What Bletchley built: the age of the computer 

Enigma was not the hardest problem at Bletchley. The hardest problem was Tunny, the British name for the traffic produced by the Lorenz SZ cipher attachment, which the German high command used for teleprinter messages between senior commanders. It was a different kind of machine, generating a stream of key characters to combine with the text, and its output carried the strategic conversations of the German army rather than tactical orders.

Its first break came from an operator's mistake. In August 1941 a long message was sent twice with the same settings and slightly different wording, and John Tiltman found an opening in it. Then William Tutte, a young chemist turned mathematician, deduced the logical structure of the machine from the statistics of the traffic without ever seeing one. Max Newman turned the resulting method into something that could be mechanised.

The first attempt, a machine nicknamed Heath Robinson, tried to compare two paper tapes running at speed. It worked badly, because the tapes tore. Tommy Flowers, a Post Office engineer at Dollis Hill, proposed replacing the second tape with electronic valves holding the pattern internally. Bletchley's senior staff doubted that fifteen hundred valves could run reliably. Flowers, who had worked with valves in telephone exchanges, knew they were reliable if you left them switched on. He built it largely on his own initiative, partly at his own expense.

Colossus ran in December 1943 and was in operational use by early 1944. A larger version, with around twenty-five hundred valves, followed in June 1944, and ten were working by the end of the war. They were electronic, digital, and programmable through switches and plugs, though not in the general-purpose sense of a modern computer with a stored program. They read tape at five thousand characters a second and reduced work that took weeks to hours. Among other things, Tunny decrypts confirmed how the German command had read the deception operation before the Normandy landings.

Then most of the Colossus machines were dismantled, the drawings destroyed, and Flowers ordered to say nothing. When the history of computing was written in the 1950s and 1960s, it was written around American machines, and Britain's electronic pioneer spent his career as a telephone engineer whose greatest work was officially unmentionable. A working reconstruction of Colossus, built over fourteen years by Tony Sale and his team, now runs at the National Museum of Computing on the Bletchley site.

What the place left behind is not only hardware. It is a working demonstration that the way to attack an impossible problem is to find the structural flaw, mechanise the tedious part, and organise thousands of people so their small contributions compound. That is a habit of mind, and habits of mind are what MindSnap is built to hand you, two minutes at a time.

Walk the grounds today and the huts are back, restored, with the noise of a reconstructed Bombe coming through a doorway. It is a museum about thinking. There are not many of those.