`Computers During World War Two`

`Alan Turing`

On September 4th 1939

Alan Turing reported for duty at the Government Code and Cypher School, which had been evacuated to the Victorian site at Bletchley Park in August. For Alan Turing, the war began on this day - as an intellectual problem. The problem was called Enigma, the encryption machine which generated the code used by the German armed forces to send their secret commands via public airwaves.

In England, teamwork

was always a determining factor in the technical developments which gave rise to the machines called computers. At a time when the complexity of the Enigma code had been increased, the Poles presented the British with an Enigma-decoding device - fittingly refered to as the "bomb" - and many of the cryptologists at Bletchley had a constructive influence on the further development of this piece of equipment.

Turing concentrated immediately on the Enigma code's logical properties (such as being autoinverse, so that individual letters cannot be used to encrypt themselves) and designed, maybe at first on paper as a Turing machine, the electrical circuitry for an elimination procedure to obtain a smaller set of possible settings. This enabled the still relatively blind searching of the Polish "bomb" to be made considerably more efficient. After this, the suggestions for improvements came thick and fast (Gordon Welchman). More and more implications were revealed, more and more complicated became the circuitry of the Bletchley bomb, which swept away "billions of false hypotheses with the speed of light." (TE p183)

Inspired by Turings work

and perhaps also by his theoretical text On Computable Numbers (the first formulation of the Turing machine), Turing's colleagues at Bletchley Park - Turing himself was in America and after his return in 1943 he no longer worked as a cryptologist - constructed a decoding machine which they christened COLOSSUS on account of its impressive size.

The COLOSSI, a working version of which was available at Bletchley Park by about June/July 1944, had 2400 tubes and ran at a speed of 5000 impulses per second (cf. BdE pp161ff). They functioned on a binary basis, their programs followed conditional jump instructions and they featured internal storage. These are all properties of modern computers.

For some people, the COLOSSI's programming options are too closely linked to the tasks of cryptology, making them not universal enough for a computer.

It wasn't until after the war

that Turing himself built a truly universal machine in the form of his Automatic Computing Engine (ACE).