Chapter 1

Discovery of Electron

Atomic Structure · Cathode Ray Experiments

The electron was discovered by J.J. Thomson at the end of 19th century during the studies of the passage of electricity through gases at extremely low pressures. These experiments were known as discharge tube experiments.

19th Century

J.J. Thomson

Discharge Tube Experiments

Discovery of Electron

A cathode ray tube is made of glass containing two thin pieces of metal, called electrodes, sealed in it. The electrical discharge through the gases could be observed only at very low pressures and at very high voltages.

When sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These were called cathode rays or cathode ray particles.

Fig. 1. Discharge tube experiment — (a) Discharge tube containing a gas. (b) Emission of cathode rays at high voltage and low pressure.

Discovery of Electron

The flow of current from cathode to anode was further checked by making a hole in the anode and coating the tube behind anode with phosphorescent material zinc sulphide. When these rays, after passing through anode, strike the zinc sulphide coating, a bright spot is developed on the coating.

Cathode Ray Tube with Fluorescent Screen

Fig. 2. Cathode rays pass through the anode and strike the zinc sulphide coating, producing a bright spot on the fluorescent screen.

Properties of Cathode Rays

Property (i)

They produce a sharp shadow of the solid object placed in the path. This shows that cathode rays travel in straight lines.

Fig. 3. Cathode rays produce a sharp shadow of a solid object placed in their path, confirming they travel in straight lines.

Properties of Cathode Rays

Property (ii)

If a light paddle wheel (e.g., that of mica) is placed in their path, the wheel begins to rotate, showing that cathode rays are made up of material particles.

Fig. 4. A light paddle wheel placed in the path of cathode rays begins to rotate, proving cathode rays consist of material particles with momentum.

Discovery of Electron

Watch the cathode ray tube experiment demonstration:

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Cathode Ray Tube Experiment
Visual demonstration of discharge tube experiments and cathode ray properties.