To see all my Chemistry videos, check out http://socratic.org/chemistry J.J. Thompson discovered the electron, the first of the subatomic particles, using the cathode ray tube experiment. He found that many different metals release cathode rays, and that cathode rays were made of electrons, very small negatively charged particles. This disproved John Dalton's theory of the atom, and Thompson came up with the plum pudding model of the atom.