Invented by the English physicist Abraham Bennet in 1786, the gold-leaf electroscope reveals the presence of electrical charges on a body. The instrument consists of a vertical brass rod with two very thin gold-leaf strips at the lower end and a small ball at the top end. A glass dome covers the lower part of the rod and the gold strips, to prevent them from being moved by air currents.

If the conductor is not charged, the strips will hang vertically because of the force of gravity. But if the ball at the top is touched with an electrically charged body such as a piece of electrified amber, part of the charge will spread to the entire conductor. As a result, the strips will acquire an identical negative or positive charge and will repel each other. The angle they form will be proportional to the electrical charge. The phenomenon is due to one of the basic principles of electrostatics: bodies with identical electrical charges repel each other; bodies with opposite charges attract each other. The two tinfoil strips on opposite sides of the dome leak the excess charge to ground.

In about 1787, Alessandro Volta developed more sophisticated devices fitted with a graduated zero-centered scale engraved on or glued to the glass dome to measure the deflection angle of the strips. All of these comparable instruments are known as electrometers.