For those interested in more than just what springs fully formed
from Cecil's mouth:

"Detecting the Earth's Electricity" by Shawn Carlson,
Scientific American, July 1999.

"Getting a Charge Out of Rain" by Shawn Carlson
Scientific American, August 1997

Read the articles, build the equipment, use it, and
decide for yourself what's going on.
73,
Tom Donaly, KA6RUH

I want to thank Tom Donaly for mentioning these articles. I worked
very hard of them back when I used to write for Scientific American and
I still think these are great resources. "Getting a Charge Out of
Rain" is actually my favorite of the projects, and it's one I invite
everyone to try. The apparatus described in the article allows one to
measure the electric charge on individual drops of rain as they fall in
a summer storm.

The instrument set a new standard for measuring charges on droplets
because the device is intrinsically calibrated. The calibration is
achieved by using a precision capacitor. Since the value of the
capacitance is precisely known, the charge on the falling drop can be
determined by simple geometry and the size of the voltage pulse
generated as it falls through the detector. As I understand it, some
folks who do these kinds of measurements professionally redesigned
their detectors to take advantage of the technique that this article
introduced.

If you can't find a copy of these articles in your local library, they
are available on The Amateur Scientist 3.0 CD-ROM, which is my personal
archive of all the articles that ever appeared in The Amateur Scientist
department of Scientific American going back to the beginning in 1927.
(Over 1000 projects.) You can find it either on Amazon or on my own
web site at http://www.brightscience.com