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.Fuly 5, 1938. > I J. E. KEYSTON ET AL 29123311 ELECTRON DISCHARGE APPARATUS Filed March 31, 1936 3 Sheets-Sheet l INVENTORS ‘ John fa’qar Keysfon Frederick Hermes Nico/l 0H0 Klemp rer EYD'OW ATTORNEY July 5, 1938- J. E. KEYSTON ET AL 29123911 ELECTRON DISCHARGE APPARATUS Filed March 51, 1936 3 Sheets-Sheet 2 6'6 , 60 6 *4: J 5/ 52 4 58 J h EdIN VENTOR-S I( f drmzauzizes?-czz Julyi, 193. .1. E. KEYSTQN ET AL. 2,1239%“ ELECTRON DISCHARGE APPARATUS Filed March 31, 1936 5 Sheets-Sheet 5 Ta v INVENTORS John Edqar lfeysfon Frederick Hermes Mcol/ O?o Br - e - I rer - ATTORNEY _ Patented July 5, 1938 2,123,011 '1 ~ 'UNITED STATES PATENT OFFICE 2,123,011 ELECTRON DISCHARGE APPARATUS John Edgar Keyston, Hayes, Frederick Hermes Nicoll, Ickenham, and Otto Klemperer, Hayes, England, assignors to Electric & Musical In dustries Limited, Hayes, Middlesex, England, a British company Application March 31, 1936, Serial No. 71,970 In Great Britain March 19, 1935 4 Claims. (Cl. 178-72) The present invention relates to electron dis ments of another conventional type of cathode charge apparatus in which a stream of electrons ray tube without the present invention. acted on by focusing means, is caused to sweep This may be more clearly understood by ref over a screen. erence to Fig. 1 of the accompanying drawings in 5 In known types of electron discharge device‘ which there is shown diagrammatically part of a commonly referred to as cathode ray tubes, elec cathode ray tube of the kind referred to above. trons are emitted from a cathode and accelerated In the bulbous part 2 of the tube there is ar towards and focussed upon a screen associated with the tube, by means of what is known as an ‘10 electron lens. An electron lens may be of the electrostatic or electromagnetic type. In such tubes means may be provided for de?ecting the beam out of a straight path to any desired degree, so that it falls on any desired spot of the sur 16 face of the tube or screen. The de?ecting means may comprise two pairs of parallel plates between which the beam passes, the plates of one pair being at right angles to the plates of the other pair. By applying suitable potential differences 20 between the plates, the beam is de?ected out of a straight path due to the electrostatic ?eld be tween the plates. Alternatively, the de?ecting system may consist of a set of electromagnetic coils, usually placed outside the tube, de?ection 25 being effected by passing a suitable current . through the coils. ranged a screen I, which is scanned by a beam of electrons 3 from a cathode ray gun (not shown). In an unde?ected position 3a the beam pa 0 is focused at a point ‘I on the screen 1. Two elec trodes 4 and 5 are provided for sweeping the beam of electrons over the screen in directions parallel to the plane of the figure. ' The beam of electrons may be considered as being pivoted at the centre 15 of the de?ecting plates 4, 5 and the focused point of the beam 3 will describe an arc of a circle shown by the dotted line 6. In the positions 31) and 30 it is clear that the beam will not be in focus on the screen I, but will fall on a consider- 20 able area of the screen as shown at 8 and 9. Thus as the beam moves over the screen the area of the scanning spot will vary. A similar disadvantage is present in cathode ray tubes in which a beam of electrons is focused 25 upon a ?uorescent screen which is plane or has The electron lens system of the tube focuses the beam of electrons to a small spot which is at a ?xed distance from the cathode. 30 The de?ecting plates or coils sweep the focal point of the beam over part of the surface of a sphere, the centre of which may be considered to therein during de?ection of the beam, though lie at the centre of the de?ecting system. There fore, in order that the beam may always be said surface or screen is not shaped in the form of part of the surface of a sphere. 35 focused on the surface of the tube or screen, the latter must also be in the form of part of a sphere with its centre at the centre of the de?ecting sys In cathode ray tubes in which the electron lens is of the electrostatic type and the de?ecting sys tem of the tube. This is often not a convenient arrangement. 40‘For example, in certain cathode ray tubes, in particular those- used for television transmitting purposes there is often employed a plane screen, the normal to which is set at an angle to the un de?ected direction of the electron beam, which 45 angle may be as great as 45°. When an electron beam is caused to scan such a screen, it is clear 2from what has been said above that the cross sectional area of the beam where it strikes the screen will vary as it moves over the screen, and consequently the size of the scanning spot on the screen will vary. Fig. 1 shows a cathode ray tube of one conven tional type without the present invention. Fig. 2 shows schematically the operative ele such curvature that the centre of curvature does not lie in the region of the de?ecting system. It is one object of the present invention to pro vide means whereby a beam of electrons remains 3o focused on a surface of the tube or screen placed tem comprises de?ecting plates to which de?ect ing potentials are applied, it is found that the electrostatic ?eld set up across the de?ecting plates, more especially the ?eld across the pair of 40 plates nearest the electron lens, distorts the focus ing ?eld in the electron lens, with the result that _the focal length of the electron lens varies, as the beam moves over the screen, in accordance with - the magnitude of the de?ecting potential. 