He was a good lecturer, encouraged his students, and devoted considerable attention to the wider problems of science teaching at university and secondary levels. Even though he was clumsy with his hands, he had a genius for designing apparatus and diagnosing its problems. In 1884 he was named to the prestigious Cavendish Professorship of Experimental Physics at Cambridge, although he had personally done very little experimental work. He was then recommended to Trinity College, Cambridge, where he became a mathematical physicist. Instead young Thomson attended Owens College, Manchester, which had an excellent science faculty. His father intended him to be an engineer, which in those days required an apprenticeship, but his family could not raise the necessary fee. Ironically, Thomson-great scientist and physics mentor-became a physicist by default. His assistant, Francis Aston, developed Thomson’s instrument further and with the improved version was able to discover isotopes-atoms of the same element with different atomic weights-in a large number of nonradioactive elements. Here his techniques led to the development of the mass spectrograph. Thomson’s last important experimental program focused on determining the nature of positively charged particles. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces.

Structure of the Atom and Mass Spectrography

When was the atomic theory discovered