What is a gold-leaf electroscope used for

This gold-leaf electroscope was used for detecting and measuring static electricity or voltage.

Developed on 1787 by British clergyman and physicist Abraham Bennet, it consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf, as the name suggests it. A disk or ball terminal is attached to the top of the rod, where the charge to be tested is applied. To protect the gold leaves from drafts of air they are enclosed in a glass bottle, usually open at the bottom and mounted over a conductive base. Often there are grounded metal plates or foil strips in the bottle flanking the gold leaves on either side.