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Re: plastic bottle caps

Subject: 
        Re: plastic bottle caps
  Date: 
        Thu, 27 Mar 1997 09:38:44 -1200
  From: 
        Ken Smith <ksmith-at-ihug.co.nz>
    To: 
        Tesla List <tesla-at-pupman-dot-com>


>Bottle capacitors can be built from glass bottles with a bolt
>down the middle.....
>Can a plastic bottle be used instead, I have these huge 2 litre 
>soft drinks bottles and was wondering if they are appropriate.
>
>Second question, what does the oil that is placed on top of the
>salt water used for ?

Kenneth,
          Hi again.  

Let me do the easy bit first.   Oil is used on top of Leyden / Bottle
caps
to stop flashover.  It is not a critical component and any *CLEAN* motor
oil
will do nicely.

Back to the bottles :  I have some words on glass first.  I spent six
months
making glass based caps and probably the best way to indicate my
experience
will be a dot list :

        Beer bottles are a good size, but often erratic quality.

        Quality is important.  Even the smallest air bubble or flaw will
cause          failure.

        Good glass makes good caps, but is hard to find and is
expensive.

        Champagne bottles are very good quality usually and emptying
them
takes         your mind off making caps.

        Kilner pickling jars make excellent caps, but you will have to
make new 
        lids unless you have glass drilling facilities / skills.

        Smooth glass is a must, patterns and etching is NO NO.

        1/4 inch copper pipe with a flattened / drilled end, hotglued in
place
        is better than bolts and can be made to length. And cheaper.

        Masking tape is fine for keeping the foil on the outside firm
against         the glass.  If you are in series then keep them at least
an
inch apart.

Ok, now onto the plastic bottles.  On the face of it they would seem too
fragile and plastic has a dielectric constant of about 2.5 as against
glass
8.0, so you would need some 4 times the area.  However these plastic
bottles
are very cheap - so a quick measure and calculate was in order.

Drinking a pint of lemonade for breakfast has not made my day, but I do
now
have a bottle on the desk.  I used a 1 litre Schweppes bottle for the
measurements and I have no knowledge if the 2 litre would be any
different
in thickness. However :

Useable height 18cms
Radius 4.5 cms
Thickness 10 mil = 0.25mm (I cut 4 squares for this to average it out
and
give me better accuracy (my guage is a little wonky on the small
numbers))

So taking a Dielectric of 2.5 a 1 litre bottle would give .005 uf and
the
wall thickness would suggest a puncture voltage of about 6,000v on the
conservative side.  This would then need 5 in series to give a realistic
30,000v to achieve a capacitance of only .0013.  Paralleling these back
up
to .01uf for say a 15000v/60ma Neon would be a lot of lemonade and a lot
of
space.  Say 30+ bottles - thats a lot of work, salt water, foil, tape
and
wiring for something that may be fragile under load. And if you get a
puncture an awful mess on the floor. So a baby bath or something would
be
needed to put them all in.

My verdict of poly drink bottles.  No thank you.

Do the same figures though for a 1 litre glass bottle (say a nice '84
Chataux Plonk).

Useable Height 18 cm
Radius  4.5 cm
Thickness 3.8mm = 150 mil

This gives a capacitance of around 0.001 uf and a puncture voltage of
30,000V (using 200 volts / mil) so there is no need for a series. 
Nominally
10 of these would give you the desired 0.01uf and you can add or
subtract
bottles from the *crate* as required.  And I do mean crate.  You can get
plastic crates with solid / waterproof bottoms - these are ideal - stops
the
mess if a bottle cracks.  Do clean the outsides carefully and wrap the
foil
tightly with a couple of bindings of masking tape.  Insulation is not a
problem, but containing the glass if it cracks in service is.

If you want to play with salt caps this is the way to go. However you do
need a stable gap since the ability of glass to survive too many surges
is a
problem.  I would recommend you make a Quick gap as a priority anyway
and
leave the bolts protruding around the outside of the plastic pipe and
then
you can use a jumper lead to adjust the number of gaps in service. 
Start
low and work up.  Also have a safety gap in place as well which will
stop a
large surge getting at the neon as well as the bottles.

Glass / saltwater is a viable capacitor, but the spark is rather reedy
and
green compared to the fat blue thumps you get with poly / oil.  As I
have
said I worked with glass for some time until I found the materials and
incentive to make a couple of decent poly /oils.  I found I wasted a lot
of
time maintaining and replacing bottles that I could have used on the
main
event.  Now I just clip on the wires and fire up....  Also two 6 inch by
2.5
ft tubes are a lot easier to move around than a crate of bottles <g>.

I hope all this helps.  It was never ment to be a treatise on salt caps
and
it scans badly, but I hope you find the details useful.


Ken


Ken Smith
Weymouth
Auckland 
http://homepages.ihug.co.nz/~ksmith
ksmith-at-ihug.co.nz