449,533

PATENT SPECIFICATION

Application Date: Oct. 24, 1934. No. 30545/34.

" " " : April 18, 1935. No. 12052/35.

One Complete Specification Left: Sept. 18, 1935.

(Under Section 16 of the Patents and Designs Acts, 1907 to 1932.)

Specification Accepted: June 24, 1936.

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PROVISIONAL SPECIFICATION

No. 30545 A.D. 1934.

Improvements in and relating to Coils for producing a Magnetic Field of approximately uniform Flux Density, for use, for example, with Cathode Ray Tubes

We, MICHAEL BOWMAN-MANIFOLD, a British Subject, of "Heathcot", Worplesdon Station, Surrey, and ALAN DOWER BLUMLEIN, a British Subject, of 32 Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to improvements in and relating to coils for producing a magnetic field of approximately uniform flux density, for use, for example, with cathode ray tubes.

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 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 fluorescent 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 beam in two co-ordinate directions, the deflection in one direction taking pace at a higher frequency than the deflection in the other direction. This is achieved in practice, in a manner well know 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 of 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 magnitude of the deflection of the ray from its mean position.

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

It is one of the objects of the present invention to provide a cathode ray tube having deflecting coils so constructed and arranged that the area of the screen scanned by the cathode ray is determined substantially solely by the nature of the scanning currents, distortion of this area due to the deflecting coils being negligible or absent.

According to the present invention, a coil such for example as a deflecting coil for use with a cathode ray tube is would to fill the whole or a part of the space defined by the surface of two imaginary cylinders of substantially the same size and shape, of which one has been displaced bodily from a position of substantially complete co-incidence with the other in a direction at right angles to the direction f the axes of the cylinders through a distance short compared to the dimensions of each cylinder in a plane at right angles to the axis thereof, the winding being carried out in such a manner that when, in operation, current flows in said coil, the direction of current flow in that part of said coil which is one side of a plane lying substantially at right angles to the direction of said displacement and containing two lines of intersection of the surfaces of said cylinders, is opposite to the direction of current flow in the part of said coil lying in the other part of said space, the coil being 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 solids of circular, rectangular or any other cross section bounded by a closed curve. When the surfaces of the cylinders intersect one another along more than two lines, the plane referred to in the preceding paragraph is the median plane; it will be found that whatever the shape of the cylinders in cross-section, no difficulty will be found in determining this media plane.

According to a feature of the invention, a cathode ray tube is associated with a deflecting coil comprising a plurality of turns, each of which is so wound that when, in operating, current flows therein, the magnetic field set up in said tube is due substantially entirely to two active portions of the turn in question, the arrangement being such that the active portions of substantially all of said turns embrace said tube; that is to say, if the active portions are straight and parallel, for example, the plane containing them intersects the tube. The coil is preferably arranged between the wall of the tube and a sheath of magnetic material, the latter offering a return path of low reluctance to the lines of force traversing said tube.

According to a further feature of the invention, a deflecting coil is disposed between the wall of a cathode ray tube and a sheath of magnetic material, and the coil is so wound that the magnetic field set up on said tube when, in operating, current flows in said coil, is due substantially entirely to active portions of the turns of said coil lying substantially parallel to the longitudinal axis of said tube; it is preferably arranged that substantially no part of said magnetic material is embraced by the turns of said coil, and the arrangement may be made such that each turn has two active portions which lie on opposite sides of a plane containing said longitudinal axis, the direction of current flow in one of said two active portions being opposite to the direction of current flow in the other.

According to another feature of the invention, a deflecting coil is arranged to fill the whole or a part of the space between the wall of a cathode ray tube and a sheath of magnetic material, the arrangement being such that substantially no part of said magnetic material is embraced by the turns of said coil. The coil may be so constructed and disposed that when, in operation, current flows in said coil, the current density in the parts of said coil in the neighbourhood of a plane which contains the longitudinal axis of the tube and cuts the lines of force in said tube substantially at right angles, is greater than the current density in the neighbourhood of a plane which also contains said axis but is at right angles to said first-mentioned plane.

According to yet another feature of the invention, a pair of deflecting coils are arranged between the wall of a cathode ray tube and a sheath of magnetic material in such a manner than substantially no part of said magnetic material is embraced by the turns of said coils, the arrangement being such that when, in operation, current flows in said coils, the magnetic fields thereby set up in said tube are substantially at right angles to one another.

