Sir John Douglas Cockcroft, OM, KCB, CBE, FRS (27 May 1897 – 18 September 1967) was a British physicist who shared the Nobel Prize in Physics in 1951 for splitting the atomic nucleus with Ernest Walton, and was instrumental in the development of nuclear power.
After service on the Western Front with the Royal Field Artillery during the Great War, Cockcroft studied electrical engineering at Manchester Municipal College of Technology. He then won a scholarship to St. John's College, Cambridge, where he sat the tripos exam in June 1924, becoming a wrangler. Ernest Rutherford accepted Cockcroft as a research student at the Cavendish Laboratory, and Cockcroft completed his doctorate under Rutherford's supervision in 1928. With Ernest Walton and Mark Oliphant he built what became known as a Cockcroft–Walton accelerator. Cockcroft and Walton used this to perform the first artificial disintegration of an atomic nucleus, a feat popularly known as splitting the atom.