The Turing-Welchman Bombe automated the search for the Enigma daily key. A crib - a guessed plaintext fragment aligned with known ciphertext - is wired into a menu on the commutator socket panels. The nine drums (three per bay) then step through rotor positions exactly as Enigma rotors do, one column per rotor, until the electrical circuit either stays consistent or signals a contradiction.
The Bombe tested rotor settings against a crib menuEach drum carries a ring of letters on its face and turns through all 26 positions. The bottom drum steps on every beat, the middle drum steps on a turnover, and the top drum follows the double-step rule - reproducing the stepping of the real Enigma so the menu wiring behaves identically to the target machine.
When the drums reach a setting that keeps the whole circuit consistent, the machine stops and the test register shows the candidate. A human then checks the candidate against the crib; most trials stop early on a contradiction, which is why the machine could search tens of millions of settings in a day - far beyond what human codebreakers could test by hand.