RE: Magnetic quenching.

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

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.

I suspected that spherical geometry of the spark gap terminal would increase
the efficiency.  I was right.  It makes a big difference (based on
observations of the spark.)  I tried the same experiment I did yesterday.
Without the magnets and with the gap set at 1/2", I turned the variac up all
the way and no spark.  I tried 4/10" and 3/10" gaps an no spark.  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.

The difference in the appearance of the spark between the magnetic and
non-magnetic quenched gaps was dramatic.  In the non-magnetic gap the spark
arched softly like a Jacob's Ladder spark.  It would begin at the closest
contact between the spheres and then travel outward toward the furthest
point of contact between the spheres.  The spark actually traveled well
beyond 90 degrees of the sphere (closer to 160 degrees).  The spark was tube
like with each end of the tube touching a sphere.

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.

Now I need to setup a Tesla coil and run it with the different types of
spark gaps to see what effect there is.

Dave

 > -----Original Message-----
 > From: Tesla list [mailto:tesla-at-pupman-dot-com]
 > Sent: Friday, March 12, 2004 5:42 PM
 > To: tesla-at-pupman-dot-com
 > Subject: RE: Magnetic quenching.
 >
 >
 > Original poster: "David Thomson" <dave-at-volantis-dot-org>
 >
 > Hi Marco,
 >
 >  > Well, I also meant "have you noticed any difference"? You are saying
 >  > that you did notice "louder operation". Good.
 >  > Now, could you check if the gap threshold voltage has changed? For
 >  > instance, you could feed the gap just straight from the NST,
 > through the
 >  > variac. Crank the voltage up until the gap fires. Do it with
 > and without
 >  > the magnets, Any difference?
 >
 > I tried your experiment.  I used just the 15kV NST, spark gap (with and
 > without magnets), and variac.  The gap was set at about 1/2".
 > Assuming that
 > my variac is accurate and that my supply voltage is 120 volt I
 > received the
 > following results:
 >
 > with magnetic quenching:
 > 	spark began at 8.505kV (68volt on the variac)
 >
 > without magnetic quenching:
 > 	spark began at 9.12kV (73volt on the variac)
 >
 > The sparking with the magnetic quench was noticeably louder with more
 > "popping" sounds.  The sparking without the magnetic quench was noticeably
 > quieter and smooth running.
 >
 > This makes perfect sense when the equation I provided is used...
 >
 > magnetic_flux / gap_potential = time_between_sparks (quench time)
 >
 > The quench time is increased as the gap potential decreases.  This causes
 > more energy to cross the gap when it does fire.  It is also
 > consistent with
 > the observation by Tesla and others that streamer length increases with a
 > lowering of secondary frequency.  As the quench time increases,
 > the gap rate
 > decreases, which in turn should lower the overall system resonant
 > frequency.
 >
 >  >  >  > Can you post more information about your magnetic gap arragement?
 >  >  >
 >  >  > http://www.tesla-coil-builder-dot-com/images/sparkgapmagnetic01.jpg
 >  >
 >  > They look like magnetron magnets, am I right? You have obviously also
 >  > tried to rotate the upper stack relative to the bottom stack (repulsive
 >  > fields vs. concurrent fields). Any difference?
 >
 > I believe the magnets are NIB (NdFeB).  I bought them from Forcefield, the
 > same place Terry buys his.  The magnets on top are three 1.5" diameter x
 > 3/8" with .5" holes, there is also a 1" diameter x 3/8" solid disk magnet
 > under the top stack which is not visible. The magnets on the
 > bottom are two
 > 1.5" x 1.5" x 3/8" squares.  I have not tried to oppose the
 > magnetic fields.
 > It's too risky without building a strong clamping device.
 >
 >  > What I am trying to understand here is, is magnetic quenching just
 >  > bettering quenching or also rising the gap threshold voltage? How the
 >  > magnetic field has to be oriented?
 >
 > It would be good for the books to try an opposing magnetic field
 > spark gap.
 > But it is not something I'm planning to do in the near future.
 >
 > It would appear that the magnetic quenching actually lowers the threshold
 > potential.  We also need to consider that this experiment was not
 > done with
 > capacitance on the spark gap terminals, as it is in a Tesla coil.
 >
 > I agree with someone else's suggestion that magnetic quenching
 > could assist
 > RSG efficiency.  I also believe that making the spark gap terminals as
 > spherical balls will also help.  As soon as I can find someone
 > with a welder
 > I'll assemble such a spark gap and test it.
 >
 > Dave