Consider now Fig. 1 of the accompanying drawing, the disposition of the closed curve 1 with respect to axes X, Y is exactly the same as that of the curve 2 with respect to axes X and Y1, and the distance X1 between the Y and Y1 axes is small compared to the distance from the origin O to any point on the curve 1.

Now the distance between the curves along any line parallel to the X axis is constant, and hence the area of the parallelogram enclosed by the curves 1 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 Y1 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 1 and 2 then represent cross sections of two identical cylinders, 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 in such a manner than 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 flow in the part of the space to the left of the plane extending vertically downwards and containing the lines of inner-section of the surfaces through E and G is opposite to the direction of current flow 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 ABDC is proportional to the length AB, it will be clear that the magneto-motive force along the chord AB is also proportional to its length. The return path of lines of force passing from A to B comprises the paths 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 X1 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 it will be clear that the flux along the line AB is constant. The same is true of any line such as AB parallel to the Y axis, and hence the flux throughout the whole coil is uniform.

In a simple case, the tunnel is rectangular in cross-section and in this case the 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 continues 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. It will be assumed that the coils are to be mounted upon the wall of a tube of circular cross-section. Reference will be made to Figs. 2, 3 and 4 of the accompanying drawing, in which Figs. 2 and 3 respectively show in cross-section suitable formers for winding the two coils, and Fig. 4 shows the tube with the coils mounted thereon, in cross-section.

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. The channel in the block 5 is formed as shown by describing two quarter circles of radius r about centres C and C1 and 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. The coil is wound of insulted 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.

Two such coils are wound, and are then mounted on the tube 3 in the manner shown in Fig. 4, where references 7 and 8 designate the two coils. The coils are connected together in series, the arrangement being such that when current flows in one direction in the left hand portions of the coils, it flow in the opposite direction in the right hand portions thereof.

Two similar coils 9 and 10 are wound on the former shown in cross-section in Fig. 3. The portion 16 is a channeled wooden bloc, the channel being of semp9crcuilar cross-section and of a radius R equal to the inner radius of the sheath 4. The cross-section of the portion 16 is determined as shown by two quarter circles of radius R1.

The coils 9 and 10 are mounted upon the coils 7 and 8, if desired with the interposition of suitable insulating material, 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 10 are both connected in series, 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 7 and 8 is at right angles to that produced by coils 9 and 10.

The parts of the turns outside the sheath 4 pass over the wall of the tube 3, and are so arranged that the 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 reflecting field in the tube. The sheath may be formed by winding over the deflecting coils a thick coil of iron wire; the shape of the sheath is of little importance, provided it 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 fill the space between the surfaces of two substantially right circular cylinders is 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.

It is to be understood that the application of coils according to the present invention is not limited to their use as deflecting coils for cathode ray tubes.

Dated this 24th day of October, 1934.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4

PROVISIONAL SPECIFICATION

No. 12052 A.D. 1935.

Improvements in and relating to Coils for producing a Magnetic Field of approximately uniform Flux Density, for use, for example, with Cathode Ray Tubes

We, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, and MICHAEL BOWMAN-MANIFOLD, of "Heathcot", Worplesdon Station, Surrey, both British Subjects, do hereby declare the nature of this invention to be as follows:-

The present invention relates to coils for producing a magnetic field of approximately uniform flux density, for use, for example, with cathode ray tubes.

In co-pending Application No. 30545/34, the construction of a set of deflecting coils for a cathode ray tube is described, the coils being adapted to be mounted on a cylindrical part of the wall of the envelope of the tube; for convenience, this part of the envelope will be referred to as the neck of the tube.

When such 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 pas unobstructed into the frusto-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 the length to diameter ration of the coils becomes relatively small, and much of the wire is therefore used up as end turns and does not contribute to the deflecting field. Further, 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 an object of the present invention to provide improved or modified coils of the kind forming the subject of co-pending Application No. 30545/34 above referred to, coils according to the present invention being suitable for use s deflecting coils in association with a cathode ray tube in which it is desired to deflect the ray through large angles.

