In the famous experiment of 1932 a lithium target was bombarded with a stream of very swift protons (hydrogen nuclei), which split the lithium nucleus into two helium nuclei. The atom-splitting protons were accelerated to the enormous velocities required by means of large demountable high-vacuum tubes, equipment only made practicable by the earlier discoveries of C. R. Burch.

Previously Allibone had been working on the production of continuously evacuated discharge tubes, intended at first for the acceleration of electrons and capable of withstanding up to half a million volts; improving on the designs evolved in this work Cockcroft and Walton built not only a discharge tube for the acceleration of positive ions but also a continuously evacuated high-voltage rectifier with which to produce a steady potential of 800,000 V starting with a transformer voltage of 200,000. The combination of these two pieces of apparatus made possible the successful results of the atom-splitting experiments.

From 1932 onwards Cockcroft contributed to most of the major achievements of the Cavendish laboratory, and in May 1939 he was appointed professor of natural philosophy at Cambridge, having been previously elected a fellow of the Royal Society. When war was declared his advice was in urgent demand by Government departments-as director first of the Air Defence Research and Experimental Establishment (later the Radio Research and Development Establishment) of the Ministry of Supply, and then of the Montreal laboratory of the National Research Council of Canada. In 1946 he was appointed director of the Atomic Energy Research Establishment at Harwell, and in 1948 he received a knighthood.

FURTHER HIGH VOLTAGE WORK
Meanwhile, in the high voltage laboratory at Trafford Park work on the mechanism of the arc discharge was initiated by Allibone in 1936, and fundamental research on lightning was carried out between 1932 and 1939, extensive field observations being made in South Africa by F. R. Perry.

A third field of investigation was that of high voltage x-ray tubes, in which continuous evacuation had great advantages. Sealed-off tubes were shortlived and expensive to replace, whereas in a continuously evacuated tube it is only necessary to replace the filament or the target; with sealed-off tubes it was also very difficult to obtain the high voltages necessary to give radiation of the intensity and wavelength required for deep therapy work. In 1930 therefore, work started with the object of producing high voltage x-ray tubes for medical work and crystallography.