On April 30, 1897, Joseph Thomson discovered the electron

This British physicist investigated the nature of cathode rays and showed that electric fields could cause their deflection. He carried out numerous experiments on its deflection, under the combined effect of electric and magnetic fields, looking for the relationship between the charge and the mass of the particles, a proportionality that remained constant even when the cathode material was altered..

On April 30, 1897, he discovered a new particle and showed that it was about a thousand times lighter than hydrogen. This particle would be baptized with the name of electron, a designation proposed years before by the Irishman George Johnstone Stoney, who had theorized about its existence. Joseph John Thomson was therefore the first to identify subatomic particles, and he came to important conclusions about these negatively charged particles with the apparatus he built obtained the relationship between the electric charge and the mass of the electron.

If you want to know more about this scientist, click on the following link: Joseph J. Thomson