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An electroscope is a sensitive instrument for detecting small electric charges. It consists of a glass-jar closed with a stopper of insulting material in which is fitted a varnished glass-tube. A rod passes through the tube. At the top of the rod there is a metal ball or disc and at the bottom of the rod two pieces of gold leaf are suspended. When a charge is brought near the electroscope, a charge of opposite sign is induced on the metal ball, and a charge of the same sign appears on the two of the gold leaves. Since, the two pieces of gold leaf now have charges of like sign they repel each other.