This is a photo of my dirt cheap bookshelf magnetometer. The white honey bucket ( drawing, 4 kb )contains a suspended bar magnet with two magnetoresistive components on the outside for measuring the twisting of the magnet in the earths magnetic field. Only one OP-AMP is needed to drive the 1 mA f.s. paper and ink recorder. At the time of taking the photo, the earth magnetic field was in a steady, peaceful state, but during a magnetic storm the recorder pen will traverse from end to end on the scale ( 1 hr/div ). A simple differentiator circuit is all thas is needed to detect potential Aurora Borealis.