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Glow tubes function on a similar principle as spark gaps. In both cases, conduction is caused by ionized gas. In the spark gap, the ionized gas is air. In glow tubes, the ionized gas is a noble gas, such as neon, at low pressure. Ignition is caused in both types of devices by supplying enough voltage across the electrodes. As the current increases, the ionization increases, causing the voltage across the electrodes to decrease, resulting in the AC resistance across the electrodes being negative. The negative resistance near the ignitin voltage is very high, and becomes less negative with increased current. As current increases, the negative resistance approaches zero. There is a delay between the current increase and the ionization increase, and this causes the maximum frequency of an oscillator using the negative resistance of a spark gap or a glow tube to be limited.
Because of the similarity between the spark gap and the glow tube, it should be possible to use a glow tube as an LC oscillator that produces sinusoidal waves. Yet, all glow tube oscillators are RC relaxation oscillators that produce triangular waves, and not LC oscillators producing sinusoidal waves.
I tried to find an E-I curve for an NE-2 bulb, but I had no luck. So, I measured the curve myself. I connected an NE-2 to a 120 kohm current-limiting resistor, and connected the bulb and the resistor, joined in series, to a variable, high voltage, power supply. The ignition voltage is 91 volts, and the voltage reduced to 66 volts when the current was 180 uA. Since the rated current is 500 uA, I did not use more than 630 uA in my test. Between 180 uA and 630 uA, the voltage across the bulb remained at 66 volts, making the AC resistance of the bulb practically zero in this region. Between just greater than zero, and 180 uA, the AC resistance is negative.
I did not have enough resolution in the adjustment of my high-voltage power supply to adjust for a stable current less than 180 uA. Perhaps some sophisticated, fast responding, current source can keep the bulb in the negative-resistance region. This would allow making a glow tube oscillator with an LC circuit.