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Harwell computer
The Harwell computer, later known as the Wolverhampton Instrument for Teaching Computing from Harwell (WITCH), or the Harwell Dekatron Computer, was an early British relay-based computer of the 1950s. From 2009 to 2012, it was restored at the National Museum of Computing,. In 2013, for the second time the Guinness Book of World Records recognised it as the world's oldest working digital computer, following its restoration, it previously held the title for several years until it was decomissioned in 1973. The museum uses the computer's visual, dekatron-based memory to teach schoolchildren about computers.〔
==Construction and use at Harwell==
The computer, which weighs two and a half tons〔(61-year-old computer springs back to life, CNN.com, Wednesday, November 21, 2012 )〕 was built and used at the Atomic Energy Research Establishment in Harwell, Oxfordshire. Construction started in 1949, and the machine became operational in April 1951. It was handed over to the computing group in May 1952 and remained in use until 1957.〔
It used dekatrons for volatile memory, similar to RAM in a modern computer, and paper tape for input and program storage. Output was to either a Creed teleprinter or to a paper tape punch. The machine was decimal and initially had twenty 8-digit dekatron registers for internal storage, which was increased to 40 which appeared to be enough for nearly all calculations. It was assembled from components more commonly found in a British telephone exchange.〔(【引用サイトリンク】publisher=University of Wolverhampton School of Computing and Information Technology )〕 The man who led the effort to rebuild the machine (see below) put it in perspective to the BBC: "All together, the machine can store 90 numbers. The closest analogy is a man with a pocket calculator," Delwyn Holroyd, who led the restoration effort, tells the BBC in a video about the restoration.〔 Although it could on occasions act as a true stored-program computer, that was not its normal mode of operation. It had a multiplication time of between 5 and 10 seconds, very slow for an electronic computer.
As Cooke-Yarborough wrote of his design in 1953 "a slow computer can only justify its existence if it is capable of running for long periods unattended and the time spent performing useful computations is a large proportion of the total time available". The design was noted for its reliability because in the period from May 1952 until February 1953 it averaged 80 hours per week running time. Dr Jack Howlett, Director of the Computer Laboratory at AERE 1948–61, said it "could be left unattended for long periods; I think the record was over one Christmas-New Year holiday when it was all by itself, with miles of input data on punched tape to keep it happy, for at least ten days and was still ticking away when we came back."〔 It was the machine's untiring durability, rather than its speed, that was its main feature. Human mathematicians (a job role called a "hand-computer") could make calculations at a similar speed, but not continuously for the same lengths of time. Dr Howlett commented:
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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