One of the team of engineers, Gerald Palmer, removing thermionic valve holders for use in the rebuilt Colossus. Picture courtesy BT.

Work is currently under way to rebuild Colossus, the world’s first production programmable electronic computer, which was used to break top-secret German codes at Britain’s WWII code-breaking base at Bletchley Park.

BT engineers are assisting the Bletchley-based Colossus Trust in the project and despite having nothing more than wartime photographs and a few ‘top secret’ wartime diagrams the machine is being rebuilt by using, amongst other things, thermionic valve holders from a cleared BT telephone exchange.

“People may not realise the link between telephone technology and the development of the computer,” said David Hay, Head of Heritage at the BT Archives Team, “but the invention of Colossus is an important part of the story of telecommunications – as well as the history of modern computing and the Second Word War.”

Just one of the computers used at Bletchley to decipher German codes, this Colossus Mk II, was developed just in time for D-Day. Picture courtesy Bletchley Park.

Colossus was originally designed and constructed by a research team led by Tommy Flowers, an engineer from the wartime Post Office research station at Dollis Hill. The first Colossus used 1500 valves and by December 1943 it was helping to decipher the codes transmitted by the German High Command.

The codes had to be cracked by carrying out complex statistical analysis on intercepted messages. Colossus could read paper tape at 5,000 characters per second, which meant the huge amount of mathematical work that needed to be done could be carried out in hours, rather than weeks.

“Many of the components of the original Colossus were sourced from telephone exchange equipment,” added David Hay. “We’re delighted that BT is able to play a part in preserving a piece of the UK’s rich heritage by supplying the necessary valve holders from obsolete exchange equipment which would be otherwise be scrapped.”

Courtesy BT.

A total of ten Colossus machines were built and used at Bletchley, however, these were later destroyed together with technical drawings and diagrams in order to keep their existence a secret.

Bletchley Park’s crucial contribution to the war effort – which only became public knowledge relatively recently, was praised by Churchill, who described it as, “the goose that laid the golden egg and never cackled.”

You can find out more about Colossus on BT’s recently re-launched online museum connected earth