Suggest a way to determine the charge sign of a charged electrometer if you have a piece of fur and an ebony stick at your disposal.

Rub the fur on the shelf. It is necessary to bring the charged stick to the charged electrometer. If the angle of deflection of the electrometer needle increases, then the electrometer is negatively charged, and if it decreases, it means that the electrometer is positively charged.

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