Re: Disectable Capacitor

["John S. Denker" <jsd@MONMOUTH.COM>]

Hi Folks --

Here's what may be going on in the Cenco dissectable capacitor.  (There's
no money-back guarantee, since I've never seen the device in question, but
this hypothesis is easy to check.)

1) Remember that a dissectable capacitor violates Kirchhoff's laws.
(That's their main purpose, AFAIK.)  I'm dubious about the pedagogic value
of such a thing.

2) The length of a spark is a poor indicator of voltage.  It depends on
(among other things) electric field, and the field depends on geometry as
well as voltage.  The violence of a spark depends on the impedance as well
as the voltage and geometry of what's driving it.

3) In the dissectable capacitor, the relationship of the insulator to the
conductors is, I believe, a distraction, so consider the following modified
version:


        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   R--------R   Q i P
        P i Q   RRRRRRRRRR   Q i P
        P i Q                Q i P
       P i QQQQQQQQQQQQQQQQQQ i P
       P iiiiiiiiiiiiiiiiiiiiii P
        PPPPPPPPPPPPPPPPPPPPPPPPPP

where all the capital letters are metal, and (i) is the insulator.  The
metal "liner" (Q) is something I've added to clarify the situation.

So....

a) Imagine that the inner plug (R) has been removed.

b) Further imagine that the (PQ) capacitor has a large charge on it.  This
is, ironically, in compliance with the Kirchhoff law that says all charges
should be confined inside capacitors.  Therefore the usual checks for
"static electricity" will indicate zero unbalanced charge.

c) Replace the plug (R).  If the (QR) gap is small, or better yet if (Q)
and (R) are in solid electrical contact, (R) now becomes one terminal of
the capacitor.  If you short-circuit from (P) to (R), then a goodly WHAM is
to be expected.

--- jsd


At 04:45 PM 3/3/99 -0600, John L. wrote:
> I recently purchased a Cenco disectable capacitor which consists of an
> outer aluminum can (no top), a plastic (beaker) which fits nicely inside,
> and an inner aluminum can which has a conducting post attached.
>
> I can easily take apart the capacitor with small (<1mm) sparks to my hand.
> I then touch all pieces which appear to have been discharged.
>
> Then I reassemble the combination as I did in the first place.  Then I =
> touched the two conductors together and WHAM, a huge spark (>1cm) passes =
> from conductor to conductor.