THE PUMPING OF THE heart is controlled by electrical signals, which doctors measure when they take an electrocardiogram. These signals also generate an irregular, ultralow-frequency electric field that extends in a circle around the body. The field is faint. However, unlike the higher-frequency signals transmitted by radio, TV, and cell phones, it can pass through almost any physical obstruction.

Scientists and military engineers at DielectroKinetic Laboratories LLC, a Washington (D.C.) startup, have exploited this field to develop a handheld sensing device to help rescuers find survivors buried under landslides or collapsed buildings. The DKL Lifeguard could also help soldiers locate enemies hiding in an urban battlefield.

The patented Lifeguard draws on 50 years of research on behavior of objects in "nonuniform" electric fields--an area of study that's already yielded pollution sensors and drug-delivery systems. The Lifeguard, however, is the first product to sense the body's field at a distance, says DKL's chief executive manager, Howard Sidman. The high-end, $14,000 model works at a range of 540 yards and is unaffected by higher-frequency radio signals. In tests, the Lifeguard's digital filters distinguished humans from other animals with 100% accuracy, says Sidman.

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