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Discharge Tubes

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To observe the effect that different gas pressures have on an electric discharge passed through a discharge tube.


The high voltage produced by the induction coil is applied across the terminals inside the discharge tubes. One plate (the cathode) becomes highly negative and releases a ray (cathode ray or electron). The electron passes through the gas in the tube and excites electrons in the atoms of the gas contained in the tube. The pressure of the gas determines the density of the atoms and therefore the nature of the collisions which take place between the electrons and atoms. Therefore, different discharge effects under different pressures can be observed.


  • Power pack
  • Two plug–plug leads
  • One set of discharge tubes(with varying pressures)
  • Induction coil
  • Two plug-clip leads


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Observe the pattern that is produced in the tubes and describe it carefully.Repeat the above procedure using each of the discharge tubes and see if you can observe streamers, Faraday’s dark space, cathode glow, Crooke’s dark space, striations and the positive column. Carefully describe each pattern identifying each of the effects mentioned.


  • The first visible effect in the tube as the pressure in the glass tube decreased was a steady luminous discharge known as a ‘glow discharge’ that appeared at the negative electrode (the cathode).
  • As the pressure was lowered further, most of the tube became occupied by a bright luminous region called a positive column. The positive column appeared to originate at the electrode connected to the positive terminal of the DC source (the anode).
  • With further decrease in pressure, the positive column was observed to break up into a series of bands or striations. At this time near the anode, a weaker glow was seen. The striations were separated by ‘dark spaces’.
  • The colours of the discharge are found to depend on the gas inside the tube. In low-pressure air which retains its approximate composition of 80% nitrogen and 20% oxygen, the positive column is a brilliant pink and the negative glow is a deep blue colour.
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Pressure in the tube (mm Hg)



Violet streamers of electrical discharge, like small blots of lightning appear in the tube.


Glow of light appears around the cathode. A pink stream of light called a positive column extends from the anode towards the cathode.


Positive column breaks into bands called striations. These appear to be like standing waves.


We may conclude that the pressure of the discharge tube will affect how the cathode rays (electrons) travel. The different observed striation patterns is due to the different pressures of the discharge tubes. We used a transformer in order to get the high voltages that are needed for this experiment. As shown in the low pressure tubes it is seen that cathode rays (electrons) can travel through a vacuum, this fact lead to the further exploration of cathode rays.

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