Robert Nansel writes:
>I wonder, though, whether you could get around the need for radioactive
>sources by periodically shorting your sense and reference plates together.
>Anybody out there know?

        I don't know why not.  Some people have connected neon lamps and
fluorescent tubes between their pickup wires and Earth which causes the
gas tube to flash each time the potential reaches whatever voltage is the
breakdown voltage for that lamp.  The ionized gas then discharges the
potential until it gets so low that the gas is no longer ionized at which
time it starts to build up again; (sort of lightning in a bottle.)  One could
measure the time between flashes and determine whether the potential was
increasing or decreasing.

Martin McCormick