US2122095A

US2122095A - Cathode ray device

Info

Publication number: US2122095A
Application number: US153412A
Authority: US
Inventors: Gabor Denes
Original assignee: Individual
Current assignee: Individual
Priority date: 1936-06-04
Filing date: 1937-07-13
Publication date: 1938-06-28
Anticipated expiration: 1955-06-28
Also published as: DE743482C; FR839869A; GB481094A

Description

June 28, 1938. D A R 2,122,095

CATHODE RAY DEVICE Filed July 13, 1937 2 Sheets-Sheet 1 INVENTOR I QM June 28, 1938. D. GABoR CATHODE RAY DEVICE Filed July 15, 1957 2 Sheets-Sheet 2 INV ENTOR W wma ATTORN EYS Patented June 28, 1938 UN STATES Dnes Gaibor,

28 Claims.

ates" to devices such as cathscxllogra'phsand cathode ray tubes for 1 television, in which oscillograms or electrically ures"ar 'e recorded by a beam of fl scent screen or other re- 'Ifhe" object of this invention is vic'e fby means of which a once ic'ture' can be made perma- 1 it is wiped out intentioncent screen, in such a way opened remains permabe', called the first, or the beam for which 6. s all be'called the second, device according to the I y ans for producing two with difierent' velocities, and a covered with insulating ls, which are in themondary electrons or of good secondary I reas of the secondurface can be maintained cans hereinafter dee considered as chargeb aims. are directed tocans are provided to evel, that initially, M or faster beam the 'slower beam are rethe, invention those mber which are hit am, are made by this second 'lectric state with state the eleca ejadmitted to the rnifof the invention member of the kind fluorescent screen, he" said member is a element bfz rsaid i m" V 1 .iaousimiaetweerrxtwo :distinc electrical states, and i sad' l atiitham es *of tri ing-overfrom} th lower .PA'TENT OFFICE CATHODE an nsvrcr.

Rugby, England Application July 13, 1937, Serial No. 153,412

In Great Britain June 4, 1936 state into the upper state involves the emission of secondary electrons.

The novel features which I desire to protect herein will be pointed out in the appended claims. The invention may be best understood by refer- 5 once to the following description and the accompanying drawings. In the drawings Fig. 1 shows a device for demonstrating the principle of the invention. Fig. 2 is a diagrammatical illustration of the processes involved. Fig. 3'shows one form of the cathode ray tube according to the invention, in which the electrostatic controlling member is itself a fluorescent screen. Fig. 4 shows a device by means of which a further application of the principle of the invention may 16 be demonstrated. Fig. 5 is a diagrammatical illustration of the action of the perforated controlling member or gate utilized therein. Fig.

6 shows a second form of a cathode ray tube according to the invention, with separate controlling member or gate and fluorescent screen. Figs. 7 and 8 illustrate the application of cathode ray tubes according to the invention to television reception. Fig. 9 is a schematic representation of an electron-optical system to produce the second or illuminating beam in television tubes, and Fig. 10 shows a system, in which the means for producing the two electron beams are united in one construction.

In Fig. 1, which is a section of a device for demonstrating and explaining the principle of the invention, I is an evacuated envelope, 2 and 3 are two indirectly heated cathodes, for producing the first and the second electron beam respectively. 4 and 5 are heating filaments, 6 and 1 accelerating electrodes, which may have a common potential. 8 is a. target on which both electron beams impinge, which may be thinly powdered with fluorescent substances, for visual demonstration of the effects in question. 9 is a collecting electrode or anode. The electrodes 2, 3, 6-1 and 9 are connected with successively increasing potentials, whereas the target 8 is connected with the same potential as the cathode 3, through a high resistance I0. Switches ll and I2 are provided for connecting the cathodes to the battery and switching in the electron beams.

If the cathode 3 is switched in by itself, no current will flow to the target and no fluorescenoe appears on it, as its potential is the same as that of the cathode 3, and it repels all electrons. If however the cathode 2 is also switched in, provided that the potentials are suitably chosen, fluorescence appears on the taras get, which persists afterthe switch II is opened again. v

This phenomenon can be best understood by reference to Fig. 2. In this diagram the voltage between the target and the cathode is called E and the current to the target I. A positive sign means electrons flowing to the target. In an equilibrium state the current flowing to the target must be equal to that leaking away through the resistance R, i. e. it must be E-/R. This is represented in the diagram by a straight line. The diagram contains further the characteristics which are obtained if one or the other of the cathodes is switched in, and the targetv is maintained at a potential Ea. I1, shown as a dotted line represents the electron current flowing to the target if only the flrst cathode, (2) is switched in, the continuous line 1: corresponds to the current from the second cathode, (3) Both curves have an s-shape, due to the secondary electron emission of the target. which sets in at a certain electron energy. The current becomes negative if more than one secondary electron is released for every impinging electron. Both curves reach zero again at or near the point Ea, which represents the potential of the collecting electrode or anode 9.

With only the second cathode working, equilibrium is possible only in the points 0, A and B, in which the characteristic I: and the resistance line intersect. Of these three points A is unstable, whereas 0 and B are stable. If the oathode 3 is switched in by itself, the target will rest at the point 0. at which it repels all electrons. If however the cathode 2 is switched in, the electrons emitted by it will impinge on the target with an energy corresponding to the voltage difference Eu between the two cathodes. This energy is sufllcient to release more than one secondary electron per impinging electron, that is, at a ratio greater than unity. The potential of the target will therefore rise until it reaches the point B at which equilibrium is again established. This point is beyond the unstable point A. If therefore the beam emitted by the first cathode is switched ofl again, the second beam will shift the equilibrium point to B. In this condition thesystem is maintained for any time, until the second beam is switched off, and the leakage takes the system back to 0.

Fig. 3 shows a longitudinal section of a cathode ray tube according to the invention, in which the above described arrangements and phenomena are utilized for obtaining on a fluorescent screen permanent records of passing electric signals. I3 is an evacuated envelope containing means for producing two electron beams of difl'erent velocities. One of these, the flrst and faster beam, is focussed on the fluorescent screen which is scanned by it in the usual way. This beam is produced by the cathode ll, modulated by the electrode i6, accelerated and focussed on the fluorescent screen in the usual way by the electrodes i6, i1 and It with successively increasing potentials. The last accelerating electrode is is.

connected with the tubular member is, which contains the electrostatic deflecting electrodes. These are arranged in four pairs, in such a way, that the deflected beam passes through an aperture 20. 0f the deflectors producing a deflection normal to the plane of the drawings only the plates II and 22 are visible. Deflection in the plane of the drawings is produced by the pairs 23-24 and 25-28, which are cross-connected in such a way as to produce opposed deflections, the

canoes anode 2!, which is shown as a conductive coating of the envelope. This anode, which has the highest potential in the whole system, corresponds to the collecting electrode in Fig. 1. One of its functions is to collect the secondary electrons emitted by the screen 30. This screen consists of fluorescent substance, capable of secondary electron emission, and is backed with a transparent metallic coating 3|, connected with the cathode 21, or with a suitable potential near that of the cathode.

