RE: Gap Question

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

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>