The Turing/Welchman Bombe
by Tony Sale
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Codes and Ciphers

Set up and Run a Bombe


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This is ATLANTA, a Bombe which was in the USA wing at Eastcote in North London. This wing was manned by US soldiers from the 6812 division who were stationed in the UK. They wrote a very extensive report on the installation, setting up, running and maintaining of "their" Bombes.

This report survived in the American National Archive (NARA) and is used as the basis for this description.

So the first thing to do is to remove all drums from the front and all cables from the rear of the Bombe.

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We are going to set up the Bombe to find the Shark Enigma configuration corresponding to the menu derived by the code breakers from the U-Boat contact signals in the film "Enigma".
The menu had been teleprinted from Bletchley Park to Eastcote as: UAFT AVFV VKZU GKET KFSS FDEU DUST FSFS SXEU YSZT and the menu redrawn by the Supervisor and given to the Bombe operator. The menu is interpreted differently by the Bombe operator compared to the code breaker. The Bombe is going to try to find a set of positions of its drums which satisfy, at the same time, all the connectivity constraints imposed by the menu.
Thus the code breakers element, A -> V at offset fv has to be interpreted as a set of three Bombe drums whith offsets z f v (top, middle, bottom) connected to the next set of three drums with offsets z z v to represent V -> K on the menu.

The Bombe operator therefore assigns a set of three drums to each link in the menu and sets their offsets to the letters on the link. The operator takes the numbering of the sets of wheels as starting at 1 in the top left hand and progressing left to right along the top row, then left to right along the middle row and along the bottom row to 36 and writes the allocated numbers on the menu.

Now the sets of three drums making up a Letchworth Enigma have to be put onto the Bombe. Start at the top left hand of the Bombe and put the drums onto their shafts, selecting drums of colour corresponding to the first wheel order to be run, 534. (Remember that the top, fast, drum on the Bombe corresponds to the slow left hand drum on the Enigma machine).
These are the drum colours and here are sets of 534 in place on the Bombe.
Now the drums have to be set to their offset positions.
Next comes the wiring up of the Bombe. This means going to the back and plugging up the 26 way cables to connect the sets of drums together to the diagonal board and to the output indicator.
So here is the back view again. The vertical strips of cable sockets are arranged in groups of three. In each group the right hand strip contains the connections to the drums, the centre strip holds "commons". These are sets of 5 sockets strapped together so that one cable coming in can be connected to up to 4 cables going out. The vertical strip on the left of the set of three is the Diagonal Board.
These sets of three are repeated three times, one set for each row of drums on the front of the Bombe. The numbering starts at 1 at the top of the left hand strip of drum connections and proceeds down to 36 at the bottom of the right hand drum connection strip.
    Here is an expanded view of part of the right hand vertical strip of drum connections. The lettered panels show which drum sets are connected and show the INput and OUTput sockets for that drum set. But also shown are bridging connectors which connect two successive drum sets together and have a socket on top so that a 26 way cable can be connected to the join between two drum sets. You can see that the output of 28 is "bridged" to the input of 29. The output of 29 and the input to 30 is not bridged, but the output from 30 and the input to 31 is.
    This is the connection diagram for the menu sent to Eastcote. The numbered drum sets are on the left and the Diagonal Board letters on the right. The allocation of menu links to drum sets is chosen so that the maximum number of drum sets are connected in sequence, with the minimum number of multiple connection to avoid the use of commons, a notorious source of bad connections. You will see that drum sets 1 to 5 can all be connected in series and commons are required on the input to 1, the join from 3 to 4 and 4 to 5 and also to join 9, 10 and 8.

    So put in the joining links between the connections for drum sets 1 to 5. Now connect a 26 way lead from the input to 1 to the top set of commons. Two leads then go from this common, one to socket A at the top of the diagonal board vertical strip of sockets, the other to the output socket from drum set 7.

    The rest of the wiring connections are then made in a similar manner and the output indicator plugged to the common between 3 and 4.

    Then the whole wiring has to be checked independantly by another Bombe operator. Absolutely no mistakes must be made otherwise vital Bombe time would be wasted.

    Lastly the switches on the side of the Bombe are set to inject a starting current at A in register K.

Run the Bombe

Click here to run the simulation of the Bombe.

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