It has sometimes been urged by those who favour the mechanical system that the cathode ray tube is a source of danger in the hands of those who use it, in that -so it is alleged - it acts as a generator of X-radiation and everyone knows that prolonged exposure to X-rays may be very harmful. There are various other advantages, but this is the principal one. The prime advantage of the cathode tube is the fact that it is electrical instead of being mechanical and the beam is virtually weightless, so that it can respond very faith.fully to the signal voltages impressed on the tube. You will easily see from this that, by causing the spot to 'scan' the screen in a regular fashion, the character of the cathode beam (which determines the brightness of the spot) being at the same time appropriately varies in accordance with incoming television signals, the television picture will be built up on the fluorescent screen. If the voltages applied to these deflecting plates are rapidly varying, the spot on the screen will shift about correspondingly. If two pairs of deflecting plates are used, and are so placed that the deflecting force exerted on the cathode beam by one pair of plates is at right angles to the deflecting force exerted-by the other pair, then clearly by adjusting the voltages across the pairs of plates in the proper way, the spot of impact of the beam on the screen can be shifted about to any desired position. The cathode beam, on its way towards the fluorescent screen, passes between certain sets of electrodes or deflecting plates, across which an electrical potential difference is established (exactly in the same way that a high-tension voltage is applied between the filament and anode of a wireless valve). In addition to this the cathode ray beam itself may be considered to be virtually without mass. So far as this part of the device is concerned, it merely involves the use of a suitable material for the fluorescent screen. If the screen is made of suitable material, so that the bright effect at any spot disappears almost instantly when the beam shifts away (or, as it is said, if there is a very small 'lag' or phosphorescence), then the arrangement capable of following very rapid movements in the cathode ray beam without blurring or confusion. The spot at which the beam strikes the screen is thereby rendered luminous and, if the point of impact shifts about, the bright spot shifts about correspondingly. At one end of the tube is a fluorescent screen upon which the cathode, beam impinges.
The cathode ray tube consists essentially of a vessel, usually of glass, evacuated to a low gas pressure and provided with electrodes by means of which an electrical discharge may be passed through it. Perhaps I should explain very briefly the particular features of the cathode ray tube which render it especially adapted for television purposes. On the other hand, there are the protagonists of the mechanical system who still maintain that this latter system is capable of fulfilling all the requirements likely to be made of it and who, moreover, urge certain objections against the cathode ray system. There is a large body of scientific opinion now inclined to the view that the cathode tube principle constitutes the only hope for any real progress in television reception. A rough analogy is provided by the high- frequency alternator for the generation of HF radio oscillations superseded by the more modern thermionic valve oscillator.
Cathode ray television series#
The building up of the picture at the receiving end by means of mechanical scanning apparatus is subject to the limitations imposed by any mechanical arrangement which has to perform a series of functions with extreme rapidity. These are, of course, rather vague distinctions, because no television system is entirely mechanical or entirely electrical, most systems being, in fact, a mixture of the two. There are various television systems now being tried out and developed in different parts of the world, and these may be divided broadly into two classes: (a) the mechanical systems, and (b) the electronic (or cathode ray) systems. Dr Roberts has had great experience with cathode ray and X-ray work generally, and was formerly at the Cavendish Laboratory, Cambridge, with Lord Rutherford and Sir J J Thomson. Our Scientific Consultant shows in this article that the possibility of appreciable X-radiation being given out from a cathode tube is remote in the extreme. Many people regard the Cathode Ray Tube as the 'white hope' of Television, but some supporters of the mechanical system have asserted that the Cathode Ray Tube is dangerous to use.
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