As chairman of the physics department at Mercer University, Randall Peters is more accustomed to helping scientists than designing commercial products. But when he developed an instrument for NASA that detects the Earth's acceleration, he found he'd invented a marketable earthquake detector (see Jan. 17, 2005, page 36). Peters' patented design is now slated to become a commercial product this year. Priced at under $500 for a bare-bones version, it will be the world's least expensive earthquake detector: The price of equally sensitive full-function seismic instruments is closer to $10,000. Peters had set up his instrument to register the Earth's acceleration — but instead it registered an incredibly large earthquake and its aftershocks. By checking the exact timing of the quakes, Peters confirmed that his instrument — which he had built for less than $200 in parts — had detected the Dec. 26, 2004, earthquake in the Indian Ocean that caused the devastating tsunami in Southern Asia. Since then Peters has used his instrument from the comfort of his Macon, Ga., lab to monitor earthquakes worldwide. Peters' design uses a novel means of varying the surface area of a capacitor. Rather than varying its gap as in standard capacitive sensors, Peters' design varies the capacitor's surface area. Because the capacitor's gap is constant, detection is not accompanied by a drop-off in sensitivity, as is the case with other capacitive sensors. Most of those become less sensitive when their gap widens. Since sensitivity and dynamic range don't have to be treated as a design trade-off — as in traditional capacitive sensors — Peters' design sets sensitivity by the constant size of the gap. By changing the surface area, it separately determines dynamic range.