45 It is a further object of the present inven tion to provide means whereby the focal length of the electron lens may be made substantially inde pendent of the potential on the de?ecting plates. In Fig. 2 of the accompanying drawings there 5 is illustrated another known form of electron dis charge device in which electrons, acted on by focusing means, are caused to sweep over a screen. Referring to Fig. 2, this known device comprises a transparent photo-electrically sensitive surface 55 2 III, on to which an optical image of the object 'to' be transmitted is thrown by means of a lens II. The photo-electrons emitted from the surface are focused upon an apertured diaphragm l2 to form an electron image thereon. The electron image is caused to sweep over the aperture I: in the dia phragm I! so that the aperture l3 scans the elec tron image. Only those photo-electrons passing through the aperture ii at any time are operative 10 in developing picture signals for transmission. A coil I 4 producing a steady and substantially uniform magnetic ?eld in the general direction of electron travel is usually employed for focusing 15 20 25 30 35 40 tus comprising a cathode, an electron lens system. for focusing electrons emitted from said cathode on to a screen and means for de?ecting said elec trons over said screen, wherein there are provided means for automatically varying the effective fo cal length of said lens system in accordance with the de?ection of said electrons whereby changes in focus of the operative electrons from said cathode which would otherwise occur as said elec trons are de?ected over said screen are removed l0 or substantially reduced. In a modi?cation of the apparatus according to the preceding paragraph, where the electron lens purposes. The electrons may be accelerated to system and the de?ecting means operate electro wards the diaphragm by maintaining the dia statically, the means for automatically varying phragm l2 at a suitable positive potential with the effective focal length of said lens system in respect to the photoelectric surface of cathode ID. accordance with the de?ection of said electrons The aperture I3 is usually formed in the centre may be such that changes of the focal length of of the diaphragm i2 and the cathode I0 and the - . said lens system which would otherwise occur as’ diaphragm 12 are usually in the form of ?at a result of the distortion of the electrostatic ?eld plates arranged parallel and facing one another in said electron lens by said electrostatic de?ect and the magnetic focusing ?eld due to the coil ing means are removed or substantially reduced. i4 is then arranged to be normal to the planes According to a further feature of the present of these two plates. A thin cylinder of metal l5 invention there is provided electron discharge ap (usually acoating upon the walls of the tube in paratus comprising a cathode, an electron lens which the electrodes are mounted) may be pro system for focusing electrons emitted from said vided extending between regions close to the cathode on to the surface of a screen, and means edges of the two plates l0 and I2 and thus sur for de?ecting the electrons emitted from said rounding the space through‘which the electrons cathode over said screen surface, the screen sur travel. This metal cylinder I5 is of high re face not being a spherical surface having its sistance and can be arranged to ensure that there centre in the region of 'the de?ecting means, is a uniform potential gradient along the space wherein means are provided for automatically between the surface In and the diaphragm l2. varying the effective focal length of said lens The electrons forming the image on the dia system in accordance with the de?ection of said phragm I! move in spiral paths, the motion being electrons in such a manner as to reduce changes revoluble into a circular motion around a line of ' in sharpness of focus of the beam with de?ection force and a straight line motion in the direction thereof. of the electrostatic ?eld between the plates. The According to the present invention in another time of ?ight of electrons between the plates HI aspect there is provided cathode ray tube ap and I2 is dependent upon the distance and the paratus comprising a cathode, an electron lens' potential difference between the plates and is in system for focusing electrons from said cathode dependent of the path followed by individual into a narrow beam upon the surface of a screen electrons. The time required by an electron to perform one revolution under the in?uence of associated with the tube and means for de?ecting the beam over the screen, the screen surface not 45 the magnetic ?eld is inversely proportional to the line integral of the magnetic field strength along the path between the plates. In the unde?ected condition of the electron image this line integral has the same value for all electrons and it can 50 therefore be arranged by suitable choice of mag netic ?eld strength and accelerating potential . difference that in the unde?ected condition all electrons have rotated through the same angle (for example they may have made one revolu 55 tion) in the time taken to reach the diaphragm i2. Under these conditions a focused electron image will be formed on the diaphragm l2. , When however, the electron image is de?ected (for scanning purposes) by suitable means which 60 may be electrostatic or electromagnetic (for ex ample two pairs of "electromagnetic de?ecting coils, one pair of which are shown in section at l6 and I1, and one member of the other pair of which is shown in dotted lines in side view at l8) , the line integral changes in value and hence the angle of revolution of electrons during their passage between the plates l0 and I2 changes. Since the time of ?ight has not changed the im age is no longer sharply focused. It is a further object of the present invention to 70 provide means whereby this change of focus as the result of de?ection in devices of this kind can be reduced or eliminated. According to one feature of the present inven tion there is provided electron discharge appara being a spherical surface having its centre in the region of the de?ecting means, wherein means are provided for automatically varying the effec tive focal length of said lens system in accordance with the de?ection of the ray in such a manner as to reduce changes in sharpness of focus of the beam with de?ection thereof. According to the present