The Harwell Dekatron computer is a 1950s computer having roughly the weight and size of a Hummer H3 and the computing power of a four-function pocket calculator. Having been restored to its original operating condition using 95 percent original parts, it is now the oldest functioning programmable digital computer in the world. Guinness might have been onto something, when, in 1973, they named the Dekatron the Most Durable Computer in the World.

The Harwell computer was built in the early 1950s for the UK Atomic Energy Research Establishment. Never intended to be a state-of-the-art general-purpose computer, the Harwell was developed to perform simple but repetitive calculations continuously and without error. Its computational rate is about 0.1 FLOPS, similar to that of a pocket calculator, but the computer operates for long periods of time without human intervention.

The Harwell computer in use at the Atomic Energy Research Establishment (Photo: National Museum of Computing)

The computational needs of the Establishment soon outstripped the Harwell, and in 1957 Wolverhampton University was offered the machine to train students about computing. The Harwell was relaunched as the WITCH (Wolverhampton Instrument for Teaching Computing from Harwell), and helped a generation of students learn about computing. But by 1974, Wolverhampton decided that the WITCH was obsolete. They donated the computer to the Museum of Science and Industry in Birmingham, where it was on display until the museum closed in 1997.

Then, once again evading disposal, the computer was disassembled and stored in the Birmingham City Council Museums Collection Centre. Rediscovered quite by accident in 2008 by Kevin Murrell, a trustee of the National Museum of Computing, it was moved to its current home at that museum, where it's been restored to full functionality.

Banks of logic relays within the Harwell computer (Photo: National Museum of Computing)

Conservationist Delwyn Holroyd led the restoration effort. He commented that the machine was "pretty dirty" when arrived at its new home. That's no idle complaint when one considers the logic operations of the Harwell are carried out by a circuit with 480 telephone exchange relays possessing over 7000 contacts, any of which can be disrupted by a speck of dust.

The operations of the arithmetic unit and the RAM storage (90 eight-digit plus sign decimal digits – about 340 bytes) of the Harwell are largely carried out by a set of 828 dekatron tubes. An additional double-length word is used as an accumulator in multiplication and division.

One of the dekatron tubes on the Harwell computer (Photo: Bad germ)

A dekatron was the simplest base-10 counter able to add and subtract while generating carry digits. Common dekatrons were based on decimal counting, and could switch reliably at speeds in excess of 10 kHz. Dekatrons are triggered to consecutively light a set of ten positions in a cold-cathode tube.

The paper tape readers that provide the Harwell computer with program instructions and data (Photo: Bad germ)

Input of programs and data are carried out by a pair of paper tape readers, and output is either through a printer or a paper tape punch.The paper tape readers of the day were rather rough on paper tape, so programs intended for multiple reuse had to be made of linen rather than paper.

Although the days of the Harwell computer as an active participant in science and engineering are long over, it stands as a rare and inspiring example of the enormous rate of change that characterizes our modern era.

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