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 the war.