Re: supercaps

[William Beaty <billb@eskimo.com>]

On Sun, 23 Feb 1997, Leigh Palmer wrote:

> The rebound phenomenon with a glass dielectric capacitor is quite
> dramatic. You can demonstrate it with a Leiden jar as well. In
> fact I do it with a disassemblable Leiden jar*. I can charge the
> jar and remove the aluminum electrodes. After touching them
> together, I replace them and a healthy arc can be drawn off the
> jar. After a minute or so I can draw another spark.
>
> It is clear that the energy is somehow stored in the glass. Is it
> chemical energy? What does it matter what it is called?

I haven't experimented with this, but I suspect that the major source of
the above effect is not dielectric polarization, but instead is surface
charges which are painted onto the glass by corona as the metal electrodes
are removed. If you charge a dissectable capacitor to many KV, and then
lower the capacitance by removing the plates or dielectric, the potential
difference will skyrocket.  The edges of the plates will spew charged wind
(same as the needle rows of a VandeGraaff), and might "paint" opposite
charges onto opposite sides of the dielectric.  Conceivably almost the
entire capacitor charge could transfer to the dielectric surface. 

As a control, don't disassemble your charged capacitor but instead try
momentarily shorting it, then see how fast the voltage comes back.  If
this rebound is lots less than the "rebound" attained during dissection
and reassembly, it points to the latter effect not being true rebound.


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