J. J. Thomson’s 1897 cathode-ray experiments identified the electron as a negatively charged subatomic particle.
Thomson studied cathode rays in evacuated discharge tubes and measured how electric and magnetic fields deflected them. The rays bent in a way consistent with negative charge, and their charge-to-mass ratio was far larger than that of any known ion. This indicated that the particles were extremely light and present in atoms of different materials.
The finding challenged the idea that atoms were indivisible. Thomson called the particles “corpuscles”; the name electron, already used in related scientific discussions, soon became standard. His work established the first clearly identified subatomic particle.
The experiment did not discover the proton or neutron. Ernest Rutherford later identified the atomic nucleus and helped establish the proton, while James Chadwick discovered the neutron in 1932. Thomson’s early atomic model, often called the plum-pudding model, was later overturned by nuclear-scattering evidence.