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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. 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. 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.
Television Systems
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. 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.
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. A rough analogy is provided by the high- frequency alternator for the generation of HF radio oscillations superseded by the more modern thermionic valve oscillator.
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. 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.
The Cathode Tube
Perhaps I should explain very briefly the particular features of the cathode ray tube which render it especially adapted for television purposes.
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. At one end of the tube is a fluorescent screen upon which the cathode, beam impinges. 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.
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. So far as this part of the device is concerned, it merely involves the use of a suitable material for the fluorescent screen.
Controlling the Beam
In addition to this the cathode ray beam itself may be considered to be virtually without mass. 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). 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.
If the voltages applied to these deflecting plates are rapidly varying, the spot on the screen will shift about correspondingly. 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.
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. There are various other advantages, but this is the principal one.
Generating Rays
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.
It is, therefore, important to consider whether there is any foundation for such a view. Let us examine the case further.
The production of X-rays is caused by the negative electrical particles striking the anti-cathode (anode), and in order that any X-rays, properly so called, shall be produced, it is necessary that the electrons strike the target with a considerable velocity, or as the phrase 'considerable velocity' may be rather vague as applied to, electrons - perhaps I should say that the production of X-rays requires a large driving voltage, many thousands, of Volts. The voltage actually applied to at modern X-ray tube is often in the region of 100,000 to 150,000 Volts that is, the voltage which is driving the electrons towards the target.
X-Rays Require High Voltages
It is not easy to define precisely the character or quantity of the X radiation in relation to the driving voltage, as the whole question is complicated by the composite character of the rays and the presence of secondary radiation and by other factors.
It is, however, definitely known that the strength of the rays increases very rapidly with the driving voltage, or, alternatively, taking actual figures, with a driving voltage of 2,000 Volts as compared with a driving voltage of 100,000 Volts, the intensity of the rays in the former case would be only a fraction of a thousandth of the intensity in the second case. As a matter of fact, the strength would probably be much less even than this, because at such relatively low voltages as 2,000 Volts the rate of variation is probably still more rapid.
In an X-ray tube great pains are taken in the design so as to focus the electron stream as much as possible on to a small spot on the anti-cathode; this is with the object of causing X-rays to issue from a point source, as far as possible, so as to increase the sharpness of the image obtained by the rays.
Soft Rays Easily Absorbed
In a cathode ray tube, however, apart from the question of the sharpness of the focus, the spot of impact is shifting about during the operation of the tube over a considerable area, with the result that, even if any rays were produced, they would in the average proceed from a very extended source and would, therefore, be enormously spread and weakened, as measured at a point away from the tube.
But probably most important of all is the question of the penetrating power or 'hardness' of the rays and their absorption. Any rays produced by the impact of the cathode stream upon the screen in a cathode ray tube will be very 'soft' owing to the extremely low driving voltage), and it is a characteristic of soft rays that they are rapidly absorbed by matter. It is, therefore, very doubtful whether any appreciable percentage of such rays would even get through the screen itself, let alone the glass wall of the cathode ray tube. In fact, very soft X-rays are absorbed by a few centimetres or even a few millimetres of air.
Summing the whole thing up, it seems to me that whatever my be said for or against the cathode ray tube for commercial purposes, including its application to television, there is not the slightest ground for alleging that it can be a source of danger owing to any harmful radiation proceeding from it.
Learning from Experience
When I was working at the Cavendish Laboratory at Cambridge I once get a bad attack of X-ray burn on the face, which was very irritating and painful for a few days - for all the world like extreme sunburn - but the tube I was using was not a cathode ray tube, it was very much ' harder' and enormously more powerful. After that, you may be sure that I took extra special care to avoid any possibility of a recurrence of the same experience, and, although I have worked with all kinds of cathode ray tubes, I have never found it necessary to take the slightest precautions against soft X-rays and I have never experienced the slightest harm.
In any case, it is a perfectly simple matter to view the cathode ray screen through a small sheet of lead glass. Since lead glass can stop the penetrating radiation from a powerful X-ray tube the use of such a simple precaution should clear away any possibility of danger from soft easily absorbed rays from the cathode tube - even assuming that such rays existed.
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