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the course of a message, since these were limited to 250 characters, or 320 in the case of Naval
Enigma.
Each wheel had twenty-six electrical contacts on each of its two faces. Fixed wiring inside
the wheel led from the contacts on one face to the contacts on the other face. The wiring was
different for each wheel. This meant that if the wheels were taken out of the machine and put
back in a different order, the machine’s wiring was different.
On the right-hand side of each wheel (when viewing the machine from the front) was a
toothed cogwheel. On the left-hand side was a metal ring or rim, with the letters A–Z on it (or
sometimes the numbers 01–26: the army and air force machine usually had numbers while the
naval machines usually had letters). This ring could be fixed in any one of twenty-six positions
by means of a clip. The ring had a ‘turnover’ notch on its extreme left-hand edge (or two notches
in some cases). The function of the ring and of the notches will be explained later.
To the right of the three wheels (again when looking at the machine from the front) was the
so-called ‘entry plate’, with twenty-six electrical contacts (depicted in Fig. 12.4). To the left of the
three rotating wheels was a fixed wheel called the ‘reflector’, with twenty-six pin contacts wired
together in pairs. Reflectors with different wirings were introduced at various times during the
war, but the most commonly used was called Reflector ‘B’ at Bletchley Park. The function of the
reflector is described below.
figure 10.1 Enigma M3.
Reproduced with permission
of the Bletchley Park Trust.