invention in a further aspect there is provided apparatus for transmit ting images of an object to a distance comprising a photo-electrically active screen, means for pro jecting upon said ‘screen an optical image of an object to be transmitted, an apertured dia phragm spaced apart from said photo-electrically active screen, an electron lens system for focus ing upon said diaphragm electrons emitted from said screen under the influence of light, to form an electron image thereon, and means for sweep ing said electron image over said aperture in such a manner that said aperture scans said electron image, wherein there are provided means for varying the effective focal length of said lens system in accordance with the de?ection of said image in such a manner that changes in the sharpness of focus of said electron image with de?ection thereof are reduced. , Further features‘ of the invention will appear from the following description and appended claims. The invention will now be described with refer 70 ‘ 2,128,011 ' ence to Figs. 3 to 10 of the accompanying dia grammatic drawings in which Fig. 3 shows a cathode ray tube embodying one feature of the present invention. Fig. 4 shows a cathode ray tube having elec trostatic de?ecting plates and embodying a fur ther feature of the present invention. ' Fig. 5 shows a modi?cation of part of the tube of Fig. 4. Fig. 6 shows a cathode ray tube having electro magnetic-de?ecting means and embodying a fea ture of the present invention. Figs. 7 and 9 are explanatory diagrams. Fig. 8 is a circuit diagram for use in carrying 15 out one feature of the present invention, and V Fig. 10 is a circuit associated with a known form of cathode ray tube for use in carrying out the present invention. Referring to Fig. 3, the envelope of a cathode 20 ray tube consists of a neck portion 23 joined to a bulbous portion 2. In the neck portion 23 are arranged a heater coil is, a cathode 20, a cathode the Junction of the neck portion 23 with the bulbous portion 2 and the loweredge of .the screen- i. >This metallic coating may be main tained at a. potential of say 500 volts positive relative to the cathode 20, and may, if desired, be connected to the second anode 24 of the main electron lens system 22, 24. _ ' ' A further ring-shaped metallized zone 20 of relatively small area is provided upon the bul bous portion2, the median plane of this zone 10 being‘ inclined to the normal to the axis of the_ tube so that portionsthereoi'lie approximately opposite the upper and lower edges of the screen i. The ring-shaped zone 28 may be maintained at a positive potential of say 1000 volts relative 15 to the cathode 20. The lower part of the zone is much closer than the upper part thereof to the edge of the 500 volt’ coating 21. The electrostatic ?eld between the two coatings is thus asymmetri cal and can be arranged to lengthenthe effective 20 focal length of the electron lens system of the tube when the ray is on the upperv part of the screen shield 2| which may in certain cases be used as a that is in position b relatively to the e?’ective modulator and an electron'lens system compris focal length when the ray is on the, lower part ' of the screen in position 0. . 25 25 ing a ?rst anode 22 and a second anode 24. These electrodes may be in the form of cylindri~ cal tubes of equal diameters. The ?rst anode 22 is provided with two apertured diaphragms 22a and 22b the diaphragm 22a being positioned at 30 the cathode end of the cylinder and the dia Clearly the ?eld in which the correction is ap plied need not be an accelerating ?eld. The ring shaped zone 28 may be maintained at a lower potential than the metallic coating 21 so that The second anode 24 consists of a metal cylinder the correction is applied in'a decelerating ?eld. 30 In another arrangement according to the in vention a suitable varying potential which maybe derived from the de?ecting circuit is'applied to without any apertured diaphragms. The form of electrode assembly in the neck 3.5 an electron lens of the tube in such a way as to alter the effective focal length of this lens in ac 35 phragm 22b being positioned about two thirds of‘ the way along the cylinder from the cathode end. portion 23 used in this embodiment of the inven tion is not of great importance and the arrange ment shown is by way of example only. In the bulbous portion 2 of the tube is arranged 40 a screen I which is to be scanned by a beam of electrons 3. The screen I may consist for ex cordance with the de?ecting current or voltage applied to the de?ecting means. Such an arrangement is illustrated in Fig. 4 of the accompanying drawings. ' Referring to Fig. 4, which represents a side view of a cathode ray tube, in the tube are ar ample of a mosaic screen comprising a number of ranged‘ a cathode 20 heated by a heater coil 19, photo-electrically active metallic elements dis a cathode shield or modulator 2|, ?rst and second posed upon a mica sheet which is backed by a 45 metal signal plate. In using such a tube, an opti cal image of an object to be transmitted, is pro jected upon'the screen, and the screen is scanned by means of the cathode ray beam 3 which is de ?ected over the screen i by means of two pairs 50 of de?ecting plates, one pair of which is shown at 4, 5. A second pair of de?ecting plates (not shown) at right angles to those shown at 4, 5 may be provided between the latter and the screen I. Picture signals for transmission are taken The 55 from the signal plate by a lead not shown. manner of operation of television transmission anodes 22 and 24, de?ecting plates 4, 5 and a screen i. These electrodes may have the form described with reference to Fig. 3. A second pair of de?ecting plates indicated at 29 are provided between the plates 4, 5 and the screen I. Surrounding the space between the ?rst and second anodes 22 and 24 is arranged a 50 cylindrical electrode ‘30. This electrode will be referred to as a “compensating electrode”. The ?rst and second anodes 22 and 24 are given suitable positive potentials relative to