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Re: Gap Question
Original poster: robert & june heidlebaugh <rheidlebaugh-at-desertgate-dot-com>
Luke: In an arc you have several conditions to consider. 1) With a low
current arc the insulation value of the gap breaks down forming a small
current path. this small current , path MAY MOVE ALONG THE CONDUCTOR BY
THERMAL AGGITATION, but remains a small current path. 2) At high current
another conditions takes place forming a plasma. A plasma forms when the
energy of the arc strips the outer shell of electrons off the atoms of the
conducting gas. At this point the plasma becomes a conductive path of a
large area with very low resistance. A gap 2 " in distance can freely
conduct 28 volts. At normal conditions 28 volts could never be conducted
across a 2" gap. In a TC we are not using low current pulses we are using
current pulses over 200 amps. An example of this action is an arc welder .
The tech moves the arc rod close to the base metal to strike the arc then
holds the arc some distance away from the base metal plate to heat a larger
area and form a metal puddel for welding. If the tech moves the arc to farr
a large round hole will form , not a single small point hole. This action is
used to scrap out sheet metal in junk yards with an arc welder set on high
currernt a 1" wide strip can be cut in old scrap for much less cost and time
than using an expensive cutting torch. 3) The plasma action can be proven in
several ways, The primary proof is the sudden drop of resistance of a gas.
this the reason flourecent lamps have a balest coil in the circuit. The
filament heats the lamp to forn a conductive vapor then the lamp conducts
and the resistance drops. The balest then provides reactance to limit the
current of the gas in the tube and you get light across the entire diameter
of the tube not just one thin line of arc. Another example is found in
inert gasses. These gasses will not form compounds and are called NOBLE
gasses. In a plasma inert gas will react with chlorine or fluorine to make a
stable salt compounds such as zenon chloride which you can buy from a major
chemical supply.
Robert H
--
> From: "Tesla list" <tesla-at-pupman-dot-com>
> Date: Wed, 25 Feb 2004 07:24:57 -0700
> To: tesla-at-pupman-dot-com
> Subject: RE: Gap Question
> Resent-From: tesla-at-pupman-dot-com
> Resent-Date: Wed, 25 Feb 2004 07:38:12 -0700
>
> 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>
>>
>>
>>
>>
>
>