Re: dielectric constant

[brian whatcott <inet@INTELLISYS.NET>]

At 16:45 1/30/01 -0800, you wrote:
> >  >A curious number can be found in many tables of dielectric constant,
> > > the DC dielectric constant of that terribly polar material, water.
> > > Sears & Zemansky give that number as 81, larger by far than any other
> > > in their table.///
> > > Can anyone think of a phenomenon which demonstrates water's very high
> > > dielectric constant? I do know that demonstrating capacitors at high
> > > voltages can be very dangerous, but does anyone know a good demo?
> > >
> > > Leigh
> >
> >
> > A thin rectangular plastic cell is placed between two capacitor plates and
> > the capacitance is noted.  Then fill the cell with water while it's still
> > sitting between the plates and watch the capacitance go way up!  Wolfgang
>
> Thanks, Wolfgang. I've thought of a variant on that which I will try.
> I'm glad someone has seen this done.
>
> After writing the paragraph above I went down to the demonstration
> room where my colleague Jeff Rudd keeps lots of sealing wax and string
> handy. In no time flat we concocted a capacitor which is an adaptation
> of Wolfgang's suggestion. It wasn't spectacular (I had hoped it would
> be), but the capacitance increased by thirty percent when the water
> was added. I will post details, with photos on my web site, tomorrow.
>
> Leigh

[Catching up on some mail...]

Prepare a round plastic tray or a round metal flat bottom pie dish.
In the former case, cover it with Reynolds heavy duty foil (25um)
The diameter is conveniently 30 cm
Cover the inside with a layer of cling foil ("Glad Cling Wrap")
of thickness 12 um
Place a circle of kitchen tissue on the film ("Bounty kitchen
 paper towel roll") of thickness 180 um

Place a circle of aluminum foil diameter 25 cm on top
Press the layers together with a flat bottom glass weight like
a coffee percolator flask.
Connect a C meter.
Pour water on the edge of the upper electrode so as to wet the paper.
Read the C meter again.

Sample readings... dry 240 pF:    wet 19960 pF
apparent dielectric const 83

(The sleight of hand result is doubtless due to a mild compression of the
 kitchen towel when wet. A stronger water vehicle would be better.)






brian whatcott <inet@intellisys.net>  Altus OK
                Eureka!