The existence of electrons was first experimentally proven in 1897 by the British physicist Joseph John Thomson (1856-1940) through his cathode ray experiments. Thomson's experiments involved passing an electric current through a cathode ray tube, a sealed glass tube with two metal electrodes at opposite ends. When a high voltage was applied between the electrodes, a beam of cathode rays was emitted from the negative electrode (cathode) and traveled towards the positive electrode (anode).

Thomson conducted various experiments to study the properties of cathode rays, including their deflection in electric and magnetic fields. He observed that cathode rays were deflected by both electric and magnetic fields, indicating that they were composed of negatively charged particles. By measuring the amount of deflection in these fields, Thomson was able to determine that these particles had a very small mass compared to atoms. He also estimated the ratio of the charge to mass (e/m) of these particles.

Thomson's experiments provided strong evidence for the existence of subatomic particles much smaller than atoms, which he proposed to be electrons. He referred to these particles as "corpuscles" in his papers but later adopted the term "electrons" suggested by his colleague G. Johnstone Stoney. Thomson's work on cathode rays and the discovery of electrons laid the foundation for the study of subatomic particles and the development of the field of particle physics.