Model Enigma Machine
rotor advance
Only the rightmost three rotors on the four rotor Enigma machine rotate automatically. If the fourth (leftmost) rotor is set to A, the fourth wheel and the reflector behave as an M3 reflector and the machine can handle traffic as if it were a three rotor machine. The rightmost wheel rotates by one notch every key press. This is simply achieved by the rightmost pawl engaging on the ratchet on the right hand side of the rightmost wheel. The second and third wheels will rotate when the notch on the alphabet ring of the wheel to the right allows the pawl to engage with the wheel ratchet.
This simple mechanical arrangement give rise to a complication - even assuming that there is only one notch per wheel. The first wheel always steps, the third wheel will step when the third pawl drops into its ratchet having fallen into the notch on the second wheel. However, the second wheel will step when either the second pawl drops into the ratchet on the right hand side or when the third pawl drops into the notch on the left hand side. After a few moment's thought, it becomes clear that these two events always happen one key press apart. The first key press cause the middle wheel to advance , one time in 26 this allows the third pawl to drop into the notch on the second wheel such that the second and third wheels rotate together one step later. This shortens the period of the machine from 26x26x26=17576 to ((26x25)+1)x26=16926 (I think!).
With the wheels β III II I and ring setting AAAA, the model shows the following positions of the rightmost wheels as each sucessive letter is typed.
ADP, ADQ, AER, BFS, BFT This is correct behaviour and shows the double step of the middle wheel described above.
View of the rotor frame and the baseplate showing the advance treadle which will be depressed by the key stems. The layout was based on the diagram on the right from Willi Korn's US patent of 1928.
The view above shows the pawls fitted roughly in position with respect to the detents. This is almost exactly as in the real machine except that in this case the reflector is retained by a fifth detent on the extreme left rather than by a special mechanism. Note that there are only three pawls since only the right hand three wheels rotate automatically - as in the real machine. The right hand picture shows the three pawls in a real machine for comparison. Note that the second and third of these pawls are double width in order to interface to the notch of one wheel and the ratchet of the adjoining wheel.
The sequence of operation can be seen in the three pictures above.
1. The treadle is forced upwards by a return sring such that the pawl is forced down into the baseplate, disengaging it from the ratchet on the wheel. This enables the wheels to be turned in either direction by the operator if required.
2. The treadle is depressed a little and the pawls rotate under gravity and engage on the wheel ratchet.
3. The treadle is fully depressed and the wheel has been rotated by one notch.