Electroscope

The first electroscope was a pivoted needle (called the versorium), invented by British physician William Gilbert around 1600.

[3] The pith-ball electroscope, invented by British schoolmaster and physicist John Canton in 1754, consists of one or two small balls of a lightweight nonconductive substance, originally a spongy plant material called pith,[4] suspended by silk or linen thread from the hook of an insulated stand.

[5] Tiberius Cavallo made an electroscope in 1770 with pith balls at the end of silver wires.

[11] It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf.

[11] 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.

These are a safety measure; if an excessive charge is applied to the delicate gold leaves, they will touch the grounding plates and discharge before tearing.

They also capture charge leaking through the air that accumulates on the glass walls, increasing the sensitivity of the instrument.

In the precision instruments the inside of the bottle was occasionally evacuated, to prevent the charge on the terminal from leaking off through the ionization of the air.

If the terminal is grounded by touching it with a finger, the charge is transferred through the human body into the earth and the gold leaves close together.