US2146822A - Television - Google Patents

Description

Feb. 14, 1939. F. c4 P. HENROTEAU TELEVISION 2 sheets-sheet 1 Filed DeC. l5, 1952 EH/E17 mms Feb. 14, 1939.

F. C. P. HENROTEALJ TELEVIS TON Filed Deo. l5, 1932 2 Sheets-Sheet 2 '.Ullllnlvlwull r9. Jr

Patentetl- Feb. 14, 1939 UNITED STATI-:s4 PATENT OFFICE i' mnvrsloN Franois Charles Pierre Henroteau, Ottawa, n-

tario, Canada, assigner, by mesne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application December 15, 1932, Serial No. 647,440

6 Claims. (Cl. TIS-7.2) Y

ordinary daylight illumination regardless, Withinv limits, of the size of these views, that isvto trans- 16 mit all views which it is possible at present to photograph with a. moving picture camera.

Yet anotherobject of the invention is to pro-` vide a, method whereby each point of the scene,

to be televised can be impressed on the photoelec- 20 tric surface for a far greater time than has hitherto been possible. Y.

A further object of. the invention is toprovide a method whereby all the points of the scene to be televised are simultaneously projected on the transmitter and not successively projected thereon, as is the case with all practical' methods' of television now in use. v

In all methods of television used at present the scene to be televised'is dividedzinto Va number of elements according to theamount of detailrequired in the scene received. Each of these elements is then successively projected on the photoelectric transmitting cell and sent to the receiving station. My method,v however, differs radically from the above methods in that all the elements of the scene are simultaneously projected on the transmitter.

If, according to known methods, it is desired to transmit an entire scene every sixteenth of a L0 second and the scene is to be divided into ten thousand elements, then eachelement lwill be projected on the transmitting screen for only one one hundred and sixty-thousandth vof a second. It is known that the electric energy liberatedby `5 photoelectric material is proportional to the amount. of light falling on the materialmultiplied by the' time during which the light acts. If, therefore, each element of the scene is to remain on the transmitter for onlyv one one- 0 hundred and sixty thousandth of a second, a very strong illumination of this scene is necessary in order that the photoelectric material may emit enough energy for transmission and, for this reason, it is at present extremely diicult to trans- 5 mit scenes under ordinary daylight illumination.

"tion becomes'still greater.Y l

`According to my method, transmission is 'divided into three or, in one casa-'fourperiods, the scene being projected on the transmitter during one of these periods. Sinceall theelements of e Y the scene are simultaneously projected on the' transmitter, the light from eachelement Aof the former falls on the'latter fLone-third *ofl vone sixteenth of a second, that is,1for'almost four v'thousand times longer than in known methods. lf, however, the. number' of elements intov which 10 the scene is divided'increases, propor- In prior arrangements which I havedevised *for carrying out my method,faneleetrostaticreproduction ofthe view has vin each`caselbeen formedi fand a photosensitive surface' thenfscanned by v erystrong light to cause'a'n emission 'of elecf f trous, the amount ofv which pickediup by the anode would be controlledyby the'electrosta-tic .reproduction Since thev light of the scanning 20 Abeam acts on each element of surface for "only, 'one ten-thousandthV (aSSuming division. intel.; A.

10.000 elements) ofthe .time#during wmcnthef tter.

reason, evenvwith a vlight l10,000 -timessibrig'ht asthat fromV the view, 'theoutput.energy-ironrthe 3 0 cell might not equal the input. vj`'.llh'uns;iwjith'finy prior methods, while the.resultsareibetterthan with other methods involvin'gfscaningloiithe o bject to be televised, they are capableof improvement. A furtherv object of the invention is,`t herefore, in a method where .all vpoints Jfthevscene are siy multaneously projected-oriv the :'transmitter, yto ensure that as large a proportion as possible ofthe energy of the scanning beam becomes effec- 40 tive for transmission. 1

A still further object ofthe invention is,. ina method as referred to in the last paragraph, to obtain an eiective energy f or transmission` as nearly as possible equal to" thefenergy liberated under the influence Vof the light from the view.

The basic difference between my method of television and' those now used lies in the formation and retention, after `they light fromthe view has been interrupted, of anelectrost'atic repro- 50 duction of the image of the view,which reproduction during subsequent scanning` modulates the method have been disclosed in my Patents 1,903,112 and 1,903,113 and in my co-pending application Serial No. 510,705. In the arrangement according to the two patents, the surface of a cathode was divided into a multiplicity of tiny photoelcctric elements insulated from one another. An image of the view, being projected on this cathode, raised the individual elements thereof to different positive potentials depending upon the intensity of the light which struck these elements. 'I'he image of the view was then shut oif and the cathode scanned by a strong beam of light, a modulated electronic current being thus received by an anode. The electrostatic reproduction of the image on the cathode was caused to disappear by projecting on a coating of photosensitive material on the interior of the cell, a beam of light of such a wave length as to cause the expulsion from the coating of relatively slow-moving electrons, which were attracted by any elements of the cathode at positive potential. Thus all these elements were brought to a uniform potential of zero or nearly zero. The arrangement shown in the copending application differed in a number of respects from that shown vin the patents. In it the cathode was uniformly photosensitive and the electrostatic reproduction of the image of the view was formed by collecting on a grid consisting of a multiplicity of mutually insulated electro-conductive elements non-photosensitive to visible but photosensitive to ultra-violet light, the electronic emission from the cathode resulting from the projection thereof of the image of the view. The grid was interposed between the anode and cathode and, during subsequent scanning of the cathode by a strong beam of light to cause the emission from it of an electronic stream, the electrostatic reproduction of the view formed on the grid modulated this stream, thus causing the current received by the anode to be modulated in accordance with the light from the view. .To cause the electrostatic reproduction to disappear, the grid was exposed to ultra-violet light which caused the elements to emit electrons until they reached a uniform and slightly positive potential.

Mry present method and apparatus differ essentially from those described above in a number of respects. According to the invention, I provide a cell having at one side thereof an opening through which the image of a view maybe focussed and at the opposite side thereof an opening through which a beam of light for causing the electrostatic reproduction of the view to disappear may be projected. Arranged in the cell between these openings are an anode and two members or plates each formed with a multiplicity of tiny openings therethrough and arranged parallel, close to and in such relation to one another that the openings of one plate are opposite the solid portions of the other. These plates are interposed between the anode and the opening for the image of the view. That nearest this opening will be referred to as the first plate and that nearest the anode as the second plate. In the preferred form of my invention the rst plate has an insulating coating on it and its surface facing the anode carries, opposite the openings in the second plate, a multiplicity of mutually insulated electroconductive elements having photosensitive surfaces. The surface of the second plate facing the opening for the light from the view is made photosensitive. Formed as a part of the cell in the side thereof out of line with the openings therein is a cathode ray oscillograph, the beam of rays from which is not designed to strike a fluorescent screen, but is designed to scan the surface of the second plate facing the anode at such an angle that it will not pass through the openings in this plate.

Transmission, using the preferred form of this 5 invention, is carried out in four operations which may be briefly described as follows:-

(a) The first plate is at a low positive potential, the second plate and the anode are grounded and no emission of cathode rays is taking place. The 1( the light from different parts of the view, differ- 1g ent areas of this photosensitive surface will emit different numbers of electrons which will be picked up by the nearest electro-conductive elements on the first plate, owing to the slight positive potential to which this plate is raised. In 21 this way the elements will be raised to varying negative potentials, and as a whole 'they will thus carry an electrostatic reproduction of the View.

(b) As the light from the view is interrupted,

the first plate is raised to a very low positive po- 2i tential, the second plate is grounded, and the anode is raised to a comparatively high positive potential. A beam of cathode rays from the cathode ray oscillograph scans the surface of the second plate facing the anode and causes a sec- 3| ondary electronic emission therefrom. The emission from any point in this surface is subjected from one direction to the attraction of the anode, and from the other direction to the attraction of the positive potential of the rst plate less the 3,

negative potential of the nearest electro-conductive element on this plate. Depending upon the potential of the various electro-conductive elements, therefore, a different number of electrons of the emission will be attracted towards the first 4 plate from different points of the second plate scanned, and thus a different number will reach the anode from these different points. In this way, the electronic emission picked up by the anode will be modulated in accordance with the 4 potentials of elements on the first plate adjacent to points of the second plate successively scanned and thus in accordance with the intensity of light of corresponding areas of the view. 'I'he modulated potential of the anode is impressed on the a grid of a three-electrode valve and transmitted.

