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The Williams tube, or the Williams–Kilburn tube after inventors Freddie Williams and Tom Kilburn,〔http://www.computer50.org/mark1/notes.html#acousticdelay Why Williams-Kilburn Tube is a Better Name for the Williams Tube 〕 developed in 1946 and 1947, was a cathode ray tube used as a computer memory to electronically store binary data. It was the first random-access digital storage device, and was used successfully in several early computers. Williams and Kilburn applied for British patents on Dec. 11, 1946〔(GB Patent 645,691 )〕 and Oct. 2, 1947,〔(GB Patent 657,591 )〕 followed by US patent applications on Dec. 10, 1947 () and May 16, 1949 (). ==Working principle== The Williams tube depends on an effect called secondary emission. When a dot is drawn on a cathode ray tube by a beam of fast-moving electrons, the area of the dot becomes slightly positively charged and the area immediately around it becomes slightly negatively charged, creating a charge well. The charge well remains on the surface of the tube for a fraction of a second, allowing the device to act as a computer memory. The lifetime of the charge well depends on the electrical resistance of the inside of the tube. The dot can be erased by drawing a second dot immediately next to the first one, thus filling the charge well. Most systems did this by drawing a short dash starting at the dot position, so that the extension of the dash erased the charge initially stored at the starting point. Information is read from the tube by means of a metal pickup plate that covers the face of the tube. Each time a dot is created or erased, the change in electrical charge induces a voltage pulse in the pickup plate. Since this operation is synchronised with whichever location on the screen is being targeted at that moment, it effectively reads the data stored there. Because the electron beam is essentially inertia-free, and thus can be steered from location to location very quickly, there is no practical restriction in the order of positions so accessed, hence the so-called ″random-access″ nature of the lookup. Reading a memory location creates a new charge well, destroying the original contents of that location, and so any read has to be followed by a write to reinstate the original data. Since the charge gradually leaked away, it was necessary to scan the tube periodically and rewrite every dot (similar to the memory refresh cycles of DRAM in modern systems). Some Williams tubes were made from radar-type cathode ray tubes with a phosphor coating that made the data visible, while other tubes were purpose-built without such a coating. The presence or absence of this coating had no effect on the operation of the tube, and was of no importance to the operators since the face of the tube was covered by the pickup plate. If a visible output was needed, a second tube connected in parallel with the storage tube, with a phosphor coating but without a pickup plate, was used as a display device. Each Williams tube could store about 1024–2560 bits of data. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「williams tube」の詳細全文を読む スポンサード リンク
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