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National Oceanic and Atmospheric Administration, (NOAA) offers a crowdsourcing smartphone app that allows you to upload magnetic field data. It's fun to use and very educational.https://www.ngdc.noaa.gov/geomag/crowdmag.shtmlhttps://www.ngdc.noaa.gov/geomag/crowdmag.shtml#appsCrowdMag project, we explore whether digital magnetometers built in modern mobile smartphones can be used as scientific instruments. With CrowdMag mobile apps, phones all around the world send magnetometer data to us. At our server, we check quality of the magnetic data and make data available to the public as aggregate maps and charts.
I hve been mystified as to why the device hanging vertically from a single string doesn't dip the expected amount but with two horizontal threads does. I think it is because the compass is suspended slightly off center and above the long axis of the screws so in the gravitational field it twists back toward the horizontal. I liken it to a pendulum rocking back and forth under gravity while subject to a magnetic field. The sum of the forces on the compass as it hangs by a single thread greatly reduces the dip.