RE: Gap Question

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

Original poster: "Luke" <Bluu-at-cox-dot-net> 

So if the current goes up the gap widens.
If the gap widens the resistance of the gap goes down.
If the resistance went down when the current went up
That is resistance not negative resistance.


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>