Gaseous discharge at various pressures:
If electric current flows through a gas, then this phenomena is known as electrical conduction or discharge in gas. At normal pressure gases are insulator, but if we apply low voltage and low pressure, gases split into ions and can conduct electricity.
A tube used to study the electric condition of gases at low pressure is known as discharge tube. It is also called cathode ray tube because cathode ray is produced here from the cathode side. It consists of a strong glass tube with electrodes and vacuum pump. The pressure inside the tube is reduced and the conduction of gas is studied at various pressures.
The discharge of electricity at various pressure is given below:
1. At 10 mm Hg:
When the pressure in the discharge tube is reduced to 10 mm Hg, the discharge of luminous streaks is formed between the electrodes called blue streamers.
2. At 5 mm Hg:
When the pressure in the discharge tube is reduced to 5 mm Hg, the blue streaks broaden into luminous column which is bright and steady. The luminous column is called Gresler’s discharge.
3. At 2 mm Hg:
When the pressure in the discharge tube is reduced to 2 mm Hg, a long luminous column appears from anode to cathode called positive column. The colour of the discharge depends on the nature of gas. It is red for air and blue for hydrogen.
4. At 1 mm Hg:
When the pressure in the discharge tube is reduced to 1 mm Hg, the positive column detaches itself from the cathode and moves towards anode. A blue luminous glow appears at the cathode called negative glow. A dark space appears between positive and negative glow called Faraday’s dark space.
5. At 0.5 mm Hg:
At 0.5 mm Hg pressure the positive column gets shortened and Faraday’s dark space is extended to greater length. The negative glow leaves cathode and a new glow appears at cathode called cathode glow and dark space between cathode glow and negative glow is called Crook’s dark space.
6. At 0.05 mm Hg:
At this pressure the positive column shortens and breaks into alternative dark and bright discs called striations.