The Dekatron computer as it is today. Photo credit: Robert Dowell
The Harwell Dekatron (aka Witch) computer will be rebooted today, becoming the world's oldest original working digital computer.The mammoth computer, from 1951, has spent the last three years undergoing restoration at the National Museum of Computing at Bletchley Park in Buckinghamshire and is now ready for action again.Museum trustee Kevin Murrell, 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 Dekatron 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 even binary, but worked in decimal – a feature that is beautifully displayed by its flashing Dekatron valves.The computer became redundant by 1957, but a scientist at the atomic establishment arranged a competition to offer it to the educational institution that could present the best case for its continued use.
Super-sized: Old-fashioned relays

Wolverhampton and Staffordshire Technical College won the competition, renaming the computer Witch (Wolverhampton Instrument for Teaching Computation from Harwell). The college used it for computer education until 1973.After a period on display in the former Birmingham Museum of Science and Industry, the computer was dismantled and put into storage.In 2008 the computer was “rediscovered” by volunteers from the National Museum of Computing. They developed a plan to restore the machine, with the aim of putting it to educational use at the museum once again.

Kevin Murrell says: “I first encountered the Harwell Dekatron as a teenager in the 1970s when it was on display in the Birmingham museum, 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 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.”
Delwyn Holroyd, a museum volunteer who led the restoration team, added: “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 computer will clatter back into action in the presence of two of its original designers, one of its first users and many others who have admired it at different times during its remarkable history.The Harwell Dekatron can now be seen in action at the National Museum of Computing.

The Witch at Wolverhampton - with some long tape reader
Harwell Dekatron/Witch timeline:

1949: Design begun
1951: First operated
1957: Moved to Wolverhampton
1973: Declared world's most durable computer
1973: Moved to Birmingham Museum of Science and Industry
1997: Moved into storage at Birmingham Collections Centre
2009: Moved to the National Museum of Computing
2012: Rebooted to become the world's oldest original working digital computer

Harwell Dekatron/Witch facts:

Power consumption: 1.5kW
Size: 2m high x 6m wide x 1m deep
Weight: 2.5 tonnes
Number of Dekatron counter tubes: 828
Number of other valves: 131
Number of relays: 480
Number of contacts or relay switches: 7,073
Number of high-speed relays: 26
Number of lamps: 199
Number of switches: 18

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