The WITCH (credit Robert Dowell)

World's oldest digital computer rebooted

The world's oldest working digital computer, the Harwell Dekatron (WITCH) computer, has been rebooted.

Now in its seventh decade and its fifth home, the computer is on display to the general public.

The 2.5 tonne, 1951 computer from Harwell, with its 828 flashing Dekatron valves, 480 relays and a bank of paper tape readers, will clatter back into action in the presence of two of the original designers, one of its first users and many others who have admired it throughout its remarkable history.

Kevin Murrell, trustee of TNMOC who initiated the restoration project, said: "In 1951 the Harwell Dekatron was one of perhaps a dozen computers in the world, and since then it has led a charmed life surviving intact while its contemporaries were recycled or destroyed.

"As the world's oldest original working digital computer, it provides a wonderful contrast to our rebuild of the wartime Colossus, the world's first semi-programmable electronic computer."

The Harwell Dekatron computer first ran at Harwell Atomic Energy Research Establishment in 1951 where it automated the tedious calculations performed by talented young people using mechanical hand calculators. 

Designed for reliability rather than speed, it could carry on relentlessly for days at a time delivering its error-free results.

It wasn't binary, but worked in decimal – a feature that is beautifully displayed by its flashing Dekatron valves.

By 1957, the computer had become redundant at Harwell, but an imaginative scientist at the atomic establishment arranged a competition to offer it to the educational establishment putting up the best case for its continued use. 

Wolverhampton and Staffordshire Technical College won, renamed it the WITCH (Wolverhampton Instrument for Teaching Computation from Harwell) and used it in computer education until 1973.

After a period on display in the former Birmingham Museum of Science and Industry, it was dismantled and put into storage, but "rediscovered" by a team of volunteers from The National Museum of Computing in 2008. 

With the blessing of the Birmingham museum and in conjunction with the Computer Conservation Society, the team developed a plan to restore the machine and to put it once again to educational use at TNMOC.

Murrell recalls its rediscovery: "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. 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."

Delwyn Holroyd, a TNMOC volunteer who led the restoration team, said: "The restoration was quite a challenge requiring work with components like valves, relays and paper tape readers that are rarely seen these days and are certainly not found in modern computers.

"Older members of the team had to brush up on old skills while younger members had to learn from scratch."

The Harwell Dekatron / WITCH computer can be seen by the general public at the National Museum of Computing.

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