JJ Thomson
(Source: Science Museum (www.nmsi.ac.uk/))

The Electron: the first sub-atomic particle

To explain the connection between electricity and matter, some scientists In the late nineteenth century argued there had to be a fundamental unit of electricity. In 1891 the Irish physicist, George Stoney, called this unit the electron.

In 1897 in Cambridge, J J Thomson experimented on cathode rays. In Britain, physicists had argued these rays were particles, but German physicists disagreed, thinking they were a type of electromagnetic radiation. Thomson showed that cathode rays were particles with a negative electric charge and much smaller than an atom. He also thought all atoms contained them. These particles were later named electrons.

Similar experiments were also carried out by Emil Weichert and Walter Kaufman. But they were less sure than Thomson that the particles they had found were smaller than an atom.

Thomson's experiments

J J Thomson investigated the invisible and mysterious cathode rays travelling through a glass bulb. He managed to change the direction of the rays using an electric field and concluded the rays had a negative electric charge.

However, Thomson also knew he could move a beam of cathode rays using a magnetic field. He arranged for the electric field to move the beam in one direction and for the magnetic field to move it in the opposite direction. Supposing the beam to be made of particles with a mass, m, and an electric charge, e, Thomson found the ratio e/m. Using a value for e measured from experiments on electrolysis, Thomson concluded the particles (or corpuscles as he called them) were hundreds of times smaller than atoms.

He had identified the first sub-atomic particle and found an important clue about the innermost structure of the atom.