It has been found that a fluorescent screen of the kind described, provided that the coating is not too thin, can be considered as built up of a very great number of elements, each of which can exhibit independently the efl'ects described in connection with Fig. 1. The resistance It is here replaced by the conductivity of the fluorescent coating between the surface exposed to the electrons and the metallic backing. It has been found that most of the usual fluorescent subs ances have sufiicientiy good secondary electron emission and sumciently low. conductivity to enable small area-elements, with dimensions comparable to the thickness of the coating, to change their equilibrium states independently of the surrounding parts by the eflects as above described. Screens with not sumciently low conductivity may be improved by applying a semiconductor of sufliciently low conductivity on the metallic coating and applying the fluorescent substance on top.

of this in single grains, which may have also an admixture of good secondary emitters.

In the device described, an oscillogram once described by the first beam is made permanently visible by the second beam which spreads out over the whole area of the screen, hits it however only on those parts where the flrst beam has passed. This device is therefore very suitable as an oscillograph for transient electric phenomena, which can be observed or photographed on the screen for any length of time.

The main application of the invention is however in the fleld of television. Cathode ray tubes for television reception could not hitherto be operated with the very great brightness as required for projection, for two reasons: The flrst is the destruction of the fluorescent screen at great inputs, the second is the difliculty of concentrating very great inputs in small writing spots. The flrst difliculty is very strongly reduced with the new cathode ray tube, for the following reasons: The destruction of the screen in cathode ray tubes is caused mainly by that the spot covers only about l/100,000 of the screen area at a time and rests on it only for about 1/100,000 of the time. The whole input, which has to cause a visual impression lasting at least for 1/50th of a second is transferred to apoint of the screen in less than a. millionth of a second. In consequence the screen is heated up for a very short time to very high peak temperatures. Artiflcial cooling is of no avail for substantially reducing these peaks, as the speed of heat conduc- 16 input densities.

tion is far ,too low for the extraordinary peak According to the present invention however the input may be averaged out over a considerable fraction of the total time. If e. g. the bombardment time by the illuminating beam is of the total time, this means that in order to create the same screen brightness and the same visual impression the input density can be reduced to 1/10,000 of its former value. Although the total input remains the same, not only the maximum but also the average temperature of the screen is now strongly reduced, as the rate of heat leakage can now easily be made comparable with the rate of energy inflow.

The second difficulty, viz. that of concentrating very strong beam currents in small writing spots, is also overcome in the tubes according to the invention, as the illuminating beam is not concentrated but diifuse, and covers a large part of the screen area.

A problem particular to television tubes, as distinct from cathode ray oscillographs is that of the modulation of screen brightness. As the trip-over process on which the invention is based, is a discontinuous process, it might seem that the usual methods of intensity modulation can not be applied. This applies however only to screens which are entirely homogeneous. In reality the unavoidable inhomogeneities of the screen, due to diflerent grain size, etc. create already a certain degree of gradation. Grains of different size, of different conductivity and of different secondary electron emissivity can be made to trip over in a certain sequence, in such a way that at a small intensity of the scanning beam only a few trip over into the upper,-permanently bright,stage, whereas at a certain higher current all trip over and saturation is reached. This effect may be utilized by mixing the fluorescent substance of grains of different properties. In order to obtain good gradation and the impression of continuously changing screen brightnesses, it is however also necessary that the spot shall cover at the same time a great number of grains of different properties.

A second method of effecting modulation according to the invention is that of building up the picture of points with maximum brightness but of difierent size, similarly as in half-tone printing. This may be effected by modulating not thebeam intensities but the beam diameter. It is moreover preferable to move the spot in the line direction not continuously but in jerks. This may be effected by superimposing on the linesweep movement a high frequency movement, with a. frequency corresponding to the desired number of picture points per second. This superimposed oscillation has preferably a saw-tooth waveform, but it has been found that a superimposed sine-wave is suflicient for resolving the line apparently into a series of sharp dots. This method of modulation is also effective only in case of independent screen-elements much smaller than the maximum area covered by the scanning spot.

The most effective method consists in a combination of the two methods above described, i. e. in a modulation both of spot size and beam intensity and the use of an inhomogeneous screen as above described.

The cathode ray tube according to the invention can be adapted to even higher screen inputs by .a modification, the principle of which can be explained with reference to Fig. 4. This device contains the same elements as the one shown in Fig. 1, but with the following difference: The electrode 32, corresponding to 8 in Fig. 1,

is made perforated, and behind it is placed a further electrode 33, with a potential higher than that of the collecting electrode 34. If the electrode 32 is in the state corresponding to "O" in Fig. 2, the current to the anode 33 due to the broad beam alone will be very small, and it can be made exactly zero by connecting 32 not to the cathode but to a somewhat lower potential. In the upper state however,-corresponding to B in Fig. 2,-the electrons of the second broad beam will shoot freely through the holes of 32. A part of them impinges on the perforated electrode and releases a sufllcient number of secondary electrons for maintaining the high potential.

This effect is utilized according to the invention as shown in Fig. 5. The grid 32 is replaced by a perforated or gauze-like structure 35, with an insulating or semiconducting coating 36. Such a structure may be called for shortness sake a "gate". It is suiflcient, or even preferable if the coating extends only over the face of the structure exposed to the electron beams, the other face being left bare. By the insulation the elements of the surface are made independent of each other. Every hole may be considered as an independent element, which may or'may not transmit electrons, independently of the state of the surrounding holes. In Fig. 5 the top opening 3'! is assumed to be at or below cathode potential, and the electrons of the illuminating, or broad, beam,starting horizontally and shown in continuous lines,are repelled. The scanning beam,shown in dotted lines,just passes the hole 38 in its upward sweep, and releases secondary electrons which are moving towards the collecting electrode. (Secondary electrons are indicated by dot-dash lines.) By this the potential is raised and the passage is opened for the illuminating beam. In the bottom opening, which has been passed by the fast beam, the illuminating beam maintains this higher potential, by releasing secondary electrons.

As rather small voltage differences are sufficient for controlling the illuminating beam current from zero to a maximum, it is advantageous to provide good secondary emitters on the surface of the insulating or semiconducting coating, i. e. substances from which electrons with rather low energy may release more than one electron. By this the voltage jump between the lower and the upper state is reduced. A high voltage jump represents in this case a waste, whereas in the device shown in Mg. 3 this voltage difference is useful, as it serves for exciting the fluorescent screen.

Fig. 6 shows a device according to the invention utilizing a gate". The means for producing the two electron beams are the same as described in connection with Fig. 3. In this device however no fluorescent screen but a gauze 40, prepared in the above described manner, is scanned by the fast beam. This gauze is held by the frame 4|. It is electron-optically imaged on the fluorescent screen 42. The electron-optical system is composed of the

electrodes

43 and 44, shown as coatings of the envelope,and the magnet coil 45.

