Joseph John Thomson. Portrait of English physicist Sir Joseph John Thomson (1856-1940), discoverer of the electron, seen as a young man. This portrait was taken in 1879 shortly before his university finals. Born near Manchester, Thomson entered Cambridge University in 1876 and remained there for the rest of his life. He showed that cathode rays are deflected by an electrical field. He was also able to measure their mass to charge ratio, deducing that they were particles much smaller than an atom. For this proof of the existence of the electron, he was awarded the Nobel Prize for Physics in 1906, and was knighted in 1908. He later demonstrated the existence of isotopes.