the cathode 20, and the potential on the second anode 55 24 is made such in relation to the potential on apparatus of this kind is well known and need 7 the ?rst anode 22 that with zero potential on not be further described. In the following description it will be assumed 60 for the sake of convenience, that ‘the view of Fig. 3 is a side view of the tube, and that the de?ecting plates 4, 5, serve to de?ect the beam 3 65 the compensating electrode 30, the beam in the unde?ected condition is focused upon the screen. The second anode 24 is connected to the de 60 ?eeting plate 5 and to earth. An oscillator in dicated at 3| generates saw-tooth oscillations which are taken from a terminal 32 and applied vertically up and down the screen I. In order to focus the beam on the screen in to the plate 4 by lead 33 for the purpose of giv every de?ected position, the present invention, ing to the beam 3 the vertical component of a 65 scanning motion. Saw-tooth potentials of a in this embodiment, provides an electrode system for varying the effective focal length of the elec tron lens system existing between the anodes 22 and 24 by the production, in the path of the ray, 70 of an asymmetrical electrostatic ?eld. To this end a sliver coating 21 is formed on the walls - of the tube, extending partly into the neck por tion 23 and partly into the bulbous portion 2. The silvering 21 extends to a plane normal to the 75 tube axis and locatedv about half way between higher frequency applied to the plates 29 give the beam the horizontal or line component of scan ning. The terminal 34 of the oscillator is con nected to earth. , Across the terminals of the oscillator 3! is con-_ nected a resistance 31, and the compensating electrode 30 is connected through a condenser 36 to a variable tapping point on the resistance 31. In this way a voltage, derived from the de?ecting 70 4 9,188,011 potential, of wave form similar to the latter and of the same sign, though of smaller amplitude, is applied to the compensating electrode 34. Now when the plate 4 is at its maximum posi tive potential, the beam 3 falls on the top of the screen, and for ,the beam to be in focus, it is neces sary for the focal length of the lens to be a maximum. This is brought about by the positive potential applied at this time to the compensat ing electrode 30. Similarly when the plate 4 is negative with respect to the plate 5, the focal length maybe at a minimum; the negative po tential then applied to the compensating elec trode reduces the focal length of the electron 15 lens between the two anodes 22 and 24. The amplitude of the derived potential applied to the compensating electrode 30 is adjusted ‘to give correct compensation by adjusting the position of the tapping on the resistance 31. The invention in this embodiment is not lim ited to the particular means described for apply ing to the compensating electrode 30 a potential derived from that on the de?ecting plate 4. Any other suitable means may be provided for ob taining the derived potential. The compensating electrode 30 may be given any suitable bias potential. As shown in Fig. 4, the electrode 30 is connected to the cathode 20 through a leak resistance 38, and thus the mean potential of the electrode 30 is the potential of the cathode 20. The electrode 30 may however, be given any desired bias for example by means of a battery inserted in series with the leak resistance 38. The potential of the electrode 30 is preferably given a bias potential not far re moved from the cathode potential in order to ensure that substantially no current is collected thereby from the electron stream. An alternative arrangement of ?rst and sec ond anodes and compensating electrode is shown in Fig. 5. In that ?gure the electron lens com prises a ?rst anode in the form of a cylinder 22 having its end remote from the cathode disposed near to but usually not quite extending to the plane of the nearer end of a larger cylinder constituting the second anode. This latter cylin der has the form of a metallic coating 24 on the tube walls and may extend to a region close to the screen which may be arranged as shown in Fig. 4. A third cylinder 30 to act as the com pensating electrode and having a diameter in termediate between the diameters of the ?rst and second anodes 22 and 24 is arranged around the ?rst anode 22 so as to overlap the latter and to project within the second anode 24. In one ex ample, the ?rst anode 22 is 1/2 inch in diameter, the compensating electrode 30, 33/; inch in diam eter and the second anode 24, 1 inch in diameter, and the compensating electrode 30 extends 8 millimetres beyond the end of the ?rst anode 22. ' The arrangement of Fig. 5 may be used in the same manner as described with reference to Fig. 4. direction normal to the plane of the ?gure, are connected in series, and de?ecting currents are passed through them from an oscillator indi cated at 43. Across the terminals 44 and 4! of the oscillator 43 is connected a resistance 44. The compensating electrode 30 is connected to a tapping point on this resistance 46 through a condenser 41. A leak resistance 48 is connected between the compensating electrode 30 and .the cathode 20. Thus a potential varying in accord ance with the currents in the coils 40 and 4| is applied to the compensating electrode 30. The grid leak 48 serves to keep the compensating elec trode 30 biased at cathode potential. The electrode arrangement described with ref 15 erence to Fig. 5 may also be applied to the case in which electromagnetic de?ecting coils are used. Where the change of focus of the beam with de?ection is due to the use of an inclined plane screen, as described in the preceding examples, 20 the compensation required is a linear function of the de?ecting current or potential. If the change of focus is due to the use of forms of screens which are other than spherical (when no correc tion is required) or planar and inclined to the mean path of the beam, the compensation re quired may be nonlinear. The desired form of compensation