As compared with the device shown in Fig. 3, this form of the invention has the advantage, that the accelerating voltage between 40 and 42 can be chosen freely, independently of the secondary emitting properties of the gate". It is therefore possible to realize by means of this device even higher screen inputs than with the first form. This device is therefore especially suited for television projection on large screens, e. g. cinema theatre screens.

In the reproduction of television pictures the difilculty arises, that if the illuminating beam is switched oif once after every frame or picture, those parts of the picture which have been traced last will emit less light than those traced at the beginning. This difficulty is overcome according to the invention with the arrangement diagrammatically illustrated in Fig. 7. In this figure 66 is the control screen such as the screen of Fig. 3 or gate 60 of Fig. 6, as the case may be, on which the potential pattern is produced. 61 represents the electron gun for producing the scanning beam. 48-46 and 606l are the two pairs of deflecting plates producing the scanning. 62 represents diagrammatically the large cathode emitting the second broad beam, which irradiates the screen on a band 55 and is deflected by the plates 53, 56. The deflections of the two beams are matched in such a way, that the illuminating beam follows the scanning beam, its upper edge moving a little ahead of it. The picture lines are indicated by dotted lines. If in this arrangement the width of the illuminating band is chosen equal to or a little less than the dark spaces following each frame, every picture point will receive equal illumination. By this method it is therefore possible to utilize about 10% of the time, as compared with about .001 by the usual method.

A further improvement is effected by the method diagrammatically illustrated in Fig. 8. This method allows utilizing almost the whole time for electron bombardment. In this figure only the screen,or gate,66 is shown, with means for producing and deflecting the scanning beam, which are the same as in the previous figure. The illuminating beam covers here the whole area of the screen, with the exception of a narrow dark band 56, which moves a little ahead of the scanning spot 51. This dark band wipes out the picture, by giving the elements of the screen or gate just sufiicient time to discharge. The previous picture 66 changes therefore almost continuously into the next picture 60, and the flicker is reduced to a minimum.

Fig. 9 is a simplified representation of an elec tron-optical device for producing full illumination of the screen, interrupted only by a thin dark band. A cathode 6i of sufilciently large area is surrounded by a guard ring 62. The electrons emitted by the cathode are accelerated by the tubular electrode 66 and thrown at the disc 66, which itself has a suitable positive potential. This disc is fitted with two

windows

66, 66, each of which has the proportions of the picture, and which are divided by a narrow cross-bar 61. The

electrostatic lens system

66, 69 is adjusted in such a way that a strongly magnified sharp image of the window with the cross-bar appears on the screen. The electrode 63, or other condensing system by which it may suitably be replaced, is arranged in such a way as to produce uniform illumination of the window and therefore of the picture. The whole image of 66 and 66 is moved during the course of the frame scan across the screen by means of the deflecting electrodes 10 and II. On the screenthere will appear therefore a dark band which wipes out the preceding picture before the next one takes its place. In order to avoid wiping out the picture during the fiy-back of the dark band, the followingv method may be used according to the invention: During the fiy-back the potential of one or both of the focussing electrodes 66. 66 is changed in such a way that the picture of the window is defocumed and the image of the cross-bar vanishes almost completely.- If however the fiy-back is sufilciently rapid, and the time of the passage of the dark strip is considerably shorter than the time constant of discharge of the screen or gate elements, this defocussing is unnecessary.

Fig. 10 shows a section of a gun construction, containing the means for producing and defiecting both electron beams, in symmetrical arrangement. This consists of the cathode I! for the first or scanning beam, with the focussing system I6, held by the insulating part I6 in the tubular member 16, which carries the deflector system for the scanning beam. v'Zll'hese are arranged in four pairs, corresponding to Figs. 3 or 6, arranged in such a way as to make the deflected beam pivot around the centre of the aperture 16.

The organs for producing the second or illuminating beam are essentially the same as in Fig. 9, and are arranged near the end of the tubular member 16. The cathode consists of an annular ring 11 of triangular cross section, enclosing the heating filament I6. Its active surface has the shape of a truncated cone, situated behind the diaphragm IS. The condensing electrode 66 is arranged in such a way, that the cathode rays emitted by 11 illuminate the window 16 uniformly. This window has the same shape as described in connection with Fig. 9 and has a thin cross bar 6i in its middle. It is followed by the focussing electrodes 6! and 66 and by the deflecting

electrodes

66 and 66.

As the potential of the cathode I2 is considerably lower than that of 11, the electrons of the scanning beam have a much higher velocity than those of the illuminating beam. They will suffer therefore only a small additional focussing and deflecting eflect by the electrodes 66-46. By suitable dimensioning of the gun system and of the applied voltages it is possible to obtain at the screen simultaneously a sharp spot for the scanning beam and a sharp image of the cross bar.

While I have shown particular embodiments of my invention, it will be understood that many modifications and applications may be made by those skilled in the art, without departing from the invention as set forth in this specification and in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a cathode ray signal reproducing tube, the combination which comprises a control screen having a conductive base and a secondary-electron-emissive charge-retaining surface thereon connected to the conductive base through resistive material, an electron gun for producing a scanning beam of electrons of relatively high velocity positioned and adapted to impinge on said control screen, deflecting means for said scanning beam, a second electron gun for producing a broad beam of electrons of relatively low velocity positioned and adapted to impinge on said control screen, said second electron gun having an electrode including a window having a cross bar thereacross adapted to divide said broad beam of electrons into two portions separated by a band free of electrons, means for electron-optically imaging said window on said control screen, a collecting anode positioned to collect secondary electrons emitted from said control screen, the

conductive base of the control screen being connected to the cathode of the broad beam electron gun so that initially, prior to the passing of the scanning beam, electrons of the broad beam will not liberate secondary electrons from the emissive surface thereof at a ratio greater than unity, the relatively high velocity electrons of the scanning beam being adapted to liberate secondary electrons from the scanned areas of the control screen at a ratio greater than unity, said secondary electrons being collected by said collecting anode and thereby raising the potentials of the scanned areas so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities and liberate secondary electrons at a ratio greater than unity to maintain the scanned areas at elevated potentials so long as electrons in the broad beam continue to impinge thereon, thereby creating a potential pattern on said control screen, means for utilizing said potential pattern to reproduce an image corresponding thereto, and deflecting means for deflecting across the control screen said portions of the broad beam separated by said band free of electrons and thereby permit the charge on the scanned emissive areas lying at said band to leak away through the resistive material and restore the areas to their initial potentials.

