J.J. Thomson Discovers the Electron
A cathode ray tube reveals a particle a thousand times lighter than the smallest known atom
Quick facts
- Discovery
- The electron, via cathode ray deflection
- Location
- Cavendish Laboratory, Cambridge University
- Key measurement
- Charge-to-mass ratio, uniform across metals and gases
- Recognition
- Nobel Prize in Physics, 1906
What happened
At Cambridge University's Cavendish Laboratory in 1897, J.J. Thomson showed that cathode rays, streams given off inside evacuated glass tubes, were deflected by electric fields, proving they were composed of charged particles rather than a wave phenomenon. By balancing the deflection from electric and magnetic fields against each other, Thomson measured the particles' charge-to-mass ratio. A Purdue University chemistry history page notes he found the same charge-to-mass ratio regardless of the metal used for the electrodes and regardless of the gas filling the tube, and concluded that the particles were a universal component of matter. A Le Moyne College historical chemistry archive adds that the small mass indicated that pieces of matter existed which were smaller, lighter, than the smallest atom yet known by a factor of 1000, and that this and the particles' universal presence together implied that even the smallest atoms have component parts, that they are not structureless or indivisible.
Why it matters
Thomson's cathode ray experiments identified the electron, the first subatomic particle ever discovered, and overturned the 19th-century assumption that atoms were indivisible. The discovery opened the door to every subsequent model of atomic structure, from Rutherford's nuclear atom to Bohr's quantized orbits, and won Thomson the 1906 Nobel Prize in Physics.
How we know
Thomson announced his conclusions at a Royal Institution lecture on 30 April 1897 and published the full experimental details in Philosophical Magazine later that year; his charge-to-mass measurements have since been repeated and refined using the same electric-and-magnetic-field deflection method he pioneered.
Sources
- Purdue University, Department of Chemistry. The Discovery of the Electron (J.J. Thomson) · Reputable sourcechemed.chem.purdue.edu · The domain "chemed.chem.purdue.edu" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Le Moyne College, Department of Chemistry (T. Giunta). Discovery of the Electron: J.J. Thomson · Reputable sourceweb.lemoyne.edu · The domain "web.lemoyne.edu" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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