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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