88 | 10 THE ENIGmA mACHINE
The wiring inside the wheels of the Enigma M3 was different from that in earlier models. The
M3 also had a ‘plugboard’ at the front. This consisted of twenty-six sockets, one for each letter
of the alphabet, again laid out in the same order as the letters on the keyboard, in the same three
rows. Cables with plugs at each end were placed in the sockets, and the plugging could be varied
in accordance with the day’s instructions for setting up the machine. Each cable connected two
selected letters together. The purpose of the plugboard was to switch the typed letter for a differ-
ent one before it entered the wheels (see Fig. 12.4). The letter then exiting from the wheels was
switched again by the plugboard before reaching the lampboard. From November 1939, most
German Enigma networks connected ten pairs of letters together at the plugboard. Previously,
fewer pairs had been connected, making messages easier to break.
When the wheels were put into the machine and the lid was shut, the serrated edge of each
wheel protruded through a slot in the lid (see Fig. 10.3). Adjacent to each slot was a small view-
ing window that displayed the topmost letter on the ring. Each wheel could be turned freely by
hand to any one of its twenty-six positions by rotating its protruding edge.
It was central to the encryption process that at least one wheel would move forward one
position each time a key was pressed at the keyboard. The right-hand wheel moved at every
figure 10.2 Inside the Enigma M3.
Reproduced with permission of the
Bletchley Park Trust.