Here's how it works:

* Basic Principle: An electroscope uses the principle of electrostatic repulsion. Like charges repel each other.

* Construction: It consists of a metal rod with two thin, lightweight leaves of gold foil or aluminum foil attached at the bottom. The rod is usually enclosed in a glass container to protect the leaves from air currents.

* Detection: When a charged object is brought near the electroscope, the charges in the object induce opposite charges on the leaves. Since the leaves have the same charge, they repel each other and spread apart. The degree of separation of the leaves indicates the strength of the charge.

There are other instruments for measuring electric charge, but the electroscope is a classic and fundamental example.