(c) Scanning is stopped. The two plates and thel anode are grounded and through the other opening in the cell the elements on the rst plate are exposed to light of a. wave length such as to l cause the emission from them of relatively very slow-moving electrons. Since the elements are at negative potentials, the electrons are picked up by other members in the cell. The insulating coating of the first plate is of such a nature that, while it opposes a high resistance to the passage of current through it from a negative to a. positive conductor, it opposes a much lower resistance to the passage of current from a positive to a negative conductor. Thus, as soon as any q element under the influence of the beam of light reaches a slightly positive potential, the leakage of current from it to the plate balances any increase in positive potential caused by the light. The elements thus all reach a. uniform and slightly positive potential.

(d) The various members in the cell are kept grounded as in operation (c) but the beam of light is shut oiI. There is then nothing to counteract the leakage of current from the elements l to the plate and they are all reduced to a unlform potential of practically zero. 'Ihe cell is then in condition for a repetition of the operations described. f

The invention will now be described in detail with reference to the attached drawings in which:

Figure 1 is a diagrammatic view of the complete transmitting arrangement.

Figure 2 is a front view of one of the plates showing in dotted lines the positions of the openings in the other plate when the two plates are properly mounted.

Figure 3 is a cross-sectional view through my preferred system of plates, showing their arrangement with reference to one another and the various coverings and layers on them, the thickness of these coverings and layers being very greatly exaggerated. l

Figure 4 is a similar view of a modified system of plates.

Figure 5 is an elevation of one of the discs used for permitting entry of light to the cell at proper intervals.

I will rst describe the apparatus as it is when completed, next the production of certain of its parts and lastly its operation.

In the drawings, I is a cell darkened to prevent the entry of light thereinto except through openings 2 and 3, the cell being suitably of cylindrical form and the openings being at each end of the cylinder. Formed at one side of the cell between its ends is a pocket-like portion 4 arranged in the form of a cathode ray oscillograph, the place of the fluorescent screen of which is taken by an opening into the cell. 'I'he oscillograph is provided with the usual anode 5, having a very small opening therein, an electron emitting cathode 6 and pairs of magnets I and 8, for oscillating the beam of cathode rays. The cell and the portion 4 are electrically shielded by a shielding 9 which is grounded at I0. Where it extends across the openings 2 and 3 this shielding is made in the form of a relatively wide mesh screen so that it will interfere as little as possible with thepassage of light. Connected and arranged parallel to and inside the shielding 9 at some distance therefrom is a metallic screen Il of relatively narrow mesh but made up of very thin wires. This screen extends over the lower part of the wall of the pocket 4 nearest the opening 3 in the cell and all around that part of the cell between the opening of the pocket 4 and the end in which the opening 3 is situated. Between the screen II and the shielding 9 and parallel therewith is arranged a grid I2 of wider mesh than the screen. The grid is insulated from the screen and the shielding and is connected to a battery I3 of relatively high-positive potential. The screen II and shielding 9 constitute a Faraday cage outside which the potential of the grid I2 has no eiect. Outside the cell and in front of the opening 2 is a lens I4 and mounted between it and the opening 2 is a rotatable disc I 5 having therein an opening I6 through which the light from the view may pass at suitable intervals. At the other end of the cell in front of the opening 3 is a lamp I1 having before it a screen I8 designed to let pass only light of such wave length as will detach from a photosensitive surface electrons of relatively very low velocity. Between the screen I8 and the opening 3 is a rotatable disc I9 similar to the disc I5 but having in it an opening extending over a lesser arc.

