Frustrated by the limitations of earlier electromechanical codebreaking devices, engineer Tommy Flowers at the Post Office Research Station championed a radically faster approach using vacuum tubes rather than relays. His design work in this period led to Colossus, operational by December 1943, which used some 1,600 thermionic valves to help Bletchley Park codebreakers crack the German Lorenz cipher used for high-level military communications.
Bletchley Park's codebreakers faced a new and more fearsome challenge in the German Lorenz cipher machine, used for the highest-level communications between Hitler and his field commanders — far more complex than the Enigma traffic that had already occupied thousands of codebreakers. Existing electromechanical tools, whirring with relays, were too slow to keep pace with the volume and complexity of Lorenz traffic intercepted daily.
Tommy Flowers, an engineer on loan from the General Post Office's research station at Dollis Hill, proposed something radical for the era: a machine built almost entirely from vacuum tubes (thermionic valves) rather than mechanical relays, sacrificing some reliability for enormous gains in speed. His superiors were skeptical that a machine with over a thousand fragile, heat-generating valves could run reliably enough to be useful, and he partly self-funded early development.
Colossus, when it became operational in December 1943, could process 5,000 characters of punched tape per second, dramatically slashing the time needed to determine Lorenz wheel settings and giving Allied commanders advance insight into German strategic thinking ahead of D-Day. Ten Colossus machines eventually ran at Bletchley. Britain kept the machines' existence secret for decades after the war, dismantling most and swearing personnel to silence, which meant Flowers and his colleagues received little public recognition for what many historians now regard as among the earliest electronic programmable computers.