Electric Discharge Through Gases:
The figure shows the arrangement used to study electric discharge through gasses at varying pressures. A glass tube of about 30 cm length and 4 cm in diameter is fitted with 2 metallic electrodes C and A which are connected to the secondary of an induction coil. A side tube T connects the discharge tube to the vacuum pump, which draws out gas to produce the desired low pressure. The pressure inside the tube is read by the manometer attached to the side. With the help of an induction coil, a high potential difference is applied across the tube.
At atmospheric pressure in the tube, there is no visible effect. The medium in the tube is then non-conducting. As the pressure in the tube is reduced, at a certain stage depending on the potential difference, sparking occurs in the form of irregular streaks of light. A crackling sound is also heard. When the pressure is further lowered, the following phenomena are seen.
(1) When pressure is about 10 mm of the mercury column, the irregular streaks of light broaden out in a luminous column of light extending from the anode to almost upto the cathode. A continuous buzzing sound is also heard. It is called a “positive column” and its color depends on the nature of the gas enclosed in the tube. It is reddish for air, bright red for neon, bluish for CO2, etc. Such glow tubes are used for advertising as neon-sign, etc.
(2) When the pressure becomes about 3-4 mm of the mercury column, the positive column starting from the anode reduces in length but the cathode is surrounded by a bluish glow. The positive column and cathode glow are separated by a dark region called Faraday dark space.
(3) At a pressure of about 1 mm of the mercury column, the Faraday dark space increases in length, and the positive column is further reduced. The cathode glow gets detached from the cathode and a new dark region called Crookes dark space appears between cathode and cathode glow.
(4) On further reducing the pressure to about 0.1 mm of the mercury column, the cathode glow and Crookes dark space increases in length while the positive column is splitted into bright and dark bands called striations.
(5) When pressure is further reduced to about 10-3 cm of mercury column, the Crookes dark space fills the entire tube. A new phenomenon starts at this stage. The walls of the tube begin to glow, the color of which depends on the nature of the glass. This glow is due to fluorescence. If the pressure is further reduced, the current through the tube decreases gradually and finally, the tube stops conducting.
The above phenomena observed in the discharge tube are based on the ionization of gas contained in the tube. Due to ionization produced by cosmic rays, some ions are always present in the tube. When the high potential difference is applied across the tube, these ions are accelerated and more ions are produced due to collisions of ions with neutral atoms of gas. Some of the items excited in the collision process emit light with colors characteristic of their structure. When pressure is further reduced, the positive ions moving towards the cathode with increasing energy strike the cathode to emit electrons from it. These electrons travel some distance (Crookes dark space) before the energy gained by them is sufficient to excite more molecules to produce light (cathode glow). The Crookes dark space corresponds to the mean free path of electrons at this pressure. The successive process of ionizing, losing ironizing power, accelerating for some distance, and again ionizing is repeated till the electrons reach the anode. Thus, we have alternate dark and bright bands or striations.
When the pressure is further lowered, the mean free path increases and becomes larger than the length of the tube, the Crookes dark space fills the entire tube, and no cathode glow or positive column is observed. The electrons coming out of the cathode, called cathode rays now strike the wall of the tube to produce fluorescence.