<P> Heaters may be energized with either alternating current (AC) or direct current (DC). DC is often used where low hum is required . </P> <P> Vacuum tubes used as switches made electronic computing possible for the first time, but the cost and relatively short mean time to failure of tubes were limiting factors . "The common wisdom was that valves--which, like light bulbs, contained a hot glowing filament--could never be used satisfactorily in large numbers, for they were unreliable, and in a large installation too many would fail in too short a time". Tommy Flowers, who later designed Colossus, "discovered that, so long as valves were switched on and left on, they could operate reliably for very long periods, especially if their' heaters' were run on a reduced current". In 1934 Flowers built a successful experimental installation using over 3,000 tubes in small independent modules; when a tube failed, it was possible to switch off one module and keep the others going, thereby reducing the risk of another tube failure being caused; this installation was accepted by the Post Office (who operated telephone exchanges). Flowers was also a pioneer of using tubes as very fast (compared to electromechanical devices) electronic switches . Later work confirmed that tube unreliability was not as serious an issue as generally believed; the 1946 ENIAC, with over 17,000 tubes, had a tube failure (which took 15 minutes to locate) on average every two days . The quality of the tubes was a factor, and the diversion of skilled people during the Second World War lowered the general quality of tubes . During the war Colossus was instrumental in breaking German codes . After the war, development continued with tube - based computers including, military computers ENIAC and Whirlwind, the Ferranti Mark 1 (the first commercially available electronic computer), and UNIVAC I, also available commercially . </P> <P> Flowers's Colossus and its successor Colossus Mk2 were built by the British during World War II to substantially speed up the task of breaking the German high level Lorenz encryption . Using about 1,500 vacuum tubes (2,400 for Mk2), Colossus replaced an earlier machine based on relay and switch logic (the Heath Robinson). Colossus was able to break in a matter of hours messages that had previously taken several weeks; it was also much more reliable . Colossus was the first use of vacuum tubes working in concert on such a large scale for a single machine . </P> <P> Once Colossus was built and installed, it ran continuously, powered by dual redundant diesel generators, the wartime mains supply being considered too unreliable . The only time it was switched off was for conversion to Mk2, which added more tubes . Another nine Colossus Mk2s were built . Each Mk2 consumed 15 kilowatts; most of the power was for the tube heaters . </P>

Who developed the first spark gap tube generator