Enthusiasts rebuild the code-breaking Bombe that won war

It looks rather like a telephone exchange constructed from a giant Meccano set by a mad scientist - which in a curious kind of way is exactly what the Turing Bombe is.

And in 1944, it was in the care of 18-year-old Wrens like Ruth Henry, working in a bare hut at Station X, the place the world knows today as the code-breaking centre at Bletchley Park.

For eight hours a day, she would stand by the wardrobe-size machine, setting its alphabet-inscribed dials and then waiting a few minutes while it whirred, clicked, clattered and eventually stopped, allowing her to read off a series of letters on a small panel. The results would be fed, by red scrambler telephone, to either Hut 6 or Hut 8 across the lawn, where the cryptanalysts worked on the top-secret German Enigma codes.

When, after each relay of letters, the word came back "Job up", it meant that particular German code had been broken and the machine could be re-set.

It also signified that, quite possibly, many thousands of lives had been saved by a single act of the machine and the course of the Second World War changed, even if only slightly.

Now Ruth Bourne, a sprightly 80-year-old grandmother from north London, has come face to face with a working replica of the Turing Bombe. It was unveiled yesterday after 10 years of painstaking research and construction work by a group of electronics and computer enthusiasts. The model is to go on display at the Bletchley Park museum next year.

"We never knew how important the work was we were doing; they discouraged us from talking about it or getting too excited because word might have got around. All we knew was that it was about German codes and they didn't tell us that until after we had signed the Official Secrets Act," said Mrs Bourne.

She added: "I didn't really learn about what had been going on until many years later and then I volunteered to become a guide here. Of course I feel extremely proud to have been a small cog in such an important wheel."

The Bombe was originally developed and built in 1940 by Alan Turing, the renowned mathematician now considered to be the father of the modern computer. He was among the group of cryptanalysts who unlocked the Enigma codes covering top-secret German military communications in one of the greatest unseen victories of the war.

The codes were often changed more than once a day and Turing and his colleagues had trouble keeping pace with manual analysis. The Bombe was designed to mechanise the task of unravelling the 158 million million million possible combinations in the code; it did not decipher the code completely, but gave a small number of options for final reading of the message. By the end of the war, there were more than 200 Bombes in use at Bletchley and its outstations around the Home Counties.

The project to rebuild a Bombe was made more difficult by the extraordinary secrecy that surrounded Bletchley, now absorbed by Milton Keynes. In 1946, when its work came to an end, Bletchley was disbanded, its workers demobbed and scattered, still bound by the Official Secrets Act. Most of its machines and paperwork, including diagrams and photographs of the Bombe were destroyed, lost or disappeared into the top secret world of GCHQ, which continued Bletchley's work during the Cold War.

The code-breaker was eventually put together using fragments from American archives, material obtained from GCHQ and the memories of those who had built or maintained the original Bombes. In so far as was possible, it was constructed using original parts, salvaged or scrounged. Other sections were made afresh in home workshops.

Simon Greenish, the director of Bletchley Park, stressed yesterday that the Bombe was an "electro-mechanical machine" and not the first computer - that was the Colossus machine, developed later to crack the even more sophisticated Lorenz cypher machine used by the Nazis.

But he added: "The Bombe was Turing's big idea and the impact on the war was enormous. It only emerged recently through the Public Record Office that Hut 6 routinely knew everything the German high command did. We still don't know the full story."

Turing committed suicide in 1953, tormented by his homosexuality, his wartime achievements largely unrecognised.