Date: Tue, 28 Sep 2004 08:09:52 -0400 (EDT)

Author: John Mocko

Subject: RE: pith balls



For pith ball demos I use two ping-pong balls coated in the same
graphite/alcohol mixture. Each ping-pong ball is glued to a thin aluminum
rod that I suspend via a paper clip from a ring hook. The ring hook is on
a wooden dowel rod that's connected to a stand yet provides electrical
isolation. I then use a wimshurst generator connected to the ring hook to
charge the balls through the rod. If the opposite sides of the wimshurst
are attached to the two balls then I have opposite charges and they
attract VERY well. It's also very easy to just move both suspended balls
to one ring hook to give both balls the same charge for repulsion.

John Mocko

On Tue, 28 Sep 2004, Dick Heckathorn wrote:

> Greetings,
> I have one centimeter smooth foam spheres that I have covered with
> graphite. I use Aqua Dag from Sgt Welch-Catalog Number 1931. It is
> graphite in Isopropyl alcohol. The alcohol tends to evaporate, but
> adding more rejuvenates it. It does rub off some but easily recoated.
> When bringing them near a charged rod the spheres may or may not repeal
> immediately. I use PVC pipe as one of the materials to charge the
> spheres. They are insulators which mean the charge while there will not
> move around the rod and thus go to where the sphere is and transfer to
> the graphite coated sphere. I and my students have to move the sphere up
> and down the rod to collect enough charges so that it is repelled. I
> have also noticed that if I replace the polyester thread with new ones,
> they tend to work better. I'm assuming that handling the thread
> contaminates it and the charges of the sphere leak off. They also work
> better when the there is less humidity. For my money, this is the
> cheapest and best.
> Dick
> Helping teachers who facilitate, motivating students who learn.
> Dick Heckathorn 14665 Pawnee Trail Middleburg Hts, OH 44130
> 440-826-0834
> Physics Teacher CVCA 4687 Wyoga Rd Cuyahoga Falls, OH 44224
> 330-929-0575 VM 120
> -----Original Message-----
> From:
> [] On Behalf Of John Welch
> Sent: Monday, September 27, 2004 8:06 PM
> To:
> Subject: pith balls
> Hi all. This is my first posting to Tap-L.
> I'm wondering if anyone has any suggestions for really good pith balls,
> either recipes for homemade/modified ones or commercial ones. The ones I
> have from Sargent Welch (the 5mm graphite coated ones) work OK, but the
> graphite rubs off quickly. The behavior I'm trying to get is that they
> repel
> immediately after touching a charged rod, and when they are less
> conductive,
> they don't seem to do this well. I've tried spray adhedsive and then
> dipping
> them in graphite powder, and that works OK, but not perfectly. Also,
> does
> anyone have a way to prevent the strings from attractinng the rod when
> the
> balls repel?
> thanks-
> John


John Mocko
Senior Teaching Laboratory Specialist (Lecture Demonstrations)
Department of Physics
University of Florida
Gainesville, Fl.
From Tue Sep 28 08:13:02 2004