can be obtained by suitable selec tion of the shape and position of the compensat ing electrode, and of the bias potential applied 30 to it. The cross section of the compensating electrode 30 in planes normal to its axis is usually circular, but the cross section in planes contain ing its axis may be chosen to suit any particular case. 35 ~ In the arrangement shown in Fig. 5, the com-' pensating electrode may have the form of an apertured diaphragm the diameter of the aper ture being intermediate the diameters of the ?rst and second anodes 22 and 24. ' 40 It is usually desirable to arrange that the focus‘ ‘compensating means shall not alter the current in the electron beam and where this is the case the compensation should be effected between the last aperture which de?nes the beam and the 45 screen because a change in the divergence of the beam will affect the number of electrons passing through an aperture arranged in the path of the beam of changing divergence. It is however not necessary that the compen sating electrode should be combined with the‘ ?nal electron lens as it may be arranged to co operate with another electron lens nearer the cathode. For example, a part of the modulating cylinder usually provided, for example in cathode ray tubes used for television reception, for con trolling the ray intensity may be insulated from the remainder and used for focal length control. The various structures above described may be used to compensate for a change in focus irre spective of the cause of the change. ' For example where the focus changes with change in beam current, a voltage or current The invention may in this embodiment also be , dependent upon the modulating potential which applied to tubes in which the beam is de?ected produces the change in beam current may be over the screen by means of electromagnetic applied to the compensating means above de de?ecting coils instead of by de?ecting plates. scribed so as to correct for the change in focus. In Fig. 6 is illustrated a plan view of the tube In certain cases it may be desirable to correct shown in Fig. 4, with the de?ecting plates 4, 5 for the variation in distance from the centre of 29 omitted. De?ection is carried out by the de?ecting system to points on a horizontal 70 ‘and means of two pairs of de?ecting coils, one pair line across the screen. If thebeam is de?ected of which is shown at 40 and 4| and the other across the screen by means of a saw tooth poten pair of which is indicated at 42. The pair of tial or current it will move relatively slowly in one coils 40, 4| which serve to de?ect the ray ver-. direction and rapidly back again. When the tlcally up and down the screen, that is in the beam is at the beginning and end of its stroke the 75 5 2,128,011 focal length of the lens system must be greater quired derived potential may be obtained by re than when the beam is half way across the screen. placing the resistance capacity circuit 55. 59 by its equivalent resistance inductance circuit. slower movement ‘of the beam across the screen, ' When correcting for changes in e?ectlve focal The correction need only be applied during the since this is the only time when the beam has any useful function in this kind of apparatus. During the quick return stroke the accuracy of focus of the beam is immaterial. Referring now length of an electron lens arising from defiect~ ing a beam of electrons over a screen the central region of which is nearer to the de?ecting system than the edges, it may be necessary to apply a to Fig. 7 at (a) is shown the wave form of a varying correcting potential which does not scanning potential which is used to de?ect the change linearly with time. Thus in certain cases 10 beam in horizontal lines across the screen. Po- _ it may be necessary to make the lines JK, KL Vtential is plotted as abscissa against time as or dinate. As the potential follows the lines PQ and RS the beam is moved comparatively slowly across 15 the screen, and over the lines QR and ST the beam is made to return quickly to the other side of the screen. Now as explained above, the focal length of the electron lens must be: greatest at the points P, Q, R and S, and shortest midway 20 between PQ and RS, that is at points V and W. Thus the compensating potential must have a symmetrical zig-zag wave form as shown at (b) etc. of the wave form of Fig. 7 (b) curved. This may be done by inserting a suitable correcting circuit between the terminal 66 of the circuit of Fig. 8 and the electrode to which the correct 15v ing potentials are applied. ' Alternatively the potential applied to the com pensatiiig electrode 30 may be given a wave form which is other than linearly related to the de ?ecting potential. The alteration of wave form 20 may be obtained by means of a suitable circuit; for example, a .circuit whereby there may be de where at the points J, L and N the compensating rived from a saw tooth oscillation an oscillating potential is such that the electron lens has a maxi 25 mum focal length and at the points K and M a potential which is substantially proportional to minimum focal length, the latter corresponding to the central position of the beam on the screen. A circuit suitable for deriving a wave form of the kind shown at (b) from a saw tooth potential as at (a) is illustrated in Fig. 8. Referring to that figure, saw tooth potentials the integral of the saw tooth wave form. Such 25 an oscillation may be obtained by feeding a saw tooth oscillation to a resistance and condenser in series. The integrated oscillations are taken from the terminals of the condenser. The time constant of the condenser and resistance is made 30 long compared with the period of oscillation of the saw-tooth oscillation. The integrated wave the terminals 5| and 52. The terminal 5| is con form then consists of two parabolic arcs for nected to the control grid of a thermionic valve 7 each cycle of saw-tooth oscillation. The major 53, the cathode of which is connected through a arc corresponds to the slowly changing part of the 35 bias