Oldest digital computer brought back to life
It was originally built in 1951, is 2 metres high and weighs 2.5 tonnes and is ready for action after a three year restoration project. It has 828 flashing Dekatron valves, 480 relays and a bank of paper tape readers.
“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, trustee of the National Museum of Computing at Bletchley Park in the UK. “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.”
When it was built the Witch worked at the Harwell Atomic Energy Research Establishment. Working 80 hours per week, the computer used its 828 Dekatron valves to keep count, as each gas-filled tube caused a neon light to rotate within it at a set frequency; the results of calculations are then stamped out onto paper tape.
By 1957, the computer was no longer needed at Harwell and a competition was set up to offer it to the educational establishment that put forward the best case for its continued use.
Wolverhampton and Staffordshire Technical College won, renamed it the WITCH (Wolverhampton Instrument for Teaching Computation from Harwell) and used it for computer education until 1973.
In 1973, it was recognised by the Guinness Book of Records as “the world’s most durable computer” and was again retired, this time to the Birmingham Museum of Science and Industry. When that Museum closed in 1997, it was put into storage, but was rediscovered by TNMOC in 2009 and a successful proposal was made to move it to TNMOC for restoration. An application has again been made to The Guinness Book of Records, but this time as ‘the world’s oldest original working digital computer’.
The machine was built using Post Office 3000 type relays, which were at the time a mature and well understood technology. Designed to operate for decades in the harsh environment of a telephone exchange, they were extremely well made.
Sequence control is entirely relay based, but arithmetic is all electronic, built using valves, trigger tubes and Dekatrons. There are also two high voltage power supply units – the rectifier unit converts the AC mains into several high DC voltages, and the stabiliser unit generates about a dozen precise voltages for use in the various circuits.
The rectifier unit has been extensively modified during the operational life of the machine, and now sports a variety of rectification technologies dating from the 1950s (copper oxide) to the present day (silicon diodes). We had to replace some of the original copper oxide rectifiers with modern components, but others had already been replaced during the machine’s time at Wolverhampton.
More information on the restoration project is available at the National Museum of Computing Website.
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