Radio Broadcast (May 1928-Apr 1929)

128 RADIO BROADCAST JULY, 1928 Pand Fare the elements of a special two-electrode Vacuum Tube THE CLARKSON TELEVISION PROJECTOR This projector makes use of a three-electrode tube, the grid of which functions to control the electron stream from the filament F. The electrons passing through the opening in the plate P are caused to scan the screen due to the action of the two coils A and B A phosphorescent screen (rather than a fluorescent screen) is used so that the screen will continue to glow for an interval after the impulse stimulus is removed. This results in considerable improvement for it reduces the amount of light required and also permits the use of a greater number of impulses so that greater detail may be obtained of seven light rays analyzing the object, instead of one, and Doctor Ives' experiment with subdivided photoelectric cell and screen. The Englishman, A. A. Campbell Swinton, in a letter to Nature, June 18, 1908, and more in detail in his Presidential Address to the Rontgen Society, November 7, 191 1, set forth the genesis of an idea along these lines but one never given publicity and never tried out. I have taken the liberty of modifying this idea and present it herewith as a last desperate hope. IS THIS THE WAY OUT? IN ALL other television devices before the pub' lie at present the method of telephotography is being used, speeded up to the tenth-second requirement. At the transmitter is a photoelectric cell. A beam of light explores the object to be "televised" and is reflected to the cell. This cell modulates the carrier wave, just as though it were a microphone. Varying light actuates it just as varying sound actuates the microphone. At the receiver, in place of the loud speaker, is a glow lamp — usually a neon tube in one form or another — which changes its brilliancy in step with the received impulses from the photoelectric cell. The light from this lamp is made to explore a screen in synchronism with the beam at the transmitter. The usual method of swinging the beams of light up and down and over the object and screen, is a mechanically revolving disc perforated spirally with holes, a device patented by Nipkow in 1884, this inventor being the first to see the advantage of breaking up a picture into lines. In the Swinton method there is no mechanically moving part. The object is illuminated strongly and we have a "television camera," let us say, which projects the image to be transmitted, not on a film, but on a composite plate made of tiny cubes of photoelectric material insulated from each other. The camera is gas tight and filled with sodium vapor, which conducts negative electrons more readily under the influence of light. Between the projecting lens and the composite plate, in the vapor chamber, is a gauze wire screen. The charge on this gauze screen modulates the transmitting tube. In effect, the gauze screen is connected by radio to a plate in the receiver projection apparatus. A beam of cathode rays is directed past this plate towards a sensitive fluorescent screen. Only when the rays are slightly bent by the repulsion of the plate can they pass through a fixed opening and actually be directed to the fluorescent screen to cause a luminous spot. At the transmitting end there is also a cathode-ray beam continually searching the composite plate of the camera but on the back side from where the image illuminates it. As this stream strikes each little photoelectric cube, it charges it negatively but the charge is dissipated unless that cube is illuminated on the front by light from the object. In the latter case, the charge of the cube will pass away through the ionized vapor along the illuminating beam of light until it reaches the gauze screen, whereupon that charge becomes an impulse carried over to the receiver projection apparatus where it charges the deflecting plate which bends the synchronized cathode ray so that a luminous spot is formed on the fluorescent screen. Each received impulse must correspond in position to the illuminated cube of the composite plate, requiring the synchronizing of the two cathode ray beams. This may be done at each end separately through the same construction as the cathode ray oscillograph, the beam being moved by the magnetic field of two coils at right angles to each other and having widely differing frequencies as 10 and 1000 cycles or 10 and 10,000 cycles. Substantially the two rays are merely tracing curves of great amplitude and rather low frequency. In this method the object itself is not explored but its projected image is automatically subdivided by the composite plate of the camera, which has no electrical connections. Only one carrier wave is required but we still have the broad band of frequencies to detect and amplify at the receiver. No mechanically moving parts are used. A telephoto lens, a wide angle lens or any usual camera arrangement may be used at will. Synchronizing presents no difficulties and the method is as adaptable to wire as to radio. But as yet it has not been found practical, the main reason being that the use of photoelectric material in the composite plate means that electrons will be given off continually as long as light falls on the plate, and in mass when the image shifts. In the writer's proposed modification of the Swinton device, the material of the composite plate is non-photoelectric but conductive. The writer uses a closed electric circuit of which the exploring electron beam is a part, the conductive cube is a part when the beam strikes it, and the ionized path in the vapor is the varying part of the circuit. An amplifying tube is readily coupled to this circuit. In the projector proposed by the writer, he suggests the use of a three-electrode vacuum tube, using a heavily biased grid, the incoming signal modulating that grid, as usual, and permitting the flow of an electron beam. The observing screen must be phosphorescent, instead of fluorescent. That is, it must glow for a time after the impulse strikes it. If any method within our knowledge has possibilities, this is it. If it fails, television will await the genius who conceives some new way of breaking up an image. There is no other hope. P,G and F are the elements of a special Vacuum Tube THE CLARKSON TELEVISION CAMERA In this arrangement a closed electric circuit exists from the source of electrons F through the beam of electrons, which act as a flexible conductor, to any conductive member of the nonphotoelectric composite plate, through the plate to the wire gauze screen, along the screen to the coupling resistance and back to the filament circuit. The object to be televised stands in front of the lens at the left