Re: Big money in the electrostatics biz

[William Beaty <billb@ESKIMO.COM>]

On Thu, 18 Jul 2002, Kossom wrote:

> I am now extraordinarily curious as to how it works. What makes it work
> differently than what Bill Beatty suggests? What is component (x)?

If I was designing a flat cardboard "zap no", then for "component x" I
would use conductive carbon ink printed on coated mylar to create a series
resistor, with the conductivity adjusted to give around 100 megohms.  For
the keychain version I'd just use a 100 meg resistor (costs about $0.05)

The series resistance value comes from a simple engineering equation.
Human-body capacitance is around 2e-10 farads, so in order to discharge a
person in under 0.1 seconds we want our current-limiting resistor to give
an exponential "RC time" constant which is several times faster than 0.1
seconds:

   R * C = 0.1 / 5

   R = .02 / C

   R = .02 / 2e-10 = 1e8 = 100 megohms

The voltage from scuffing on carpets is 0K to 10KV, so with 1e8 ohms, the
current is limited to 100 microamps (way below the threshold for pain from
electric current.)  Voltage from getting out of a car can exceed 50KV, so
we might want to use 500 megohms rather than 100M for the keychain
version, otherwise the person might feel their muscles twitch (which feels
weird but is definitely not painful.)  But with 500M, the person's body-
voltage might not be eliminated unless they carefully held the device
against the car door rather than just bumping it on the metal.  We have to
choose our resistor value to give fast discharge but to keep the peak
current as low as possible.

I needed to accomplish just such a task back around 1988 for a science
museum exhibit.  I wanted to painlessly remove a human body charge from a
person who had just touched a VandeGraaff machine (300KV machine, so the
"victim" was charged to 100KV or so).  I just did the obvious thing, and
used a few 10 megohm resistors in series to limit the current to below a
milliampere.  In the end I used other techniques: giving the person a
metal rod and letting them touch either the VDG or ground...  or letting
them touch a wooden rod which had been painted with slightly conductive
carbon paint, with a ground connection on the other end of the rod...  or
I reduced the system voltage to zero while the person was still touching
the VDG machine:

  Taming your VDG
  http://www.amasci.com/emotor/vdgtame.html

There is a competitor for "zap no" which uses an entirely different
scheme, and which sells for $5US.  I thought their price was WAY too high,
considering the low manufacturing cost of their product.  That's one
reason why my jaw totally dropped when I saw the price on the "zap no."


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William J. Beaty                            SCIENCE HOBBYIST website
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