Virtual Wartime Bletchley Park
by Tony Sale

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The Bombe and the Ringstellung problem

You may recall that the wheel of an Enigma machine contained a ring or tyre round the core which held the scrambled cross wiring. This ring could be turned and then clipped into one of 26 positions. (The Ringstellung). The ring also had one or two notches cut into it which allowed the carry of its movement to the next wheel on its left.

Thus the core cross wiring position of an Enigma machine was arrived at through two components, the amount the ring had been turned relative to the core and then the amount the joined ring and core had been turned to set the wheel start letter. To be able to decrypt all the intercepted messages for a 24 hour period the code breakers had to find the Ringstellung for that period but the Bombe could only find the core wiring positions which satisfied the menu. The convention was to assume a constant Ringstellung of ZZZ for the Bombe.

In deriving the menu it was assumed that the cipher/crib pair started at ZZZ and incremented going to the right, thus offsets like ZAG. When the Bombe stopped its drums would indicate a core position of say GUD as being equivalent to the first element in the menu. But to check the stop against the menu, an Enigma machine needed to be set up with the correct wheels and Ringstellung ZZZ. Then the wheel starts turned to GUD and then further turned by the menu offset to reach the point in the cipher/crib length that was to be tested. This was quite a process and liable to error so when the Checking Machine was devised to handle large numbers of stops more quickly a different approach was used.

The Bombe was provided with three extra drums to the right of the centre row of Enigmas. These extra readout drums were lettered so as to show not the core wiring position but 26-(core position), i.e. the alphabet went round in the opposite direction to that on the Enigma drums. The reason for doing this was so that rotatable discs on top of the drums used on the Checking Machine could be set to this reciprocal set of letters and then the drums could be just turned to the offset letters of the menu in order to check the derived Steckers. (the extra discs just did the letter offset calculation for the operator).

Unfortunately in some 1940's reports the term 'Ringstellung' is used for the Bombe stop read out from these extra drums. It is not anything to do with the true Enigma Ringstellung the code breaker needs.

There is yet another complication with Bombe readouts. In 1939 the core wiring of wheels 4 and 5 was measured electrically on an incorrect datum. This caused the first set of Zygalski punched sheets to be incorrect for wheels 4 and 5 and caused some consternation. The error persisted and the wiring on Bombe wheels 4 and 5 is also out by one letter for wheel 4 and two letters for wheel 5.

The Bombe simulator calculates the correct core wiring positions using the correct wheel wiring datum for all wheels. This is what is shown as 'wheel letters' on various windows on the displays. But the US Report on the British Bombe gives the extra 3 wheel reciprocal readouts as the stop so the simulator calculates these from the core result and shows them as the 'Stop'.

For the record the formula is:

stop:=25-core;
IF wheel="4" THEN
stop:=stop-1;
ELSIF wheel="5" THEN
stop:=stop-2;
END;
IF stop<1 THEN
stop:=stop+26;
END;

I have avoided this problem on the rest of the codesandciphers web site by just using core wiring references and not showing the actual Bombe readout on the Javascript Bombe simulator. However having to recreate the actual stops shown in the US Bombe Report forces this explanation.

The simulation of the Checking Machine is programmed to work correctly with the Bombe stops as shown on the three output drums.

But remember that when setting up a stop on a real Enigma machine the stop needs to be converted to Enigma wheel positions.

After this slight diversion proceed to derive the Steckers from a stop using the Checking Machine and some more complex menus.

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This page was originally created by the late Tony Sale
the original curator of the Bletchley Park Museum

Virtual Wartime Bletchley Park by Tony Sale

The Bombe and the Ringstellung problem

Virtual Wartime Bletchley Park
by Tony Sale