiiculty which has been encountered in designing deflecting coils for cathode ray tubes will now be considered. Such tubes usually comprise an envelope having a frustro-conical portion joined to a cylindrical portion, and the coils have usually been mounted on the cylindrical part of the envelope of the tube; for convenience, this part of the envelope will be referred to as the neck of the tube.

When a set of coils is required to deflect the cathode ray through large angles, it is necessary to make the diameter of the neck of the tube relatively large, in order that the deflected beam shall not hit the neck but shall pass unobstructed into the frustro-conical portion of the tube. For large angles of deflection, therefore, it is necessary to make the diameter of the coils relatively large; inefficient coils are thus obtained, not only on account of the large diameter, but also since it may be necessary to make the length to diameter ratio of the coils relatively small; with a coil of which the length to diameter ratio is small, the uniformity of the field obtained is found to be poor.

It is one of the objects of the present invention to provide a cathode ray tube apparatus wherein the deflecting coils are so constructed and arranged that the deflection of the ray in each co-ordinate direction depends only on the deflecting current fed to the coils, so that distortion of the area scanned due to the deflecting coils is avoided. This will be realized if the set of coils for deflecting the ray in each direction produces a magnetic field which, in every plane at right angles to the axis of the cathode ray tube, is of uniform strength and direction over the area which the cathode ray beam traverse.

Another object of the invention is to provide a cathode ray tube wherein the deflecting means produce a magnetic field substantially only where it is required, without the expenditure of magnetizing energy in producing a field elsewhere, thereby reducing the magnetizing energy which is necessary to produce a field of the required flux density. This arrangement is adaptable to deflect the cathode ray beam through large angles. The deflecting coils according to this invention is wound about a space of cylindrical or frustro-conical shape, and adapted to produce within said space a magnetic field substantially at right angles to the axis of said space, said coil lying wholly outside the cylinder or cone of which said space fills a part, wherein said coil comprises a plurality of turns having active portions which lie close to said space and approximately parallel to the axis thereof, and wherein said active portions are so arranged that when current is passed through said coil, the magnetic field set up within said space, in a plane at right angles to the axis thereof, is substantially uniform. v

According to a feature of the present invention, a coil is wound to fill the whole or a part of the space defined by the surfaces of two imaginary solid figures both in the form of cylinders or conical frustra of substantially the same size and shape, of which one has been displaced bodily from a position of substantially complete coincidence with the other in a direction at right angles to the direction of the axes of the said figures through a distance short compared to the dimensions of each figure in a plane at right angles to the axis thereof, the winding being carried out in such a manner that when, in operation, current is caused to fiow in said coil, the direction of current fiow in that part of said coil which is on one side of a plane of intersection lying substantially at right angles to the direction of said displacement and containing two lines of intersection of the surfaces of said figures is opposite to the direction of current fiow in the part of said coil lying in the other part of said space; the coil may be located within and close to a sheath of magnetic material which serves as a return path for the magnetic flux.

In the preceding paragraph, the term cylinder is to be understood to mean the solid figure generated by a straight line which moves so that it is always parallel to a fixed straight line, and so that the ends trace out a closed curve. The term thus includes solid figures of circular, rectangular or any other cross section bounded by a closed curve. When the surfaces of the figures intersect one another along more than two lines, the plane of intersection referred to in the preceding paragraph is the median plane; it will be found that whatever the shape of the figures in crosssection, no difiiculty will be found in determining this median plane.

The term conical frustrum is intended to cover a frustrum of the solid figure generated by a line-which is usually straightand which moves so that one point in it remains fixed, while any other point therein traces out a closed curve such, for example, as a circle or a rectangle.

The axis of a solid figure, for the purpose of this specification is intended to mean a line running longitudinally through the figure, but not intersecting the surface thereof, the axis lying substantially in a median position; for example, the axis in the case of a right circular cylinder or frustrum is the axis of symmetry of the figure.

