Background to the discovery
The Electron: the first sub-atomic particle
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.
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.
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