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RE: Magnetic quenching.



Original poster: "David Thomson" <dave-at-volantis-dot-org> 

Hi Marco,

 >  > Today I built a spark gap out of two 3/4" diameter tungsten carbide
balls
 >  > (ordered through McMaster Carr).  The balls were brazed to 1/8"
thoriated
 >  > tungsten welding rods.  I have a photo of the spark gap, but my host is
 >  > being hit with a denial of service attack.
 >
 > Great setup! I would like to find here in Finland tungsten carbide balls
that easily.
 > Would you please tell me what alloy did you use for the brazing? Did you
do it yourself?

I used standard brass brazing rods with flux.  I did the brazing myself and
it was the first time I have ever tried brazing.  The braze came out really
nice.  Fortunate for me, there is a retired veteran engineer from Argonne
Labs living across the street from me who led me through the whole process.

 > <SNIP>
 >  > At 2/10" gap it fired at 85 volt on the variac.  The spark didn't
 >  > extinguish until the potential was down to 48 volt on the variac.
 >  >
 >  > I then tried the experiment with the magnets.  At 2/10" the gap fired
at 82
 >  > volt on the variac and extinguished at 55 volt.  The spark window is
much
 >  > smaller with the magnets.
 >
 > I imagined the magnets oppose to the spark formation, therefore
 > facilitating its quenching but also rising the triggering voltage of the
 > gap. You experienced just the opposite of this: a decreased voltage
 > threshold.

Yes, I was surprised myself at first.  Then when I looked at the equation I
was using, the potential is in the denominator.  This causes the quench time
to increase as potential decreases.  The data is evidence in favor of the
equation (so far), but much work remains to be done.

 >  > With the magnetic quenched gap, the spark looked like a circular plane.
The
 >  > perimeter of the circle moved up and down while the center remained
more or
 >  > less between the closest points of the spheres.  The circular plane was
a
 >  > light blue color.  In the center of all this there were two distinct
yellow
 >  > sparks that joined parallel to each other between the spheres.  In
other
 >  > words, there were two distinct types of sparks occurring between the
spheres
 >  > at the same time.
 >
 > I guess the plane was parallel or perpendicular to the magnets
 > faces, was it?

The spark plane was more or less parallel to the magnets.

At first I was thinking the magnets were squeezing the spark, but that
couldn't be true since all the magnetic flux is oriented in one direction.
It must be that the electrons themselves are forced to become polarized such
that their north pole is facing the south pole magnet and the south pole of
the electron is facing the north pole magnet.  Each electron would then have
its angular momentum equally aligned in the same plane, more or less.  This
in turn would cause the spark to spread out in a plane.

The potential difference of the spheres is what drives the spark across the
gap.  The magnetic flux merely causes the spark to flatten out by
magnetically aligning the electrons.  In the center of the magnetic flux,
which is strongest, the spark is more stable than on the outer edges of the
circular plane.  This is likely what causes the circular plane to "pop up
and down around the circumference".

 > Interesting stuff, I'll try to replicate myself your experiment, even if
 > I have only two of those ring magnets available. I want to try with a
 > diverging field also.

Here is a link to the tungsten balls I'm using.
http://www.mcmaster-dot-com/param/asp/psearch.asp?FAM=tungsten&FT_128=558&FT_101
=11283&FT_121

But you will need to go to the home page and search for "tungsten ball" to
order them...
http://www.mcmaster-dot-com/

I don't suppose it is very important what the rod is made from.  I just
happened to have thoriated tungsten rod handy.  Perhaps brass rod will work
fine?  Maybe the thoriated tungsten rod is necessary for the brazing part,
though.  I'm not familiar enough with brazing to guess whether brass rod
would melt before the ball could be brazed.  Others on this list might be
able to help us out here.

Dave