Sir John Douglas Cockroft was born on May 27, 1897, in Todmorden, Yorkshire, England. He entered the University of Manchester to study mathematics in 1914, but soon abandoned his studies to serve in the British Army during World War I. Starting as an enlisted person in the Royal Field Artillery, Cockcroft emerged at the end of the war a second lieutenant with several decorations. After World War I, he worked as an apprentice in an engineering firm in Manchester. His technical talents were quickly recognized, and the company sponsored him to study electrical engineering at the Manchester College of Technology.
Upon completing his engineering degree, Cockcroft worked for two years at the Metropolitan Vickers Electric Company. He left it to study at St. John’s College, Cambridge. In 1924, he received a degree in mathematics from that institution. He then joined the staff of Ernest Rutherford’s Cavendish Laboratory as a researcher. At first, he collaborated with the Soviet scientist Peter Kapista (1894–1984) in the production of intense magnetic fields and low temperatures.
In 1928, Cockcroft turned his attention to the acceleration of protons by high voltages. By good fortune, he was soon in collaboration with the Irish physicist, Ernest Walton (1903–1995). In 1932, they were able to induce the first artificial disintegration of a nucleus by constructing a linear accelerator capable of bombarding a lithium nucleus with protons accelerated across a potential difference of some 600 kilovolts. As the machineaccelerated protons smashed into the thin lithium target, Cockcroft and Walton observed a characteristic pair of energetic helium nuclei (alpha particles) emerging from each disintegrating lithium nucleus. Their analysis of this first artificially induced nuclear reaction confirmed the energy-mass equivalence principle (E = mc2) postulated by Albert Einstein’s relativity theory.
The Cockcroft-Walton accelerator was an important new tool for nuclear scientists and soon led to other, more powerful “atom-smashers.” For their pioneering research involving the transmutation of atomic nuclei by artificially accelerated atomic particles, Cockcroft and Walton would share the Nobel Prize in physics in 1951.
In 1929, Cockcroft was selected as a fellow in St. John’s College at Cambridge. He progressed through other academic positions, including demonstrator and lecturer. He became Jacksonian Professor of Natural Philosophy in 1939 but soon departed the college to support the British war effort during World War II. In September 1939, he accepted appointment as assistant director of scientific research in the Ministry of Supply and used his technical expertise to apply radar for coastal and air defense. In 1944, Cockcroft went to Canada to take charge of the Canadian Atomic Energy project, and he served as the director of the Montreal and Chalk River Laboratories until 1946. He then returned to Great Britain to become director of the Atomic Energy Research Establishment at Harwell.
Cockcroft was knighted in 1948 and was named knight commander of the Bath in 1953. From 1954 to 1959, Sir John Cockcroft served as a scientific research member of the United Kingdom Atomic Energy Authority. In 1960, he was elected master of the newly established Churchill College at Cambridge. He died on September 18, 1967, in Cambridge.