Theory:

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:
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.
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Structure of atom