J. J. Thomson

British physicist, discoverer of the electron in 1897, who built an early positive ray apparatus around 1912 to 13 that passed charged particles through electric and magnetic fields to sort them by mass, producing the first mass spectra and the first evidence that some elements exist as a mixture of isotopes. He won the 1906 Nobel Prize in Physics for his earlier work on the conduction of electricity through gases, not for the mass spectrograph itself.