Joan Clarke - Mathematician and Codebreaker

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The ornate main building at Bletchley Park.

Joan Clarke was an English mathematician who became one of the few women to work on the Enigma Codes at Bletchley Park during the Second World War. There she became a friend of the noted computer scientist Alan Turing. In later life she turned to the study of coins.


Joan Clarke was born on 24 June, 1917 in London to William Kemp Lowther Clarke and Dorothy Clarke, nee Fulford. She was the youngest of their five children - she had three brothers and a sister. Her father was a clergyman who had studied at the University of Cambridge in his youth. Joan was also academic - she won a scholarship to study at Newnham College, Cambridge in 1936. She was among the highest-achieving students when she graduated in 1939 but was not formally awarded a first class degree because of being a woman1 (she was awarded a 'titular degree' instead).

One of Joan's tutors had been Gordon Welchman, a mathematician who was working at Bletchley Park in the Government Code and Cipher School (GC&CS). He arranged for Joan to join GC&CS in 1940. Although she had been recruited for her mathematical talents, her first role was in the clerical section of Hut 8 with other female staff. However, during her first months in the role she did get to use a 'Bombe' machine. One of the main tasks for GC&CS was to decode messages intercepted from enemy forces that had been encrypted using Enigma machines. These machines generated thousands of different ways to encrypt messages, depending on the initial settings that were used. The Bombes were large (1m × 2m × 2m) electromechanical machines2 that tested the various combinations of settings, enabling numerous encrypted messages to be decoded.

Joan was not able to be formally promoted to the role of Cryptanalyst because it was not a job that was open to women. Instead she was given the title of Linguist so that her salary could be increased, even though she knew no languages except English. She began working with Alan Turing and seven other men on the 'Banburismus' technique for reducing the number of Enigma machine settings that needed to be tested by a Bombe before a message could be decrypted. Their work helped to reveal information that saved many ships from being sunk by enemy U-Boats.

Joan was promoted to Deputy Head of Hut 8 in 1944 and held the role until Bletchley Park was vacated in 1946. She transferred to the Government Communications Headquarters (GCHQ), which had replaced GC&CS.


Joan Clarke and Alan Turing had met before joining GC&CS as he was a friend of one of her brothers. They found they had a lot in common, including an interest in mathematics, chess and botany. In 1941 Alan proposed to Joan and she accepted. The following day, he admitted he had a 'homosexual tendency', but, as Joan went on to say in a 1992 interview, 'We carried on.' They introduced each other to their families and went on holiday together to Wales that summer, but agreed to discontinue their engagement soon afterwards. After they left Bletchley Park, they wrote letters to each other until his death in 1954.

At GCHQ, Joan met a retired army officer, Lieutenant-Colonel John Murray. They married in 1952. Jock (as he was known) became too ill to work, so they moved to Scotland, where they studied history together - in particular, the history of Scottish coinage. Jock recovered by 1962 so they both returned to work at GCHQ in Cheltenham.


Jock retired from GCHQ in 1971, and Joan retired in 1977 at the age of 60. They again worked together on numismatics (the study of coins) and wrote papers for the British Numismatic Society. After Jock died in 1986, Joan continued studying coins in collaboration with other members of the Society.

By 1991 Joan was not in the best of health, having pain and stiffness in her right arm. She moved to Oxfordshire to be closer to her family (and also close to resources in the museums and libraries of Oxford). She continued contributing to the work of the British Numismatic Society until 1995, and died on 4 September, 1996 at the age of 79.


Joan Clarke was awarded an MBE in 1947 for her contribution to the war effort. In 1986 she was awarded the John Sanford Saltus Medal by the British Numismatics Society for her contributions, including studies of several coin hoards and work on the chronology of coins made by Scottish Mints. A Blue Plaque commemorating her work as a cryptanalyst and numismatist was installed on her Oxfordshire house on 27 July, 2019.

Joan contributed to books and articles about the role of cryptanalysis in the Second World War, including the book Codebreakers: The Inside Story of Bletchley Park (F Hinsley and A Stripp, 2001). An article she wrote in the 1970s was analysed in 'From the Archives: A Lady Codebreaker Speaks: Joan Murray, the Bombes and the Perils of Writing Crypto-History From Participants' Accounts' (C Burke, 2010). She inspired the character of Pat Smith in the 1987 play 'Breaking the Code'. She appeared on television in the BBC Horizon interview in 1992. In The Imitation Game (2014), a film based on the life of Alan Turing, she was portrayed by Keira Knightley.

1It was not until 1948 that Cambridge began awarding degrees to women as well as men, even though women had been admitted to the university since 1869.2Ancestors of electronic computers.

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