Across the cell parallel to its ends and approxiside layers 53 of silver.

mately in line with the wall of the portion 4 nearest the opening 2 are two very thin members or plates 2l and 2|y mounted very close to one another `and in such a way that the surface of plate facing the opening 2 lies in the focal plane of the lens I4. Mounted in the cell between the opening 3 and the plates 20 and 2l is an anode 22. The plate 2l is grounded at 23, while the plate 2|, anode 22 and electron emitting cathode 5 of the cathode ray oscillograph 4 are connected to rotating commutators 24, and 26 respectively, having therein electro- conductive sectors 21, 2l and 23 respectively, extending over slightly less than 120 of arc. The three commutators 24, 25 and 25 and the rotatable discs I5 and I 9 are preferably mounted for rotation upon a common shaft 23a. YArranged around each commutator are three contacts spaced from each other by 120 of arc. In the case of the commutator 24, the contacts 38, 3| and 32 are connected to the positive terminal of a battery 33 of low voltage, the positive terminal of a battery 34 of even lower voltage and ground 35 respectively. In Vthe ease of the commutator 25 the contacts 35 and 38 are connected to grounds 39 and respectively, while the contact 31 is connected to the grid 4I of a thermionic tube 42 and also through a resistance 43 to the positive terminal of a battery 44 of relatively high voltage. In the case of commutator 28, the contacts 45, 46 and 41 are connected to ground 48, the negative terminal of a battery 49 of high voltageand ground 50 respectively.

In Figures 2 and 3 the construction and arrangement of the plates 20 and 2| are shown in detail. As shown in Figure 2 they are so arranged with respect to one another that the openings of one are opposite solid portions of the other. In this figure the openings of plate 2l are shown in full lines and those of plate 20 in dotted lines. As will be seen from Figure 3, the plate 20 is composite and is made up of sections 5I, 52 and 53, all perforated correspondingly. The section 52 is offset with respect to the sections 5I and 53 in a direction towards the opening of the cathode ray oscillograph portion 4 so that in this direction the openings 54 through the plate are about half as long as in the direction perpendicular to it. The angle of incidence of the beam 55 of cathode rays on the surface of the plate 20 is so chosen that this beam will always strike the surface 55 of the section 52 and never the side walls 5'I of its openings. It is of course desirable that the angle of incidence of the beam of cathodel rays be as small as possible in order that at any one time the beam may cover as small an area of the plate as possible. For this reason, while the sections 5I and 52 should be made as thin as possible the section 53 may be made thicker than these sections so that the angle of incidence of the beam of cathode rays may be reduced without, however, the beam being able to penetrate through the openings in the plate.

As shown in Hgure 3 the plate 2| is entirely coated with a layer of aluminium oxide 58, overlying a layer of pure aluminium 59. On the surface of this plate facing the plate 20 and covering areas the full size of the openings 60 in the section 5I of the plate 2l are formed mutually insulated electroconductive elements designated generally by the numeral 6I. These elements 6I have photosensitive surfaces 62 of caesium, intermediate layers 53 of caesium' oxide silver and in- The surface of the plate 2@ facing the View, and incidentally also the walls of the openings il in the section 5|, has a photosensitive coating 65 of caesium and underneath it an intermediate layer 66 of caesium oxide silver and an inside layer 51 of silver.

The reason for the particular formation of the plate 2l and for choosing the angle of incidence of the beam of cathode rays so that it will never strike on the surface 51 of the section 52, is to prevent destruction of the photosensitive surfaces 62 of the elements 6|. It is necessary, to ensure not only that the beam of cathode rays does not strike these elements directly, but also that they are not struck by any fast moving electrons of the secondary emission from the plate 20 caused by the beam. Since the fast moving electrons of the secondary emission, being substantially en' tirely unaffected by the attraction of the anode, move in straight lines, it will be seen from an inspection of Figure 3 that none of them can strike the surfaces 62 of the elements 5|.

I shall now describe the method f producing the plates 20 and 2l in the form in which they appear in the completed cell.

' I'he plates 20 and 2| are preferably made of pure copper, for example, electrolytic copper which can be produced in sheets having a. thickness as small as one two-thousandths of an inch. At this thickness the plates may easily be perforated with a number of openings up to ten thousand or more to the square inch by means of photo-engraving, as used in the preparation of ordinary half-tone plates. After being perforated, the plates are placed in a tube heated to red heat and degasied by means of the formation of a high vacuum in the tube.

