Colossus Computer Built

In 1943, engineers in Britain completed Colossus, a room-sized machine built to help break encrypted German military messages at Bletchley Park. Often described as the first programmable electronic digital computer, it showed that thousands of vacuum tubes could work reliably together at high speed, and it gave Allied commanders a steady stream of intelligence about German high command thinking. The claim to be "first" comes with caveats, since Colossus was programmed by switches and plugs rather than a stored program, and it was designed for a narrow task rather than general computing.

The background lay in the German use of several cipher systems. Enigma, the best known, was used for much tactical traffic. Higher-level communications between Hitler's headquarters and army group commanders were sent by teleprinter using a more complex machine the British called Tunny, the Lorenz SZ40/42. Codebreakers at Bletchley Park, led in this effort by figures such as Bill Tutte, worked out the logical structure of the Lorenz machine without ever having seen one. Mathematician Max Newman's section then built early mechanical aids, but these were slow and prone to errors from paper tape stretching or misaligning.

Tommy Flowers, a telephone engineer at the Post Office Research Station at Dollis Hill, proposed using electronics instead. Many officials doubted that a machine with well over a thousand valves would function, since valves were believed to fail when switched on and off. Flowers argued that if the machine was left running continuously, it would be dependable. He and his team built the first Colossus, which reportedly went into operation at Bletchley in early 1944 after construction and testing during 1943. An improved Mark II followed around June 1944, shortly before D-Day.

Colossus read encrypted message tape at high speed and counted the results of Boolean logic tests against possible settings of the Lorenz wheels, helping analysts find the starting positions. It did not decrypt messages by itself; people still carried out much of the later work. Around ten Colossus machines were eventually in use. The intelligence they supplied is widely credited with helping the Allies gauge whether German deception and defence plans were working, including in the period around the Normandy landings.

After the war, most machines were dismantled and documentation destroyed, and the project stayed secret for decades. As a result, Colossus was long left out of accounts of computing history, while American machines such as ENIAC received the recognition. Only from the 1970s did its existence become public. A working reconstruction, built by volunteers, is now displayed at The National Museum of Computing at Bletchley Park. Historians still debate how directly Colossus influenced later computer design, but its engineering proved that large-scale electronics were practical.