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Electrical Discharge through Gases - Discharge Tube

Electrical Discharge through gases.

The flow of electric current through a gas.
At NTP, gases are insulators of electricity as molecules do not have free charge particles for migration at this condition.
At the high voltage and low pressure, gases act as conducting medium due to the formation of ions.

Discharge Tube.

The cylindrical tube of length 30 cm and diameter 4cm closed at both ends in which electric discharge is studied.
Consists of two platinum electrodes near the ends of the tube.
The vacuum pump and manometer are also connected.
The potential difference across the electrodes is about 50kV.

Gaseous Discharge at different pressure.

At 10mm of Hg.
  • Discharge starts.
  • Luminous streak in between the electrodes.
  • Violet irregular sparked called a blue streamer (single rope) arises.
  • Produces craking sound.

At 5mm of Hg.
  • Blue streaks broaden.
  • The discharge becomes bright and steady.
  • A luminous column called positive column appears with the buzzing sound.
  • Also called Geissler's discharge.
  • Color depends on gas, used.
    Red for air & blue for hydrogen.
At 2mm of Hg.
  • The positive column started from anode occupies the greater part of the tube.
  • The positive column detaches from the cathode and moves towards the anode.
  • Luminous glow at cathode called negative glow.
  • Space between the positive column and negative glow is called Faraday's dark space.
At 0.4mm of Hg.
  • Positive column shortens and breaks into alternative bright and dark disk-like structures called striations.
  • Faraday's dark space increases.
  • Negative glow leaves cathode and another glow called cathode glow appears.
  • Space between the cathode and -ve glow is Crookes dark space.


At 0.01mm of Hg.
  • Striations, faraday's dark space, the negative glow disappears.
  • A tube filled with Crooke's dark space.
  • The Wall of the tube will glow with certain color fluorescence.
  • Because certain rays are emitted the cathode strikes the wall.
  • These invisible rays are called cathode rays.
At 10-4mm of Hg.
  • No discharge passes through the tube as there are very few gas molecules for conduction.

Cathode Rays.

When a discharge tube is exhausted to 0.01mm of Hg, the whole tube is filled with Crooke's dark space and the wall to the tube starts to glow. It indicates that the thing which comes from the cathode strikes the wall and causes the wall to glow.
This something from the cathode as a stream of visible rays is called cathode rays.
These fast-moving invisible particles are called electrons.

Properties of Cathode Rays.

The properties of Cathode rays are listed below:

  • It is emitted from the cathode normal to the surface of the cathode.
  • The direction is independent of the position of anode due to the presence of gases.
  • It travels in a straight path and cast shadow.
  • It produces fluorescence when they fall on a certain substance.
  • It induces chemical change and affects photographic plates.
  • It produces X-rays when they strike the metal of high atomic number.
  • It produces heat when stopped by matter.
  • It exerts mechanical pressure and possess large potential and kinetic energy.
  • It carries -ve charge.
  • It is deflected by both electric and magnetic fields.
  • Its speed 107 m/s.
  • It Can ionize the gas through which they pass.
  • It can penetrate through a thin sheet of paper.