According to the present invention, the whole or a part of a coil of a kind which is a modification of that referred to in the preceding paragraph is wound so as substantially to fill the whole or a part of the space defined by the surfaces of two imaginary conical frusta of substantially the same size and shape, of which one has been displaced bodily in a transverse direction form a position of substantially complete coincidence with the other through a distance short compared with the dimension of the frusta. By transverse direction is meant a direction which, in the case of right circular frusta, is at right angles to the axes; in this case, the displacement is made short compared with the dimensions of the frusta in a plane at right angles to the axes.

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

Coils according to this invention have certain features in common with the coils which form the subject of the above-mentioned co-pending application. Other features of coils according to the present invention will be apparent from the following description.

A cathode ray tube has a glass envelope comprising a cylindrical portion of abut one inch diameter closed at one end, and having the other end joined to a right circular conical frustum 2Ĺ inches in length; the frustum has a diameter at is larger end of 2 inches, and this larger end is joined to the smaller end of a second right circular frustum which flares either at the same rate as or at a different rate from the firs frustum and is closed at its larger end by an end wall. A fluorescent screen is mounted on this end wall.

Within the cylindrical portion is mounted an electron gun which may be of any suitable kind, and comprises essentially a cathode, a first anode and a second anode. It is arranged that the second anode lies near to the smaller end of the first frustum.

Mounted upon the outer wall of the first frustum are two pairs of deflecting coils: these coils are of a firm very similar to that of the coils described in the co-pending application referred to above, but instead of being wound to fit a cylindrical portion of the tube, they are wound to fit a frusto-conical portion. The coils are conveniently wound on formers such as are described in the co-pending application, but differing in that the winding boundaries are not cylindrical, but frusto-conical. 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. 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 having a conical hole therein may, if desired, be mounted as a snug fit over the scanning coils, 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 of the drawing accompanying co-pending Application No. 30545/34. 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 due to these portions produces little effect on the deflecting field.

The part of the envelope on which the coils are mounted may be of other than right-circular frusto-conical shapes; for example, the tube may have a part in the shape of a frustum of a pyramid. 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 frusto-conical portion having a fluorescent screen at its larger end, and the coils being arranged partly on each of the two portions. The coils may the 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 frusto-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.

Dated this 18th day of April, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4

COMPLETE SPECIFICATION

Improvements in and relating to Coils for producing a Magnetic Field of approximately uniform Flux Density, for use, for example, with Cathode Ray Tubes

We, MICHAEL BOWMAN-MANIFOLD, a British Subject, of "Heathcot", Worplesdon Station, Surrey, and ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, do hereby declare the nature of this invention, and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to improvements in and relating to coils for producing a magnetic field of approximately uniform flux density, for use, for example, with cathode ray tubes.

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 desire path on a fluorescent or other screen associated with the tube. The deflecting means often comprise two deflecting coils, or two pair 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 co-ordinate 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 on the initial position of the scanning spot where the cathode ray strikes the screen.

In the deflection of the beam in either co-ordinate 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, different portions of the beam will be differently 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 difficult to provide deflecting coils capable of operating without introducing distortion of the kind discussed above.

A further difficulty which has been encountered in designing deflecting coils for cathode ray tubes will now be considered. Such tubes usually comprise an envelope having a frusto-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 frusto-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 coils suitable for use, for example, as deflecting coils for a cathode ray tube which 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 realised 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 traverses.

Another object of the invention is to provide a coil adapted to produce a magnetic field substantially only where it is required, without the expenditure of magnetising energy in producing a field elsewhere, thereby reducing the magnetising energy which is necessary to produce a field for the required flux density.

It is a further object of the invention to provide coils which are suitable for use, for example, as deflecting coils in association with a cathode ray tube in which it is desired to deflect the ray through large angles.

It should be borne in mind that coils according to this invention, while they are well suited for use with cathode ray tubes, are not so limited in their application, but can be employed for many other purposes in which a field of very uniform flux density is required.

The present invention accordingly provides a coil wound to lie wholly outside a space of cylindrical or frusto-conical shape and to produce within the space a magnetic field substantially at right angles to the axis thereof, wherein said coil comprises a plurality of turns having active portions which are disposed close to the surface of said space and are arranged to lie substantially in planes containing said axis, and wherein there is provided around and close to said space a sheath of magnetic material arranged to provide a return path of relatively low reluctance between the points where the lines of force of said magnetic field leave said space to the points where these lines of force re-enter said space, the dispositions of said active portions being such that, when current is passed through said coil, the strength of said magnetic field is substantially uniform over any plane at right angles to said axis and within said space.

