Thomson’s experience consisted in studying beams of cathode rays passing through a system of parallel metal plates that created an electric field and systems of coils that created a magnetic field.
It was found that the beams were deflected under the action of both fields separately, and at a certain ratio between them, the beams did not change the direct trajectory. This field ratio depended on the particle velocity. After a series of measurements, Thomson found that the speed of particles is much lower than the speed of light – thus it was shown that the particles must have mass.
Further, an assumption was put forward about the presence of these particles in atoms and a model of the atom was proposed, subsequently developed in the experiments of Rutherford.
The essence of the experiments and the hypothesis of the existence of matter in a state of even finer fragmentation than atoms, Thomson outlined at the evening meeting of the Royal Society on April 29, 1897. An extract from this message was published in the Electrican on May 21, 1897.
For this discovery, Thomson received the Nobel Prize in Physics in 1906.