electroscope

The electroscope is a simple means of detecting electric charge. The metal foil leaves diverge when a charge is applied to the metal sphere.

Apparatus for detecting electric charge. The simple gold-leaf electroscope consists of a vertical conducting (metal) rod ending in a pair of rectangular pieces of gold foil, mounted inside and insulated from an earthed metal case or glass jar. An electric charge applied to the end of the metal rod makes the gold leaves diverge, because they each receive a similar charge (positive or negative) via the rod and so repel each other.

The polarity of the charge can be found by bringing up another charge of known polarity and applying it to the metal rod. A like charge has no effect on the gold leaves since they remain repelled. An opposite charge neutralizes the charge on the leaves and causes them to collapse.