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 > > > > _____________________________________________________________________ > > > > Public Seismic Network Mailing List (PSN-L) > > > > To leave this list email listserver@.............. with the body of the > > message: leave PSN-L > > _____________________________________________________________________ > > Public Seismic Network Mailing List (PSN-L) > > To leave this list email listserver@.............. with the body of the > message: leave PSN-L _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>