Ans. ELECTROSCOPE
A gold leaf electroscope is a sensitive instrument used for detecting charges.
Working Principle
It consist of a brass rod with a brass disk at the top and two thin leaves of gold foil hanging at the bottom. The rod passes through an insulator that keeps the rod in place and also retains the charges. Charges can move freely from the disk to the leaves through the rod. A thin aluminium foil is attached on the lower portion of the inside of the jar. Usually, the aluminium foil is .grounded by connecting a copper wire. This protects the leaves from the external electrical disturbances.