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Publish On: 2019-05-21

Steeb more

Total Post: 416

Question: Explain the concept of discharge of electricity through gases at low pressure

Reply On: 2014-01-07

lala singh

Total Post: 22

ANS: Explain the concept of discharge of electricity through gases at low pressure

An interesting experiment on phenomena of electric discharges through gases at reduced pressures may be performed by the use of the apparatus. A glass tube about 3 ft long is connected to a vacuum pump. Cylindrical aluminum electrodes sealed into the ends of the tube are attached to the terminals of a source of high voltage, such as induction coil. When the gas in the tube is at atmospheric pressure, the sparks will pass across the short air gap between the terminals of the induction coil. As the gas pressure in the tube is continuously reduced, the discharge beings to pass through the long tube, in preference to the shorter path between the terminals of the coil in air atmospheric pressure. The gas in the tube emits light of a color characteristic of the particular gas use. When air is used, the first discharge to appear consists of long, spark like streamers emitting bluish-violet light. As the pressure is reduced further, a glow appears on the cathode, and a pinkish glow appears throughout most of the tube.

Each of the electrodes is covered by a velvety glow known, respectively, as the cathode and anode glow. A comparatively dark space near the cathode is called the Crookes dark space. Near it is a short region of light known as the negative glow; this being followed by another darker portion designated the Faraday dark space. The major portion of the tube is filled with a striated series of bright and dark regions called the positive column.

If the pressure of the tube is lowered below the optimum value for the type of discharge just described, it will be observed that the Crookes dark space becomes larger, finally filling the whole tube. At this stage most of the ions traverse the whole length of the tube without colliding with a molecule of the gas. Consequently, there are few ions produced, and the discharged current decreases. The voltage required to maintain the discharge rises rapidly until finally, at very high vacuum, the discharged be comes nearly impossible.

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