Harwell computer

The Harwell computer, later known as the Wolverhampton Instrument for Teaching Computing from Harwell (WITCH), or the Harwell Dekatron Computer, is 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 decommissioned in 1973. The museum uses the computer's visual, dekatron-based memory to teach schoolchildren about computers.

The computer, which weighs two and a half tons 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 eight-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. 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.

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