rotor steps, takes middle rotor (II) one step further, which is now in its own E—notch position — normal step of right rotor, double step of middle rotor, normal step of left rotor — normal step of right rotor Fourth rotor The introduction of the fourth rotor was anticipated because captured material dated January 1941 had made reference to the development of a fourth rotor wheel; indeed, the wiring of the new fourth rotor had already been worked out. On 1 February 1942, the Enigma messages began to be encoded using a new Enigma version that had been brought into use. The previous 3-rotor Enigma model had been modified with the old reflector replaced by a thin rotor and a new thin reflector. Breaking Shark on 3-rotor bombes would have taken 50 to 100 times as long as an average Air Force or Army message. It seemed, therefore, that effective, fast, 4-rotor bombes were the only way forward. Encoding mistakes by cipher clerks allowed the British to determine the wiring of the new reflector and its rotor. References External links enigvar2.pdf enigmabombe.htm ultraenigmawirings.htm g-312.zip Enigma machine