Here I have attempted to derive some basic numbers
for the capacitance and time constant of a cable.
I admit I am somewhat out of practice so someone
scream if I have made any major errors.

r = radius of the wire in meters
b = distance of wire to ground in meters
e = 8.854 * 10^-12 F/m


                     2 * pie * e
C/m = --------------------------
               b + (b^2 - r^2)^0.5
         ln [ -------------------- ]
                             r

I get about 8pF / meter of capacitance between a
small bare wire suspended 1 meter above ground
and ground.

The resistivity of qood quality plastic dielectrics are commonly
10^16 ohm cm and can reach 10^19 ohm cm for teflon.

For a 0.1 mm insulator and a 1mm diameter wire I get
about 0.01 cm^-1 for 1 meter of wire.

Time constants of a wire.

10^14 ohms * 8pF = 800 seconds.
10^17 ohms * 8pF > 9 days

I am making a lot of assumptions here but I could see
where you could achieve some very high time constants.

Many plastics will absorb water so these numbers can differ
by orders of magnitude.

---------------------------------------------------------------------
| Dr. Kevin Dale Kirmse
| PhD Electrical Engineer
|
| King of Prussia, PA 19406
| kirmse@netaxs.com
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