In order to prepare the insulating coating on the plate 2|, the plate is placed in a tube under high vacuum and is coated with aluminium by the well known explosion method, such as is u sed for silvering the interior of photoelectric cells and the like. Care must be taken that the vacuum is high in order to prevent oxidation of the aluminium. The coated plate is then used as the anode in an electrolytic bath in which the electrolyte is, for example, a concentrated solution of borax and boracic acid. When a gradually increasing potential is applied between the cathode and anode in the electrolytic bath, a uniform coating 58 of aluminium oxide forms on the aluminium-coated copper plate, the thickness of this coating being proportional to the potential applied between the electrodes. For my purposes, the potential may be gradually increased to 350 volts and then retained at this point for some time in order to insure absolute uniformity of the coating. After its coating with aluminium oxide, the plate 2| is placed in a tube under a high vacuum and in front of it is placed an auxiliary plate perforated and arranged -in exactly the same way with respect to it as the section of the plate 20 in the final cell. A bead of silver is now exploded through the openings in the auxiliary plate ontothe plate 2| from a point relatively remote therefrom, so that a multiplicity of mutually insulated silver spots 64 are formed on the plate 2|. Pure oxygen at about 1 millimetre pressure is then admitted to the tube and the plate is made the anode and bombarded with electrons so that the silver re-acts with the oxygen. This reaction is carried on until the elements show a bluish colour, the layer of silver oxide on the silver being then about atoms thick. This method of oxidation is Well known and needs no further explanation.

when the plate has been degasined as above described, the section 5| thereof is placed in a high vacuum tube and silver exploded onto it without, however, any plate being interposed between it and the source of the explosion. The section has, thus, a uniform coating 61 of silver over one surface and probably also incidentally along the walls of the openings 60 through it. A coating of silver oxide is then formed on the silver in the manner described withreference to the plate 2|.

When the two plates have been thus prepared, they are mounted in the nai tube in the relation to each other described above and shown in the drawings, the silver oxide elements on the plate 2| facing the silver oxide coated surface of the plate 20 and the plates being about two-thousandths of an inch distant from each other. The interior of the cell is now heated to a temperature of about 125 C. and at the same time a pellet 69 of a mixture of caesium dichromate and silicon contained in a side pocket 10 of the cell I is heated and caesium vapour evolved. The tube is during this time under a high vacuum and is kept thereunder by a pump attached at 1| (which is shown in the drawings as closed o in the iinal tube). At the temperature of the cell the caesiumy vapour reacts with the silver `oxide with the formation of caesium oxide silver, which is referred to thus since it is not well known whether the caesium oxide and silver are in distinct layers or not. On the outside of this layer of caesium oxide silver is formed a layer of pure caesium which, though only one atom thick, is extremely tenacious. The surplus caesium is partly removed by the pump and, after the reaction with the silver oxide is complete, the tube is heated to a temperature of 250 C., at which temperature the remaining caesium vapour is fixed by a getter such, for example, as lead glass. The cell is then sealed oif at 1| and is ready for use.

In Figure 4 I have shown a system of plates somewhat different from that of my preferred system. 'I'his system includes, as in my preferred system, the plates 20 and 2|, the plate 2| having, however. no insulating coating around it. Instead of this coating on the plate 2| an unperforated transparent plate 12 of insulating material such as. for example, mica, is interposed between the plates 20 and 2|, and on the surface of the plate 12 facing the anode are formed the mutually insulated electroconductive elements 6|, these elements having, as in the preferred form, photosensitive surfaces 62 of caesium, intermediate layers 63 of caesium oxide silver, and inside layers 6l of silver. The elements 6| are formed opposite solid portions of the plate 2| and the openings Elin the section 5| of the plate 20. The insulating plate 12 may be made as thin as possible, and is arranged at the same distance from the plate 2|! as is the plate 2| in my preferred system. It performs the same function as the coating 58 of aluminium oxide on the plate 2| used in the latter system, and owing to the fact that it is transparent so that the light from the view may pass through it, no openings need be made in it. In this modified system the plate 20 is arranged in exactly the same way as in my preferred system.

