RE: Gap Question

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

On 25 Feb 2004, at 7:24, Tesla list wrote:

 > Original poster: "Luke" <Bluu-at-cox-dot-net>
 >
 > Sorry, when I wrote gap in this post I should have said arc.
 > So when the term negative resistance is used what is meant is that the
 > resistance value is changing in a negative direction (getting less)?
 > Is that a correct statement?

Yes.

 > An actual resistance value that could be measured in the negative is
 > the picture the words negative resistance brings to my mind (sort of).
 >  Just seems like a misleading term at lest to someone new to this
 > concept like me.
 >
 > Luke Galyan
 > Bluu-at-cox-dot-net
 > http://members.cox-dot-net/bluu

In fact such things can be created using active (opamp/transistor)
circuitry. Non-linear resistance is possibly a more useful term but
covers the lightbulb as well as the sparkgap.

Malcolm

 > -----Original Message-----
 > From: Tesla list [mailto:tesla-at-pupman-dot-com]
 > Sent: Tuesday, February 24, 2004 5:19 PM
 > To: tesla-at-pupman-dot-com
 > Subject: RE: Gap Question
 >
 > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >
 > On 24 Feb 2004, at 13:09, Tesla list wrote:
 >
 >   > Original poster: "Luke" <Bluu-at-cox-dot-net>
 >   >
 >   > So if the current goes up the gap widens.
 >
 > The arc channel increases in diameter.
 >
 >   > If the gap widens the resistance of the gap goes down.
 >
 > If the arc channel increases in diameter, the resistance of the gap
 > goes down (assuming the arc length remains constant).
 >
 >   > If the resistance went down when the current went up
 >   > That is resistance not negative resistance.
 >
 > No. With a "normal" resistance, as current through the resistance goes
 > up, the voltage across it goes up and vice-versa. In the case of an
 > arc, the voltage is almost constant regardless of the current. Hence,
 > the resistance changes with current instead of the voltage. Resistance
 > in a spark gap is parametric variable.
 >
 >        As Matt said, at any point plotted on a V/I curve of the gap,
 > you will find power is being dissipated. Hence at the chosen point,
 > one can derive a resistance value. The resistance value will be
 > different at different points along the curve.
 >
 > Malcolm
 >
 >   >
 >   > Luke Galyan
 >   > Bluu-at-cox-dot-net
 >   > http://members.cox-dot-net/bluu
 >   >
 >   > -----Original Message-----
 >   > From: Tesla list [mailto:tesla-at-pupman-dot-com]
 >   > Sent: Tuesday, February 24, 2004 6:52 AM
 >   > To: tesla-at-pupman-dot-com
 >   > Subject: RE: Gap Question
 >   >
 >   > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >   >
 >   > On 23 Feb 2004, at 20:51, Tesla list wrote:
 >   >
 >   >   > Original poster: "Luke" <Bluu-at-cox-dot-net>
 >   >   >
 >   >   > If the current goes up and the channel widens would that give
 >   the >   > channel less resistance because it now has a larger cross
 >   section >   to > travel through? > > Like a piece of 28Awg wire
 >   having a higher >   resistance than a piece of > 10Awg wire.  The
 >   larger cross
 > sectional
 >   >   area decreases its
 >   > resistance.
 >   >
 >   > Exactly right ;)  The point is that the resistance of the gap is a
 >   > *dynamic* value since a current dependency is factored in and if
 >   the > current waveform is a sinusoid as it is in a TC primary
 >   ........ > > Malcolm > >   > Thanx >   > >   > >   > Luke Galyan >
 >   > Bluu-at-cox-dot-net >   > http://members.cox-dot-net/bluu >   > >   >
 >   -----Original Message----- >   > From: Tesla list
 >   [mailto:tesla-at-pupman-dot-com] >   > Sent: Monday, February 23, 2004 5:01
 >   PM >   > To: tesla-at-pupman-dot-com >   > Subject: RE: Gap Question >   >
 >   >   > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> >
 >   > >   > Hi Luke, >   >           Nice try: >   > >   > On 23 Feb
 >   2004, at 7:28, Tesla list wrote: >   > >   >   > Original poster:
 >   "Luke" <Bluu-at-cox-dot-net> >   >   > >   >   > Bart: >   >   > I thought
 >   about what you said with the negative resistance >   thing. >   May
 >   > I lay something out that does not use real values >   but values >
 >    > >   >   arbitrarily picked out to make the example simple.  I may
 >   be
 > way
 >   > off
 >   >   >   > base but would like some in put.  I think the idea has
 >   merit. >   > > >   I was thinking it may appear to be negative
 >   resistance but >   may not >  > >   >   really be.  Maybe it is like
 >   having a gap that has different >
 >  >
 >   >     dimensions when it heats up. > > Let me try to explain.  And >
 >     before >   anyone starts quoting some actual > values try to look
 >   >   past that > for >   >   about 30 seconds just to get the >   >
