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How did J.J. Thomson discover the electron?

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Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

In 1897, English physicist J.J. Thomson discovered the electron by investigating the nature of cathode rays, which occur when an electric current is passed through a vacuum tube.12

While previous researchers had observed these rays, there was a long-standing controversy over whether they were waves or charged particles. To resolve this, Thomson repeated an experiment by Heinrich Hertz using a significantly improved vacuum to ensure that residual gas did not interfere with the results. He directed the cathode rays between two parallel aluminum plates and observed their deflection when an electric field was applied.12

By observing both magnetic and electric deflections, Thomson confirmed that the rays were composed of negatively charged particles. He further calculated their mass-to-charge ratio, discovering that these particles—which he originally called "corpuscles"—were nearly 2,000 times lighter than a hydrogen ion. This proved that atoms were not homogeneous, indivisible particles but possessed a complex internal structure, establishing the electron as the first identified subatomic particle.12