Using the system of plates shown in Figure 3, transmission is carried out in four operations, while with the system of plates shown in Figure 4 transmission is in three operations, all of which take place Within one-sixteenth of a second. The division of the total time between the various audace operations will depend upon circumstances, but in general, with either system of plates the rst two operations will each last l5@ second, while with the preferred system the last two operations will each last l/m second, and with the modified system the third'operation will last .360 second. If, however, the light from the view is weak it may be found desirable to lengthen the time ofthe first operation at the expense ofthat of the second, since if the variation of potential of the diil'erent elements 8| is too small a certain part ofthe energy released by the beam of cathode rays is wasted. This additional energy may therefore be sacriced by shortening the scanning period to allow more time for the electrostatic reproduction of the view to build up. In the case of weak light from the view the length of time for the iirst operation might, for example, be increased to iH00 second, and that for the second' operationl reduced to M00 second. It may also be found desirable, when using the preferred system of plates, to lengthen the time for the third operation at the expense of that for the fourth operation. Itis to be noted in this connection that the term-substantial used in referring to the length of time for the fourth operation ls to be taken in its relative rather than its absolute sense. Thus. if the exposure to infra-red light is stopped 1,400 or even 00 second before the first operation is begun again. this time is substantial in relation to the total time for all operations, i. e. 1,4 second, as it constitutes approximately yone-sixth or one-twelfth of this period.

I shall now describe the transmission operations when the preferred system of plates, as shown in Figure 3, is used:-

(a) Through the rotation of the shaft 29a carrying thecommutators `24, 25 and 26 and the discs I5 and I9; the opening I6 in the disc I5 is brought into registry with the lens I4 and opening 2 in the cell." At the same time the plate 2| is connected to pcsitvevbattery 33 of low voltage through the sector 21 of commutator 24 and the contact 30, the anode 22 is grounded through the sector 28 of commutator 25 and the contact 3G, and the hot cathode 6 is likewise grounded through the sector 29 of commutator 26 and the contact 45. Through the opening I6 in the disc I5 the image of the view is focussed by the lens I 4 on the photosensitive surface 65 of the plate 20. Under the influence of and in accordance with the intensity of the light from the view striking diierent parts of the surface 65, different numbers of electrons are emitted from these parts, and owing to the positive potential of the plate 2|, the electronic emission from each part of the surface E5 is attracted to the nearest element 6|, this element being thus raisedto a certain negative potential. The potential of plate 2| should behigh enough adequately to attract electrons to the elements but not so high as to cause any substantial leakage Aof current toit from the elements when charged. V`".lisuitable potential would be approximately two volts. Since the potentials to which the various'elements Bi'are raised depend upon the amounts ofthe electronic emissions from corresponding parts of the surface 65, and since the amounts of these emissions depend in turn upon the intensity o f the light from the view striking the parts from which they are emitted, the negative potentials of the various elements 6| will be proportional to these intensities. On the elements 5| as -a whole, there will therefore be formed an electrostatic reproduction of the image of the view.

(b) Through the rotation of the shaft 29a, the light from the view is interrupted, plate 2| is connected to battery 34 of very low positive potential, the anode 22 is connected through the contact 31 to the grid 4| of the thermionic valve 42 and through resistance 43 to battery 44 of relatively high positive potential, while the hot cathode 6 is connected through contact 48 to battery 49 of high negative potential. The positive potential of the plate 2| in this operation must be at least slightly higher than the maximum negative potential of any element and may suitably have a value of from one-half volt to two volts. By reason of the connection of its cathode 6 to the high negative potential, the cathode ray oscillograph portion 4 is caused to emit a beam 55 of cathode rays, which is caused by the magnets 'I and 8 to scan the surface of the plate 20 facing the anode. the beam striking this surface at approximately the angle shown in Figures 3 and 4. Under the influence of the beam a secondary electronic emission is produced from every part of the plate 20 which is struck. As is known, the greater part of the secondary emission is made up of relatively slow moving electrons which, on being emitted, are subjected to two attractions acting in opposite directions. The first of these is that of the anode 22 and the second is that of the positive potential of the plate 2| less the negative potential of the nearest element 6| on this plate. Thus, of the secondary electronic emission from any area of the surface of plate 20 struck by the cathode ray beam, that part which consists of slow moving electrons will be divided in accordance with the relative intensities of the two attractions to which it is subjected. The electronic current reaching the anode from any area of the surface of the plate 20 will therefore depend upon the negative potential of the element 6| nearest this area, which potential is proportional tothe intensity of the light from a corresponding part of the view. Thus the electronic current reaching the anode during the scanning operation will be one modulated by the electrostatic reproduction of the image of the view. The variations of potential caused in the anode by the reception of the modulated electronic. current are impressed on the grid 4| of the thermionic valve 42, whence transmission takes place by known methods.

