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Electron generator:

When a discharge tube containing air or any gas at ambient pressure is exposed to a high electric voltage of \(10,000\) volts or more, no electricity flows through the air. When a high voltage of \(10,000\) volts is applied to the electrodes of a discharge tube containing air or any gas at an external pressure of about \(0.001\) mm of mercury, a greenish glow is observed on the discharge tube walls behind the anode.
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Electron generator 
Note: Air is a good conductor of electricity.
  • This observation demonstrates some invisible rays coming from the cathode.
  • Hence, these rays are referred to as cathode rays.
  • They were later given the name electrons.
Electron emission
Electrons are subatomic particles that revolve around the nucleus of an atom. They usually are negatively charged and much smaller than the nucleus of an atom.
Structure of atom