[Prev][Next][Index][Thread]

[Fwd: Re: corona (fwd)]



-- BEGIN included message







----------
> 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






-- END included message