In a had discharge tune, gas (e.g. nitrogen, helium, neon, argon) is introduced to the vacuum. The free electrons in the tune collide with the had molecules, and discharge their electrons leaving a positive ion. This process is called ionization. The force required by the high speed electrons to cause ionization of the gas molecules will be obtained from the anode cathode voltage. There is a definite voltage voltage value existing for each type of had to be ionized. The gas atoms ionize and emit coloured light as they conduct the current along the tube. The light originates from the change in the energy levels of the ionized atoms. By coating the inside of the tube with a phosphor, invisible emitted radiation can produce visible light. This is the principle of the fluorescent lamp.
This tube , like the diode, also allows current flow in one direction. The gas discharge tune may also be used in converting an AC to a DC. Its construction is very similar to that of other high vacuum valves, except that the cathodes, grids, and anodes in it are usually larger, so that it can carry heavier currents. It is usually referred to as a Thyratron, since an excessive increase in Temperature affects the performance of all valves, but the gas in this tube helps to decrease the temperature thereby improving ionization. The small dot within the circle is generally used to indicate that the valve is a gas discharge tube.
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