An early electric instrument that can be used to determine the polarity of the electrostatic charge of an object is the ____.

• A. Ammeter
• B. Voltmeter
• C. Electroscope
• D. Galvanometer

Answer: (C)

An electroscope senses static electricity and can reveal its sign. Its classic form has a metal rod with two light leaves that diverge when the instrument carries a net charge. When a charged object is brought near the top, charges inside the electroscope rearrange and the leaves repel, showing that there is a charge and giving a sense of its magnitude by how far they move. To tell polarity, you can use a grounding step: briefly connect the knob to Earth and then remove the connection. Depending on whether the inducing object is positive or negative, charge will flow to or from the electroscope during grounding, leaving the leaves with a net charge of a particular sign after grounding is removed. The resulting pattern of movement or final deflection lets you infer whether the object was positively or negatively charged. In contrast, ammeters, voltmeters, and galvanometers measure current or potential difference and aren’t used to determine the polarity of a static charge in air.