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“The electroscope is an instrument for detecting and testing electrification. A metallic rod passes through the cork of a glass vessel, and terminates on the outside in a ball or a disk. The lower end of the rod carries two strips of gold-leaf or of aluminum foil that hang parallel and close together. When an electrified object is brought near the knob or into contact with it, the metal strips below become similarly charged and are, therefore, mutually repelled.” — Avery, 1895


Elroy M. Avery School Physics (New York: Sheldon and Company, 1895) 417


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