Harwell Dekatron: rebooting the most dependable computer ever

The National Museum of Computing has done it again, bringing yet another historical computer back to life. But where the Colossus and Tunny were war-time codebreakers, brought back to life through scraps of information, the Harwell Dekatron was merely in pieces in storage, with two of the three original developers still alive.

The 2.5-tonne beast was built in 1951 at the Atomic Energy Research Establishment in Harwell, making it the “oldest working digital computer,” according to TNMOC. It was out of date by 1957 — people wanted the latest technology even back then — so following a competition, it was handed to what was then the Wolverhampton College of Technology, where it was renamed Wolverhampton Instrument for Teaching Computation from Harwell (WITCH).


The Harwell was used in teaching there until 1973, when it was declared the world’s most durable computer, and moved to Birmingham’s Museum of Science and Industry. After more than two decades of astounding visitors with its multitude of blinking lights and futuristic whirring — the museum never actually got it properly working — it was dismantled and put in storage.

Handily, that museum doesn’t dispose of its old exhibits, storing them in an epic warehouse that sounds like good fun to pick through — which is exactly what TNMOC volunteers did. TNMOC Trustee Kevin Murrell spotted the computer, which he’d visited as a teenager, in a photo of stored equipment, kicking off the restoration process.

So, unlike previous TNMOC rebuild projects — such as the Colossus and Tunny — the volunteer engineers had all of the parts, full schematics, and even two of the three original designers, Dick Barnes and Ted Cooke-Yarborough, both of whom attended the Harwell Dekatron’s reboot at Bletchley Park. The pride at seeing their creation brought back to life was clear on their faces.


The device was mainly used to help mathematicians run calculations — “terribly tedious work” they otherwise had to do on hand calculators, the 93-year-old Cooke-Yarborough explained. To do this, the Harwell uses Dekatron tubes for volatile memory, storing the numbers necessary for the calculation.


“All the very early computer projects, despite the obvious problems of building them, had the problem of how do we store data and the instructions,” Barnes said. “At Manchester and Cambridge, the development of storage devices was a major project in its own right. But thanks to Ted having spotted the Dekatron, here was something that was a ready-made storage device for one decimal digit.”

Slow mover

While it’s truly an impressive-looking machine — it’s impossible not to be intrigued by the clanking noises and blinking lights — it was slow even for its time. Bart Fossey, one of the academics using the computer, is famous for “racing” the computer back in the 1950s. As it turns out, it wasn’t a planned race, he merely ran a few of the calculations by hand at the same time as the machine, to double check that an alteration to how his program dealt with rounding numbers was working. Still, he did manage to keep up for half an hour, showing either how quick he was or how slow it was. “The machine was known to be not very fast… so it wasn’t altogether surprising that I got that result,” he noted.

Fossey repeated the “race” for a few minutes at the reboot event, managing to keep up with a hand calculator – despite not having used such a device for several decades.


While some computation was actually too advanced for the Harwell, it was good at repeating the same operations over and over again. Instead, the Harwell Dekatron was special because it kept going, unattended: Fossey said it was left running for ten days straight over Christmas one year — it must have been a fair bit of tape.

And that’s the other interesting aspect of the Harwell Dekatron rebuild: alongside the parts in the museum warehouse, volunteers uncovered boxes of tape — the very programs academics had created to run on the computer. They didn’t come with any instructions, so volunteers are still working to unpick their purpose.

The reboot

Delwyn Holroyd, the leader of the restoration team, rebooted the machine, walking attendees through a basic program. First, the code is typed into a perforator machine — the last bit of the Harwell to be uncovered at the museum, on the last shelf at the very back, of course — which punches the holes onto the tape. That’s fed into the paper tape readers; the machine uses six, in order to break a program up into individual subroutines — and to run loops. “The term “loop” [in programming] is literally a loop in paper tape – a continuous piece that goes around, to do repetitive calculations,” he noted.


The Harwell uses the same relays as telephone exchanges because they’re reliable and well-built, but because they’re mechanical it takes time for the contacts to move, making the machine a bit slow. The Dekatron tubes store the numbers, allowing the machine to do addition and subtraction, as well as multiplication and division in a single instruction — apparently quite the achievement in the 1950s.

The numbers being stored in the Dekatrons are clearly visible on the lit up tubes, meaning you can actually watch what it’s doing, Holyrod noted, adding that some of the Dekatrons are the original component from the 1950s. Then, the results are printed out on one of two peripherals, either a printer or a perforator — both from the 1940s.


You can watch the reboot here via a TNMOC video from YouTube — you’ll want your headphones in for the wonderful clunky sounds:

Looking forward

Barnes was asked if, while working on the Harwell Dekatron, he could have imagined the way computers would have progressed. “I think it is impossible that people could imagine the future because the real breakthrough came several years later with first the transistor and then the integrated circuit,” Barnes said, marvelling at the evolution of “things like this that aren’t really transportable to those that can fit in your hand”.

That doesn’t mean the 92-year-old no longer uses computers. He’s at his Dell desktop or laptop on a daily basis, he told me, while his wife prefers a tablet. “But we don’t know how they work inside,” he laughed.

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