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At very low pressure gases become conducting in nature,as evident in cathode rat experiment.but upon still decresing the pressure the electric discharge stops.why does this happen?

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In order to have a discharge, you need to ionise matter (gas) so that the motion of the ionized matter (electrons and cations) carries the discharge current. Usually, the accelerated electrons collide with neutral atoms/molecules to produce even more ionised matter. If the gas density becomes too low between cathode and anode, there are not enough atoms/molecules left to be ionized with certainty, and the process cannot continue. Usually this happens when the mean free path becomes of the order of magnitude of the distance between the electrodes (give or take a few orders of magnitude).

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  • $\begingroup$ So that means the discharge will disappear automatically after a certain time, when all some have been ionized ,right? $\endgroup$ – user150854 Dec 19 '17 at 5:26
  • $\begingroup$ @user150854: no of course not. At the electrodes, the atoms are neutralized again/electrons are absorbed to transmit the current from the gas to the electrodes. The process can go on as long as there is a source of potential difference. $\endgroup$ – entrop-x Dec 19 '17 at 12:58

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