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Re: More Wimshurst questions

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  • mebikerider
    Thanks for the tips. I have an old flourescent tube from a copier machine, so I tried having the Wimshurst light it. I had some success, but had to go do some
    Message 1 of 4 , Dec 1, 2008
      Thanks for the tips.

      I have an old flourescent tube from a copier machine, so I tried
      having the Wimshurst light it. I had some success, but had to go do
      some family thing and so didn't have the time to do much with it.

      Last night I made a first attempt at a corona-measuring device:

      Connect a sewing needle to one end of a 66 meg resistor (from an old
      TV receiver) on a long wire. Connect the wire through a diode over-
      voltage clamp circuit through 10 megs to ground. Connect the top of
      the 10 meg to the gate of an MPF102 amplifier. Connect the MF102
      output to adjust the tone of a 555 oscillator which drives a speaker.

      This way, I can point the needle all around the Wimshurst, and the
      speaker tone changes depending on both the strength and polarity of
      the corona discharge.

      As crude as the first model was, it was able to immediately tell the
      polarity of each side of the Wimshhurst, because positive raises the
      tone and negative lowers it.

      But the tone is kind of harsh, and the positive-negative swing isn't
      balanced very well, so there's more to do to refine this circuit.

      Eventually I'd like to have something that can measure the field at
      any point, nondestructively, as this circuit draws off some charge
      in the process of measuring the corona. But this may be a handy
      circuit to have around.



      --- In VanDeGraaffGenerator@yahoogroups.com, "acmqsoueu" <acmdq@...>
      wrote:
      >
      > --- In VanDeGraaffGenerator@yahoogroups.com, "mebikerider"
      > <david_beals@> wrote:
      > >
      > > I don't understand what my Wimshurst machine is doing - can
      anyone
      > > help explain it's behavior?
      > >
      > > I've only had it for a couple of weeks, and it's been working
      great,
      > > but over the last few days it hasn't been sparking as
      consistently.
      >
      > Humidity and dirt on the insulators, specially on the disks, are
      the
      > most common problem.
      >
      > > I've noticed a funny thing about it, though. If I steadily turn
      its
      > > crank, I can hear it hissing and crackling, and smell its ozone,
      but
      > > there are no fat blue sparks between the Leyden jar discharge
      balls.
      > >
      > > But if I bring my wrist near the charge collector bar on one
      side,
      > > the bar with the needle that connects to the Leyden jar, and let
      a
      > > thin pale spark jump to my wrist, immediately there is also a
      giant
      > > fat blue spark between the two Leyden jar discharges. This is
      very
      > > consistent; if I leave my wrist near and draw a steady stream of
      > > thin sparks, then we get a steady series of giant fat sparks. If
      I
      > > remove my wrist, the giant fat sparks stop.
      > >
      > > This doesn't happen with the other needle-bar. I can pull a thin
      > > spark off it, but it does not cause the giant fat spark.
      >
      > When you draw a spark from one terminal to ground, the voltage at
      the
      > other terminal suddenly rises, due to the voltage on the Leyden
      jars
      > that canĀ“t change immediately. This causes breakdown in the
      surface of
      > the opposite terminal, triggering a spark.
      > But this works only if the terminals are with a certain polarity,
      > probably negative, I don't remember, at the side from where you
      draw a
      > spark.
      >
      > > Last night I used a paper napkin to gently rub down all the
      plastic
      > > parts, thinking that maybe dust had collected and was creating
      > > leakage paths, and after that, the behavior reversed sides - the
      > > machine would spark if I brought my wrist near the righthand
      side
      > > instead of the lefthand side.
      >
      > The machine started with opposite polarity.
      >
      > > What is going on? Why would a discharge from one area trigger a
      > > spark from another area?
      >
      > This does not happen with any machine. I have several Wimshurst
      > machines, and only a small one shows this behavior.
      >
      > > I'm guessing that somehow corona leakage over dirty surfaces is
      > > unbalancing the machine, but what are the physics that result in
      > > what I'm seeing?
      > >
      > > There's another thing I don't understand. Sometimes after I'm
      done
      > > cranking the machine, I'll take a screwdriver and bring the
      metal
      > > blade near the discharge balls to zap off one final spark. But
      > > often, what happens is I'll bring the blade near both balls,
      when
      > > suddenly there will be a fat blue spark between the balls, not
      to
      > > the screwdriver blade at all.
      >
      > The sharp tip of the screwdriver can emit some corona and trigger
      a spark.
      >
      > > I'm guessing that the presence of the metal screwdriver blade
      > > increases a low current corona discharge into clouds around both
      > > balls, to the point where the clouds are drawn between the two
      > > discharge balls and an ionized path forms allowing high current
      flow
      > > between the balls, sort of how lightning supposedly sends down a
      > > number of smaller leaders before the massive strike occurs. Does
      > > that sound right?
      >
      > Yes.
      >
      > > Is there any way to detect this invisible corona discharge? I've
      run
      > > the machine in a dark room, and the most vigorous sparking
      areas,
      > > like near the brushes, emit light, but I'm guessing that most
      corona
      > > discharge does not emit light. Does it make noise? Would a
      > > microphone at the end of along plastic tube pick it up? Would
      some
      > > kind of high impedance needle probe be able to pick up and
      detect
      > > corona current, without drawing off a massive spark?
      > >
      > > Has anyone done any experimenting with this sort of thing?
      >
      > You can measure corona current with a microamperimeter, and if you
      > operate the machine in the dark (complete darkness) and wait until
      > your eyes adapt to the darkness, you will see the corona easily.
      >
      > Antonio Carlos M. de Queiroz
      >
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