Date: Sat Feb 10 08:11:03 2007

Author: Urs Lauterburg

Subject: Re: dipole


I think this is pretty much what is going on here. Reading your
explanation freed me to think it throughly through myself. We have a
demo to demonstrate the behavior of an induced dipole in an
inhomogeneous E-field. It's a small piece of wood about the size of
a match which is suspended on a fine thread between a flat plate and
a small metallic sphere much the same way Sam has hung his mylar
stick. Applying a pure DC high voltage from a high voltage supply
will first rotate the wood to aline the induced dipole along the
E-field. Then increasing the voltage further the stick is attracted
towards the sphere where the E-field is denser.

However the wood is suspended in a way which does not allow the match
to touch the sphere. Sometimes however when humidity is high enough
it can happen that some charge is transferred to the wood to make it
immediately ''jump'' away.

Manipulating charge in material is indeed interesting and sometimes
tricky theme.

Thanks for the nice movie Sam and the towels for my tears. I
specially like the background music ;-)


Urs Lauterburg
Physics demonstrator
Physikalisches Institut
University of Bern

> Here is what I think is happening. The straw is initially drawn
>to one plate by induction. As it nears, charge begins to leak off
>the straw because the end is like a point. The charge leakage
>reduces the inductive attraction, and the straw stops short of the
>plate in an equilibrium position. When you stop cranking the
>Wimshurst, the charge on the plates start dropping. When it drops
>below the threshold voltage where the plate can draw charge off the
>straw, the straw is drawn closer to the plate, again by inductive
>attraction, and touches it. This could perhaps be tested by
>cranking the Wimshurst much slower to keep the voltage below the
>threshold level. If that proves difficult, maybe a variable HV
>supply would be better...
>At 06:31 PM 2/7/2007, you wrote:
>>... To answer Ziggy's/Adam's questions - the straw does not
>>oscillate as far as I can tell by standing close, and it never
>>touches the plate that it's near. As the Whimshurst slows, the
>>straw actually touches the nearest plate before being repelled to
>>the other plate. That's when this thing behaves normally.