Re: Earth's E Field Meter

[William Beaty <billb@ESKIMO.COM>]

On Thu, 24 Jun 1999, Bob Sciamanda wrote:

> The current (July,'99) Sc. Am. Amateur Scientist column describes a novel
> instrument for measuring the electrostatic field in the atmosphere. You can
> also get to the article at www.sciam.com.
>
> I am concerned that the configuration of metal used in the construction of
> the instrument will significantly disturb the field to be measured (a
> difficulty common to all such atmospheric E measurement techniques).  Any
> comments?  It would make a good construction project for students/faculty.

I think that the device is just a voltage-divider based on capacitors.
The particular configuration of the metal simply determines the division
ratio, and so it would be "calibrated out".  The spinning disk acts as a
"chopper" to convert the DC output into AC (which eliminates the zeroing
problem and the drift caused by leakage across the input impedance of the
actual voltmeter.)

In the early days of FM ham radio, experimenters would attach one plate of
a 2-plate capacitor to a loudspeaker, then use this to frequency-modulate
the main oscillator of their transmitter.  This was called a "wobbulator".
A field mill is a type of Wobbulator.  :)

I think that the sci-am device is a bit too complex.  Any small antenna
which has a spinning or vibrating ground electrode near it will serve as
an e-field transducer.  If the frequency of the grounded chopper is set to
60hz, then any normal voltmeter can measure its output.  If the output of
the device is too low for the meter, then an audio amplifier can be used
to crank it up.

I've been meaning to build one of these because I've always wanted to
measure the potential on a VandeGraaff machine.  If the "field mill" is
locked in place near a VDG machine, and if we first calibrate it by
placing 10KVDC on the VDG sphere via a known DC power supply, then we will
obtain a non-contact VDG output meter.  Only we need to stand far away
while making measurements, so our body capacitances don't become part of
the series capacitor which forms the step-down capacitive divider.  Also,
perhaps the charged wind that exits from the VDG sphere will mess up any
readings.

PS  Isn't the device in the SciAm article upside down?  If I read it
correctly, the author wants us to point it towards the ground, rather than
at the sky.

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William J. Beaty                                  SCIENCE HOBBYIST website
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