In 1892, German physicist Heinrich Rudolf Hertz (1857–1894) published a major experimental paper in Annalen der Physik (Vol. 45) titled “Ueber die Durchgang der Kathodenstrahlen durch dünne Metallschichten”.

Hertz demonstrated that cathode rays pass directly through thin aluminum, gold, and silver foils inside vacuum discharge tubes without rupturing the metal.

Experimental Discovery & Foil Penetration

Hertz mounted a thin leaf of aluminum foil inside a discharge tube between the cathode and a fluorescent glass screen:

  1. Observation: When cathode rays struck the aluminum foil, they passed through it, causing the glass screen behind the foil to glow brightly with fluorescence!
  2. Hertz’s Conclusion: Hertz incorrectly inferred that cathode rays were ethereal wave vibrations rather than particles, reasoning that solid metal atoms would block material particles.

Enabling Philipp Lenard & J.J. Thomson

Hertz instructed his assistant Philipp Lenard to construct a vacuum tube with a thin aluminum window (Lenard Window, 1894), allowing cathode rays to emerge into open air.

In 1897, J.J. Thomson realized that cathode rays penetrate metal foils because cathode rays consist of Electrons—subatomic particles $1,836$ times smaller than a hydrogen atom!

Key Takeaways

  • Year: 1892
  • Key Figure: Heinrich Hertz (German Physicist)
  • Core Discovery: Demonstrated that Cathode Rays penetrate thin metal foils (aluminum, gold).
  • Experimental Legacy: Led directly to the Lenard Window (1894) and Thomson’s electron discovery (1897).
  • Quantum Relevance: First experimental evidence of subatomic particle scattering through atomic crystal lattices.