resistance 54 to the terminal 52. The anode saw-tooth wave form and this part of the inte of the valve 53 is connected through a resistance grated wave form may in suitable circumstances 55 to terminal 55 which is connected to the posi— be fed to the compensating electrode 30. tive terminal of a source of potential (not shown). Two connecting potentials may be applied to 40 The terminal 52 is connected through lead 51 to the compensating electrode 30, each derived from 40 the terminal 58 to which is connected the nega the corresponding de?ecting'potential, to correct tive terminal of the source of potential. for changes in focus of the beam in both direc Between the anode of valve 53 and the lead 57 tions. is connected a condenser 59. Between the ter As already mentioned in the introduction to 45 minal 56 and the cathode of valve 53 is con this speci?cation, variations in the focus of the nected a variable resistance 60, by means of which beam occur in cathode ray tubes employing elec the bias on the grid of valve 53 may be varied. trostatic electron focussing and electrostatic de The operation of the circuit above described is flection due to the potential on the de?ecting~ . of the kind shown at (a) in Fig. 7 are applied to as followsz- .. The bias of the grid valve 53 is so adjusted that - _' the valve passes no current when the potential of the applied wave form (a) of Fig. 7 falls below a value denoted by the line 0 O in that ?gure. While the valve 53 is nonconducting the conden 55 ser 59 is charged through the resistance 55, and it - is arranged that the time constant given by the product R1C1 where R1 is the value of resistance 55 and C1 the capacity of condenser 59 is large compared with the oscillation period of the saw 60 tooth'wave form applied across the terminals ‘ 5|, 52. Condenser 59 will then change linearly. When the valve 53 begins to conduct, the poten tial of the condenser 59 will fall in sympathy with the potential on the grid of valve 53. Thus the 65 potential of the condenser 59 will follow a wave form as shown at (b) in Fig. 7. This wave form is in antiphase to the potential applied to the grid of the valve 53. To reverse the phase of the potential on the condenser 59, there is provided 70 a valve 63 which, with its associated circuits com prises an ordinary resistance-capacity coupled phase-reversing stage. The output terminal 66 of this phase reversing stage may be connected to 75 the compensating electrode 30 of Fig. 4, 5 or 6. In the above described arrangement, the re plates distorting the focussing ?eld. This di?iculty may be overcome, according to 50 the present invention, by methods similar to those described above for compensating for varia tion in the distance of different points on the screen. Thus a varying potential derived from the saw-tooth deflecting potentials may be fed to 55 the compensating electrode 30, the amplitude of the potential being suitably chosen so that the e?ect of distortion of the de?ecting potentials on the focussing ?eld is eliminated. An alternative arrangement by means of which 60 elimination of distortion may be effected will be described with reference to Figs. 9 and 10. Referring ?rst to Fig. 10, a cathode ray tube is shown having a neck portion 23 and a frusto conical portion 10. At the base of the frusto 65 conical portion 10 is a ?uorescent screen 7!. In the neck portion 23 is arranged an electrode sys tem comprising a cathode 20 heated by a heat ing coil I9, a cathode shield 2|. an accelerator electrode 61, a modulator electrode 68, a first 70 anode 22 and second anode 24. The accelerator electrode is given a potential between that of ‘the anode and cathode. The modulator may be biased at cathode potential, and modulating po tentials in the negative direction applied to it 75 6 2,198,011 by a suitable circuit not shown serve to modulate the intensity of the beam. The modulating po tentials may consist of received television‘ sig nals. It will be assumed that the screen ‘II is formed on part of a spherical surface the centre of which lies in the region of the de?ecting sys tem 4, 5, 29, in order that variations of focus of the beam due to configuration of the screen need , '10 ‘is not be considered in describing this embodiment of the invention. The second anode 24 and the de?ecting plate 5 are connected together and maintained at, say, 2000 volts positive relative to the cathode 28. In order to cause the cathode ray beam to scan the screen, saw tooth potentials, one at frame frequency and the other at line frequency, are applied to the de?ecting plates. It is found as already mentioned, that, given constant voltages on the anodes 22 and 24, the cross sectional area 20 of the beam on the screen ‘II where in the pres ent case it forms a spot of light, varies accord ing to the potential applied to the electrode 4. It is also found that for any potential on the plate 4, there is a corresponding potential of 25 the ?rst anode 22 at which the spot size is a mini mum. In Fig. 9 is a curve showing the relation between the potentials on the ?rst anode 22 plotted as abscissae against potentials on the plate 4 plotted as ordinates for which the size of 30 the spot on the screen 1| is a minimum. It .will be seen that the curve has an exponential ' form. It will be clear that by applying to the ?rst anode 22 of the tube of Fig. 10 a potential which varies with the potential of the plate 4 according to the curve of Fig. 9, the beam may be focussed on the screen as a spot of substantially constant size. The circuit of Fig. 10 by means of which this varying voltage is applied will now be de 40 scribed. Two resistances 12 and 13 are connected in ' series. One end 14 of the combination is earthed directly to the earthed lead 89. One end of a third winding 9| of the transformer 85 is con nected to lead 81 and to terminal 92, and the other end to terminal 93. Across the anode and cathode of the valve 8| is connected a condenser 94. The anode is also connected to the de?ecting plate 4 of the cathode ray tube. through a condenser 95‘ and a circuit comprising an inductance 98 shunted by a con denser 91 and resistance 98 in series. The plate 10 4 is connected to the platevi through a leak resistance 99 of high value.