An electronic special-purpose digital “computer” that was built in great secrecy by the Post Office Research Station in London and began useful work at the government establishment at Bletchley Park, Buckinghamshire, in late 1943. It contained 1500 vacuum tubes (valves) and could operate at high speed. The strategy or “program” was controlled from patchboards and switches. The faster Mark II machines, operating by mid-1944, contained 2500 tubes. Both versions were used for code-breaking purposes during World War II.