Sir Joseph John Thomson (born: 18 December 1856 / died: 30 August 1940)
Sir Joseph John Thomson often known as "JJ", was an English physicist, the discoverer of the electron. 

Thomson was born in 1856 near Manchester, England, of Scottish parentage. He studied engineering at Owen's College, Manchester, and moved on to Trinity College, Cambridge. In 1884 he became Cavendish Professor of Physics. In 1890 he married Rose Paget, and he had two children with her. One of his students was Ernest Rutherford, who would later succeed him in the post. 

Thomson's most important line of work, interrupted only for lectures at Princeton University in 1896, was that which led him, in 1897, to the conclusion that all matter, whatever its source, contains particles of the same kind that are much less massive than the atoms of which they form a part. They are now called 'electrons', although he originally called them corpuscles. His discovery was the result of an attempt to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out. Nearly all German physicists of the time held that these visible rays were produced by occurrence in the ether--a weightless substance then thought to pervade all space--but that they were neither ordinary light nor the recently discovered X rays. British and French physicists, on the other hand, believed that these rays were electrified particles. By applying an improved vacuum technique, Thomson was able to put forward a convincing argument that these rays were composed of particles. Furthermore, these rays seemed to be composed of the same particles, or corpuscles, regardless of what kind of gas carried the electric discharge or what kinds of metals were used as conductors. Thomson's conclusion that the corpuscles were present in all kinds of matter was strengthened during the next three years, when he found that corpuscles with the same properties could be produced in other ways--e.g., from hot metals. Thomson may be described as "the man who split the atom" for the first time, although "chipped" might be a better word, in view of the size and number of electrons. Although some atoms contain many electrons their total mass is never so much as 1/1000 that of the atom.

Prior to the outbreak of World War I, he made another ground-breaking discovery: the isotope. In 1918, he became Master of Trinity College, Cambridge, where he remained till his death. He died in 1940 and is buried in Westminster Abbey, close to Isaac Newton.