In the present specification, the term cylinder is to be understood to mean the solid figure generated by a straight line of finite length which moves so that it is always parallel to a fixed straight line, and so that its ends trace out a closed curve or polygon. The term thus includes solid figures of circular, rectangular or any other cross section bounded by a closed curve or polygon, the figure of circular cross-section being referred to as a circular cylinder.

The term conical frustum is intended to cover a frustum of the solid figure generated by a line - which is usually straight - and which moves so that one point in it remains fixed, while any other point therein traces out a closed curve or polygon 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 intersection the surface thereof, the axis lying substantially in a median position; for example, the axis in the case of a right circular cylinder or frustum is the axis of symmetry of the figure.

The invention further provides a coil wound substantially to fill the space swept out by the surface of an imaginary cylinder or conical frustum when displaced in a direction at right angles to its axis through a distance short compared to its dimensions in a plane at right angles to said axis, wherein the turns of said coil are so disposed that when, in operation, current is caused to flow ins aid coil, the direction of current flow in all the active parts of the turns of said coil which lie on one side of a lane which is at right angles to the direction of said displacement and bisect said space, is opposite to the direction of current flow in the active part of the turns of said coil lying on the other side of said plane, and wherein there is provided around the close to said coil a sheath of magnetic material arranged to provide a return path of relatively low reluctance between the points where the lines of force of the magnetic field leave the space within the coil and the points where these lines of force re-enter this space.

In a modification of the present invention as defined above, the shape of the space within the coil is substantially that of a circular cylinder or circular conical frustum, and the sheath of magnetic material is omitted.

Before particular embodiments of the invention are described, reference will be made for purposes of explanation to Fig. 1 of the drawing accompanying the provisional specification of application No. 30545/34; the disposition of the closed curve 1 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 Y1, and the distance X1 between the Y and Y1 axes is small compared to the distance from the origin O to any other point on the curve 1.

Now the distance between the curves along any line parallel to the X axis is constant and equal to X1, and hence in the limit the area of the parallelogram enclosed by the curves 1 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 X and Y1 axes, the area of the crosshatched 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 1 and 2 then represent cross sections of two identical cylinders having as axes lines drawn perpendicularly downwards into the paper through points O and O1, 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 in such 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 flow 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 flow 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. Assuming that the return path of the lines of force passing from A to B comprises AD and BC, and a path through the surrounding iron, it will be clear that the M.M.F. around this loop is proportional to the area ABCD, which in turn is proportional to the length AB. The lengths AD and BC are both equal to the displacement X1 which will always be small so that the lengths AD and BC are small compared with AB for most values of AB. If the path through the iron is of negligible reluctance, as will generally be the case, it will then be approximately true to say that the reluctance of any flux path through the coil is proportional to the length AB in that path. It follows then that the flux density along the line AB is substantially independent of the length of that line. The same is true of any line such as AB parallel to the Y axis, and in any plane within the coil at right angles to the axis of the coil, and hence the flux throughout the whole space within the coil will be substantially uniform. Actually the lines of force might be expected to follow paths approximately radially through the coil rather than the paths AD and BC but provided that X1 is small compared with the dimensions of the coil this will not appreciably modify the conditions stated above.

In a simple case, the tunnel is rectangular in cross-section and in this case the 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 loosely 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 4 of the drawing of the provisional specification of application No. 30545/34, and to the drawing accompanying the present specification, which is numbered Fig. 5; Figs. 2 and 3 respectively 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 or a right circular cylinder of wood. The radius r of the portion 6 is the radius of the other 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 circles or radius r1 about centres C and C1; 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 or 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 7 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 shown 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 10 are wound on the former shown in cross-section in Fig. 3. The portion 15 is a channeled 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 16 is determined as shown by two quarter circles of radius R1.

