In the view of Professor Archie Howie, the present head of the Cavendish Laboratory, the significance of Cockcroft's work lies not only in his atomic experiment, momentous though it was. It lies also in the way he and Walton opened up a new scientific area by devising an accelerator as a means of investigating nuclear physics.

''The whole idea of using accelerators and manipulating nuclei artificially dates from the 1932 experiment, and that was a really significant legacy,'' says Professor Howie. ``Cockcroft and Walton transformed the nuclear physics of Rutherford into an engineering subject in which matter could be manipulated. Rutherford had to make do with what nature provided.''

Professor Howie believes that Cockcroft's genius lay in discernment, in seeing an opportunity. He had a very important meeting with George Gamov in 1929, when he realised that Gamov's theory about collisions between atoms and fast protons - namely that particles could get into the nuclear of an atom by quantum mechanics - opened the way to an important experiment.

''Cockcroft saw that it should be possible to get a nuclear reaction going without resorting to the use of very high voltages to accelerate the protons, as had previously been thought. He foresaw the possibility of managing with perhaps half a million volts in the accelerator instead of having to use about five million, which was unthinkable in those days.''

Ernest Rutherford, then Professor of Physics at the Cavendish Laboratory, had in 1919 made the first nuclear transformations, using particles emitted by natural radioactive nuclei. But even after Cockcroft and Walton's experiment he was sceptical about any practical use for nuclear energy.

History has shown otherwise. Not only did Cockcroft's work contribute directly to the success of the atomic bomb but also to a host of beneficial applications, especially in medicine and in the manufacture of semiconductor devices.

''Nuclear medicine is entirely dependent on the nuclear industry, either in the production of X-rays by collisions of particles with nuclei or in the production of the nuclear isotopes which are used in all kinds of medical treatments nowadays,'' says Professor Howie.

''Magnetic resonance imaging, for example, is entirely dependent on the properties of nuclei and if people had not been uneasy about the word 'nuclear', this scanning technique would probably still be known, as it was initially, as nuclear magnetic resonance imaging.''

In material science there are many nuclear processes, basically using atom smashing techniques, says Professor Howie. ``In the manufacture of silicon chips, crucial stages are often done with ion beams to etch different areas. And when doping atoms are inserted into semiconductors, this is done by firing them in with an accelerator not unlike the one designed by Cockcroft and Walton.

"In addition to being the source of all these techniques, nuclear physics underpins a whole variety of methods for precise radioactive dating of the remote past.''

During the 1939-45 war John Cockcroft was influential in the development of radar. Later he was appointed director of Britain's Atomic Energy Research Establishment. He retired from this post in 1960 to become the first Master of Churchill College, Cambridge. His energy was undiminished. He saw his mastership as an opportunity not only to build an impressive new college, a process in which he became deeply involved, but also to establish the college as a centre for the training of scientists and engineers. He broke down conventional ways when they seemed to be a barrier to progress, creating a college that attracted academics of distinction from many parts of the world.

Cockcroft was quiet and laconic but people who knew him sensed they were in the presence of a remarkable man. "He had a great force of personality,'' says Professor Howie. "He had a very dry style and did not believe in wasting words, but people understood what he wanted. And he had the ability not to listen to something he did not want to hear. He somehow inspired a great deal of loyalty and admiration, even though people never really felt they knew him very well. They could sense an integrity in him.''