 >   idea >   >   > I am getting at.  Then fire away with all the real
 >   value >   stuff. > >   > Let's pretend for the example that: >
 >   1:	The >   breakdown voltage of >   air is 100 volts per inch. And >
 >   2:
 > That an
 >   >   arc has a
 >   > certain amount
 >   >   >   of resistance per length. >  But that resistance follows a >
 >     >   logarithmic curve or spiral. >  What I mean by this is like
 > that
 >   >   of >   a logarithmic spiral. >  Where the curvature gets tighter
 > and
 >   >   >   tighter as the spiral >  Curves inward. >  Now draw a line
 >   from >   the >   center outward.  Measure the distance >  from
 >   center to >   where the >   line intersects each turn of the >
 >   spiral. >  So say >   0.5=3, 1=6, >   1.5=12, 2=24, 2.5=48, 3=96,
 >   3.5=192 etc. > > > >   Imagine only the >   spark gap the primary
 >   coil and the capacitor.
 >  >
 >   >   And all this is at >   the time of break down for the gap. > >
 >   Ok >   say we have a gap of 3" >   that would give 300 volts for
 >   break
 > down
 >   >   > and >  >From the curve >   above the resistance of that arc
 >   would >   be 96 ohms. > Using plain > old >   >   ohms law you would
 >   get a current of 3.125 amps. > > Now let's >   say >   the
 >   electrodes get hot.  Instead of just looking at it > >   like The >
 >    voltage breakdown got lower, lets assume it acted as >   though >
 >   it >   made the distance between the electrodes closer, >   which
 >   would have > a >   > lower breakdown voltage.  I think of it >
 >   like the hot air/ions > whatever >   > might act as an extension of
 >   >   the electrodes making them have a >   larger > diameter and >
 >   therefore be closer together. > > So lets say >   that the heat >
 >   involved made the gap ACT as though the > distance > was >   >
 >   2.5" even though the measured distance might actually be > 3".
 >  >
 >   >   > >   This would give a break down voltage of 250 volts and a >
 >    resistance >   of > 48 ohms.  So the current would be 5.2 amps.  So
 >   >   the voltage > the >   >   cap > charged to was 50 volts lower
 >   but the current went up. >   Not >   because > of >   > negative
 >   res. but because the electrodes >   in a way are now closer. >   > >
 >   So is it that the gap has a >   negative resistance? > Or is it >
 >   that >   >   the heat makes the gap act as though it has, >
 >   different >   dimensions >   (a closer spacing)? > so the gap acts
 >   different? > > >   I know this >   might be reaching a little but I
 >   think there is
 > some
 >   >   > logic >   > in it I would like some opinions on. >   > >   >
 >   If >   what I am saying had some truth to it then if one did not
 >   take > >   into >   > account the gap acted as if the distance were
 >   closer it >   would > seem >   >   > that there was indeed negative
 >   resistance. > > Any thoughts? >   And >   keep in mind I am no math
 >   whiz.  Just laying out a >
 > concept
 >   >   and >   hoping to get some qualified people thinking / talking
 >   so > >   I can >   hear bout it. > > What really happens is that as
 >   gap >   current goes up, the width of the > arc increases. A spark
 >   tries to >   keep to as narrow a channel as > possible at "normal"
 >   atmospheric >   pressure. The width of the channel > at >   > a
 >   particular current is defined by the number of ions (current > >
 >   carriers) that can be formed in the channel cross-sectional area, >
 >    the > molecular density of air being the arbiter. In effect, the
 > arc
 >   > behaves
 >   >   > in a saturable manner, extending no wider than it has to. If
 >   the
 >  >
 >   >   available current goes up, the channel widens to boost the
 >   cross- > >   sectional area. So it is the fact that the channel is
 >   able to widen >   > without limit that gives an arc the
 >   negative-resistance > >   characteristic. In an inductor analogy, it
 >   is as though the
 > inductor
 >   >   > core increased in cross-sectional area as the applied
 >   magnetizing >   > force tried to take it beyond saturation. In a
 >   camera flashtube > >   however, the ability of the arc to widen is
 >   restricted by the > diameter >   > of the flashtube (ions that can
 >   be formed per unit area) so after >   > initially exhibiting a
 >   negative resistance, the tube truly >   saturates > and stays there
 >   meaning that beyond a certain current, >   the arc > reverts >   >
 >   to a positive or normal characteristic, highly desirable since
 > you
 >   >   > don't want the storage capacitor seeing voltage reversals. > >
 >   >      The short distances involved in Tesla spark gaps don't have a
 >   > >   huge influence on the gap dissipation. > > Malcolm > <snip> >
 >   > > >
 >
 >
 >