The Turing-Welchman Bombe machine was an electro-mechanical device used to break Enigma-enciphered messages about enemy military operations during the Second World War. The first Bombe - Victory - started code-breaking on Bletchley Park on 14 March 1940 and by the end of the war almost 1676 female WRNS and 263 male RAF personnel were involved in the deployment of 211 Bombe machines.

Alongside the working reconstructed Bombe you can see a Checking Machine, also used in the process of Key recovery, as well as an engima which can be demonstrated for visitors.

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The Bombe

It was the task of the Bombe to discover the daily key - wheel order, wheel settings and plugboard configuration - to enable the 3-5,000 Enigma messages intercepted each day to be deciphered. Some keys would be broken within 2-4 hours, some would never be broken – speed was always of the essence.

Prior to the war, in the 1920s, three Polish mathematicians were the first to break the Enigma cipher. Using a Bomba machine in 1938, they supplied valuable pre-war information from the simpler Enigma cipher techniques then being used.

With the outbreak of war in 1939, ‘Y’ Intercept stations around Britain, and indeed in many parts of the world, intercepted all sorts of communications, including Enigma messages. At Bletchley Park, attempts to decipher messages began.

At the Park, Alan Turing was asked to find a way to break Enigma messages. Because of changes to the German operating procedures and the introduction of extra wheels, the Polish Bomba was now obsolete. Turing's attack was based on the use of ‘cribs’ (comparing patterns of the encrypted message and a known portion of plain text) to break the key. This approach was aided by the fact that no letter on the Enigma could be represented by itself in an enciphered message.

Turing realised that his approach was capable of being mechanised, and his invention of the Bombe, together with Gordon Welchman's diagonal board, (which dramatically reduced the number of invalid stops - false positives) increased throughput to the point that the Bombe became a major success.

The first Turing/Welchman Bombe based machine, known as Agnus Dei or simply Agnes, became operational in August 1940.

The engineering and construction of the original Bombes was the work of the British Tabulating Machine Company – BTM (which was later responsible for the 1950s HEC computer now at TNMOC).

Throughout the war the operation built around 211 Bombe machines and broke many keys on a daily basis. Huge amounts of intercepted traffic were deciphered, supplying invaluable information about enemy operations.

Many of the machines were located at out-stations, including Stanmore and Eastcote.

After the war, about 50 Bombes were temporarily retained, and some, according to official documents, continued to run. The rest were dismantled.

Reconstructing the Bombe

Forty years after the war, GCHQ released some 2000 BTM documents and drawings about the Bombe. As a tribute to the Bletchley Park codebreakers, John Harper, a retired engineer, decided it was timely to reconstruct a Bombe in the park which had just been saved from redevelopment.

BTM, the original Bombe manufacturers, through a series of mergers, became ICL in the late 1960s. As a former employee of ICL, John was able to make contact with some of the original engineers. Recruiting volunteers, at one time numbering 60, the task of reconstructing the one-ton Bombe with its countless components and moving parts began.

Although a few parts were sourced from the period, most were re-manufactured from the original drawings. As the mammoth jigsaw of information from several sources began to take shape, AutoCAD was used to prepare a complete set of parts drawings, together with a new set of derived assembly drawings.

In September 1997, the reconstructed steel frame was installed in Hut 11, one of the original Bombe locations, and over the next five years the machine with its 12 miles of insulated wire was painstakingly assembled. In April 2002, the mechanical phase was completed, and the machine was moved to Block H.

In June 2003, the electrical phase began, then the manufacture of more than 200 drums and in 2004 the Bombe was moved from Block H to Block B where it was commissioned in 2006/7.

The support from the British Computer Society, Quantel, Nortel, other companies and of course the veteran engineers and others had paid off and the Bombe was officially unveiled by HRH The Duke of Kent in July 2007.

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Crowdfunders finance the move

The re-organisation of the Bombe exhibits on the Bletchley Park Trust site led to the move of the Bombe in April 2018 to The National Museum of Computing to sit close to the reconstruction of Colossus, the sister machine that had accelerated the deciphering of the Lorenz messages of the German High Command.

Funded by the crowdfunding generosity of private individuals and companies, the move was achieved expertly and speedily in April 2018 and the new gallery was opened in June 2018.

Today the Bombe sits in Block H, the home of the wartime Colossus machines.

Enigma

Enigma was the brand name of a commercial cipher machine which began to be used by German armed forces and intelligence in the mid-1920s to encrypt messages to be sent via Morse communications. A plugboard was added to Enigmas used by the military to greatly enhance the security.

The Army and Air Force Enigmas chose three wheels from a set of five, whereas the Naval Enigmas chose from a set of eight wheels. The wheel settings were changed frequently according to ‘key sheets’, different ones being provided to different networks. A keyboard and a lampboard lighting up letters were used to encipher and decipher messages using the key sheets.

There is a working Enigma in the Bombe gallery which can be demonstrated upon request.