Re: Sparks - Bright in the middle, how to verify it.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mike Harrison by way of Terry Fritz <twftesla-at-uswest-dot-net>" <mike-at-whitewing.co.uk>

On Sat, 20 Jan 2001 11:33:06 -0700, you wrote:

>Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<jimlux-at-earthlink-dot-net>
>
>The same phenomenon, in a slightly different form, is also evident on Van
>deGraaf generators.  Sparks tend to grow from one polarity better than the
>other.  When you run your coil on AC, the polarity of the top is reversed
>every half cycle.  On one half cycle, the spark is growing from the ground
>rod towards the terminal, on the other half cycle, it is the opposite.  If
>your spark length is 2/3 the length of the gap between the terminals, in the
>middle third, you'll see twice the brightness.
>
>I'd love for someone who has a very fast video camera (one of those 1000
>frames/second jobs) to take a look at TC sparks.  When I worked in the movie
>business, we used to do shoots where for super slow-mo they used to run
>cameras at 300 fps or faster (thousands of frames/second), but I couldn't
>ever work a deal where they'd film my little coil as an experiment.  There
>was always a schedule conflict, or I didn't have the cash for the film and
>processing. At $1/foot, 300 frames (around 25 feet) costs about $25, so
>burning a minute of film is a pretty pricey experiment.  Production
>companies may be profligate with star salaries and cushy seats for the
>advertising agency guys, but they're pretty stingy with the below the line
>costs like film, processing, and staff.
>
>However, there are a lot of the new Kodak and NAC motion analysis systems
>with very fast CCD cameras hooked to a computer around now and the rental
>prices are coming down.  Maybe next time there is a "teslathon" in the area,
>I will rent one.
>
>You might also be able to improvise something using the fast shutter mode on
>consumer video gear.  It has a shutter speed of 1/10,000 second (or so), but
>the frame rate is going to be 60 Hz.  You might get useful data if you could
>slowly vary the phasing of the power driving the coil relative to that in
>the camera (actually, the camera probably isn't that stable, so it would
>drift by itself). You're not going to get the ideal, a series of frames on
>the same spark, but, you might get a feel for spark growth in general, as a
>function of time past "bang".

I thought of doing this a while ago as I have a camera with an
external sync input, which could be locked from a mains derived source
with an adjustable delay. The problem, however, with both film and
video methods is sensitivity, or lack of it. When you take a video
camera down to 1/10,000 sec, you need an awful lot of light, and would
need to look at pretty big sparks to get a useable picture.  This is
not helped by the fact that as sparks are a 3-d phenomenon, you need a
reasonable depth-of-field, and so you may also need to reduce the
aperture to get a sharp picture.

One possibility I did think of, is that webcam type devices are
getting very cheap nowadays, so it might be feasible to use several of
these to take a small sequence of pics in rapid succession with
appropriate syncing. 3-d spark pics would also be possible with such
an array. A bunch  of disposable film cameras could also be a
possibility.....

I've also frequently noticed the 'bright centre' phenomenon on several
coils. I also saw the opposite effect, a dark centre, when playing
with a piezo gas lighter recently. 
 
>A DC coil running off a well filtered supply might also be easier to
>synchronize to
>----- Original Message -----
>From: "Tesla list" <tesla-at-pupman-dot-com>
>To: <tesla-at-pupman-dot-com>
>Sent: Saturday, January 20, 2001 8:36 AM
>Subject: Re: Sparks
>
>
>> Original poster: "Charles Hobson by way of Terry Fritz
><twftesla-at-uswest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>
>>
>> Hello all,
>>
>> Placing a grounded rod about 300mm from the terminal of my coil I get nice
>> sparks similar in appearance to lightning. At the center of these sparks
>> there is a 25mm segment which is much brighter than the rest. I would be
>> most grateful for an explanation for this phenomena.
>>
>> Chuck
>>
>>
>>
>>
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