Cracking the Lorenz cipher
The centerpiece of The National Museum of Computing is the rebuilt Colossus, the machine that broke the German High Command's Lorenz SZ40/42 cipher during World War II. This was not the famous Enigma code used by field units; Lorenz was a far more complex 12-wheeled cipher machine used for strategic communications between Hitler and his generals. The breakthrough in cracking it came from Post Office engineer Tommy Flowers and his team at the Dollis Hill Research Station.
The original Colossus Mark 1, operational at Bletchley Park in February 1944, contained 1,600 thermionic valves (vacuum tubes). The improved Mark 2 arrived on June 1, 1944—just in time for D-Day, and used 2,400 valves. Colossus read encrypted messages from a punched paper tape at a rate of 5,000 characters per second., This speed was a physical limit; any faster and the paper tape would disintegrate. By the war's end, ten Colossus machines were operational, having decrypted 63 million characters of German intelligence. The existence of these computers was kept secret until the 1970s, and all the machines and most of their blueprints were destroyed. The functional rebuild in the museum, led by Tony Sale, began in 1993 and was completed in 2007.
From Dekatrons to desktops
Beyond Colossus, the museum has the world's largest collection of working historic computers. One of the oldest is the Harwell Dekatron, or WITCH, first operational in 1951. Recognized by Guinness World Records as the world's oldest working digital computer, this 2.5-metric-ton machine was built at the Atomic Energy Research Establishment. It was not fast, taking between 5 and 10 seconds to multiply two numbers, but it was incredibly reliable. Its memory consisted of 828 glowing Dekatron tubes, a form of cold-cathode gas-filled tube.
The museum's galleries display the history of computing through the massive mainframes of the 1960s and 70s to the rise of personal computers in the 1980s. Visitors can follow the entire timeline of computing history in one location, from the secret codebreaking machines of the 1940s to the familiar desktops that eventually appeared in homes and offices. The museum itself is located in Block H on the Bletchley Park estate, the same building constructed in 1944 to house six of the original Colossus computers.