________________
. 61. Satish K. Gupta, op. cit. pp. 1059, 1060, 1062—
“Discharge Through Gases "The phenomenon of discharge gases at low pressure is of great importance, as it provides a great deal of information about the structure of atom. At atmospheric pressure, dry and dust free air is found to be a bad conductor of electricity. It is because, under such condition, air does not have free positive and negative ions. The ionisation of air may be caused by heat radiation, ultraviolet light, X-rays, radio-active rays (alfa, beta, and gamma-rays), cosmic rays, etc. However if a potential difference of about 30 kV is applied across two electrodes held at a distance of 1 cm in dry air at S.T.P., then a spark is found to occur between the electrodes. The value of potential difference applied across the electrodes depends upon the shape and size of electrodes, distance between the electrodes, state of dryness and pressure of the air (or the gas enclosed). Discharge tube. A discharge tube consists of a glass tube about 40 cm long and 3-4 cm in diameter. The tube is provided with two aluminium disc-electrodes. It contains the gas through which electric discharge is to be studied. The pressure of the enclosed gas can be reduced with the help of a vacuum pump by connecting it to a side tube. A high potential difference of the order of 10 to 15 kV is applied across the electrodes with the help of an induction coil. As the pressure is gradually decreased, the following phenomena are observed, as explained below : 1. At a pressure above 10 mm of mercury. When pressure of
the gas is above 10 mm of mercury, no discharge passes through the gas. If a milliammeter is connected in the circuit, it will not record any current. At a pressure of 10 mm of mercury. When pressure of the gas is reduced to 10 mm of mercury, the discharge passes through the gas between the electrodes with a cracking sound. At a pressure of 5 mm of mercury. As the pressure is reduced to 5 mm of mercury, the discharge broadens out and becomes bright. It extends from cathode to anode. At a pressure of 2 mm of mercury. On decreasing the pressure of the gas to 2 mm of mercury, long luminous column from the anode up to almost the cathode appears. It is called positive
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