PSN-L Email List Message

Subject: Re: Worldwide magnetic recordings of satellite reentry
From: "Raul J. Alvarez" ralvarez@........
Date: Thu, 03 Feb 2000 17:36:16 -0700

Thanks for your offer to place the .gifs on your page.

Raul Alvarez

BOB BARNS wrote:

> Cap,
>   Many thanks for your very lucid description of the magnetometer--it
> sounds like a good thing to build.
>   If you mail me the drawings, I will make them avail. as gifs on my web
> site.
> Bob Barns
> 63 Martins La.
> Berkeley Heights, NJ 07922
> CapAAVSO@....... wrote:
> >
> > In a message dated 02/03/2000 10:19:54 AM Eastern Standard Time,
> > roybar@........ writes:
> >
> > << Is there a link to construction details, etc. about McWilliams
> >  magnetometer?
> >  Bob >>
> >         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > Hi Bob,
> >     There is no link but I can tell you briefly how it works. Later I could
> > send you detailed drawings and instructions how to build it. These are from
> > the August 1998 Solar Bulletin published by the American Association of
> > Variable Star Observers (AAVSO). I would need a postal address or FAX number.
> >     The basic design is a torsion magnetometer consisting of a bar magnet
> > suspended on a torsion wire that can be a piece of guitar string 0.2 mm
> > diameter and about 25-40 cm long. The bar magnet should be about 6 mm
> > diameter and 5 cm long. The magnet is mounted with epoxy in the middle of a
> > thin wooden vane about
> > 1cm wide and 20cm long. The torsion wire can be epoxied to the center of the
> > magnet so the vane hangs horizontal. The upper mount for the torsion wire is
> > then rotated to provide enough torque to make the wooden vane point east and
> > west instead of its preferred north-south direction that it seeks as a
> > compass. The vane is now torsion balanced against the Earths magnetic field
> > and will rotate in response to changes in the strength of the field due to
> > magnetic storms. Beneath one end of the wooden vane are two Radio Shack
> > photocells and above it is a 12-volt automobile bulb, the kind with a
> > straight filament. The bulb and the photocells are arranged so the shadow
> > vane shades one half of each photocell from the lamp above. The linear design
> > of the photocells should be oriented so the lines are perpendicular to the
> > edge of the shadow vane. The photocells are variable resistors. their
> > resistance varies in direct relation to how much light falls on them. If they
> > are equally shaded their resistance is equal. They are made two legs of a
> > four-leg Wheatstone bridge. The other two legs can be 5000 ohm 1/4 Watt
> > resistors. A regulated 9-volt power supply is connected across the bridge and
> > also lights the 12-volt automobile lamp (9-volts is plenty for this
> > application). A chart recorder or A/D converter and computer is connected
> > between the centers of the two legs of the bridge. The two resistors in
> > series have 9-volts across them but at the center the voltage is half as much
> > or 4 1/2-volts. There is also 9-volts across the two photocells that are also
> > connected in series. At the center where they are connected together the
> > voltage is also 4 1/2-volts provided the shadow vane shades the photocells
> > equally which is the way it should be set initially. The recorder is
> > connected between these two 4 1/2-volt points and since they are of equal
> > potential, no current flows and the recorder reads zero (adjust it or bias it
> > so the zero point is in the middle of the chart). When the strength of the
> > Earth's magnetic field changes in response to a solar flare of coronal mass
> > ejection, the shadow vane's balance by the torsion wire becomes unbalanced
> > and rotates the position of the shadow vane above the photocells. this
> > increases the resistance of one photocell and lowers the resistance of the
> > other and unbalances the Wheatstone bridge so current flows in the recorder.
> > It moves it up or down depending on whether the flare-induced current in the
> > Earth's magnetosphere adds to or subtracts from the normal earth magnetic
> > field strength.
> >     The beautiful part of the McWilliams magnetometer is its simplicity. It
> > produces recordings equal to those of professional flux gate magnetometers at
> > the USGS magnetic observatories. Furthermore it needs no amplification and
> > can easily drive a 0 to 1 milliamp recorder directly and is sensitive enough
> > to spread a magnetic storm over the full chart.  Most AAVSO magnetic storm
> > observers use Rustrak strip chart recorders running 1/4 inch/hr but it can
> > also be recorded on a computer using Windaq. For details on recording with
> > Windaq email Jerry Winkler at . He will be glad to help
> > you. I should mention that the shadow vane must be damped like a seismograph
> > and for the same reason. A damper in an oil cup under the shadow vane will do
> > the trick. Don't use magnetic damping for obvious reasons and place it as far
> > as possible from big steel objects.  Place the magnetometer far from
> > driveways. Cars passing by at a distance of ~40m will put tiny blips on the
> > chart but moving a car in your driveway will send it off scale. The
> > McWilliams photocell sensor would probably work quite well on a seismograph.
> > Have fun,
> > Cap
> >
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> >
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Larry Cochrane <cochrane@..............>