2. In a cathode ray signal reproducing tube, the combination which comprises a control screen having a conductive base and a secondary-electron-emissive charge-retaining surface thereon connected to the conductive base through resistive material, an electron gun for producing a scanning beam of electrons of relatively high velocity positioned and adapted to impinge on said control screen, deflecting means for said scanning beam, a second electron gun for producing a broad beam of relatively low velocity electrons positioned and adapted to impinge on said control screen in a band covering a plurality of scanning lines, a collecting anode positioned to collect secondary electrons emitted from said control screen, the conductive base of the control screen being connected to the cathode of the broad beam electron gun so that initially, prior to the passing of the scanning beam,'electrons of the broad beam will not liberate secondary electrons from the emissive surface thereof at a ratio greater than unity, the relatively high velocity electrons of the scanning beam being adapted to liberate secondary electrons from the scanned areas of the control screen at a ratio greater than unity, said secondary electrons being collected by said collecting anode and thereby raising the potentials of the scanned areas so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities and liberate secondary electrons at a ratio greater than'unity to maintain the scanned areas at elevated potentials so long as electrons in the broad beam continue to impinge thereon, thereby creating a potential pattern on said control screen, means for utilizing said potential pattern to reproduce an image corresponding thereto, and deflecting means for deflecting the broad beam across the control screen and thereby permitting the charges on the scanned areas from which the broad beam has been deflected to leak away through the resistive material and restore the areas to their initial potentials.

3. In a cathode ray signal reproducing tube. the combination which comprises a perforated conductive control grid having on the surface thereof secondary-electron-emissive charge-retainingeiements k H vhrous resistive mafiefla eh l ctrqasun or: P du i a scanning be'ai'n o'f electrons o relativelyqhigh velocity positioned 3 and adapt to impinge ,con said control'grid,deflecting me ns ,said scan- 5 ning beam, a second electrongun;;-for, producing a broad beam ofelectrons of, ;relatively low1velocity positioned and adapted to impinge on .said control grid'infa band covering a pluralitysofm scanning lines, acollecting anode positioned; and 10 adapted to collect secondary electronmemitted from said control grid, a luminescent screenpositioned to receive electrons or -said broad-beam passing through" said a, control, grid and adapted; to be rendered luminescent thereby the. con- 15 ductive control grid being connected to the, cathode of the, broad beam electron'gunsothatinitial- 1y, prior to the passing oftheiscanningbeam the potentialsof the emissive elements ;thereof wi1l 1 substantially prevent the. electrons, of the, broad beam from impinging on said luminescent screen and electrons in the broad ,beam,wil1 ;not-.;li b erate secondary electrons fro t ments at a ratio' greater than; ly high velocity electronsfof t being adapted to'fliber'ate from, the scanned areas at a unity, said secondary electro by said collecti g anodefand potentials of the scanned are of the broad; beam tn 'raftej scanned areas at increased "velo secondary 'electroi isfata ratio to maintain the scannedeleme tentials so long as electrons i continue to impinge "there tentials permitting electrons to pass through corresponding'are trol grid and impinge on the lumin and defiectingf meansf'for' fsaid deflecting the beam away fr and thereby permitting the' ha sive elements to lealraway thr material and restoring the initial potentialsi 4. In a cathode-ray *signal conductive control; grid? h'avin thereof secondary retaining element A p H through resistive' *rnatferi producing a scanning n atively high velocity po'sition impinge 'onisam centre for said 'scanning' bea for producing a broad atively low velocity po dow having =a'cros's bar dividesaid broad be means for electron-optically imagi'n dow at saidwcontrol grid;. a-*colleetin' ridcanda-adaptedst erebmthe: Conductive m1 h l I a on said ent creen and electrons in the broad beam will not liberate secondary electrons from the emissive elements at a ratio greater than unity, the higher velocity electrons of the scanning beam being adapted to liberate secondary electrons from the scanned areas at a ratio greater than unity which are collected by the said collecting anode, the potential of the collecting anode being sufficiently high so that the potentials of the scanned areas are raised by said secondary electron emission so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities and liberate secondary electrons at a ratio greater than unity to maintain the scanned elements at elevated potentials so long as electrons in the broad beam continue to impinge thereon, said elevated potentials permitting electrons in the broad beam to pass through corresponding areas of the control grid and impinge on the luminescent screen, and deflecting means for deflecting across the control grid said portions of the broad beam separated by said band free of electrons and thereby permit the charge on the scanned emissive elements lying at said band to leak away through the resistive material and restore the elements to their initial potentials.

5. In a cathode-ray signal reproducing tube,

the combination which comprises a perforated conductive control grid having on the surface thereof secondary-electron-emissive charge-retaining elements connected to said grid through resistive material, an electron gun for producing a scanning beam of electrons of relatively high velocity positioned and adapted to impinge on said control grid, deflecting means for said scanning beam, a second electron gun for producing a broad beam of electrons of relatively low velocity positioned and adapted to impinge on said control grid in a band covering a plurality of scanning lines, a collecting anode positioned to collect secondary electrons emitted from said control grid, a luminescent screen positioned to receive electrons of said broad beam passing through said control grid and adapted to be rendered luminescent thereby, the conductive control grid being connected to the cathode of the broad beam electron gun so that initially, prior to the passing of the scanning beam, the potentials of the emissive elements thereof will substantially prevent the electrons of the broad beam from impinging on said luminescent screen and electrons in the broad beam will not liberate secondary electrons from the emissive elements at a ratio greater than unity, the higher velocity electrons of the scanning beam being adapted to liberate secondary electrons from the scanned areas at a ratio greater than unity which are collected by the said collecting anode, the potential of the collecting anode being sufilciently high so that the potentials of the scanned areas are raised by said secondary electron emission so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities and liberate secondary electrons at a ratio greater than unity to maintain the scanned elements at elevated potentials so long as electrons in the broad beam continue to impinge thereon, said elevated potentials permitting electrons in the broad beam to pass through corresponding areas of the control grid and impinge on the luminescent screen, and deflecting means for said broad beam for deflecting the beam away from the scanned areas and thereby permitting the charge on said emissive elements to leak away through the resistance material and restoring the elements to their initial potentials.

6. A cathode ray signal reproducer tube which comprises an electron gun for producing a scanning beam of relatively high velocity electrons, a second electron gun for producing a broad beam of relatively low velocity electrons, a secondary-electron-emissive charge-retaining lumii nescent screen positioned and adapted to be impinged by said broad beam and said scanning beam of electrons, and a collecting anode positioned and adapted to collect secondary electrons emitted from said screen, the velocity of the electrons in said broad beam being initially too low to strike said luminescent screen and render it substantially luminescent and too low to liberate secondary electrons therefrom at a ratio greater than unity, the electrons in said scanning beam being of sufllciently high velocity to liberate secondary electrons from the scanned areas at a ratio greater than unity, said secondary electrons being collected by said collecting anode at a sumciently high potential to thereby raise the potentials of the scanned areas so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities to render said scanned areas luminescent and to liberate secondary electrons therefrom at a ratio greater than unity to maintain the scanned areas at increased potentials and thereby maintain said luminescence so long as electrons in the broad beam continue to impinge thereon.

