World's oldest digital computer restored to former glory

November 20, 2012 | 10:02

Tags: #computer-history #dekatron #harwell #kevin-murrell #the-national-museum-of-co #witch

Companies: #bletchley-park #the-national-museum-of-computing #tnmoc

The National Museum of Computing (TNMOC) at Bletchley park has announced the switching on of the world's oldest working digital computer, the Harwell Dekatron, following the successful completion of a three-year restoration project.

Built in 1949 by the Atomic Energy Research Establishment in Harwell, Oxfordshire and first used in 1951, the Dekatron is the oldest surviving digital computer. Constructed from 828 Dekatron valves and 131 other valves, 480 physical relays, a total of 7,073 contacts, 199 lamps, 18 switches, and a bank of paper tape readers, its 2.5 tonne chassis is barely recognisable as a computer. Even its method of operation is unique in the modern world: unlike today's computer systems, which operate in binary, the Harwell Dekatron operates in base 10 - the same decimal system as adopted by most human societies, in deference to our ten fingers.

Designed to automate atomic energy calculations previously worked out laboriously by hand, with the staff who operated the hand-cranked mechanical calculators enjoying the job title of 'computer,' the Harwell Dekatron was designed with reliability in mind. Not the fastest system in the world, it was nevertheless capable of running flat-out for days - something rare to see in the days of unreliable valves and large, hot-running components.

By 1957, the Harwell Dekatron had become too slow for the calculations being made by the Atomic Energy Research Establishment. Rather than scrap the device, a fate to which that all too many irreplaceable systems from throughout the history of computing were resigned, one scientist suggested the system be offered as-is for educational use. Having put forward the best use case Wolverhampton and Staffordshire Technical College were the lucky recipients of the machine, rebranding the system to the Woverhampton Instrument for Teaching Computation from Harwell, or WITCH.

As the Harwell WITCH, the Dekatron would continue to be used right through to 1973 as a means of teaching digital computation. As binary computers became increasingly popular, however, its ability to prepare students for real-world computation decreased, and the system was retired in 1973. The Harwell WITCH was displayed as a non-working artefact at the former Birmingham Museum of Science, but the closure of the museum in 1997 combined with the WITCH's unwieldy size led to it being dismantled and being placed into storage.

Fast-forward to 2008, when the Harwell WITCH was rediscovered by a team of volunteers from the National Museum of Computing. 'I first encountered the Harwell Dekatron as a teenager in the 1970s when it was on display in the Birmingham Museum of Science and Industry - and I was captivated by it,' recalls Kevin Murrell, TNMOC trustee and initiator of the restoration project. 'When that museum closed, it disappeared from public view, but four years ago quite by chance I caught a glimpse of its control panel in a photograph of stored equipment. That sparked our ideas to rescue it and we hunted it down.

'The TNMOC restoration team has done a superb job to get it working again and it is already proving to be a fascination to young and old alike. To see it in action is to watch the inner workings of a computer - something that is impossible on the machines of today. The restoration has been in full public view and even before it was working again the interest from the public was enormous.

The Harwell WITCH, in its fully-functioning form, is now on display at the National Museum of Computing at Bletchley Park near Milton Keynes, where it will serve as a tool to teach computational thinking and computer history as well as a contrast to the previously-rebuilt Colossus system, the world's first semi-programmable electronic computer, which can also be seen at the museum.
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