The Colossus Rebuild

In 1991, some colleagues and I started the campaign to save Bletchley Park from demolition by property developers. At this time I was working at the Science Museum in London restoring some early British computers. I believed it would be possible to rebuild Colossus, but nobody else believed me. In 1993, I gathered together all the information available. This amounted to no more than eight 1945 wartime photographs of Colossus (some of which are printed in this book), plus brief descriptions by Flowers, Coombs, and Chandler, and—crucially—circuit diagrams which some engineers had kept, quite illegally, as engineers always do! I spent nine months poring over the wartime photographs, using a sophisticated modern CAD system on my PC to recreate machine drawings of the racks. I found that, fortunately, sufficient wartime valves were still available, as were various pieces of Post Office equipment used in the original construction. In July 1994, His Royal Highness the Duke of Kent opened the Bletchley Park Museum and inaugurated the Colossus rebuild project. At that time I had not managed to obtain any sponsorship for the project, so my wife Margaret and I decided to put our own money into it, to get it started. We both felt that if the effort was not made immediately there would be nobody still alive to help us with memories of Colossus. Over the next few years various private sponsors came to our aid and some current and retired Post Office and radio engineers formed the team that helped me in the rebuild. In 1995, the American National Security Agency was forced by application of the Freedom of Information Act to release about 5000 Second World War documents into the US National Archive. A list of these documents was put onto the Internet. When I read it I was amazed to see titles like ‘The Cryptographic Attack on FISH’. I obtained copies of these documents and found that they were invaluable reports written by American servicemen seconded to Bletchley Park when America entered the war. I was also fortunate enough to be given access to the then still classified General Report on Tunny (parts of which are published for the first time in this book).