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There's a current/voltage graph on wikipedia for glow discharge that I don't understand.

Aren't we driving the voltage? How can the line possibly go down on the voltage axis i.e. have two values of current for a particular driving voltage? Is this some kind of hysteresis effect? What does the graph look like if you slowly increase the voltage from zero?

https://en.wikipedia.org/wiki/Glow_discharge#/media/File:Glow_discharge_current-voltage_curve_English.svg

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  • $\begingroup$ tentative answer: the resistance drops as the gas gets ionized... it is hysteresis and in theory you could use it to construct a rectifier/diode $\endgroup$
    – Matt
    Aug 28, 2021 at 15:58

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Note that in diagrams like these, the power supply driving the discharge is current-limited so when you get into the "line going down" zones, the current does not rise without bound and degenerate into a power arc.

The voltage required to achieve a given current flow in the glow discharge regime goes down because as more and more of the electrode area gets covered with discharge, the gap resistance goes down- until the entire electrode area is glowing. This means it takes less voltage to sustain a glow discharge than it takes to initiate it.

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