Reference: EM39
Category: Electromagnetism
Purpose: Used to measure potential difference.
Description: This electroscope measures the potential difference (between the leaves and the case), and not charge directly. Connect the plate of an electroscope and its case to two parallel brass plates (call them (A) and (B)) mounted on vertical insulating pillars. Charge a Perspex rod by rubbing it with silk (or a Teflon rod with rabbit skin.) Stroke it against plate A. The leaves diverge. Next stroke it against plate B. The leaves collapse, more or less. Now touch B (or the plate of the electroscope) momentarily. The leaves immediately spring apart. This experiment dramatically disproves erroneous notion that earthing the plate of the electroscope removes its charge and the leaves must therefore collapse. We see them doing exactly the opposite on being earthed. If the plates are covered with insulating varnish, rub the rod across the insulating terminals behind them.
