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Re: bottle capacitors
From: Esondrmn-at-aol-dot-com [SMTP:Esondrmn-at-aol-dot-com]
Sent: Wednesday, November 12, 1997 11:10 AM
To: tesla-at-pupman-dot-com
Subject: Re: bottle capacitors
In a message dated 97-11-11 01:51:48 EST, you write:
<< From: ghunter-at-mail.enterprise-dot-net[SMTP:ghunter-at-mail.enterprise-dot-net]
Sent: Monday, November 10, 1997 3:36 PM
To: Tesla List
Subject: bottle capacitors
I've been experimenting with capacitors made from 750ml wine
bottles. I found a case of empty bottles at a "boot" sale in East
Anglia for the princely sum of two pounds (that's roughly $3.20).
They are clear, colorless glass, with straight, parallel sides ending
in abrupt, round shoulders. I covered them with foil up to the
junction of the neck & shoulder. I filled them with very strong
salt water topped off with USP mineral oil. I used 12" x 1/4"
threaded brass rods through plastic champagne stoppers for top
terminals.
I measured the output of my Transco neon transformer at 15,200v and
32ma with a Fluke multimeter. This is at 50Hz, as I'm using UK
mains power and a stepdown autotransformer.
I found that individual bottles could not withstand 15kv. Two of four
bottle caps I tested failed instantly with a loud "snap" as the
voltage punched right through the glass. I have plenty of bottles,
so I continued testing.
I found that two bottles in parallel drew 19ma of current from the
neon. Three caps pulled 25ma. Four caps pulled the max rating of
32ma. Apparently, two or more parallel bottles pull enough current
to drop the voltage to a level that can't puncture the glass. I put
two bottles in series (for about 950pf) and ran them for several
minutes with no failures. I switched the wall current on and off
several times to create voltage spikes, but they didn't fail.
The bottle caps have much higher capacitance than I expected. All
of the www Tesla design software I have tried predicted much
lower capacitance for my bottle caps than I actually measured. The
real-world value seems to be around 1900pf per bottle, with a spread
of 1770pf to 2015pf. The puncture voltage is also much lower than I
expected. I wouldn't have believed that 15kv could break them down
if I hadn't seen it myself. On a broken bottle I measured the
sidewall glass thickness to be from 1/16th to 3/32nd of an inch (the
glass is a bit wavy).
I'm going to employ six or eight of these on my 4 1/2" dia. Tesla
coil (3 in series with 3, or 4 in series with 4). It will be
interesting to see if they can hold up to the harsh, flywheeling
voltage swings of a primary tank circuit.
Greg
>>
Greg,
Good information. You will get more capacitance out of your bottle caps if
you set them in a metal pan (or plastic pan lined with foil) and fill the pan
with salt water just as you did the inside - including oil on the top. The
water will form a much more intimate surface contact with the glass than you
can get from the foil. It would be interesting to see from your testing just
how much difference this makes. You would need several bottles in the pan
with all the center contacts in parallel. The pan itself becomes the other
contact. Then, as you say, you would need another container with another set
of bottles to connect in series with the first.
Nikola Tesla also had a problem with breaking bottles. He ordered bottles by
the hundreds and ended up with many in parallel/series connections to get the
values he needed at the appropriate voltage levels.
Ed Sonderman