As mentioned above, not all the electronic emission caused by the cathode ray beam consists of electrons having a velocity low enough to be materially affected by the attractions of the anode 22 and plate 2|. The electrons of1 high velocity not so aiected will tend to move from the surface of the plate 2U in straight lines, and a large proportion of them will have such velocity that if they were to strike some other surface in the cell they would themselves cause the emission of further secondary electrons of lower velocity, which would be picked up by the anode 22 and thus cause distortion of the image currents transmitted. It is to prevent this that I have provided the screen II and grid I2. 'Ihe grid I2 is connected to battery I3 of a positive potential equal to or even greater than that of the battery 44 to which the anode is connected, but since the screen I and shielding 9 together constitute a Faraday cage the potential of the grid l2 has no influence outside the screen EI. Fast moving electrons detached from the surface of the plate 2@ will pass through the screen il, and in the great majority of cases through the grid l2, to strike the shielding 9 and cause the emission o f further electrons of lower velocity. 'Ihe great majority of these latter electrons will be captured by the grid l2. If some of them escapeit and pass outside the Faraday cage they will merely strike the shielding 9 again at some other point, and the process of capture by the grid l2 of the electrons which they emit will be repeated. Since the area of the anode 22, which is in the form of a wire loop, is extremely small in relation to the area of the rest of the cell, the number of fast moving electrons which will either strike it directly or pass so close to it as to be captured by it will be negligible.

(c) Through further rotation of the shaft 29a the opening in the disc I9 is brought into registry with the opening 3 in the cell, and the hot cathode 6, anode 22 and plate 2| are grounded at 50, 40 and 35 respectively through contacts 41, 38 and 32 respectively. Through the opening in the disc I9 the light of the lamp I1 which passes through the screen I8 is admitted to the cell and strikes the phtosensitive surfaces 62 of the elementsSl on plate 2|. This light is of such frequency that it will detach from the photosensitive surfaces 62 electrons of low velocity, and it isfpreferably in the infra-red region. Since the elements 6| are at various negative potentials the rest of the members of the cell are positive with respect to them and will thus attract any electrons which they emit under the iniuence of the light striking them. The coating of aluminium oxide on the plate 2| has, as is Well known, the property of opposing a resistance approximately a thousand times as great to the passage of current from a negative element to a positive element as from a positive element to a negative element. Thus while the elements 6| are negative there will be substantially no passage of current from them to the plate 2|. As soon, however, as they reach a positive potential by reason of the detachment of electrons from them under the iniluence of the light, the leakage current from them to the plate 2| will become very great and thus any further increase in their positive potential will be counteracted. At the. end of this operation all the elements will be at a uniform and slightly positive potential.

(d) Through further rotation of the shaft 29a the opening in the disc I9 moves out of registry with the opening 3 in the cell, and the light from the lamp |1 is cut off. Since, however, the arc over which the opening in the disc I9 extends is substantially less than 120, this will happen while the metallic sectors 21, 28 and 29 of the commutators 24, 25 and 26 are still passing over the contacts 32, 38 and 41 respectively, so that the potential of the various members in the cell will remain as in operation (c), but the cell will be completely dark. There will now be nothing to counteract the leakage currents from the elements 6| to the plate 2|, and these elements will thus all be reduced to a uniform potential of practically zero. At this point the cell is ready for a repetition of the operations described.

The only difference between the operations with the system of plates shown in Figure `3 and that shown in Figure 4 is that with the latter system the eelectrostatic reproduction of the view is caused to disappear in one operation which lasts for the same length of time as operations (c) and (d) described above. The one operation is as followsz- (c) The members in the cell are all grounded armena as in operation (c) above, and the light of the lamp I1 is admitted to the cell through the opening in the disc I9. Under the influence of the light electrons are emitted from the elements 6| on the plate 12 and are picked up by the members in the cell. The elements are thus all brought to a uniform and slightly positive potential, which potential will not be exceeded owing to the increased leakage of current above it from the elements to the plate, and also owing to the fact that above it electrons emitted will be re-attracted by reason of the higher positive potential.

