Designed by British telephone engineer Tommy Flowers, Colossus was built to speed up code-breaking of the complex Lorenz cipher, used in communications between Hitler and his generals during World War II.
Colossus occupied the size of a living room (7ft high by 17ft wide and 11ft deep), weighed five tonnes, and used 8kW of power. It incorporated 2,500 valves, about 100 logic gates and 10,000 resistors connected by 7 km of wiring.
The 70th anniversary celebrations were held at Bletchley Park, bringing together some of the machine's creators and operators at The National Museum of Computing (TNMOC).
The machine's code-cracking prowess was demonstrated on the day using the museum's rebuilt Colossus.
Now widely recognised as the first electronic computer, Colossus was kept a secret for 30 years because of the sensitive work it did during World War Two to crack German codes, the BBC reported.
The first prototype was built out of parts from telephone exchanges including 1,600 valves. Later versions used even more valves and by the end of the war 10 of the machines were in use in the UK.
During wartime, about 550 people worked in the Bletchley Park unit that ran Colossus.
Most of the machines were broken up and the plans destroyed after the war in an attempt to keep the work secret and to conceal the fact Britain was still using two of the machines to read Soviet messages.
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