What is a gold leaf electroscope?
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. 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 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.
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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.  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 could accumulate on the glass walls, and increase the sensitivity of the instrument. In precision instruments the inside of the bottle was occasionally evacuated, to prevent the charge on the terminal from leaking off through ionization of the air.
When the metal terminal is touched with a charged object, the gold leaves spread apart in a 'V'. This is because some of the charge on the object is conducted through the terminal and metal rod to the leaves.  Since they receive the same sign charge they repel each other and thus diverge. 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.
The electroscope can also be charged without touching it to a charged object, by electrostatic induction. If a charged object is brought near the electroscope terminal, the leaves also diverge, because the electric field of the object causes the charges in the electroscope rod to separate. Charges of the opposite polarity to the charged object are attracted to the terminal, while charges with the same polarity are repelled to the leaves, causing them to spread. If the electroscope terminal is grounded while the charged object is nearby, by touching it momentarily with a finger, the same polarity charges in the leaves drain away to ground, leaving the electroscope with a net charge of opposite polarity to the object. The leaves close because the charge is all concentrated at the terminal end. When the charged object is moved away, the charge at the terminal spreads into the leaves, causing them to spread apart again.
The gold leaf electroscope is a device that measures static electricity.
The animation below shows how the gold leaf electroscope works.
The electroscope is made of metal and metals are good conductors of electricity, so that electrons can move freely in it.
(The atoms of the metal are made of protons, neutrons and electrons. Electrons - negative charges- can move freely; protons -positive charges- and neutrons don't move around. So, electrons are responsible for the flux of charges in an electroscope , a wire and so on...)
The machinery of the electroscope
The gold leaf is the mobile part which you see above.
When the positive rod approaches, the electrons flow towards it (positive charges attract negative charges), leaving the area close to the gold leaf positively charged (because it is deprived of electrons). The positive charges that become exposed repel each other, causing a macroscopic effect which is the movement of the gold leaf. When the positive rod is taken away, things return to what they were before, e.g., the gold leaf comes back down.
If the positive rod touches the electroscope, electrons are removed from the electroscope as they pass on to the rod. As a result the electroscope becomes permanently charged (because of the lack of electrons).
Analogously, a negative rod would leave the electroscope negatively charged by donating electrons to it.