Re: PHYS-L Digest - 24 Jun 1999 to 25 Jun 1999

["Glenn A. Carlson" <gcarlson@MAIL.WIN.ORG>]

I wondered about this myself.  On flat ground away from objects and
structures, the electric field near the surface should be nearly
perpendicular to the ground, and, over a short lateral distance, should
be nearly constant (though time-varying).

It appears that the design attempts to minimize disturbing the electric
field by a generous use of shielding (of the electric motor and wiring),
non-conducting spacers (at the motor and plates), and by grounding the
sensor plate and the inner trash can.  Close spacing of the plates will
also reduce fringe effects.

One variation I have already considered is replacing the direct drive
with a belt drive.  The motor could then be placed some distance away
from the sensor, and sensor rotating speed could be varied by varying
the diameter of the drive pulleys.  (Would buildup of static charge on
the belt would be a problem?)  A belt drive would also permit deleting
the outer trash can.  Note that the height of the sensor portion of the
device is only a small fraction of the total height, so, if the drive
unit could be placed away from the sensor unit, the device could be
calibrated much more accurately since larger, closely-spaced calibration
plates could be used.

I hope to find time in the next few months to build this device.  I also
hope to vary the design a bit to observe how the field measurements are
affected.  If I get good results, I may offer it as a student project in
my physics class.

The January 1999 issue of Scientific American described how to measure
the earth's magnetic field using a torsion balance.  It would be
interesting to correlate measurements of the earth's electric and
magnetic fields using these two devices.

Glenn A. Carlson, P.E.
St. Charles County Community College
St. Peters, MO
gcarlson@mail.win.org


Bob Sciamanda <trebor@VELOCITY.NET> wrote:
> Subject: Earth's E Field Meter
> Date: Thu, 24 Jun 1999 23:41:27 -0400
>
> 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.
>
> -Bob
>
> Bob Sciamanda
> Physics, Edinboro Univ of PA (em)
> trebor@velocity.net
> http://www.velocity.net/~trebor