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From: irwin@m.cs.uiuc.edu
Newsgroups: sci.electronics
Subject: Re: St Elmo's Fire - 100 v/m ?
Message-ID: <21000089@m.cs.uiuc.edu>
Date: 25 Jul 90 21:03:00 GMT
References: <26@<5498>
Lines: 47
Nf-ID: #R:<5498:26:m.cs.uiuc.edu:21000089:000:1916
Nf-From: m.cs.uiuc.edu!irwin    Jul 25 16:03:00 1990


/* Written  2:29 pm  Jul 25, 1990 by kenny@m.cs.uiuc.edu in m.cs.uiuc.edu:sci.electronics */
>...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
/* End of text from m.cs.uiuc.edu:sci.electronics */


I hobby in radio contolled model aircraft, and recall that about
15 years ago, a Maynard Hll from JPL labs, who also flies R/C
models, built an electrostatic auto pilot. He used op amps, the
sense elements on the wing tips tied to the op amps was elements
removed from a draftman's brush, obtained at a local drafting
supply store. I do not recall what the material was.

The op amp output was tied into the servos such that the R/C
system could over ride the auto pilot, but the autopilot would
maintain straight and level flight, by measuring the voltage
in the air at the wing tips. He also had one on the tail of the
aircraft.

This is the same Maynard Hill that flew models of the 747 and
shuttle at JPL labs to test the piggy back set up in model form,
before they tried it full scale. There are video tapes available
that our model club has checked out for viewing, showing this
testing.

Al Irwin
irwin@cs.uiuc.edu