Question

working principle of discharge tube

Answer 1

Hi guys ,
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(this answer is dedicated to my sis NAVATIKA.Missing you di very much)
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Here is your answer,
There is a long glass tube connected with two terminals, as known as cathode and anode. The pressure inside the tube can be reduced by pumping. At the beginning the discharge tube is full of gas and a high voltage of current was passes through it. No noticeable change happens. but since the pressure is decreased and the voltage is increased noticeable changes occurs in it.
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Answer 2

To give an idea of how to interpret the graph, note that atmospheric pressure is 760 mm Hg (760 Torr). Therefore a vertical line corresponding to 76 on the pressure × distance axis gives the breakdown voltage for a gas between a pair of flat electrodes separated by 1 mm. For air, this gives a breakdown voltage of about 5 kV DC (3.5 kV RMS AC). Thus the air curve can (for example) be used for working-out the voltage ratings of air variable capacitors; although a safety margin is normally included to allow for surface roughness and sharp edges, which can reduce the breakdown voltage considerably.
     Electrodes, incidentally, are not essential for gas breakdown. Any method that produces a strong-enough electric field, with or without electrodes, will also cause the gas (or regions therein) to glow; which is why discharge tubes light up when placed near resonant radio antennas and coils.
     Once a glow discharge between electrodes has initiated; a low resistance current-path is formed, and passing a current sustains the discharge. Thus the tube resistance remains low, and the glow will continue, until the current falls below a value called the 'extinction point'. The voltage at the extinction point is usually considerably lower than the strike voltage, i.e., the discharge is difficult to get going, and then difficult to stop once started.
    A gas discharge tube, once it has struck, usually has a negative resistance characteristic. This means that the product of the voltage across the tube and the current through it diminishes as the current is increased. Thus the current is effectively unlimited, and must be controlled by the external circuitry if it is not to lead to the destruction of the tube or the power supply. Some tubes are intended to be operated in a range where the voltage remains almost constant as the current increases (the glow-discharge region). One such application is as a voltage regulator, the current being limited by a series resistor in much the same way as for a Zener diode. Tubes can also be designed to work in a range where the voltage falls as the current increases (the arc discharge region), and while this is often an incidental feature, such tubes can be used to make oscillators that work in the same way as the tunnel diode (Esaki diode) oscillator. Note that, for tubes in which the running voltage increases as the current is reduced, the extinction voltage is higher than the lowest running voltage.
    Since the tube running voltage is considerably lower than the strike voltage, tubes with an applied voltage above the running voltage but below the strike voltage can be triggered. This can occur in various ways, such as by illumination of the cathode by ionising radiation, passage of radioactive particles through the gas, or by the use of a triggering electrode.