GREENBERG | 89
keystroke, while the movement of the middle and left-hand wheels was determined by the
position of the ring. As a consequence, the electrical circuit through the machine was dynamic,
changing every time a key was pressed. Because of this, typing the same letter again and again
at the keyboard would produce a stream of different letters at the lampboard.
Once the wheels were clamped together in the machine, between the entry plate and the
reflector, there were twenty-six parallel electrical circuits running between them. When a key
was pressed at the keyboard, a current passed from the key through, in turn, the plugboard,
entry plate, right-hand wheel, middle wheel, left-hand wheel, and reflector, and then (due to
the wiring of the reflector) back again through the left-hand wheel, middle wheel, right-hand
wheel, entry plate, and finally the plugboard, eventually reaching the lampboard (again see
Fig. 12.4). As the current passed through the machine from point to point, the letter continually
changed identity. By the time the bulb lit up on the lampboard, the letter could have switched
its identity up to nine times.
By 1939 Naval Enigma operators had a box of eight wheels, from which they chose three.
Initially, the M3 had been supplied with five cipher wheels, which were compatible with the
wheels of the Enigma Model I. This meant that the navy’s Enigma machines could exchange
messages with army and air force Enigmas. In 1939, however, three extra wheels, used exclu-
sively by the navy, were added to the M3’s original five (the new wheels were named VI, VII, and
VIII). German U-boats used the M3 until the North Atlantic U-boat fleet was equipped with
the four-wheeled M4 in February 1942.^6
The M4, backwards compatible with the M3, was a modified three-wheel machine. The
width of the reflector was halved and the remaining space taken up by a fourth wheel. (There
were two different versions of this extra wheel, known at Bletchley Park as ‘Beta’ and ‘Gamma’.)
figure 10.3 A window adjacent to the edge of each wheel shows the wheel’s current position.
Reproduced with permission of the Bletchley Park Trust.