Responding to Malcolm Watts & Antonio de Queiroz
Malcolm & Antonio & others-
Thanks for your comments. My sense of the fundamental purpose of the top
electrode has been that it provides a conducting surface whose minimum
radius facing space is sufficiently large to allow voltage to build up on
it to the desired level prior to discharge. I would almost think that
that is the >only< essential purpose. Without such a radius present at
the top of the coil, the voltage will not have a chance of building up
before a discharge occurs.
The secondary/top-electrode system is a simple resonant oscillatory
circuit. Current surges up & down within the coil (I like to think in
terms of electron flow) to & from the top electrode. During one
half-cycle, electrons flow upward from ground, crowding onto the top
electrode. During the other half cycle, electrons flow downward to
ground, depleting the quantity of electrons residing on the top
electrode. Eventually, as the voltage rises, either too many or too few
electrons become present on the top electrode for its minimum radius
facing space, or ground, to withstand and a discharge occurs, electrons
initially either flowing from space or ground to the electrode's
minimum-radius spot or from that spot. As I see it, there will then
ensue an oscillatory discharge, i.e. an ac spark, until the potential on
the top electrode diminishes sufficiently. The discharge will always
commence at the peak of a half-cycle since clearly the peak of the
previous half cycle was just too low in voltage for a discharge to have
I've noticed, with my FET-driven coil, that if I induce a spark to a good
ground the secondary-voltage drop is very abrupt--just a few microseconds
to zero v. This is with a 4" c.s.d. x 14" o.d. toroid producing 14" or
so sparks to nowhere in particular and 18-20" sparks to a ground rod.
That seems to indicate that the impedance to "nowhere in particular" is
considerably higher than that to the ground rod. I don't know the
significance of that as regards the appearance of much larger sparks than
I can yet generate.
Observing the secondary's voltage rise on the 'scope, I happen to have a
way to go before it tops out, so my next step (before the step of
doubling the voltage with more FETs) is to go to a larger c.s.d for the
toroid so as to allow the higher voltage. Perhaps a greater o.d. will be
indicated, also, although I still think that what fundamentally matters
is the electrode's minimum effective radius: there's always room for more
electrons as long as they don't "see" a sharp radius to fly off of (they
don't, after all, take up all that much space!).
Perhaps this is all old hat to everyone but newbies like myself. I
welcome comments or correction.
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