Joseph John Thomson, English physicist, c 1900.
© Science Museum / Science & Society Picture Library
Thomson (1856-1940), discoverer of the electron, photographed at his desk. After graduating from Trinity College, Cambridge, Thomson (as Cavendish Profesor) turned the Cavendish Laboratory, Cambridge, into the most important centre for physics research in the world. In 1896, he began experiments on cathode rays, proving that they are particles with a negative charge and much smaller than an atom - particles later renamed 'electrons'. In 1912 he went on to develop mas spectrometry.