This is a device to measure or compare static charge. It consists of a metal cap with a rod, with a thin gold leaf attached to it, electrically isolated from an earthed case with a black wax plug. When a charged object is brought near to it, the leaf diverges. This one has a scale to measure the angle of divergence. This happens because electrons can move through the metal and either collect at the cap if the object is positively charged, or on the stem and leaf if negatively charged (electrons either repelled or attracted.) In either case, the charges on the leaf, being the same, will repel each other. The closer the charged object is brought to the cap, the greater the divergence. The device effectively behaves like a voltmeter measuring the p d between leaf and case. See here for an animation.