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When was the Television Invented?
Television is older in its idea than one may suspect. It depends on the sensitivity of selenium to light, from which it produces an electric current; this was discovered by May in 1872. Eight years later Shelford Bidwell had produced a crude but workable system of picture telegraphy. In those days there were no amplifiers and the transmission was necessarily over wires; Bidwell's method of building up the picture was by mechanical synchronization of apparatus at the transmitting and receiving ends. This mechanical exploration of the picture was known as scanning. In 1926 the first demonstration of true television - by which is meant the transmission of live pictures and their reception without intervening wires - was made by John Logic Baird (1888-1946), using a scanning method devised by Nipkow in 1884. Baird's demonstration was possible only by reason of developments in thermionic amplifiers and light-sensitive photo-cells since Bidwell's wired attempts in 1881. It was true television, but in quality it left much to be desired. From 1929 to 1935 the British Broadcasting Corporation transmitted experimental programmes on this system, in which the picture was scanned in only thirty lines at the very low frame frequency of twelve-and-a-half pictures per second. This gave an unpleasant flicker.
As early as 1906 von Leiben suggested that the cathode-ray oscillograph, in which a beam of electrons can be controlled by electromagnetic fields in such a way that they leave on a fluorescent screen a trace which gives a record of the controlling forces, should be used for reproducing pictures; variations in picture brightness were to be the main controlling force. On 18 June 1908 Nature in London published a remarkable letter from A. A. Campbell-Swinton. He declared that scanning and reception by cathode-ray oscillograph held out the only hope of effective television owing to the complete absence of inertia with the cathode rays, which could thus sweep and be swept at speeds that would be unthinkable in any mechanical device. In the Baird mechanical system a picture three inches by two inches required, forresolution equivalent to that of a newspaper illustration, over a quarter of a million signals per second. In a cathode-ray system a rate many times this value presents no great difficulty.
By 1911 it was realized that the cathode ray might be so controlled that it would go fast over the dark parts of the scene to be produced and slowly over the light parts, although at the time there was no means of exercising such control. The end of the cathode-ray tube is coated with a fluorescent material that glows for a short time with an intensity proportionate to the time it is under bombardment by the ray. If the ray sweeps over it very fast that part of the screen would therefore hardly glow at all and would look dark, whereas a slow-moving ray would make the screen look bright. This suggestion, made by Rosing in 1911, became practicable when in 1931 von Ardenne designed suitable circuits and amplifiers, which depended upon intervening developments in thermionic valves.
The first regular commercial television by cathode-ray means was started in Great Britain in November 1936. From that time onwards, interrupted only by the Second World War, high-definition television transmissions have continued. The Marconi-E.M.I. system adopted by the B.B.C. in 1936 was based on scanning the picture in 405 lines and transmitting fifty frames per second. Compared with the original Baird system of thirty lines and twelve-and-a-half frames, the result was about sixty times as effective, but Great Britain's pioneering success was hardly an unmixed blessing, since the existence of many thousands of sets pegged the standard to the 405-line picture, which is less finely detailed than it need be. Practical 1000-line systems have been devised.
Baird's mechanical system had severe limitations, but he demonstrated a number of refinements which are just the same, in principle, as those possible to cathode-ray television. Thus, in 1927 he demonstrated 'noctovision' using invisible infra-red rays to activate the photocells. He also experimented with big-screen television, colour television (which is fairly simple in theory, but requires three times the number of signals and triple synchronization in practice) and projections.