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Re: corona (fwd)
Thomas McGahee wrote:
>
> ----------
> > From: Tesla List <tesla-at-pupman-dot-com>
> > To: tesla-at-pupman-dot-com
> > Subject: corona (fwd)
> > Date: Monday, March 03, 1997 11:32 PM
> >
> >
> >
> > ---------- Forwarded message ----------
> > Date: Mon, 03 Mar 1997 15:01:47 -0700
> > From: Dave Burman <dburman-at-ix-dot-netcom-dot-com>
> > To: Tesla List <tesla-at-poodle.pupman-dot-com>
> > Subject: corona
> >
> > I recently took some photos of my coil in operation. When I got the
> > photos back I noticed that all the pictures show a blue ring of corona
> > on the ground ring around the primary and comming down from the top of
> > the secondary. I have not noticed this while running the coil but it
> > shows up on all of my photos.
> >
> > I understand that the corona from the top may be from having the
> > torroid too high. Does any one have any ideas what may cause the corona
> > on the ground ring? (It looks cool in the pictures but I doubt if it's a
> > good thing)
> >
> > My coil configuration is as follows:
> >
> > 2 15kva neons
> > 2 air wound and 2 ferrite core chokes
> > copper tube vacuum quenched spark gap (8 tubes, .030 spacing)
> > Primary -1/4" copper pipe with 3/8" spacing 12 turns
> > Secondary - 6" acrylic wound with 22" of 24ga magnet wire
> > Torroid - 4" dryer duct around a 24" pizza pan
> > ground - very heavy welding cable (1/2" dia) connected to 1 8' copper
> > plated ground rod
>
> Dave,
> Most film is sensitive to ultraviolet, even though the human eye is not
> very sensitive to it. The "blue corona" you see in your photograph appears
> much brighter than it does in real life for two reasons. 1) The ultraviolet
> component is invisible to you, but shows up as blue/purple/white on color
> film. 2) The shutter speed affects the amount of light gathered. The longer
> the exposure time, the greater effect any type of light will have on the
> film (all other factors being the same, of course!)
>
> It is my guess that you have taken a time-averaged exposure that has
> enhanced elements that are normally too weak for the naked eye to take
> notice of. The blue is particulary "enhanced" by the photo both because of
> the exposure time and its inclusion of ultraviolet light. In your post you
> did not mention f stop, film speed, or shutter speed, so this is just a
> guess. You probably notice that the photo has also enhanced the streamers,
> right? That is typical of a time exposure. You may only see a few streamers
> in person, but on film it can look like the whole toroid is aburst with
> hundreds of streamers in a time exposure. That's one reason why photos of a
> coil can be somewhat deceiving!
>
> The corona you noticed is not really a cause for alarm. By looking at the
> shape of the blue corona field you can map out the relative voltage
> distribution. If it is sort of like an inverted parabola in shape, then you
> have pretty good distribution.
>
> By the way, a *really* long time exposure with *all* the lights out and the
> spark gap fully covered can help you pin point sources of corona loss that
> you would never see with the naked eye. How long is really long? try about
> ten times what your previous exposure was. Make sure your camera is on a
> tripod for stability.
>
> Fr. Tom McGahee