[Vo]: Voltage versus field, and the electrophorus

[Stephen A. Lawrence]

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.

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...