The first indication of the complex nature of atom was obtained from a study of conduction of electricity through gases at reduced pressure by Julius Plucker (1859). He took a cylindrical glass tube, called discharge tube which was 30 cm long and 4 cm in diameter, closed at both ends, provided with metallic electrodes and a side tap meant for connecting the tube with a high vacuum pump and low pressure gauge. To obtain very low pressures, molecular pump and charcoal cooled by liquid air that has the property of absorbing most of the residual gas were used. The two electrodes were connected to a high voltage battery (voltage was of the order of 10,000 volts and more). The electrode connected to the negative terminal of the battery is known as cathode (-) while that connected to the positive terminal is called anode (+) or anti-cathode. Plucker pumped out most of the gas of the tube by means of high vacuum pump and low pressure gauge and thus reduced the pressure of the gas in the tube to about 0.001 mm Hg. Now he observed that when he applied a high voltage of the order of 10,000 volts across the metallic electrodes. the walls of the tube began to glow with a bluish or greenish light depending on the composition of the glass og which the tube is made. Ty thus appeared that the invisible rays of light were emitted from the cathode and moved towards anode at right angles and with large velocity. These rays were called cathode rays by Goldstein (1876), since these originated from the cathode.
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