from one rotor to the next. However, rather than looking for the one correct rotor setting based on the indicators, as the Bomba did, Turing's would look for all the rotor settings that allowed the cipher to match the assumed plain text. Or, more correctly, it searched all the settings and disregarded those that were incorrect. For example, if the assumed letter was 'G' and the corresponding cipher letter was 'L,' Turing's test register ignored any results that did not allow the electrical current to pass from 'G' to 'L.' By disproving thousands of rotor settings, those left were possible correct settings.

While Turing developed plans for his cryptanalytic machine, Gordon Welchman also thought about the Enigma problem. Though GC&CS assigned him to work in traffic analysis, a field that involves the externals of a message and not the message itself,(5)he contemplated ways to break Enigma messages more easily. On his own, he reinvented the series of perforated sheets that Henryk Zygalski had developed for the Poles. Poland had turned this achievement over to Britain at the same time as the Bomba, and BP was already creating new sheets for five rotors.(6)

Undeterred, Welchman began working on another complication on the Enigma, the plugboard. Because the plugboard uses a cable to connect one letter to another, it automatically connects the second letter back with the first. If A is plugged into E, E is plugged into A. Knowing this, Welchman designed a board that connected each letter with every other letter. The wires created a pattern of diagonal lines. He created a 'diagonal board.'

Gordon Welchman showed his design to Alan Turing, who agreed it would greatly enhance his machine. Although simple in design, combined with Turing's test registers, the number of possible rotor settings decreased from thousands to only a few. Analysts could easily test these few solutions on an Enigma duplicate or analog.