HV phenomena

[Andy Kay]

Whilst playing around with HV I've noted three different kinds of phenomena:

1. The basic spark.. small lightning strike and accompanying sharp crack
2. Beautiful violet arc that may be silent and may hiss
3. Cream-coloured arc that looks more like a flame and sounds more like a hum or buzz

Can anyone enlighten me as to the physics of these phenomena -- i.e. what is happening at the molecular level? I'd like to know what to expect under what conditions.

Thanks for reading.

[StaticBuildup]

I have noticed this too and it appears to be down to voltage, current and frequency.

Type 1 is usually a discharge of static electricity, or a low frequency spark which is essentially a series of very short discharges.
(Van de Graaff generator, Wimshurst machine, some induction coils)

Type 2 is a DC or high frequency AC arc. From what I have observed, the thin arc and violet colour are associated with low current.
(CRT flyback transformer, ioniser, plasma ball)

Type 3 is a low frequency AC arc with high current. The flame appearance is because the heat caused by the high current pulls it up into a flame shape. The hum or buzz is the 50Hz or 60Hz frequency of the mains.
(Neon sign transformer, microwave oven transformer)

[Andy Kay]

I have noticed this too and it appears to be down to voltage, current and frequency.

Type 1 is usually a discharge of static electricity, or a low frequency spark which is essentially a series of very short discharges.
(Van de Graaff generator, Wimshurst machine, some induction coils)

Type 2 is a DC or high frequency AC arc. From what I have observed, the thin arc and violet colour are associated with low current.
(CRT flyback transformer, ioniser, plasma ball)

Type 3 is a low frequency AC arc with high current. The flame appearance is because the heat caused by the high current pulls it up into a flame shape. The hum or buzz is the 50Hz or 60Hz frequency of the mains.
(Neon sign transformer, microwave oven transformer)

Thanks SB.
That would seem consistent with my experience... I get sparking from a ten-stage voltage multiplier but a violet plasma arc when current limited using ten 720k resistors in series, and I get the cream-coloured flame on a Jacob's Ladder driven by a flyback transformer (the driver draws around 5A). I was wondering what is happening to the air (oxygen and nitrogen molecules) to give these different phenomena.

[Uspring]

The different colors are due to the differing mechanisms which create the light. For high power arcs that is thermal radiation from the plasma. Typical temperatures are from around 4000K, which corresponds to a bright orange up to 10000K (white blueish) in an arc lamp. Often evaporating substances contribute to the color of the arc.

In low power discharges, e.g. corona or from power supplies with a low duty cycle or in low pressure, there is not enough power to heat up the gas significantly. The light comes from excited atoms or molecules, which were hit by high speed electrons causing the excitation. The color depends on the properties of the involved gas. It is often a line spectrum, which differs considerably from the smooth spectrum emitted from hot objects.

[Andy Kay]

The different colors are due to the differing mechanisms which create the light. For high power arcs that is thermal radiation from the plasma. Typical temperatures are from around 4000K, which corresponds to a bright orange up to 10000K (white blueish) in an arc lamp. Often evaporating substances contribute to the color of the arc.

In low power discharges, e.g. corona or from power supplies with a low duty cycle or in low pressure, there is not enough power to heat up the gas significantly. The light comes from excited atoms or molecules, which were hit by high speed electrons causing the excitation. The color depends on the properties of the involved gas. It is often a line spectrum, which differs considerably from the smooth spectrum emitted from hot objects.

That makes sense. Violet arcs are cooler than whitish arcs then, which would be consistent with the Jacob's Ladder drawing 5A from a 15V supply and the voltage multiplier drawing 0.25A from a 13V lead-acid battery. Thank you.