A coil according to a further feature of this invention comprises a plurality of turns each having two active portions arranged substantially parallel to one another and to the active portions of the other turns, and is so constructed that when, in operation, current flows in said coil, the

current per unit angle in the active parts of the turns of said coil in the neighbourhood of a plane which contains the longitudinal axis of the coil and cuts the lines of force inside the coil substantially at right angles, is greater than the current per unit angle in the neighbourhood of a plane which also contains said axis but is at right angles to said first-mentioned plane. By the expression current per unit angle is meant the current which fiows through the part of the crosssection of the coil bounded by two straight lines which intersect at the axis of the coil and make a unit angle with one another.

The longitudinal axis of the coil, referred to in the above paragraph, is a line lying within the coil in a substantially median position, which does not intersect the active parts of said turns but is substantially parallel to said active parts.

Our invention will best be understood by reference to the accompanying drawings in which Fig. 1 is an explanatory curve, Figs. 2 and 3 are indicative coil formers, Fig. 4 is a cross-sectional view of a coil mounting, and Fig. 5 is a longitudinal view of a coil mounting.

Before particular embodiments of the invention are describedreference will be made for purposes of explanation to Fig. l of the accompanying drawings; the disposition of the closed curve I in this figure with respect to axes X and Y, is exactly the same as that of the curve 2 with respect to axes X and Y, and the distance X between the Y and Y axes is small compared to the distance from the origin 0 to any point on the curve l. r

. Now the distance between the curves along any line parallel to the X axis is constant and equal to X, and hence the area of the parallelogram enclosed by the curves l and 2 and any pair of lines, parallel to the X axis and separated by an infinitely small distance, is constant. Thus it will be clear that if a line such as AB is drawn parallel to the Y and Y axes, the area of the cross-hatched portion ABCD between the two curves is proportional to the length AB.

Let it now be assumed that the line CDEFG represents the boundary of a tunnel, of constant cross section, cut in a block of magnetic material such as iron, the tunnel extending downwards into the paper. The curves l and 2 then represent cross sections of two identical cylinders having as axes lines drawn perpendicularly downwards into the paper through points 0 and O, and a coil of wire of uniform cross section is according to this invention wound in the space enclosed by the surfaces of these cylinders; the winding is carried out insuch a manner that the parts of the turns lying in the space between the surfaces of the cylinders run parallel to the axes of the cylinders, the arrangement being such that when a current is passed through the coil, the direction of current fiow in the part of the space to the left of the plane extending vertically downwards and containing the lines of intersection of the surfaces through E and G is opposite to the direction of current fiow in the part of the space to the right of that plane. There is thus a uniform distribution of current over the whole of the space between the cylinders.

Now since the cross-hatched area ABCD is proportional to the length AB, it will be clear that the magnetomotive force around the loop ABCD is also proportional to the length AB. The return path of lines of force passing from A to B comprises the path AD and BC and a path through the surrounding iron, which will be assumed to be of zero reluctance. The lengths AD and BC are both equal to the small displacement X and are thus small compared with AB for most values which AB may have. The-whole return path may thus be assumed. to be of zero reluctance. The reluctance of the air path from A to B is proportional to the length AB, and thus the reluctance of the loop ABCD is also proportional to the length AB. It follows then that the flux density along line AB is independent of the length of that line. The same is true of any line such as AB parallel to the Y axis, and hence the flux throughout the whole space within the coil is uniform.

In a simple case, the tunnel is rectangular in cross-section and in this casethe coil comprises two uniform layers of straight, parallel wires arranged along opposite sides of the tunnel and constituting the active portions of the coil. The iron in which the tunnel is cut constitutes the return path for the flux, and in practice preferably comprises a sheath, of any desired suitable shape, fitting closely around the coil. The active portions of the latter are connected by end portions arranged outside the sheath.

The construction and method of mounting of a pair of coils according to the present invention, for use as the deflecting coils of a cathode ray tube, will now be described by way of example. Reference will be made to Figs. 2 to 5 of the accompanying drawings, of which Fig. 1 has already been referred to; Figs. 2 and 3 respectively to the drawings show in cross-section suitable formers for winding a pair of coils according to the invention, Fig. 4 shows in cross-section a cathode ray tube with the coils mounted thereon, the section being taken at right angles to the longitudinal axis of the tube, and Fig. 5 illustrates another embodiment of the invention.

