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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