The bullet has a small magnet inside that lets it generate AC current as it spins and a microprocessor that measures those current oscillations to derive how far it's traveled.
子弹藏有一块小磁铁和一颗芯片，子弹旋转带动小磁铁产生直流电，芯片通过电流波动感知射程的远近。
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Meanwhile, the company that pioneered much of today's electric-vehicle knowhow, AC Propulsion of San Dimas, California, has steered clear of permanent-magnet technology.