Referring to Fig. 4, the coils are mounted between the wall of the tube 3 and a ring 4 of magnetic material; the inner coil is shaded vertically, and the outer is shaded horizontally. The inner coil is wound on the former shown in Fig. 2, which comprises a wooden block 5 having a channel or groove cut in it and a portion 6 in the shape of a half of a right circular cylinder of wood. The radius r of the portion 6 is the radius of the outer wall of the cathode ray tube 3, Fig. 4. The channel in the block 5 is cylindrical in shape, and of the cross section which is defined by describing two quarter cir-.

cles of radius T1 about centres C and C, the channel is slightly flattened at its bottom. The portion 6 is also slightly flattened, and the two portions are held together in the positions shown by a releasable connection (not shown). The coil is wound of insulated wire so as to fill the Whole of the space between the portions 5 and 6 of the former, the wire being carried continuously around the portion 6, down one side and back along the other.

Two such coils are wound, and are then mounted on the tube 3 in the manner shown in Fig. 4, where references 1 and 8 designate the two coils which together constitute the inner coil. The coils are connected together in series, or parallel, depending on the inductance desired, the arrangement being such that when current flows in one direction in the left hand portions of the coils, it flows in the opposite direction in the right hand portions thereof.

Two similar coils 9 and I0 are wound on the former shown in cross-section in Fig. 3. The portion I5 is channelled wooden block, the channel being of semi-circular cross-section and of a radius R equal to the inner radius of the sheath 4. The cross-section of the approximately cylindrical portion I6 is determined as shown by two quarter circles of radius R1.

The coils 9 and I 0 are mounted upon the coils 1 and 8, if desired with the interposition of suitable insulating material, or of suitably shaped separating members, and the whole coil assembly is surrounded by a magnetic sheath 4 which may be built up from semi-circular or circular laminations. The coils 9 and ID are connected in series or parallel, and it is arranged that when current flows in one direction in the upper halves of these coils, it flows in the opposite direction in the lower parts thereof. The resultant field due to coils 'l and 8 is at right angles to that produced by coils 9 and ID. The overall length of the outer coils may be made rather less than that of the inner coils, if desired.

The parts of the turns outside the sheath 4 pass over the wall of the tube 3, and are so arranged that they lie in planes substantially at right angles to the longitudinal aXis of the tube: the magnetic field due to them thus produces little or no effect on the action of the deflecting field in the tube. The sheath may be formed by winding over the active portions of the turns of the deflecting coils a thick coil of iron wire; the shape of the sheath is of little importance, provided its provides flux-return paths of a reluctance which is negligible compared with the reluctance across the tube.

It is found that a coil of the form last described above, that is to say a coil wound to flll the space between the surfaces of two substantially right circular cylinders displaced from one another, is capable of producing a uniform field in the absence of an iron sheath. It is also found that in the absence of the iron sheath, the inductance of such a coil is approximately halved.

Referring to Fig. 5, a cathode ray tube has a glass envelope comprising a cylindrical portion I! of about one inch diameter closed at the left hand end, and having the other end joined to a right circular conical frustrum l8 2 inches in length; the frustrum l8 has a diameter at its larger end of. 2 inches, and this larger end is joined to the smaller end of a second right circular frustrum I9 which flares at a greater rate than the first frustrum and is closed at its larger end by an end wall 20. A fluorescent screen is mounted on this end wall.

Within the cylindrical portion l 1 is mounted an electron gun which may be of any suitable kind, and comprises essentially a cathode and modulator assembly 2|, a first anode 22 and a second anode 23. It is arranged that the second anode 23 lies near to the smaller end of the first frustrum.

Mounted upon the outer wall of the first frustrum I 8 are two pairs of deflecting coils indicated by reference 24; these coils are of a form very similar to that of the coils described with reference to Fig. 4, but instead of being wound to flt a cylindrical portion of the tube, they are wound to fit a frustro-conical portion. The coils are conveniently wound on formers such as are described in Figs. 2 and 3, but differing in that the winding boundaries are not cylindrical, but frustro-conioal. The formers are so constructed that the depth of the winding space decreases towards the wider end, and the nature of the resulting coil is therefore such that its thickness decreases as its diameter increases, as shown in Fig. 5. This is necessary in order that the same number of turns shall be accommodated at the narrower end as at the wider end.

