Plural of electroscope; multiple instruments for detecting electrical charge through observable physical movement.
From electroscope + -s (plural marker). The plural form reflects widespread use of these instruments in 18th and 19th-century scientific laboratories.
In the 1700s-1800s, electroscopes were in almost every physics classroom and research lab because they were the window into understanding electricity before we had meters and electronic equipment—they're still used in education today.
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