On December 30, 1895, 25-year-old French physicist Jean Baptiste Perrin (1870–1942) published a paper in Comptes Rendus (Vol. 121) titled “Nouvelles propriétés des rayons cathodiques”.
Perrin provided the world’s first direct experimental proof that cathode rays carry negative electric charge.
The Faraday Cup Discharge Experiment¶
Perrin designed a vacuum discharge tube containing a hollow shielded brass Faraday cylinder (Faraday Cup) connected to an external quadrant electrometer:
- Deflection & Collection: When cathode rays were directed straight into the open aperture of the Faraday cylinder, the electrometer registered a rapid accumulation of negative electric charge.
- Magnetic Deflection: When a magnet deflected the cathode ray stream away from the aperture, the electrometer registered zero charge accumulation.
Perrin concluded that cathode rays and negative electric charge are inseparable physical entities.
Nobel Prize in Physics (1926)¶
Perrin’s experiment provided the indispensable experimental foundation for J.J. Thomson’s 1897 charge-to-mass ratio ($e/m$) measurement. Perrin was awarded the 1926 Nobel Prize in Physics for his work on the atomic structure of matter and Brownian motion.
Key Takeaways¶
- Year: 1895
- Key Figure: Jean Baptiste Perrin (French Physicist)
- Core Discovery: Proved Cathode Rays carry negative electric charge using a Faraday cup electrometer.
- Nobel Laureate: Awarded the 1926 Nobel Prize in Physics.
- Quantum Relevance: Direct experimental proof of subatomic negative charge particles ($e^-$).