- The anode of the valve 8| is also connected through a condenser |8‘8 and resistance | 8| to the lead 19. The operation of the circuit is as follows. 15 The valve 8| and associated screening grid and control grid circuits form a normal blocking oscillator, the operation of which is well known and need not be fully described. It is su?lcient to say that the valve becomes alternately con ducting and non-conducting. The frequency of these changes is controlled by impulses ap _ plied across the terminals 92 and 93. These im pulses may have the form of square topped pulses which'are obtained from the synchronizing im pulses of the received television signal. While the valve BI is non-conducting, the con denser 94 charges up through the impedance of the Winding 82 of transformer 'l'l. When the valve 8| conducts, the condenser94 is rapidly dis charged. The impedance of the winding 82 and the capacity of the condenser 94 arepreferably made such that the curve showing the potential across the condenser 94 against time during the charging process has the exponential form of the curve shown in Fig. 9. Thus the wave form of the oscillations generated in the anode circuit of the valve 8| have the form shown in Fig. 7 (a), modi?ed in that the lines PQ and RS con form to an exponential curve. These oscilla tions are passed to the ?rst anode 22 through winding 88 of transformer 11 and lead 18. The source of high potential. The ?rst anode 22 of the tube is connected through lead ‘I6 and one winding 88 of a transformer 11 to a tapping on resistance 12. The cathode 28 of the tube is con connections to the winding 88 are so arranged that the oscillations passed to the ?rst anode 22 are in phase opposition to those passed to the de?ecting plate 4. ,The oscillations also pass through condenser 95 and the circuit 96, 91, 98 to the plate 4. The circuit 98, 91, 98 is a ?lter circuit so designed that the exponential saw nected through variable resistance 18 and lead tooth wave form is converted into a straight line ' and connected to the negative terminal of a source of potential (not shown). The other end 45 15 is connected to the positive terminal of the Thus the saw-tooth wave form of the kind shown in Fig. anode 22 is ‘given a positive potential relative 7 (a). Thus the beam of the tube is de?ected in the usual fashion over the screen 1|, and by ap 19 to a tapping on resistance 19. to the cathode 28. The potential on the acceler ator electrode 61 may be derived in a similar way. The cathode shield may also be connected to a tapping on resistance 13. In order not to com plying the correcting voltage of exponential form to the ?rst anode 22, variations in focus of the spot on the screen are substantially removed. ' A screen-grid valve 8| has its anode connected Usually a change in voltage on the ?rst anode 22 will result in a change in the current ?owing to the second anode 24, and hence in a varia tion in brightness of the spot on the screen ‘II. through winding 82 of transformer TI to terminal To avoid this, a part of the voltage applied to plicate the drawings, however, the connections to the accelerator 81 and cathode shield 2| are omitted. 83' which is connected to a source of positive the first anode 22 is also applied, in phase op potential (not shown) the negative terminal of which is earthed. The screening grid of the position, to the cathode 28. Thisvoltage is fed to the cathode 28 through the condenser valve 8| is connected through a winding 84 of a transformer 85 to a terminal 88 which is con | 88 and resistance |8|. nected to a suitable tapping on a source of po tential. This_ may be the same source which is connected to the terminal 83. One end of a 70 winding 81 of transformer 85 is connected to the control grid of the valve 8| through a condenser 88, and the other end is connected to an earthed lead 89. The grid of the valve 8| is also con nected to this lead through a variable resistance 98. The cathode of the valve 8| ‘is connected _ ‘Preferably there is connected a decoupling con denser |82 between the tapping on resistance 12 and the earthed lead 89. In the operation ' of the above described arrangement, the voltage applied to the ?rst anode 22 is of exponential ' 70 saw-tooth wave form. However, a considerable improvement in the constancy of spot size will be obtained if a linear saw-tooth wave form voltage is applied to the ?rst anode. Thus the value of the condenser 94 may be made suf 75 2,128,011 ?ciently large to cause a linear saw-tooth wave form to be developed across it. In this case the diaphragm IZ- facing the cathode III, or both ‘?lter circuit 96, 91, 98 may be'omitted. If desired, in either of these arrangements the resistance In both the examples given above, the desired e?’ect is obtained by providing a suitably inhomo these surfaces, are concave. _ llll may be connected to the cathode shield 2!, geneous magnetic or electric ?eld. A similar re or in cases where a grid is placed between the cathode and ?rst anode, the resistance IDI may sult can also be obtained by producing variations in the ?eld so that the ?eld strength varies in be connected thereto. The time constant of the circuit comprising the condenser I00 and re 10 sistances llll and 18 should be su?lciently large to prevent any phase distortion of the impulses is active upon the scanning aperture l3. In such passed through it. , . . If desired the circuit of Fig. 10 may be used with a cathode ray tube having a compensating 15 electrode of the form described with reference _ to Figs. 4, 5 or 6, the lead ‘I6 being connected thereto instead of to the ?rst anode 22. In accordance with the part of the image which cases the ?eld may be homogeneous. The de sired variations in the ?eld ‘may be obtained (in the case where the ?eld to be varied is the elec trostatic ?eld) by superimposing upon the nor