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From: kenny@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: St Elmo's Fire - 100 v/m ?
Message-ID: <21000088@m.cs.uiuc.edu>
Date: 25 Jul 90 19:29:00 GMT
References: <26@<5498>
Lines: 22
Nf-ID: #R:<5498:26:m.cs.uiuc.edu:21000088:000:829
Nf-From: m.cs.uiuc.edu!kenny    Jul 25 14:29:00 1990


...A foot from your nose, the air is at a potential of +200V.

..Just curious...
..Why can't this be measured with a DVM?

.Because that 10 megohm input DVM is also a much better conductor than air.

.Maybe with something like an electrometer... ??

Yup.  There's an article in an old Amateur Scientist column (back in
the Stong era) about this.  The circuit presented is ridiculous by
today's standards -- lots and lots of vacuum tubes.  I bet someone
could do a real nice job with an instrumentation-grade FET-input
opamp.  Have to be really careful about common-mode noise elimination.
Also would have to bleed the charge off the antenna periodically.  I
may design such a beast eventually; a measure of atmospheric charge is
a valuable datum for short-term weather forecasting.

Kevin, KE9TV
kenny@cs.uiuc.edu
ke9tv@uiuc.edu