7. A cathode ray signal reproducer tube which comprises an electron gun for producing a scanning beam of relatively high velocity electrons, deflecting means for said scanning beam, a reproducing screen having a conductive base and secondary electron emissive charge retaining elements on the surface thereof connected to said conductive base through resistive material, said reproducing screen being adapted to be rendered luminescent by electrons impinging thereon, a second electron gun for producing a broad beam of relatively low velocity electrons positioned and adapted to impinge on said reproducing screen in a band covering a plurality of scanning lines produced by said scanning beam, a collecting anode positioned and adapted to collect secondary electrons emitted from said screen, the conductive base of said reproducing screen being maintained at a potential such that the electrons in saidbroad beam have velocities initially too low to strike said reproducing screen and render it substantially luminescent and too low toliberate secondary electrons therefrom at a ratio greater than unity, whereas the electrons in said scanning beam have suillciently high velocities to liberate secondary electrons from the scanned areas at a ratio greater than unity, said collecting anode being maintained at a sufllciently high potential to collect said secondary electrons to raise the potentials of the scanned areas so that electrons of the broad beam thereafter impinge on the scanned areas at increased velocities to render said scanned areas luminescent and-to liberate secondary electrons therefrom at a ratio greater than unity to maintain the scanned areas at increased potentials, thereby maintaining said luminescence so long as electrons of the broad beam continue to impinge on the scanned areas, and deflecting means for said broad beam for deflecting the beam away from the scanned areas and thereby permitting the charges on the scanned elements to leak away through the rel sistive material to the conductive base and restore the elements to their initial potentials.

8. In cathode-ray tube signal reproducing apparatus in which the reproduction persists after the signal has passed, the combination which comprises means for producing a concentrated electron scanning beam and a broad electron beam of different velocities, a secondary-electron emissive surface positioned to be impinged by said scanning and said broad electron beams, means for impressing upon said emissive surface an initial potential such that prior to the passing of said scanning beam over elemental areas thereof the electrons insaid broad beam do not raise the potentials of said elemental areas, whereas electrons in said scanning beam do raise the potentials of the scanned elemental areas by secondary electron emission therefrom to an extent suflicient to cause electrons in said broad beam to thereafter impinge on the scanned areas at velocities suflicient. to maintain the scanned areas at elevated potentials by secondary electron emission therefrom so long as electrons in the broad beam continue to impinge thereon.

9. In cathode-ray tube signal reproducing apparatus in which the reproduction persists after the signal has passed, the combination which comprises means for producing a concentrated electron scanning beam and a broad electron beam of different velocities, a secondary-electron emissive surface positioned to be impinged by said scanning and said broad electron beams, elemental areas of said surface being charge-retaining, means for impressing upon the elemental areas of said emissive surface initial potentials such that prior to the passing of the scanning beam thereover the electrons in said broad beam do not liberate secondary electrons therefrom at a ratio greater than unity, whereas electrons in said scanning beam do liberate secondary electrons at a ratio greater than unity and thereby raise the potentials of the scanned areas an extent suflicient to cause electrons in said broad beam to thereafter impinge on the scanned areas at velocities sufficient to maintain the scanned areas at elevated potentials by secondary electron emission at a ratio greater than unity so long as'electrons in the broad beam continue to impingethereon.

10. In cathode-ray tube signal reproducing apparatus in which the reproduction persists after the signal has passed, the combination which comprises a secondary-electron emissive screen, elemental areas of which are charge-retaining; means for producing a potential pattern on said screen corresponding to the signal to be recorded comprising means for producing a scanning beam of relatively high velocity electrons and a broad beam of relatively low velocity electrons, said scanning beam being adapted to impinge on said screen in an elemental area and said broad electron beam being adapted to impinge on said screen over a relatively large area, means for impressing upon elemental areas of the emissive screen initial potentials such that prior to. the passing of the scanning beam thereover the electrons in said broad beam do not liberate secondary.electrons therefrom at a ratio greater than unity whereas electrons in said scanning beam do liberate secondary electrons at a ratio greater than unity and thereby raise the potentials of the scanned areas an extent sufiicient to cause electrons in said broad beam thereafter to liberatesecondary electrons from the scanned areas at a ratio greater than unity and maintain thescanned areas at elevated potentials so long as electrons in the broad beam continue to impinge thereon, thereby producing a potential pattern on said screen; and means for utilizing the potential pattern so produced to reproduce an image corresponding thereto.

11. In. cathode-ray tube reproducing appa-' ratus for producing on a luminescent screen records of electric signals which persist after the signals have passed, the combination which comprises an electron gun including a cathode and an anode co-operating therewith to define an electron scanning beam of elemental cross-section, a second electron gun including a cathode and an anode co-operating therewith to define a broad electron beam of relatively large cross-section, means for maintaining the cathode of the first electron gun at a lower potential than the cathode of the second electron gun, a control screen positioned in the paths of said electron beams, said control screen being conductive and having thereon a layer of substantially discrete secondaryelectron emissive elements connected to the screen through resistive material, a collecting anode positioned to collect secondary electrons emitted from said elements, a luminescent screen positioned and adapted to receive electrons of said broad beam under the control of said control .screen, means for maintaining the conductive screen at a potential with respect to said cathodes such that initially, prior to the passing of the scanning beam, the electrons in the broad beam will be substantially prevented from producing an image on said luminescent screen and the potential difference between the emissive elements and the cathode of the second electron gun will be less than required for the emission of secondary electrons from the elements in a ratio sufficient to raise the potential of the elements, whereas the potential difference between the emissive elements and the cathode of the first electron gun, in conjunction with the potential of the collecting anode, will cause electrons in the scanning beam to liberate secondary electrons from the scanned emissive elements in a ratio sufiicient to raise the potential of the scanned elements to at least the potential level at which electrons from the second cathode also release secondary electrons in a ratio suflicient to balance the leakage current from the scanned elements in the conducting screen, thereby maintaining the scanned emissive elements at at least said potential level so long as electrons in the broad beam continue to impinge thereon, means for modulating the scanning beam to create a potential pattern of elements maintained at at least said potential level corresponding to the signal to be recorded, said potential level admitting electrons of said broad beam to said luminescent screen to produce an image thereon.

12. In apparatus for producing an optical image including a recording screen adapted to be rendered luminescent by electrons impinging thereon under the control of a potential pattern created on a secondary electron emissive chargeretaining surface, the method of creating said potential pattern which comprises initially exposing said secondary-electron emissive chargeretaining surface to a relatively broad uniform stream of electrons whose velocities are too low to strike said surface and emit secondary electrons at a ratio greater than unity, and bombarding elemental areas of said surface by a scanning beam of relatively high velocity electrons to raise the potentials of said areas by secondary electron emission therefrom to an extent suiiicient to cause electrons of said broad beam to thereafter impinge on said areas at increased velocities sumcient to maintain said areas at raised potentials so long as the exposure of the scanned areas to said broad beam is continued.

