A spark should be a continuous line, jagged but without breaks. Right? So I thought too until you look closely at single sparks when you can see gaps.

The photos above show sparks from my ignition coil setup with perhaps 10 sparks per second. This single brief impulse means that there is not a second spark going down the heated channel of the first. You can see gaps in the spark channel which is viewed here end on and magnified. Sometimes there is a hazy glow in the gap but often not.

Here is another example with the magnified view. The left electrode is negative in this DC spark taken through my 300 kV diode so this is a negative side effect.

The photo above shows the Crooke’s space in a neon tube at reduced pressure. So there is a precedence for gaps in the spark channel.
Sparks don’t have uniform brightness as well which may be a related effect. About one third of the negative end of the spark is brighter as well. Although this is a spark from a coil it is being driven by a DC pulse and can be regarded as polarized.

           

I therefore suspect that this is a feature of the spark structure, presumably analogous to the Crooke’s spaces seen in lower pressure discharges. Since you often see this with Tesla coils maybe the discharge is polarized there too.

               

Above shows the bright area in the spark seen in a Tesla coil discharge. Generally seen when sparks are low power and purple rather than hot white. Sort of suggests that a Tesla coil sparks that only just connect are from a primarily positive discharge from the toroid.

The spark above is viewed in two directions at the same time using a mirror at 90 degrees angle. 3 of the 5 sparks have gaps and they are present in both views. The negative end is brighter as well.

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Photo Date: 2006