World's oldest original digital computer WITCH returns to life

The WITCH computer, which began life in 1951, has been restored by a team of volunteers at the UK's National Museum of Computing based at Bletchley Park.
Written by Jon Yeomans, Contributor on

A team of volunteers has completed a three-year restoration of the world's oldest working digital computer, the Harwell Dekatron, also known as 'WITCH'.

The WITCH computer at its new home, the National Museum of Computing. Image: TNMOC

The 2.5-tonne machine, which first entered operation in 1951, went on display at the UK's National Museum of Computing (TNMOC) at Bletchley Park on Tuesday.

"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," said Kevin Murrell, a trustee of TNMOC who initiated the restoration project.

The computer's rather chequered existence began at the Harwell Atomic Energy Research Establishment, where it was put to work performing routine calculations. Already redundant by 1957, the machine transferred to the Wolverhampton and Staffordshire Technical College, where it was renamed WITCH (Wolverhampton Instrument for Teaching Computation from Harwell) and was used until 1973.

After that, WITCH went on display at the Birmingham Museum of Science and Technology, before being put into storage. It was rediscovered in 2008, when Murrell indentified its control panel in a photograph of stored equipment. "That sparked our ideas to rescue it and we hunted it down," Murrell said.

Dekatron valve
A Dekatron valve on WITCH. Image: TNMOC

WITCH has 828 flashing Dekatron valves, 480 relays, 18 switches, and consumes 1.5kW of power. Remarkably, the restoration team said that the majority of parts in the machine are original.

"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," said Delwyn Holroyd, a TNMOC volunteer.

WITCH will now go on permanent display at TNMOC. The museum is dedicated to preserving Britain's computing history and already houses a faithful reconstruction of the Colossus Mark 2, the world's first programmable, digital, electronic computing device, which helped break German encryptions during World War II.

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