A laminated iron yoke 25 having a conical hole therein may, if desired, be mounted as a snug fit over the scanning coils 24, and a cross-section through the coils in a plane at right angles to the longitudinal axis of the tube appears much as is shown in Fig. 4. The end portions of the turns of the coils are arranged to lie in planes substantially at right angles to the longitudinal axis of the tube, and thus the magnetic field clue to these portions produces little efiect on the defleeting field.

The part of the envelope on which the coils are mounted may be of other than right-circular frustro-conical shapes, for example, the tube may have a part in the shape of a frustrum of a pyramid and the coils may be wound on this part. The coils may also be wound on a portion of the tube which is in part of one shape, for example, cylindrical, and in part of another, for example conical. Thus the envelope of the tube may comprise a cylindrical portion flaring out into a single frustro-conical portion having a fluorescent screen at its larger end, the coils being arranged partly on each of the two portions. The coils may then be wound on formers so arranged that the winding space is of constant depth along the cylindrical parts, and tapers along the conical parts in the manner already described.

In an arrangement such as that last described, the inner pair of coils only may lie on both the cylindrical and the conical parts of the tube, the outer coils being arranged to lie over the cylindrical portions only of the inner coils. The end turns of the inner coils may be spread along the surface of the frustro-conical part of the tube, thus giving the maximum length for the outer coils and the maximum length for the magnetic yoke if one is employed.

As has already been stated, the application of the invention is not limited to coils for use in association with cathode ray tubes; furthermore, the invention is not limited to the embodiments described. Many other uses for coils according to the invention, and many modifications thereof within the scope of the appended claims will ocour to those versed in the art.

What we claim is:-

l. A cathode ray tube including means for developing a cathode ray beam and fluorescing means adapted to respond to the impingement of said cathode ray beam thereon, an envelope, and means for controlling the deflection of the cathode ray beam comprising at least one coil wound about a solid figure the shape of which is substantially that defined by a straight line which rotates about another nonintersecting straight line, the two straight lines lying always in the same plane, for producing within said space a magnetic field at right angles to the axis of said space, said coil comprising a plurality of turns lying wholly outside the solid figure of which said space occupies a part, each of said turns comprising active. portions lying substantially parallel to said axis, said active portions being so distributed that when current flows in said coil, the magnetic field at all points in a plane at right angles to said axis is substantially uniform, and mounted on said envelope so as to produce a substantially uniform field at right angles to the axis of said tube.

2. A cathode ray tube including means for developing a cathode ray beam and fluorescing means adapted to respond to the impingement of said 'cathod ray beam thereon, an envelope, a sheath of magnetic material arranged around the surface of said envelope and spaced apart therefrom, means for controlling the deflection of the cathode ray beam comprising a coil wound about a solid figure the shape of which is substantially that defined by a straight line which rotates about another nonintersecting straight line, the two straight lines lying always in the same plane, for producing within said space a magnetic field at right angles to the axis of said space, said coil comprising a plurality of turns lying wholly outside the solid figure of which said space occupiesa part, each of said turns comp-rising active portions lying substantially parallel to said axis, said active portions being so distributed that when current fiows in said coil, the magnetic field at all points in a plane at right angles tosaid axis is substantially uniform, said coil filling at least partly the space between said tube envelope and said sheath.

3. A cathode ray tube including means for developing a cathode ray beam and fluorescing means adapted to respond to the impingement of said cathode ray beam thereon, an envelope, a sheath of magnetic material arranged around the surface of said envelope and spaced apart therefrom, means for controlling the deflection of the cathode ray beam comprising a pair of coils each wound about a solid figure the shape of' which is substantially that defined by a straight line which rotates about another nonintersecting straight line, the two straight lines lying always in the same plane, for producing within said space a magnetic field at right angles to the axis of said space, each of said coils comprising a plurality of turns lying wholly outside the solid figure of which said space occupies a part, each of said turns comprising active portions lying substantially parallel to said axis, said active portions being so distributed that when current flows in said coils, the magnetic field at all points in a plane at right angles to said axis is substantially uniform, said coils filling at least partly the space between said tube envelope and said sheath, so that the magnetic field of each of the coils is substantially at right angles to that of the other.

MICHAEL BOWMAN-MANIFOLD. ALAN DOWER BLUMLEIN.