Kevin Owens · Enigma and Turing's machine · The story · Enigma · The machine · 3D view

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The test register's lamps: the wires live on the central letter's bus; one lit, or all but one, is a stop108 drums, 36 Enigma scramblers: top II maroon (left rotor, the fast drum), middle V brown, bottom III green; 13 are wired to the menuThe menu on the side plate: letters joined by crib positions; its two loops (bright) are what make a stop rareThe open bay: the motor's belt drive, and on the back the scrambler jacks cabled to the diagonal boardPaletteThe machine was designed by Alan Turing in 1939, with Gordon Welchman's diagonal board added in 1940, and built by the British Tabulating Machine Company at Letchworth; each machine was about 7 ft wide, 6 ft 6 in tall and 2 ft deep and weighed about a ton. Its front carries 108 drums in three banks of twelve vertical triplets, 36 Enigma scramblers; each drum is colour-coded by the rotor it imitates (I red, II maroon, III green, IV yellow, V brown) and wired like it, and the top drum stands for the Enigma's left rotor. All the top drums turn together, at 120 rpm on the later machines; at each full turn the middle drums step once, and at each turn of the middle drums the bottom ones step, so the run visits all 26 × 26 × 26 = 17,576 positions, here 5.6 minutes at full speed. The menu (on the side plate) comes from a crib, guessed plaintext set against the ciphertext: each crib position joins its plain and cipher letters through a scrambler whose bottom drum is set that many places on, so the bottom drums of the 13 wired scramblers show their offsets while every top drum and every middle drum shows the same letter. At each position the test register puts current on one wire of the central letter's bus (the hypothesis 'E is steckered to E'); the current spreads through the scramblers and, with the diagonal board, through every reciprocal pair (X steckered to y means y steckered to X). If all 26 wires come alive every hypothesis is contradicted and the drums move on; if one, or all but one, stays different, the machine stops. On this job, a weather report beginning WETTERVORHERSAGE enciphered on II-V-III with rings BUL, start RKX and ten plug pairs, the sweep stops twice: at DBC with 25 live (E steckered to M, which the checking machine then rejects) and at QQM with 1 live, the true core position (window RKX less rings BUL), E steckered to E. Without the diagonal board, Turing's 1939 design, the same sweep stops 707 times. The drums, letters, lamps, stops and steckers on this page are computed by the same tested model that drives it. Simplifications, as in that model: a stop is a core position (the machine knows nothing of ring settings), the menu assumes no middle-rotor turnover inside the crib, and the real machine over-ran a stop and backed up to it, drawn here as an instant halt. The open bay at the lower right is this figure's own: on the real machine the scramblers' jacks and the diagonal board are on the back of the machine, and the motor, its 2:1 belt and the guard are drawn as an illustration, not from a drawing of the machine; the source gives only the drum speeds (50.4 rpm on the first machines, 120 rpm later).

Running the machine · wheel order II-V-III · menu from the crib WETTERVORHERSAGE · central letter E