The bright beam of a laser pointer completes the instrument. (Radio Shack sells one pointer for less than $30.) Position the laser so that the beam shines through the glass case and bounces off the mirror and onto a distant wall. Ripples in the earth's magnetic field will show up as deflections of the beam.
Although at this point you're ready to do real science, Baker has improved the device further. By wrapping wire onto a rolled-oats container, he has fashioned a pair of coils to calibrate his creation. He uses the so- called Helmholtz arrangement of two circular coils (ones separated by half their diameter) to make the magnetic field in the center uniform. Such Helmholtz coils can also be combined with photocells and electronic feedback to keep the laser beam fixed as the earth's field changes. Measuring the current needed to null the signal in this way shows the size of the magnetic fluctuations. Using a computer to monitor the current will produce a stream of measurements that you can record and later analyze.
This article was originally published with the title Taking the Earth's Magnetic Pulse.