Chemistry / Chem 0193 · Atom · ~20 seconds
J. J. Thomson and the Discovery of the Electron (1897)
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In 1897, J. J. Thomson used a cathode ray tube to show cathode rays were negatively charged particles lighter than any atom.
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Deflecting the ray with electric and magnetic fields let Thomson calculate a charge-to-mass ratio, identical no matter which metal formed electrodes.
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The particle was not a smaller atom; being far lighter than any atom marked it a new subatomic constituent.