13. In cathode-ray tube apparatus for producing images by a flow of electrons under the control of a potential pattern created on a secondaryelectron emissive surface, elemental areas of which are charge-retaining, the method of creating said potential pattern which comprises creating a broad beam of electrons and directing the beam toward said secondary-electron emissive surface, the velocities of said broad beam electrons being initially insufficient to raise the potentials of said elemental areas, and raising the potentials of elemental areas of said emissive surface by bombarding said areas, in the presence of said broad electron beam, with a relatively high velocity scanning beam of electrons to an extent such that electrons of the broad beam will thereafter impinge on the scanned areas at increased velocities suflicient to maintain the scanned areas at elevated potentials by secondary electron emission therefrom so long as the flow of electrons in the broad beam to the scanned areas is continued, thereby producing a potential pattern on said emissive surface.

.. 14. In cathode-ray tube apparatus for producing images by a flow of electrons under the control of a potential pattern created on a secondaryvelectron emissive surface, the method of creating said potential pattern which comprises initially exposing said secondary-electron emissive surface to a broad beam of electrons having velocities too low to strike said emissive surface and liberate secondary electrons at a ratio suflicient to raise the potentials of elemental areas of said surface, and scanning elemental areas of said surface in the presence of said broad beam with a scanning beam of electrons having velocities sufliciently high to liberate secondary electrons to raise the Potentials of the scanned areas to such an extent as to cause electrons of the broad beam to thereafter impinge on said scanned areas at velocities suflicient to liberate secondary electrons to maintain said areas at raised potentials so long as the flow of the electrons in the broad beam to the scanned areas is continued, thereby producing a potential pattern on said emissive surface. a

15. The method of producing an image by a cathode-ray tube which comprises creating a broad beam of electrons for producing an image, controlling the production of said image by said broad beam by means of a secondary-electronemissive surface positioned in the path of said broad beam, initially maintaining said emissive surface at a potential such that electrons of the broad beam will be substantially prevented from producing an image, and scanning elemental areas of said emissive surface with a scanning beam of relatively high velocity electrons to raise the potentials of the scanned areas by secondary electron emission to a degree such that electrons of the broad beam will thereafter impinge thereon with suillciently high velocities to maintain the scanned areas at raised potentials so long as the flow of the electrons in the broad beam to the scanned areas is continued, the raised potentials of said scanned areas permitting the electrons of the broad beam to produce an image corresponding thereto.

16. The method of producing an image by a cathode-ray tube which comprises creating a broadbeamofrelativelylowvelocityelectronsfor producing an image on a recording surface, controlling the production of said image by said broad beam by means of a secondary-electrom emissive charge-retaining surface positioned in the path of said broad beam, initially maintainingsaidemissivesurface atapotentialsuchthat electrons in said broad beam will be substantially prevented from producing an image and will not raise the potential thereof by secondary-electron emission, and scanning elemental areas of said emissive surface in the presence of said broad beam with a scanning beam of relatively high velocity electrons to raise the potentials of the scanned areas by secondary electron emission therefrom to an extent such that electrons of the broad beam will thereafter impinge thereon with sufliciently high velocities to maintain the scanned areas at raised potentials by secondaryelectron emission therefrom so long as the flow of electrons in the broad beam to the scanned areas is continued, the raised potentials of said scanned areas permitting electrons of the broad beam toproduce an image on the recording surface.

17. The method of producing an image on a recording surface by a cathode-ray tube which comprises initially exposing to a relatively broad beam of electrons a screen having secondaryelectron-emissive charge-retaining elements on the surface thereof, the velocity of said electrons being too low to raise the Potentials of said elements by the emission of secondary electrons at a ratio greater than unity, scanning elemental areasof said screen duringsaidexposurebya scanning beam of relatively h velocity electrons to emit secondary electrons at a ratio greater than unity, collecting said secondary electrons at a potential suiiiciently high to cause the potentials of the scanned elemental areas to be raised by said secondary electron emission to an extent sufficient to cause electrons in said broad beam to thereafter emit secondary electrons at a ratio greater than unity and thus maintain said scanned areas at elevated potentials so long as the flow of electrons in the broad beam to the scanned areas is continued, thereby producing a potential pattern on said screen, and utilizing said potential pattern to control the flow of elecimage by electrons impinging thereon.

18. The method of producing an image on a recording surface by a cathode-ray tube which comprises creating a broad beam of relatively low velocity electrons for producing an image on a recording surface, controlling the admission of electrons of said broad beam to said recording surface by meansof a secondary-electron-emissive screen positioned in the path of said broad beam, elemental areas of said screen being charge-retaining, initially maintaining said elemental areas at such low potentials that electrons in said broad beam will be substantially prevented fromproducing an image on said recording screen and will not raise the potentials of said areas by secondary electron emission therefrom, scanning elemental areas of said screen in the presence of said broad beam by a scanning beam of relatively high velocity electrons to emit secondary electrons at a ratio greater than unity, collecting said secondary electrons at a potential sufiiciently high to cause the potentials of the scanned elemental areas to be raised by said secondary electronv emission to an extent sufilcientto cause electrons in said broad beam to thereafter emit secondary electrons at a ratio greater than unity and thus maintain said scanned areas at elevated potentials so long as the flow of electrons in the broad beam to the scanned areas is continued, the raised potentials of said scanned areas permitting electrons of the broad beam to produce an image on the recording screen corresponding thereto.

19. The method of producing an image on a recording surface by a cathode-ray tube which comprises creating a broad beam of relatively low velocity electrons for producing an image on a recording surface, controlling the admission of electrons of said broad beam to said recording surface by means of a secondary-electron-emissive screen positioned in the path of said broad beam, elemental areas of said screen being charge-retaining, initially'maintaining said elemental areas at such low potentials that electrons in said broad beam will be substantially prevented from producing an image on said recording screen and will not raise the potentials of said areas by secondary electron emission therefrom, scanning elemental areas of said screen in the presence of said broad beam by a scanning beam of relatively high velocity electrons to .emit secondary electrons at a ratio greater than unity,

collecting said secondary electrons at a potential sumciently high to cause the potentials of the scanned elemental areas to be raised by said secondary electron emission to an extent sufiicient to cause electrons in said broad beam to thereafter emit secondary electrons at a ratio greater than unity and thus maintain said scanned areas at elevated potentials so long as theflow of electrons in the broad beam to the scanned areas is continued, the raised potentials of said scanned areas permitting electrons of the broad beam to produce an image on the recording screen corresponding thereto, and periodically restoring the elemental areas of said emissive screen to substantially their initial low potentials for rescanning thereof.

20. In a cathode-ray tube signal reproducing apparatus for producing, screen, records of electric signals which persist after the signals have passed, the combination which comprises means for producing a scanning electron of elemental cross-section and a broad electron beam of relatively large crosssection, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a secondary-electron emissive reproducing screen adapted to reproduce images by the impingement of electrons thereon, said electron beams being directed toward said reproducing screen, means for scanning elemental areas of said reproducing screen with said scanning beam in the presence of said broad beam, means for maintaining elemental areas of said screen initially, prior to the passing of the scanning beam thereover, at such potentials that electrons in said broad beam substantially do not reproduce an image on said areas and do not raise the potentials thereof by secondaryelectron emission therefrom, whereas at said potentials the higher velocity electrons in the impinge thereon at increased velocities sufilcient on a reproducing to maintain said areas at at least said potential level by secondary electron emission therefrom at a ratio greater than unity so long as electrons in said broad beam continue to impinge thereon. said increased velocities of the electrons in the broad beam being suiiicient to reproduce an image on said reproducing screen.