The coils 9 and 10 are mounted upon the coils 7 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 build up form semi-circular or circular laminations. The coils 9 and 10 are connected in series or parallel, and it is arranged that shown 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 7 and 8 is at right angles to that produced by coils 9 and 10. 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; it is found in practice that by suitable disposition of the end turns, the magnetic field due to the end turns produces substantially no adverse effect on the uniformity 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 it 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 fill the space between the surfaces of two substantially 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 17 of about one inch diameter closed at the left hand end, and having the other end joined to a right circular conical frustum 18 2Ĺ inches in length; the frustum 18 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 frustum 19 which *** at a greater rate than the fires frustum 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 17 is mounted an electron gun which may be of any suitable kind, and comprises essentially a cathode and modulator assembly 21, 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 frustum.

Mounted upon the outer wall of the first frustum 18 are 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 fit a cylindrical portion of the tube, they are wound to fit a frusto-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 frusto-conical. The formers are so constructed that the depth of the winding space decreases towards the winder 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 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 due to these portions produces little effect on the deflecting field.

The part of the envelope on which the coils are mounted may be of other than right-circular frusto-conical shapes; for example, the tube may have a part in the shape of a frustum 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 frusto-conical portion having a fluorescent screen at is 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 tapes 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 frusto-conical part of the tube, this giving the maximum length for the outer coils and the maximum length for the magnetic yoke of 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 occur to those versed in the art.

Having now particularly described and ascertained the nature of our said invention, and in what manner the same is to be performed, we declare that what we claim is:-

  1. A coil wound to lie wholly outside a space of cylindrical or frusto-conical shape and to produce within the space a magnetic field substantially at right angles to the axis thereof, wherein said coil comprise a plurality of turns having active portions which are disposed close to the surface of said space and are arranged to lie substantially in planes containing said axis, and wherein there is provided around and close to said space a sheath of magnetic material arranged to provide a return path of relatively low reluctance between the points where the lines of force of said magnetic field leave said space and the points where these lines of force re-enter said space, the dispositions of said active portions being such that, when current is passed through said coil, the strength of said magnetic field is substantially uniform over any plane at right angles to said axis and within the coil.
  2. A coil wound substantially to fill the space swept out by the surface of an imaginary cylinder or conical frustum when displaced in a direction at right angles to its axis through a distance short compared to its dimensions in a plane at right angles to said axis, wherein the turns of said coil are so disposed that when, in operation, current is caused to flow in said coil, the direction of current low in all the active parts of the turns of said coil which lie on one side of a plane which is at right angles to the direction of said displacement and bisects said space, is opposite to the direction of current flow in the active parts of the turns of said coil lying on the other side of said plane, and wherein there is provided around the close to said coil a sheath of magnetic material arranged to provide a return path of relatively low inductance between the points where the lines of source of the magnetic field leave the space within the coil and the points where these lines of force re-enter this space.
  3. A coil according to either of the preceding claims, wherein the space within the coil is other than rectangular shape.
  4. A coil according to any of the preceding claims, comprising a plurality of turns each having their active portions lying substantially parallel to said axis, said active portions being connected by end portions which are so placed that the coil lies wholly outside the cylindrical or frusto-conical space surrounded thereby and wholly outside prolongations of this space in both directions.
  5. A coil according to any of the preceding claims, wherein substantially only the active portions of said coil are embraced by said sheath.
  6. A coil constituting a modification of that claimed in claim 1 or 2, in which the shape of the space within the coil is substantially that of a circular cylinder or a circular conical frustum, and in which said sheath of magnetic material is omitted.
  7. A coil according to any of the preceding claims, and so constructed as to be wholly or in part of frusto-conical shape, wherein the thickness of the coil is less at the wider end of the frustum than at the narrower end thereof.
  8. A cathode ray tube having mounted so as t fit closely on the envelope thereof a coil or coils according to any of the preceding claims.
  9. A cathode ray tube according to claim 8, wherein there are provided two coils or pairs of coils, the disposition of which is such that one of said coils or pairs of coils surrounds the other and produces a magnetic field substantially at right angles to that produced by the other.
  10. A cathode ray tube having mounted on the outer wall of the envelope thereof two coils or pairs of coils according to any of claims 1 to 3, the disposition of which is such that one of said coils or pairs of coils produces a magnetic field substantially at right angles to that produced by the other, the outer surface of the envelope of said tube and the inner surface of said sheath being of substantially the same shape, and said two coils or pairs of coils being so arranged as substantially to fill the space between said tube and said sheath.
  11. A coil substantially as herein described with reference to the drawings.

Dated this 18th day of September, 1935,

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. - 1936.