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Question

Explain how cathode rays are formed from the gas taken in the discharge tube.


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Solution

In 1859, Julius Plücker started the study of the conduction of electricity through gases at low pressure in a discharge tube.

When a high voltage of the order of 10,000 volts or more was impressed across the electrodes, some sort of invisible rays moved from the negative electrode to the positive electrode. Since the negative electrode is referred to as cathode, these rays were called cathode rays.

Cathode rays are deflected by the electric and magnetic fields.

When the cathode rays are passed between two electrically charged plates, these are deflected towards the positively charged plate.
They discharge a positively charged gold leaf electroscope. It shows that cathode rays carry a negative charge.

The nature of the cathode rays is independent of:

  • The nature of the cathode.
  • The gas in the discharge tube.

Thomson proved that whatever the gas is taken in the discharge tube and whatever the material of the electrodes, the value of e/m is always
the same.


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