I am researching the Bombe Machine, having just completed a project on the Enigma Machine. I visited The Rutherford Journal - The Turing Bombe, and it claimed that

The scales on the three indicator drums are designed to show the ‘rotor core starting positions’ at a ‘stop’ when measured in terms of the corresponding Enigma ring-settings for the Enigma rotor settings: ZZZ. This is the reason why the letters on the scales on the indicator drums are in the reverse order to those on all the other drums.

I don't understand this explanation, though. Aren't the indicator drums used to determine the rotor core offset? And I thought that the rotor core offset was calculated by the rotor position minus the ring setting. As the Bombe tests the rotor positions from A to Z, shouldn't the indicator drums also run from A to Z, since the Ringstellung is assumed to be a constant ZZZ?


1 Answer 1


I emailed someone, and it turns out that the rotor positions found by the Bombe are used as the ring setting in the checking machine. Because the ring setting is the "negative" of the rotor position, the indicator drums spin backwards, so that it shows the correct ring setting that corresponds to the rotor positions for the checking machine.


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