21. In cathode-ray tube signal reproducing apparatus in which the reproduction persists after the signal has passed, means for producing a potential pattern for controlling the reproduction of, an image corresponding thereto which comprises, in combination, means for producing a scanning electron beam of elemental crosssection and a broad electron beam oi relatively large cross-section, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a control screen having a secondary-electron emissive surface thereon elemental areas of which have substantial resistance therebetween, means for scanning elemental areas of said emissive surface with said scanning beam in the presence of said broad beam, means for impressing upon elemental areas of said emissive surface initial potentials such that prior to the passing of the scanning beam thereover the electrons in the broad beam do not raise the potentials thereof by secondary electron emission therefrom, whereas at said potentials the higher velocity electrons in the scanning beam liberate secondary electrons from the scanned areas at a ratio greater than unity, and means for collecting. the secondary electrons liberated by the scanning beam at a potential suficiently high so that the potentials of the scanned areas are raised to at least the level at which electrons in the broad beam thereafter impinge thereon at increased velocities sufhcient to maintain said scanned areas at at least said potential level by secondary electron emission therefrom at a ratio greater than unity so long as electrons in the broad beam continue to impinge thereon, whereby a potential pattern may be created on said control screen.

22. In a cathode-ray tube signal reproducing apparatus for producing, on a luminescent screen, records of electric signals which persist after the signals have passed, the combination which comprises means for producing a scanning electron beam of elemental cross-section and a broad electron beam of relatively large cross-section, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a secondary-electron emissive surface, said electron beams being directed toward said emissive surface, a luminescent surface adapted to be rendered luminescent by electrons impinging thereon and positioned to be impinged by electrons of said broad beam under the control of said emissive surface, means for maintaining elemental areas of said emissive surface initially, prior to the passing of the scanning beam thereover, at such low potentials that the slower electrons of the broad beam do not release secondary electrons therefrom at a ratio greater than unity and do not produce substantial luminescence of said luminescent surface, whereas at said potentials the faster electrons of the scanning beam release secondary electrons from the scanned areas at a ratio greater than unity and thereby raise the potentials of the scanned areas to a level at which the electrons of the broad beam also release secondary electrons at a ratio greater than unity and suiiicient to maintain said scanned areas at at least said scanning electron beam of elemental cross-section, a second electron gun adapted to produce a relatively broad electron beam, said electron guns having respective cathodes, means for maintaining the cathode of the first electron gun at a lower potential than the cathode of the second electron gun, a control screen positioned in the path of said electron beams having a conductive member and a secondary-electron emissive surface exposed to said electron beams, elemental surface areas of said emissive surface having substantial resistance therebetween and being connected to the conductive member through substantial resistance, a collecting anode positioned to collect secondary electrons emitted from said emissive surface, a luminescent screen positioned and adapted to receive electrons of said broad beams under the control of said control screen, means for scanning elemental areas of said emissive surface with said scanning beam in the presence of said broad beam, means for maintaining the potential of said conductive member at a potential near that of the cathode of the second electron gun such that initially, prior to the passing of the scanning beam over elemental areas of the emissive surface, the broad beam electrons are substantially prevented from producing an image on said luminescent screen and the potential difference between the elemental surface areas and the cathode of the second electron gun is less than required for electrons in-the broad beam to raise the potentials of said areas by secondary-electron emission therefrom at a ratio greater than unity, the potential of the cathode of the first electron gun being selected with respect to the potential of the conductive member so that electrons in the scanning beam liberate secondary electrons from scanned areas at a ratio greater than unity, the potential of the said collecting anode being maintained sufllciently high so that the potentials of the scanned areas are raised by the said liberation of secondary electrons therefrom to at least the potential level at which electrons in the broad beam also liberate secondary electrons in a ratio greater than unity and sufficient to balance the leakage current from the scanned areas to the conducting member, thereby maintaining the scanned areas at at least said potential level, so long as electrons in the broad beam continue to impinge thereon, said potential level admitting electrons of said broad beam to said luminescent screen to produce an image thereon, and means for periodically discontinuing the flow of electrons in the broad beam to the scanned areas to permit the charge on the scanned areas to leak away to the conductive member and restore the scanned areas to substantially their initial potentials at which the electrons in the broad beam are substantially prevented from producing an image on the luminescent screen.

24. In cathode-ray tube signal reproducing apparatus for producing, on a luminescent screen,

records of electric signals which persist after the signals have passed, the combination which comprises a control screen having a secondary-electron emissive surface elemental areas of which have substantial resistance therebetween, means for producing a scanning electron beam of elemental cross-section and a broad electron beam of relatively large crosssection, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a luminescent surface adapted to be rendered luminescent by electrons impinging thereon and positioned to be impinged by electrons of said broad beam under the control of said emissive surface, means for scanning elemental areas of said emissive surface with said scanning beam in a plurality of lines in the presence of said broad beam electrons, said broad beam covering an area of said emissive surface including a plurality of scanning lines, means for maintaining elemental areas of said emissive surface initially, prior to the passing of the scanning beam thereover, at such low potentials that the slower electrons of the broad beam do not release secondary electrons therefrom at a ratio greater than unity and do not produce luminescence of said luminescent surface, whereas at said potentials the faster electrons in the scanning beam liberate secondary electrons from the scanned areas at a ratio greater than unity, means for collecting the secondary electrons liberated by the scanning beam at a potential sufllciently high so that the potentials of the scanned areas are raised to at least the level at which electrons in the broad beam thereafter impinge thereon at increased velocities sumcient to maintain said scanned areas at at least said potential level by secondary electron emission therefrom at a ratio greater than unity, whereby the raised potentials of the scanned areas persist after the scanning beam has passed on so long as electrons in said broad beam continue to impinge thereon, said raising of the potentials of the scanned areas admitting electrons in the broad beam to said luminescent surface to produce a luminescent image thereon, and means correlated with the deflection of the scanning beam for deflecting the broad beam in a direction substantially perpendicular to the direction of the scanning lines to cause electrons of the broad beam to impinge on the scanned areas when they are scanned and to periodically discontinue the flow of electrons oi the broad beam to scanned areas and thereby permit the scanned areas to be restored to their initial potentials prior to rescanning.

