Bruce Cannon writes:
>difference >between your feet and your head (usually a couple hundred volts
>over six feet).
>
>This is really interesting!
>
>But if it's a couple of hundred volts, why do you need a sensitive
>instrument amp?

        You need that because this voltage has almost no current at all,
thank goodness.:-)   Any conductivity in your measuring device will suck
down the potential to nothing so that one will not see it.

        Very high-impedance amplifiers will let enough potential build up
to detect.

        The gradient varies with humidity and weather conditions.  It is even
known to reverse polarity during thunderstorms and lightning is simply
this gradient carried to extreme so a strong dose of common sense is in order
if one wants to experiment with it.

        I once had a wire amateur radio antenna about 20 feet above ground
and forgot to connect it to Earth when a thunder head began to rumble in the
distance.  The lightning was many miles away, but the static gradient was
so high that sparks began arcing the approximately 8-millimeter distance between
the center conductor of the coaxial cable connected to the wire and the
shell of that connector which was ground.  It was just like listening to
an electronic fuel igniter.  I grounded the connector, but one should never
wait until things get that far along before doing something about them.
What I did was flat dangerous and stupid.  It was also really neat to observe.

        There is probably not that much to worry about on a clear day except
for damage to FET's, but be sure to look out for
unsettled weather so you can live to tell about it.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group