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7.
There appears a glow on the cathode, called negative glow. Between the positive column and the negative glow, the gap appears comparatively dark. At a pressure of 1 mm of mercury. As the pressure is further decreased to 1 mm of mercury, the negative glow leaves the cathode and another glow called cathode glow appears on the cathode. The space between the cathode glow and the negative glow appears dark. At a pressure of 0.01 mm of mercury. As the pressure is reduced further dark space increases in length. At a pressure of 0.01 mm of mercury, the whole tube is then filled with dark space. At this stage, a stream of invisible particles is emitted from the cathode. Such rays are called cathode rays. The portion of the glass tube opposite to the cathode starts glowing as the rays fall on it. This glowing of the tube is due to fluorescence produced by the cathode rays. At a pressure of 104 mm of mercury. As the pressure is reduced below 0.01 mm of mercury it becomes difficult to maintain the discharge and at a pressure 10-4 mm of mercury, discharge stops to pass through the gas. The discharge tube phenomenon has been utilised in making fluorescent tubes, neon signs, flood light mercury lamps, sodium lamps etc. Cathode Rays When a potential difference of 10 to 15 kV is applied across the two electrodes of a discharge tube and pressure is reduced to 0.01 mm of mercury, the rays known as cathode rays are emitted from the cathode. These rays are independent of the nature of the gas in the discharge tube and their direction of propagation is not affected by the position of the anode. Cathode rays ionise the gas through which they pass. Cathode rays can excite fluorescence. When they fall on certain substances, the substance start glowing. The colour of fluorescence depends upon the nature of the substance.
सम्पर्क सूत्र : जैन विश्वभारती संस्थान (मा.वि.) MIST - 341 306 (7767YTA)
CAT 451 3701 - fratre, 2003 -
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