Discovery of Electrons and Protons
Cathode Ray Experiment – Discovery of Electron (J.J. Thomson, 1897)
J.J. Thomson conducted the Cathode Ray Tube Experiment to study the nature of electric discharge in gases.
Experiment Setup:
A glass tube was partially evacuated and sealed.
Electrodes (cathode and anode) were placed inside the tube.
A high voltage was applied across the electrodes.
A fluorescent screen was placed to detect the rays emitted from the cathode.
Observations:
A beam of rays emerged from the cathode (negative electrode) and moved toward the anode (positive electrode).
The rays were deflected by electric and magnetic fields, indicating they carried a negative charge.
When an obstacle was placed in the path, a shadow was cast on the opposite side, proving that the rays traveled in straight lines.
Conclusion:
The rays consisted of negatively charged particles, later named electrons.
The charge-to-mass ratio (e/m ratio) of electrons was calculated.
Diagram: Cathode Ray Tube Experiment by J.J. Thomson
(A labeled diagram of the cathode ray experiment showing the cathode, anode, and deflection of electrons.)
Goldstein’s Experiment – Discovery of Proton (1886)
E. Goldstein performed the Anode Ray Experiment (also called Canal Ray Experiment) to investigate the presence of positively charged particles in an atom.
Experiment Setup:
A discharge tube with a perforated cathode was used.
A low-pressure gas was filled inside the tube.
A high voltage was applied across the electrodes.
Observations:
A new type of rays, called anode rays or canal rays, was observed traveling in the opposite direction of cathode rays.
These rays were positively charged and moved toward the cathode.
The mass of these particles was much greater than electrons and depended on the gas inside the tube.
Conclusion:
The positive particles were named protons.
The charge of a proton was equal and opposite to that of an electron.
Diagram: Anode Ray Experiment by Goldstein
(A labeled diagram of the anode ray experiment showing the perforated cathode and movement of canal rays.)