mal steady potential difference between the cathode l0 and the diaphragm 12 a compensat 16 ing potential difference varying in a suitable manner in accordance with the de?ection. For such a case, no variations in beam current occur example where the de?ection is effected in known and variation of the potential applied to the The circuit I00, llll manner with the aid of two electrical oscilla tions of saw-tooth wave form, one at line scan may therefore be omitted. There will now be described methods of com ning frequency and the other at picture fre quency, the varying potential differences re-' quired for focus compensations may be derived 20 cathode then unnecessary. pensating for change of focus due to de?ection of an electron image over a screen, as described and illustrated in the introduction to this speci?ca ~ tion with reference to Fig. 2. Referring again 20 from these two saw-tooth oscillations. These de rived corrective potential di?erences will nor 25 mally be arranged to be constituted by compo nents of twice line frequency and twice picture frequency, and may have the wave form’ indi cated in Fig. 7 (b). They may be derived from the saw-tooth scanning currents by means of 30 to Fig. 2 one way of doing this is to superim pose upon the magnetic focusing ?eld due to the coil‘ M an inhomogeneous steady magnetic ?eld so that the composite magnetic ?eld is stronger in the central than in peripheral regions. circuits of the form illustrated inFig. 8. Clearly, If the aperture l3 in the diagram 12 is centrally ’ if correction is to be applied in both scanning disposed, the centre of the image will be operative ‘components, one such circuit will be needed to . upon the aperture l3 in the unde?ected condition derive compensating potentials/from the scan 35 and corners of the image will be operative on ning oscillations at line frequency, and another 85 the aperture l3 with maximum de?ection. Thus to derive compensating potentials from the scan v30 with no de?ection the operative electrons travel in_a relatively strong magnetic ?eld and with maximum de?ection the operative electrons travel from a region of relatively weak magnetic ?eld into the stronger region. The latter elec trons therefore travel in a path along which the line integral of -the magnetic ?eld strength is ‘ smaller than in the case of the former electrons. The time of revolution of the electrons which is 45 inversely proportional to the line integral is there fore greater with de?ection than without. By suitable arrangement the distribution of mag netic ?eld strength may thus be made such that, in the case of all‘ operative electrons, the time 50. of ?ight is substantially equal to the time re ' quired for one revolution and the focus is thus maintained during the scanning process. The inhomogeneous magnetic ?eld may be produced by placing a permanent magnet on the 55 axis of the coil i4 and some distance away from it. The magnet is arranged with its magnetic 60 05 70 ~75 ning oscillations at frame frequency. The two compensating potentials are then combined and applied to the cathode ID or diaphragm H in suitable phase relationship to the scanning os cillations. ‘ We claim: 1. Electron discharge apparatus comprising a cathode, a screen, an electrostatic electron lens system for focussing electrons emitted from said cathode on to said screen, said electron lens sys tem cemprising two cylindrical lens electrodes juxtaposed co-axially and being associated with means for maintaining a potential difference be tween said lens electrodes, means for de?ecting 50 said electrons over said screen, a compensating electrode positioned in the neighbourhood of the region of juxtaposition of said lens electrodes, said compensating electrode being associated with compensating means for varying the poten tial thereon relatively to that of said lens elec trodes according to one or both of the two vary axis on the axis of the coil M and so placed that ing oscillations which, in operation, produce the its ?eld enhances that due to the coil H. Al de?ection of the electrons, thereby changes in ternatively the permanent magnet may be re focus of the operative ‘electrons from said cath 60 placed bya further coil energized by a suitable ode ‘which would otherwise occur as said elec current. This coil may take the form of a rel- ‘ trons are de?ected over said screen are removed ativelyv short coil arranged coaxial with the main or substantially reduced. focusing coil at some distance away from it. 2. Electron discharge apparatus as claimed in Another way in which the change of focus can claim 1, wherein means are provided for generat be corrected is by making the accelerating elec ing and applying, to effect de?ection of the elec trostatic ?eld between the two plates stronger trons, a de?ecting saw-tooth wave form, and in peripheral‘ than in central regions. In this wherein said compensating means for varying way the time of flight is made shorter for elec the potential on said compensating electrode trons which are operative in the de?ected condi comprise a circuit for developing from the saw .70 tion than for electrons which are operative in tooth oscillation and feeding to the said com the unde?ected condition. The required elec pensating means a potential of substantially trostatic ?eld distribution may be obtained by symmetrical zig-‘zag wave form having a fre arranging that the surface of the cathode l0 quency equal to that of the saw-tooth oscilla facing the diaphragm l2 or the surface of the tions. 76 8 ~ 2,190,011 ' 3. Electron discharge apparatus as claimed in claim 1 wherein said lens electrodes are equal in diameter and said compensating‘electrode is annular and surrounds the space between said lens 5 electrodes. 4. Electron discharge apparatus’ as claimed in claim 1 wherein said lens electrodes are of dif- Ierent diameters, and the compensating electrode overlaps one of said lens electrodes and projects within. the other of said lens electrodes. ' JOHN EDGAR KEYSTON. FREDERICK HERMES NICOLL. OTTO KIEMPERER.