The frequency band necessary for transmission might be narrowed if, instead of using only one apparatus where the whole cycle of operations is completed in one sixteenth of a second. three apparatuses were used. In this case while the ilrst apparatus was performing operation (a) the second would be performing operation (h) and the third operations (c) and (d). Thus. though a picture would be transmitted every 116 second each operation could last 1*.,- instead of only 1,?,0 of a second. This would be of importance in transmitting the picture since transmission could then be spread over three times the period which could be allowed when only one apparatus was used and the frequency band for transmission thus narrowed.

Very strong, enlarged and contrasted images of very faint, still objects could be produced with my system owing to electrical amplification. The disc I would be brought to a position to allow the view to fall on plate 20 for a suitable length of time. The shaft 29a would then be rotated to a position to allow scanning of the screen and this scanning could be repeated without any intermediate operation for as long as desired since the electrostatic image of the object would always remain on the elements 6|. My device could, for instance, be attached to a powerful astronomical telescope and, while the image focussed would be large and weak, a very vivid strong image would appear on the receiver. The device could also be attached to a microscope and permit hitherto invisible objects to be seen.

Various modifications may be made in the invention without departing from the spirit thereof or the scope of the claims and, therefore, the exact forms shown are to be taken as illustrative only and not in a limiting sense, and I desire that only such limitations shall be placed thereon as are imposed by the prior art or are specifically set forth in the appended claims.

What I claim as my invention is:

1. Apparatus for the electrical transmission or images, which comprises means for impressing an image upon a photosensitive surface, means for scanning a surface other than said photosensitive surface to cause the emission of an electronic stream of substantially constant intensity from said other surface, means for causing the formation at one time of an electrostatic reproduction of the whole of said image to modulate said stream, an anode for picking up the modulated stream, and means for thereafter causing said electrostatic reproduction to disappear.

2. Apparatus for the electrical transmission of images, which comprises means for impressing an image upon a photosensitive surface, means for scanning a surface other than said photosensitive surface to cause the emission of an electronic stream of substantially constant intensity from said other surface, means for causing the formation at one time of an electrostatic reproduction of the whole of said image to modulate said stream, an anode for picking up the modulated stream and means for thereafter exposing said electrostatic reproduction to infra red light.

3. Apparatus for the electrical transmission of images, which comprises means for impressing an image upon a photosensitive surface, means for scanning a surface other than said photosensitive surface to cause the emission of an electronic stream of substantially constant intensity from said other surface, means for causing the formation at one time of an electrostatic reproduction of the whole of said image to modulate said stream, an anode for picking up the modulated stream, means for thereafter exposingl said electrostatic reproduction to infra red light, and means for stopping said exposure a substantial time before an image is again impressed upon the photosensitive surface.

4. Apparatus for the electrical transmission of images, comprising a cell, a photosensitive surface within said cell, means for impressing an image on said surface, means in the form of a multiplicity of mutually insulated elements for retaining an electrostatic reproduction of said image, said elements being formed on the surface facing said photosensitive surface of a member interposed between said photosensitive surface and the view.

5. Apparatus for the electrical transmission of images, comprising a cell, a photo-sensitive surface within said cell, means for impressing an image on said surface, means in the form of a multiplicity of mutually insulated elements for retaining an electrostatic reproduction of said image, said elements being formed in the surface facing said photosensitive surface of an opaque member interposed between said photosensitive 5 surface and the object of which an image is projected on said photosensitive surface and formed with a multiplicity of openings therethrough.

6. Apparatus for the electrical transmission of images, comprising a cell, an opening in said cell 1 for the projection therethrough of an image, an anode in said cell, a member between said anode and said opening having amultiplicity of openings therethrough and having a pluotosensitive surface facing said opening, another member be- 1 tween said opening and said first mentioned member and having on its surface directed towards said lfirst mentioned member a multiplicity of mutually insulated elements arranged opposite the openings in said first mentioned member, 2 means for impressing an image on said photosensitive surface, means for causing'said multiplicity of elements to retain an electrostatic reproduction of said image and means for scanning the surface of said rst mentioned member fac- 2 ing said anode to cause an electronic emission from said surface and means for picking up said emission, modulated by the electrostaticl reproduction, at the anode.

3 FRANCOIS CHARLES PIERRE HENROTEAU.

Images