RE: Gap Question

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

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>
 >
 >
 >
 >