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End-on spark view - broken sparks

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LAN_403

Tesladownunder

Sat Sept 02 2006, 05:51AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Here are some interesting shots of sparks taken end on. I used my MO-SIDAC-ignition coil setup which gives 4 inch sparks.
With 2 fine electrodes and aligning the camera down the axis you get some unusual effects.

Peter

Part Scavenger

Sun Sept 03 2006, 01:01AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Cool. Reminds me of medusa.

Tesladownunder

Sun Sept 03 2006, 03:04AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I was wondering about the discontinuity of the sparks with apparent gaps. Others have observed this and commented on it in Tesla shots.
I offer this explanation. An area of hot gas will act as a lens (concave type) due to the central rarefied area being of lower refractive index. This will act in mirage like fashion to diffuse the spark area beind it so that you get to see only a fraction of what you might otherwise.
This effect will be seen more on longitudinal shots which is exactly what you are seeing here and much more than in the standard right angle view.
The second pic shows the faint connecting segment. So this is the real black plasma

Any comments? I haven't seen this lensing effect given before.

Peter

1157252650 10 FT15549 Hvmosparksendonsegment2

Bjørn

Sun Sept 03 2006, 12:15PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Take two pictures at the same time from different angles.

Electroholic

Sun Sept 03 2006, 08:21PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

great idea but how?
oh heres and idea, how about a mirror?
that would give you another angle,
not exactly at the same time, but should be close enough.

Steve Conner

Sun Sept 03 2006, 09:18PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think it's because of the timing of the spark interacting with the timing of the camera shutter. This can lead to some weird results in cameras with focal plane shutters (a travelling slit that doesn't expose the whole frame simultaneously) I've posted about this effect on the TCML quite a few times.

Of course, if it was a time exposure, please ignore me :P

Bjørn

Sun Sept 03 2006, 09:37PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I was thinking of a mirror.

These pictures seems to be taken with a 2 second exposure time. The shutter may still generate some artifacts if something happens at just the right time.

HV Enthusiast

Sun Sept 03 2006, 11:06PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Definitely not the shutter. If the camera captures the arc from beginning to end, and the gap is in the middle, than it definitely isn't the shutter.

What is happening is as TDU stated. When an arc channel forms, it superheats the air leaving a nice heated air channel which rises and thus blocks light from new arcs causing them to appear "broken" and "segmented." With larger Tesla coils (bigger arcs), the effect also occurs, but since the arc length is very large and not confined to such a small area, you don't see it as readily.

With some single-shot discharge photos i've taken, i've actually captured the shadow of the channel (after the arc occurred), on the wall behind it with the flash.

Tesladownunder

Mon Sept 04 2006, 12:06AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

On further reflection, I don't think lensing is the effect since it occurs at only one point along the spark.
I now suspect that it is just a Crooke's space effect in a predominantly DC spark.

Note the brightness variation and where the dropout occurs.

Now also note the same effect with a TC. which to me implies some constant effect of polarisation of the spark. ie only positive or negative strokes make it. That is not surprising as a spark from smooth surface to point is affected by polarity by a factor of two if I recall. I did some measurements on this a couple of years ago.

Peter

1157328404 10 FT15549 Teslaphysicsrun5secs

HV Enthusiast

Mon Sept 04 2006, 01:12AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Interesting effect, although I am still not convinced its an actual effect occuring within the arc. Do you have other cameras you can use to attempt to capture this?

I've taken literally tens of thousands of high voltage images ranging from low voltage DC discharges, to spark gap coils, DRSSTCs, high power Marx Generators, to lightning, and never have seen this effect even once.

Seems weird you have caught it numerous times, and in the last three photos all to the right side of the image.

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