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RE: Gap Question
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?
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
-----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>
>
>
>
>