Image: DANIELS & DANIELS

In January of last year I described a delightful device for detecting microfluctuations in the earth's magnetic field. The instrument was a sensitive torsion balance consisting of two small rare-earth magnets affixed to a taut nylon fiber with a tiny mirror attached to the fiber to reflect a laser beam onto a distant wall. When the instrument was properly nulled with additional magnets to cancel the earth's average magnetic field, an infinitesimal change in the earth's field rotated the rare-earth magnets and deflected the laser beam.

Originally developed by Roger Baker of Austin, Tex., this homemade magnetometer created quite a stir in the amateur community. But the device required constant visual monitoring to collect data, so it wasn't really suitable for serious science. Baker, however, suggested how someone could convert his unit into a research-grade instrument. This month I'm delighted to report that Joseph A. Diverdi, a chemist in Fort Collins, Colo., has met that challenge brilliantly.

Following Baker's suggestions, Diverdi placed the magnetometer at the center of a pair of Helmholtz coils, a special electromagnet that produces an extremely uniform magnetic field. Diverdi also designed a detector that could sense tiny displacements in the laser beam's position, and he developed a feedback circuit that runs just enough current in the coil to create a countermagnetic field that precisely cancels any external shift. The current necessary to keep the beam fixed thus tracks the changing field, and a personal computer can record these measurements directly through an analog-to-digital converter.

You'll find a complete description of the Baker-Diverdi magnetometer on Diverdi's Web site. I've reserved this column to give an overview of the device and to offer some fine-point kibitzing.

Image: DANIELS & DANIELS

For the Helmholtz coils, Diverdi started with two identical stiff cardboard rings about 6.5 centimeters (2.6 inches) in diameter, but you can use larger rings cut from a cylindrical oatmeal container. Mount the rings on a wooden base parallel to each other at a separation equal to their diameter [see illustration].

Next, purchase a spool of 30-gauge enamel-coated magnetic wire from an electronics supply store and neatly wrap each ring with 40 turns of the wire. Use one continuous length for both coils so that the same current passes through them and wrap them both in the same direction, either clockwise or counterclockwise. Secure the wire loops with a liberal dose of hot glue.

Diverdi soldered two lead wires to the coils and insulated the joints with shrink-wrap tubing. He hot-glued the tubing to the base of the assembly, leaving some slack so that the wires wouldn't break while the coils were being handled. Because the magnetic field is most uniform at the center of the Helmholtz assembly, be certain to position the rare-earth magnets of the magnetometer there.

To detect minute changes in the laser beam's position, Diverdi has devised a clever solution. He shines the beam on a small slide of frosted glass with two cadmium sulfide photoelectric cells (Radio Shack part no. 276-1657) positioned a few centimeters behind. The glass spreads the beam, which illuminates the photocells. If the beam is centered directly between the cells, equal amounts of light will shine on each of them. If, however, the beam is displaced even slightly, the output of the photocells will change measurably.

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