Joseph John Thomson, English physicist, c 1900.

UNITED KINGDOM - AUGUST 08: Thomson (1856-1940), discoverer of the electron, photographed at his desk. After graduating from Trinity College, Cambridge, Thomson (as Cavendish Professor) 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 mass spectrometry. (Photo by SSPL/Getty Images)