Re: pure water capacitor?
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Wednesday, February 16, 2000 4:42 PM
Subject: pure water capacitor?
>Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
>I'm a very new member to the TC community. I've been interested in
>building one, or two for quite some time, but have never had the
>opportunity ($$$, tools, ect) until recently.
>I'm a senior majoring in Engineering Phyisics (don't ask....) and am
>involved with the SPS (Society of Physics Students). The SPS occasionally
>gives physics demos to local schools. We have a POS 250kV 1/2 wave TC that
>must be 20 years old. I thought that a nice medium-sized 1/4 TC might make
>an excellent "gift" to the SPS as a going-away gift.
>I've been trying to do some research regarding design issues, when I came
>across a list of dielectrics for some materials for use in capacitors. The
>dielectric constant for pure water is around 85 with an infinite breakdown
>voltage! This means that two 12" x 12" plates spaced 1/8" apart will give
>a capacitance of .020uF! I thought it would be a pretty sweet idea: a pure
>water cap in a plexiglass (or lexan (?)) box. But, I have NEVER seen this
>mentioned anywhere I've looked. The only problem I can see is
>contamination: everything has to be 110% clean before adding water. This
>Are there any reasons why I shouldn't try to use this?
Water has a high dielectric constant, but also intolerably high leakage
current at TC frequencies. Water works as a fine dielectric in those 1 nSec
pulsed power system, but as the frequency starts to drop, the current starts
to rise. I think someone has also tried glycerine as a dielectric (high
epsilon, too, for the same reason: it is highly polar)
As soon as any bias is applied across the cap, the water starts to
dissociate, and whatever you used as electrodes starts to dissolve.
>For the record, I'm looking at a 15kV/120mA+ NST.