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calculated gunnery data for the U.S. Navy, weighed 5 tons and was more than 50 feet long. Its miles of wiring linked together more than 500,000 electronic components, including more than 3,000 relays, but for all its massiveness, the Mark I did not support conditional jumps either.

A relay is not the only or best kind of controllable on-off switch. The same function can be performed with a vacuum tube, a device, patented in 1904, that was an outgrowth of Thomas Edisons work with incandescent lighting.A vacuum tube is an evacuated glass enve- lope that contains electrodes.Without air molecules to interfere, elec- trons stream through space from electrode to electrode, carrying information and electrical power.This device initiated the electronic age because it could control and amplify electrical signals, making it indispensable for radio and television as well as for video and audio reproduction.

Like a relay, a vacuum tube can switch current on and off, but without mechanical parts, the tube is faster and more reliable than any relay.The first Boolean circuit with tubes was made in 1939, and in 1943, British engineers built the Colossus.With several thousand vacuum tubes, this special-purpose computer analyzed German mili- tary codes as part of the famous Enigmacode-breaking effort at Bletchley Park in England. The first full-featured electronic digital computer followed in 1946: the electronic numerical integrator and computer (ENIAC) built by J. Presper Eckert and JohnW. Mauchly at the University of Pennsylvania. Its 18,000 vacuum tubes used many kilowatts of electrical power to determine artillery trajectories at a rate of thousands of calculations a second.

ENIAC served its military purpose and was also used for scien- tific calculations, but its hardware connections had to be tediously set by hand. Other machines of the era entered programs on punched paper tape and were no great advance either over the Jacquard cards that Babbage had envisioned.The idea that made computers infinitely more flexible is usually ascribed to the brilliant Hungarian-born mathematician John von Neumann, although there is evidence that Eckert, Mauchly and others entertained a smiliar approach. In 1945

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