Most folks think about the earth's magnetism only when they need to find their way in the wilderness. When we look at a compass, the earth's magnetic field appears to be a steady guide. But in reality the magnetic field is anything but stable. Ephemeral undulations, called micropulsations, ripple about the ionosphere and generate magnetic disturbances that reach down to ground level. Although they are common and sometimes last from seconds to minutes, these magnetic disturbances are hard to detect, having barely one ten-thousandth the strength of the earth's average magnetism.

REFLECTIVE BEAM ...from this laser pointer will oscillate visibly when the earth's magnetic field in the horizontal direction varies by even a tiney amount--as it will when ionospheric storms create magnetic micropulsations. To investigate the field's vertical component the equipment must be mounted with the magnets parallel to the ground. (Note: In this diagram the plane of the nulling magnets, which should be parallel to the plan that the penny lies in, has been rotated slightly to provide a better view of the equipment.)

For decades, the high cost of sensitive magnetometers has made tracking these signals the exclusive privilege of professionals. But now, thanks to the creative genius of Roger Baker, a gifted amateur scientist in Austin, Tex., anyone can easily study magnetic micropulsations. Baker's magnetometer costs less than $50, yet it can easily capture those tiny pulsations as well as the occasional dramatic effects of a magnetic storm high in the ionosphere.

Baker's device employs one of the most sensitive instruments in science. It's called a torsion balance, and it measures a force by using it to twist a fine filament. The thread gently resists rotation with a torque that grows until it just balances the torque created by the applied force. The resulting angle of deflection, which is found by bouncing a light beam off a small mirror attached to the filament, is proportional to the force under study. With the beam from a laser pointer and a match-head-size mirror, one can in principle resolve deflections as small as one ten-millionth of a degree.

Most professional torsion balances use fine quartz fibers, which are incredibly strong and insensitive to changes in humidity and temperature. Sadly, quartz fibers are difficult for amateurs to come by. But Baker has found that nylon fibers also work quite well. Start with silky, multifilamented nylon twine, which you can purchase at any hardware store, and cut a 30-centimeter (one-foot) length. Next, gently unravel it and use tweezers to select the finest strands, which should be about 25 microns (0.001 inch) thick.

Baker installs the nylon filament into a simple case made from window glass. Cut two strips five centimeters wide and 15 centimeters long using a glass cutter. (Most hardware vendors sell glass and will also cut it for a nominal fee.) These pieces serve as the vertical walls of the case. Then cut eight glass strips one centimeter wide by five centimeters long and use silicone cement to glue pairs together back to back. Finally, glue one pair of these small glass strips to the top and one to the bottom of each of the longer glass walls. The smaller pieces will act as spacers between the walls [see illustration above].

When the glue sets, cover the horizontal spacers of one of the walls with a layer of stretchy, black vinyl electrical tape. The tape prevents the glass from cutting the fiber. Next, lay one end of the fiber across the center of the top spacer and tack it in place with a small dollop of epoxy; secure it with another strip of tape. The epoxy will keep the thread from slipping over time. Baker generates the necessary tension in the fiber by dangling four nickels attached temporarily to the end. He then epoxies and tapes the lower end of the thread into place against the bottom spacer, locking in the tension.