THE WORLD OF SCIENCE

Te Awamutu Courier, Volume 53, Issue 3828, 2 November 1936, Page 2

THE WORLD OF SCIENCE

WHAT TELEVISION OWES TO EARLY DISCOVSRV (By Professor Kerr Grant) In 1602, or thereabouts, a shoemaker of thd town of .Bologna, in Italy, made an observaci m of a curious optical effect, which, after thiec centuries of quiet incubation, promises to-day to emerge as an essential element in the practical realisation of television. Having heated a piece of the mineral now known as “heavy spar”— a sulphate of the metal barium —in an oven until it was partly calcined, he then observed that, after exposure to sunlight, it shone brightly in the dark. As far as I can find, this is the first recorded case of phosphorescence in an inorganic substance, though the use of the word “il fosforo,” by wjhieh it was described at that time, a word whicn is merely an Italian form of the Greek “phosphor” (light-bearer), suggests that similar luminescence had already been observed in other things. The element “phosphorous” owes its name, of course, also to the fi.ct that it shines in the dark, though, in this cases the cause—nr.mfely, '-slow spontaneous combustion—is entirely different. OBJECT OF INTEREST. This “Bologna Stone,” as it was called speedily became an object cf scientific interest Robert Boyle studied it, and found that diamonds had the same property—but no con? sidcirable extension of .knowledge concerning it was made until the experiments of two Frenchmen, some hundreds years ago, showed that the property of phosphorescence was common to a whole class of chemical compounds, namely, the sulphides of the group of metals to which barium belongs—calcium and strontium are its nearest fellows and to many other chemical substances as well. (The calcination of the heavy spar conveirts it, partially at least, to the sulphide.) But the power of storing up light and subsequently re-emitting it posse ised by those “phosphors” was still curiously capricious until the lengthy researches of Professor Phillipp Lenard of Heidelberg gave an explanation of this variability. Lenard found that absolutely pure chemicals were/ devoid of phosphorescence!, but that the addition of minute traces of certain heavy metals, such as copper, bismuth or antimony, combiner! with a special method of preparation, was sufficient to confc? this power. INTERESTING PROBLEM. Sii’ George Stokes, of Oxford, found that, for a large class of substances, which he termed “fluorescent,” the light emisison is simultaneous with the excitation, and that this excitation is the more powerful the shorter the wavelengths of the light. Fluorescence is, in fact, sucn a common property of organic substances under ultra-violet illumination that we may regard it as a universal property of these, and perhaps of all liquids and gases, whatever their nature.

Since, Stokes’s day, half a century ago, hundreds of investigations have been made on this most interesting problem, and out of these to-day mtny important practical results are emerging. The use of fluorescent screens has from the first been a most important aid in X-ray diagnosis, for X-rays, which are light of extremely short wave length, have also the power to excite phosphors—calcium tungstate, zinc sulphide, or barium platino-cy-anide are those chiefly employed—to vigorous luminescence, and ths shadow of a bone or other object denser than the flefeh shows up clearly on the luminous background of the screen. But it is not light alone that can stimulate this luminescence. The impact of fast-flying atoms, especially if electrically charged, is equally, or even morel effective. Professor Braun, of Strasburg, who shared the Nobel prize with Marconi many years ago, applied this effectively in the construction of a vacuum tube in which a fine pencil of cathode rays or electrons impinges upon a thin coating of fluorescent material on the inner wall of the glass bulb, and produces there a brightly luminous spot. USE IN TELEVISION. This tube, known as the “cathoderay oscillograph,” has been, and is being ever more extensively employed in the detection or measurement of rapid changes of voltage in an electrical circuit—such as result, for example, from a lightning flash on a transmission line—for a stream of electrons is far more mobile! than the lightest mechanical device. In the modern problem of television the cathode-ray oscillograph promises to furnish by far the most effective means of converting a varying electrical impulse received by wire or wireless from a distant station, in a picture corresponding faithfully in detail with that at the transmitting station by which these impulses are controlled, and it thus becomes not merely an instrument for scientific measurement, but an element in a device of immense industrial and commercial importance. Hence it is that at the present day the, research laboratories of great

eledtrical undertakings, as well as many private investigators, are devoting much attention to the problem of making highly efficient phosphors for coating the necessary luminescent screen on which the televised picture will appear as the electron beam sweeps to and fro over its surface. It is essential not merely that the luminous spot should be a bright one, but that its duration should be short- —not more than one-twenty-fifth of a second—in order that, in a rapidly changing picture, the present should not be blurred by persistence of the past (a. principle which has far reaching implications in other spheres of thought and action). Colour of the light is also important, -for the pleasure of listening to a singer might not be enhanced if her face upon the screen before us appeared as a ghastly green or fiery red. RECENT REMARKABLE PROGRESS. From recently published reports of the researches carried out by Manfred von Ardenne in Germany an# by Dr Ldvy and Mr West in England, it appears that very remarkable by these investigators in preparation progress has recently been achieved of suitable phosphors. The best results are given by a mixture of zinc sulphide with the double sulphide of zinc and cadmium with a metallic (copper). A screen of this material can be made to give a luminosity better- than that of the ordinary home cinema, the colour of the fluorescent light is white, and undue persistence of phosphorescence can be entirely eliminated as Dr Levy and Mr West have discovered —by the addition of less than onemillionth part of metallic nickel to the mixura. The size of the screen is small, it is true less than a square foot—but size is mainly a question of the amount of money which the> purveyors of television are prepared to spend.-y-K.G.