RE: Quick and dirty capacitors
In response to renewed tesla list interest in salt water caps, I made a
couple of measurements myself.
First, I immersed a "Miller Genuine Draft" 12oz. beer bottle in salt water
up to the point where the neck meets the shoulder. This is a clear glass
bottle, somewhat shorter than the longneck bottle. I filled the bottle to
the top with strong brine. The cap function on my cheap DMM read this one
at 930pF (.00093uF) after discarding the capacitance of the test leads.
Next, I wrapped the same bottle in kitchen foil made sticky with a shot of
spray glue. It was a smooth, tight covering, including the bottom of the
bottle and extending up to the neck/shoulder junction. I burnished it good
with the back of a spoon until it was slick as chrome. Topped up with
brine, this "dry" cap read 890pF after subtracting the stray capacitance.
Although my meter is a cheapie, I have verfied its accuracy by testing
several commercially made caps of known value with it. It reads right on
the button on every scale.
FYI, I routinely use .005uF as the approximate value for a six pack of 12oz
beer bottle or wine cooler bottle caps. The above measurements confirm
that my guesstimate is plenty close enough for hobby coiling. Our overseas
cousins can probably use the same value with 330ml beer bottles--it will be
I have read web & list literature concerning the puncture voltage & "K"
value of various dielectrics. According to this literature, my glass
bottle caps should fail when used with my 15kv NST. However, I have never
experienced a HV puncture. I have also used clear 750ml wine bottles
(1300pF each) with the same NST, and none of them have punctured either. I
think the puncture voltage for glass listed in standard dielectric tables
is too conservative. Experience shows they can handle 15kvac operation
with no trouble.