Stephen A. Lawrence wrote:
There's been a little confusion over field strength versus voltage
recently on this list. Here's a cool gadget that helps to illustrate
the difference. Most likely most readers are already familiar with it,
but perhaps it will be new to some. (General description obtained from
"Models for Experiments in Physics", by A. D. Bulman, no doubt out of
print for 30 years at this point.)
(BTW I've been planning to build one of these for the last few decades
and haven't _quite_ gotten around to it just yet -- maybe before this
winter's over...)
Start with two shallow tin trays, baking pan size or a bit smaller, with
rolled edges.
Glue an insulated handle to the smaller one. Recommended: An empty
detergent bottle, with cap glued to pan, so the handle may be unscrewed.
(Remove label first, he sez; maybe paper isn't so hot in the
"insulator" department?)
Glue a sheet of "good insulator" to the larger pan. Recommended
sources: Bottom of an old plastic dishpan, or a cylindrical polyethelene
bottle, top and bottom cut off, cylinder slitted and "unrolled" so it's
flat; heat it to flatten it, he sez. Not too hot or it will stink, sez I.
Now warm everything up (why warm it? dunno) and then charge the
insulating sheet by "flicking" it with a piece of flannel, or rubbing it
with a "dry dust cloth" or piece of fur.
You're done. (Does the large pan have to be grounded? Book doesn't
say; I think the answer is no, it must float, but I'm not sure.)
Now, to use it, holding the smaller pan by its handle, place it on the
insulator on the larger pan (insulator must be bigger than smaller pan,
did I neglect to mention that?). Touch the small pan with a finger to
ground it. Now lift it up by the handle, being careful not to touch it.
Finally, bring your knuckle near the small pan, which you are holding
by the handle; supposedly, if all goes well, you can pull a spark an
inch long from it ... despite the fact that it was at exactly the same
potential as your finger mere moments before, and no charge has been
added or removed since.
You can do it again, as often as you like, _without_ recharging the
insulator, until the charge on the insulator finally bleeds off through
the air (this is presumably best done in the winter).
Anyhow so says the text. As I said, I keep meaning to build one of
these, but we use our old tin baking pans for baking things, I don't
have any cylindrical polyethylene bottles lying around the house, and I
just never got around to scaring up alternatives.
Now, the point of this is that the _field_ between the small and large
pans is strongest when they're closest together, yet the _voltage_
between the small pan and ground is largest when it's farthest from the
large pan. The voltage is boosted mightily by the work done in pulling
the pans apart, while the actual field between them is either unchanged
or actually decreases in intensity.
Minor addendum/correction: The field is small, _UNTIL_ you bring your
finger or other grounded pointy thing near the pan; at that point the
field between the point and the pan will get very large, which is what
finally breaks down the air to form the spark.
This suggests that you must lift the gadget straight up (don't tilt it),
to avoid having the charge on the pan "slosh" to one edge due to the
(approximately vertical) field from the insulator, thus forming a "hot
spot" in the charge density which would break down the air around it and
short out before you got them far enough apart to be interesting.
Does gluing a needle to the small pan make it possible to pull a longer
spark? Dunno. Sigh... I've gotta finally spend the time to build one
of these things...
Anybody cares to find out if this gadget actually works, I'd love to
hear about the results -- or, as I said, maybe I'll finally get around
to building one this year, before the weather gets warm again and the
humidity goes up...
