Re: Gap Question

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

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Bart,

On 25 Feb 2004, at 21:24, Tesla list wrote:

 > Original poster: Bart Anderson <classi6-at-classictesla-dot-com>
 >
 > Hi Luke, All,
 >
 > This has been a good discussion!
 >
 > Tesla list wrote:
 >
 > >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?
 >
 > That's how I'm coming to visualize from what the others have posted
 > here.
 >
 > Here is what I'm seeing: The voltage is very high and when an arc path
 > is forced. The large potential pushes a large current across the path
 > and takes the path of least resistance. In the case of a high current
 > arc, the path of least resistance isn't a single initial arc path. It
 > takes less energy to force the surrounding air around the initial arc
 > path to become conductive than it is to wait for the current to cross
 > this small pathway. Thus, the resistive portion around the initial arc
 > path is forced into conduction. The area of conduction increases and
 > the resistive region decreases. There is a maximum at which the VI
 > product can force the surrounding resistive gas into conduction. Once
 > this maximum is reached, because the voltage is decreasing, the
 > resistive portion begins increasing as the area of conduction
 > decreases.

It is important to understand that the lower limit on R is actually a
reflection of the ability of the arc path to expand. One thing that
stops it expanding is a limit on current, the other, as in the
flashtube example, a physical limit. One might see the finite size of
the electrodes in a small sparkgap imposing such a limit.

 > I have a question for the list.
 >
 > I can't visualize the current remaining constant and is what I thought
 > I read in one of the posts (maybe that was just for conceptualizing
 > the process?). As the conductive area changes throughout the process,
 > the current should also change concentrically with the resistance as
 > the voltage drops. If the current is constant, I don't understand how.

It is a limit imposed by circuitry external to the discharge, for
example a resistor or an inductor (i.e. the internal impedance of the
power source). In the case of an oscillating primary, the current
peaks and then falls again, then rises again (in the opposite
direction etc.) as the circuit oscillates. It is the change in
current which causes the change in spark cross-sectional area. The
voltage across the gap is a consequence.

 > A larger number of pathways should increase the current value at any
 > point in time.

But one doesn't normally get multiple pathways in an arc, at least at
STP does one? A conduction channel of hot ions is favoured. A glow
discharge in a rarified gas is another matter. Unlike a welder, the
gap goes out periodically which doesn't favour a plasma type
discharge like that you can draw from an arc welder although I
suppose one could argue that a plasma is present while the primary is
ringing and the gap is alight. For nature's best demonstration of
spark behaviour, lightning formation warrants some attention. I
cannot agree that pipe spark gaps simultaneously conduct along their
whole length as at least one person has said.

Malcolm