The gold-leaf electroscope was developed in 1787 by British clergyman and physicist Abraham Bennet, as a more sensitive instrument than pith ball or straw blade electroscopes then in use.*[Anon.] (2001) "Electroscope", Encyclopaedia Britannica It consists of a vertical metal rod, usually brass, from the end of which hang two parallel strips of thin flexible gold leaf. 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. 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.
