Discharge Tube

Gaseous Discharge at Various Pressure

If an electric current flows through a gas, it is known as electrical conduction or discharge in the gas. At normal temperature and pressure, the gases are good insulators of electricity. We can make a gas conducting

1. By applying very high voltage
2. By reducing its pressure to a very low value.

Discharge Tube

A tube used to study the electrical conduction through gases at low pressure known as discharge tube. The figure shows the experimental set up to study the discharge phenomenon. It consists of a strong glass tube about 0.5 m long and 0.04 m diameters, closed at both ends and provided with two platinum electrodes A and C, called anode and cathode respectively. The two electrodes are connected to secondary terminates of a powerful induction coil. The discharge tube in connected with a vacuum pump to reduce the pressure inside the tube is gradually reduced, using a pump the following phenomenon are observed as explained below.

1. At a pressure of about 10 mm of Hg:
   

   

   When the pressure in the discharge tube is reduced to about 10 mm of Hg, the discharge is in the form of luminous streaks between the electrodes called blue streamers and produces cracking noise as shown in a figure.

2. At a pressure of about 5 mm of Hg:
   

   

   When the pressure in the discharge tube is reduced to about 5 mm of Hg, the blue streaks broaden out into a luminous column which bright and steady. The luminous column is called Geissler’s discharge. Colour of the discharge depends on the nature of the gas.

3. At a pressure of about 2 mm of Hg:
   

   

   When the pressure in the discharge tube is reduced to about 2 mm of Hg, a long luminous column appears from an anode to cathode called the positive column as shown in a figure. Colour of the discharge depends upon the nature of the gas e.g. it is red for and blue for hydrogen.

4. At a pressure of about 1 mm of Hg:
   

   

   When the pressure in the discharge tube is reduced to about 1 mm of Hg, the positive column detaches itself from the cathode and moves towards the anode. A blue luminous glow appears at the cathode called negative glow. A dark space appears between a positive column and negative glow called Faraday’s dark space as shown in the figure.

5. At a pressure of about 0.5 mm of Hg:
   

   

   When the pressure in the discharge tube is reduced to about 0.5 mm of Hg, the positive column gets shortened, the Faraday dark space extending to a greater length. The negative glow leaves the cathode and another glow appears on cathode called cathode glow. The negative glow moves towards the anode. The space between cathode glow and a negative glow appears dark and is called Crooke’s dark space as shown in the figure.

6. At a pressure of about 0.05 mm of Hg:
   

   

   In this range of pressure, the positive column shortens and breaks into alternative bright and dark disc called striations as shown in the figure.

7. At this pressure of about 0.01 mm of Hg:
   

   

   At this pressure first striations disappear, the negative and cathode glow vanishes and the whole tube is filled with Crooke’s dark space. At this stage, luminous rays are seen to come out of the cathode. These rays are called cathode rays. Cathode rays are the electrons travelling from the cathode to the anode. Now the walls of the tube are seen to glow with a blue or green light depending on the nature of glass. This is called the fluorescence. If the pressure in the tube is reduced further, to maintain discharge very high voltage is required, and finally the tube stops conducting.

Reference

Manu Kumar Khatry, Manoj Kumar Thapa,et al. Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.