World's oldest original digital computer WITCH returns to life

World's oldest original digital computer WITCH returns to life

Summary: 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.


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.

Topics: Hardware, United Kingdom, After Hours

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  • A dozen of them in 1951?

    I thought someone important back then (perhaps in the late 40s) predicted that total world demand would only be about 6 or 7 of these big and powerful beasts.

    Who the hell would need all that computing power anyway?
    • One attribute

      "I think there is a world market for maybe five computers."
      Thomas J Watson, President of IBM, 1943
  • No mention of Microsoft or Apple in the article

    Too bad. Then there will probably be no entertaining comments from Loverock (Hi! Hello there!!!) et al. here. Too bad...

      Even when I don't comment I still get mentioned. Thanks for being a fan!
      Loverock Davidson-
  • Apparently It Was Built For Reliability, Not Speed

    A fast human computer* could just about keep up for a few minutes, but not for the hours on end that the machine could manage.

    *That's right, "computer" originally meant a person who did computations, not a machine.
  • Bletchley Park & CSIRAC

    I've seen the 1949 CSIRAC computer on display at the Melbourne Museum, but it's no longer working. The Wikipedia says they have no plans to repair it.

    It'd be too cool to see WITCH actually running. Next time I visit the UK I'll definitely visit Bletchley Park. (I visited the area in the early 1990s, but there didn't seem to be any sort of museum that I could find.)
    • Bletchley Park

      I live nearby and the visitor experience has improved dramatically in the last decade, guided tours by volunteers who worked there in the war, and the Computing Museum is pretty cool too. My favorite object was a solid state 16k RAM that looks just like a Borg cube!
    • Top Secret?

      If it was used at Bletchley Park, we are fortunate that it was not dismantled and the plans destroyed after the war, as happened to the other top secret ("Most Secret" as the Brits say) computing machines used to crack the German codes. Probably not the most sophisticated player in the game, and the Cold War rivals probably all had that technology anyway.
  • As soon s they turned it on...

    it started sending out e-mails about a great new deal you can get on bulk purchases of Viagra!
    Tony Burzio
  • Dekatron! Memories! (Human type, that is)

    The Dekatron valve ("tube" to us Yanks) was described in one of my engineering textbooks as already obsolete but interesting. It was a neon-based device which used ten intermediate electrodes connected to a common "trigger" input to transfer the role of active cathode around a circular array of ten cathodes (and transferring the "glow" along with the current), using the voltage/current hysteresis of near-vacuum ionized gas to do the "logic". When operating at low speed or in single-step, the user could read the current digit value in each tube. Presumably, a pair of Dekatrons, a pair of "Hexatrons" and a "Dodekatron" could build a direct reading time of day clock, if such variant tubes existed (possibly a configuring pin for each value may have been available to allow jumping past unneeded digits). Eventually binary flip-flops won out over decimal-counting devices.

    One neon tube that I DO remember seeing in the early 1960 was used for display ONLY; the Nixie had ten digit-formed wire cathodes, spaced by inert screen-grid electrodes, and a common anode. One cathode at a time would be grounded by a logic gate, causing the glow to surround that wire and display a digit. An additional anode displayed the decimal point. Again, unlike the Dekatron, it had no logic capability, it displayed the output of other logic circuits. In 1964, my high school electronics lab had ONE rack-mountable (about 20" wide by 6" high by 18" deep) 1950's era digital VOM using the Nixie display, which we were told cost about $2000 (probably donated). Today you can get the equivalent hand held meter with LCD display for under $20!
  • Dekatron tube is correct in the UK as well

    When I wrote a piece about the restoration three years ago, TNMOC's Kevin Morrell told me they were Dekatron tubes: "they've never been called Dekatron valves, so please don't try to correct me!"
    Jack Schofield
    • Tube not Valve

      Probably because the American company that made them (I forgot which) trademarked the entire phrase "Dekatron Tube". Generically, though, tubes were called "valves" by the original British inventor of the rectifying tube (later known as a diode), John Fleming, because they were the first known "one-way valves" for electric current. Americans, noting that the Fleming Valve, and its successors starting with the Lee De Forest Audion (triode; the first "transconducting" or amplifying device), were housed in modified light bulbs, i.e. glass tubes, just began calling all such devices "tubes". Like bonnet/hood, boot/trunk, lift/elevator, etc., one of the many examples of being "divided by a common language"!