J. J. Thomson and the Electron

Sir Joseph John Thomson (1856-1940) played a pivotal role in developing our understanding of the electron. In the 1890's, cathode ray tubes had been developed in which a luminous beam could be produced in a partially evacuated glass tube, directed from the negative electrode (cathode) to the positive (anode). A narrow luminous beam could be produced by using an aperture near the cathode, and this beam could be deflected by either an electric field or a magnetic field. Thomson showed that with the application of both electric and magnetic fields, he could balance the deflections and obtain a straight beam. This same principle is presently used in velocity selectors for mass spectrometers. Using this apparatus, Thomson determined the charge-to-mass ratio of the electron, e/m.

The determination of the charge of the electron awaited the work of Millikan who measured the electron charge in 1909 with his oil drop experiment. With the combined results of Thomson and Millikan, a value for the electron mass was obtained - a value far below that of atoms. It was known that electrons could be removed from atoms, and that they became positive ions as a result. From the perception of solid matter, it was presumed that this positive residual matter filled the entire space of the atom, giving rise to what has been called the "Thomson model" of the atom.