Introduction

The discovery of the electron marks one of the most significant breakthroughs in modern physics. In the late 19th century, scientists conducted various experiments using cathode ray tubes, which ultimately led to our understanding of the first subatomic particle - the electron.

What Are Cathode Rays?

Cathode rays are produced in partially evacuated discharge tubes called Crookes tubes. These tubes consist of two electrodes:

When a high potential difference is applied between these electrodes in a near-vacuum environment, electrons flow from the cathode to the anode. These moving electrons, then unknown to scientists, produced a visible glow when they struck fluorescent material coating the tube's glass surface.

The Great Debate: Waves or Particles?

Scientists initially disagreed about whether cathode rays were waves or particles. Here are the key properties they observed:

Particle Properties:

Wave Properties:

Key Experiments

1. The Maltese Cross Experiment

Setup: Scientists placed a metallic cross (Maltese cross) in the path of cathode rays.

Observation: A sharp shadow of the cross appeared on the fluorescent screen.

Significance: While this demonstrated that cathode rays travel in straight lines, it didn't conclusively prove whether they were waves or particles, as both could produce shadows.

2. Electric and Magnetic Field Deflection

Setup: Cathode rays were passed through uniform electric and magnetic fields in separate experiments.

Results:

Mathematical Description: The force on a charged particle in an electric field is given by:

$\vec{F} = q\vec{E}$

where $q$ is the charge and $\vec{E}$ is the electric field.

3. The Paddle Wheel Experiment

Setup: A freely rotating glass paddle wheel was placed in the cathode ray path.

Observation: The wheel rotated when struck by the rays and moved toward the cathode.

Significance: This demonstrated that cathode rays possess:

Resolution: J.J. Thomson's Contribution

J.J. Thomson's measurement of the charge-to-mass ratio ($\frac{e}{m}$) of cathode rays definitively proved their particle nature. This groundbreaking work led to:

Historical Significance

These experiments revolutionized our understanding of atomic structure and laid the foundation for: