Re: Gap Question

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

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:

>Original poster: "Luke" <Bluu-at-cox-dot-net>
>So if the current goes up the gap widens.
>If the gap widens the resistance of the gap goes down.
>If the resistance went down when the current went up
>That is resistance not negative resistance.
>
>Luke Galyan
>Bluu-at-cox-dot-net
>http://members.cox-dot-net/bluu
<SNIP>

Hi Luke,

In a normal resistor, the voltage drop increases linearly as you increase 
current since the resistance is a constant (not a function of current), so 
that doubling the current simply doubles the voltage drop. This is defined 
as positive resistance characteristic.

The resistance of an unconfined arc will tend to decrease as you increase 
current since the conductive channel becomes fatter. The effective 
resistance of the arc no longer a constant, but instead is a function of 
the applied current. The voltage drop across an arc that bridges a fixed 
gap length rapidly stabilizes to a comparatively low steady state "burning 
arc" voltage (~20 volts to ~150 volts depending on the electrode materials).

As you indicate, the overall resistance of the arc is always positive. 
However, an arc has a "negative resistance characteristic" since an 
increase in current causes a rapid DECREASE in the arc's resistance such 
that the arc's voltage drop tries to approach the burning arc voltage for 
the gap.

Macroscopically, a spark gap can be envisioned as a pair of back to back 
Zener diodes with a 100-150 volt drop (Egap). Adding more gaps in series 
will add more discrete voltage drops into the primary LC circuit. Each 
additional spark gap adds an increment of loss (Iprimary*Egap), so multiple 
gaps will tend to be lossier than single gaps (all other things being the 
same). Multiple gaps often handle high power levels more effectively 
without needing to resort to a heroic level of forced air cooling. However, 
a well designed high velocity single-gap system (such as Gary Lau's vortex 
vacuum gap) will be more efficient than a multi-gap system assuming that 
they both style gaps are capable of quenching at the same primary current 
notch.

Best regards,

-- Bert --
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