Technology History | Block H, Bletchley Park — The First Building Made for Computers

Block H was built to hold computers, which makes it the world's first purpose-built computer centre — a claim it can make only because the machines inside it stayed secret for thirty years. Colossus arrived at Bletchley Park in January 1944 and did its first job on 5 February, reading the Lorenz traffic that carried messages between the German high command rather than the Enigma traffic the huts are famous for. Lorenz was the bigger prize and the harder problem. By the end of the war Bletchley was running ten Mark 2 machines, and this block was where they stood. Then Churchill ordered them broken up and the drawings destroyed, and the room went quiet for half a century. What runs here now is a rebuild, standing in the position Colossus number 9 occupied. It exists because Tony Sale refused to accept that the machine was unrecoverable. Sale had eight photographs taken in 1945 and some fragments of circuit diagrams that engineers had kept, illegally, as engineers do. He gathered them in 1993, started the project with his own money, and had it inaugurated by the Duke of Kent in July 1994. The optical tape reader defeated him until he tracked down Arnold Lynch, the man who had designed it in 1942, and asked him how it worked. A functioning Mark 2 was running by 1 June 2004, the sixtieth anniversary of the original Mark 2's first day, and the rebuild was finished in 2007. It had taken about as long as Sale had left; he died in 2011. The machine's public humbling came in 2007, when the museum staged a cipher challenge and a German radio amateur running his own software on a laptop beat it to the answer. That is the right result to sit with. Colossus was not fast by any standard we now use — it was fast enough, thirty years before anyone outside this room could have known it existed. In the photograph the patch panels are labelled CHI, MU and PSI, after the three sets of wheels inside the Lorenz machine the operators were trying to set; the tape transport stands to the right. The rebuild runs for visitors in the Colossus Gallery at The National Museum of Computing, which occupies Block H and is ticketed separately from the rest of Bletchley Park. Sources: The National Museum of Computing on the rebuild and the block; Tony Sale's own account of the reconstruction at codesandciphers.org.uk; contemporary reporting of the 2007 cipher challenge. Cover photo: Farai Gandiya, CC0, via Wikimedia Commons.