25. In cathode-ray tube signal reproducing apparatus for producing, on a luminescent screen, records of electric signals which persist after the signals have passed, the combination which comprises a control screen having a secondary-electron emissive surface elemental areas of which have substantial resistance therebetween, means for producing a scanning electron beam of elemental cross-section and a broad electron beam of relatively large cross-section, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a luminescent surface adapted to be rendered luminescent by electrons impinging thereon and positioned to be impinged by electrons of said broad beam under the control of said emissive surface. means for periodically scanning a selected area of said emissive surface with said scanning beam in a plurality of lines in the presence of said broad beam electrons, saidbroad beam substan- I release secondary electrons therefrom at a ratio greater than unity and do not produce substantial luminescence of said luminescent surface, whereas at said potentials the faster electrons in the scanning beam liberate secondary electrons from the scanned elemental areas at a ratio greater than unity, means for collecting the secondary electrons liberated by the scanning beam at a potential sufllciently high so that the potentials of the scanned elemental areas are raised to at least the level at which electrons in thebroad beam thereafter impinge thereon at increased velocities suflicient to maintain said scanned elemental areas at at least said' potential level by secondary electron emission therefrom at a ratio greater than unity, whereby the raised potentials of the scanned elemental areas persist after the scanning beam has passed on so long as electrons in the broad beam continue to impinge thereon, said raising of the potentials of the scanned elemental areas admitting electrons in the broad beam to saidluminescent surface to produce a luminescent image thereon, and means correlated with the deflection of the scanning .beam for periodically deflecting sa d broad beam in a direction substantially perpendicular to the direction of the scanning lines at substantially the same velocity as that of the scanning beam in that direction to cause electrons of the broad beam to impinge on the scanned elemental areas when they are scanned and to cause said band free of broad beam electrons to move across the selected area and thereby periodically permit the scanned elemental areas to be restored to their initial potentials prior to rescanning.

26. In cathode-ray tube signal reproducing apparatus for producing, on a luminescent screen, records of electric signals which persist after the signals have passed, the combination which comprises a control screen having a secondary-elec tron emissive surface elemental areas of which have substantial resistance therebetween, means for producing a scanning electron beam of elemental cross-section and a broad electron beam of relative large cross-section, the electrons in.

i scanning elemental areas of said emissive surface with said scanning beam inthe presence of said broad beam electrons, means for maintaining elemental areas of said emissive surface initially,

prior to the passing of the scanning beam thereover, at such low potentials that the slower electrons oi. the broad beam do not release secondary electrons therefrom at a ratio greater than unity and do not produce substantial luminescence of said luminescent surface,'whereas at said potentials the faster electrons in the scanning beam liberate secondary electrons from the scanned areas at a ratio greater than unity, means for collecting the secondary electrons liberated by the scanning beam at a potential sumciently high so that 'the potentials of the scanned areas are raised to at least the level at which electrons in the broad beam thereafter impinge thereon at increased velocities suificient to maintain said scanned areas at at least said potential level by secondary electron emission therefrom at a ratio greater than unity, whereby the raised potentials of the scanned areas persist after the scanning beam has passed on so long as electrons in said broad beam continue to impinge thereon, said raising of the potentials of the scanned areas admitting electrons in the bread beam to said luminescent surface to produce a luminescent image thereon, and means for modulating the crosssectional area of the scanning beam at the control screen in accordance with the signal to be reproduced.-

27. In cathode-ray tube signal reproducing apparatus for producing, on a luminescent screen, records of electric signals which persist after the signals have passed, the combination which comprises a control screen having a secondary-electron emissive surface elemental areas of which have substantial resistance'therebetween, means for producing a scanning electron beam of elemental cross-section and a broad electron beam of relatively large cross-section, the electrons in the scanning beam having higher velocities than the electrons in the broad beam, a luminescent surface adapted to be rendered luminescent by electrons impinging thereon and positioned to be impinged by electrons of said broad beam under the control of said emissive surface, means for deflecting said scanning beam to scan said emissive surface in lines composed of a series of substantially discrete points, the scanning. oi elemental areas oi said emissive surface being in the presence of electrons of said broad beam, means for maintaining elemental areasof said emissive surface initially, prior to the passing of the scanning beam thereover, at such low potenials that the slower electrons of the broad beam do not release secondary electrons therefrom at a ratio greater than unity and do not produce substan- 2 tie] luminescence of said luminescent surface,

whereas at said potenials the faster electrons in the scanning beam liberate secondary electrons from. the scanned areas at a ratio greater than unity, means for collecting the secondary electrons liberated by the scanning beam at a potential sufiicientlyhigh so that the potentials of the scanned areas are raised to at least the level at which electrons in the broad beam thereafter impinge thereon at increased velocities sumcient to maintain said scanned areas at at least said potential level by secondary electron emission therefrom at a ratio greater than unity, whereby the raised potentials of the scanned areas persist after the scanning'beam has passed on so long as electrons in said broad beam continue to impinge thereon, said raising of the potentials of the scanned areas admitting electrons in the broad beam to said luminescent surface to produce a. luminescent image thereon, and means for modulating the scanning beam in accordance with the signal to be reproduced.

28. In cathode-ray tube reproducing apparatus for producing on a luminescent screen, records of electric signals which persist after the signals have passed, the combination which comprises an electron gun adapted to produce a scanning electron beam of elemental cross-section, a second electron gun adapted to produce a relatively broad electron beam, said electron guns having respective cathodes, means for maintaining the cathode of the first electron gun at a. lower potential than the cathode of the second electron gun, a control screen positioned in the path of said electron beams having a conductive member and a secondary-electron emissive surface exposed to said electron beams, elemental surface areas 01' said emissive surface having substantial resistance therebetween and being connected to the conductive member through substantial resistance, a collecting anode positioned to collect secondary electrons emitted from said emissive' surface, a luminescent screen positioned and adapted to receive electrons of said broad beam under the control of said control screen, means for deflecting said scanning beam to scan said emissive surface in lines composed of a series of substantially discrete points, the scanning of elemental areas or said emissive surface being in the presence of electrons of said broad beam, means for maintaining the potential of said conductive member at a potential near that of the cathode of the second electron gun such that initially, ,prior to the passing of the scanning beam over elemental areas of the emissive surface, the broad beam electrons are substantially prevented from producing an image on said luminescent screen and the potential difference between the elemental surface areas and the cathode of the second electron gun is less than required for electrons in the broad beam to raise the'potentials of said areas by secondary-electron emission therefrom at a ratio greater than unity, the potential of the cathode of the first electron gun being selected with respect to the potential oi the conductive member so that electrons in the scanning beam liberate secondary electrons from scanned areas at a ratio greater than unity, the potential oi the said collecting anode being maintained suiliciently high so that the potentials of the scanned areas are raised by the said liberation of secondary electrons therefrom to at least the potential level at which electrons in the broad beam also liberate secondary electrons in a ratio greater than unity and sumcient to balance the leakage current from the scanned areas to the conducting member, thereby maintaining the scanned areas at at least said potential level so long as electrons in the broad beam continue to impinge thereon, said potential level admitting electrons of said broad beam to said luminescent screen to produce an image thereon, means for modulating the cross-sectional area of the scanning beam at the control screen in accordance with the signal to be reproduced, and means for periodically discontinuing the flow of electrons in the broad beam to the scanned areas to permit the charge on the scanned areas to leak away to the conductive member and restore the scanned areas to substantially their initial potentials at which the electrons in the broad beam are substantially prevented from producing an image on the luminescent screen.

palms 0.4503.