Electroscope

An electroscope is an early scientific instrument that is used to detect the presence and magnitude of electric charge on a body. It was the first electrical measuring instrument. The first electroscope, a pivoted needle called the versorium, was invented by British physician William Gilbert around 1600.[1] The pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscope that are still used to demonstrate electrostatics. A type of electroscope is also used in the quartz fiber radiation dosimeter.
Electroscopes detect electric charge by the motion of a test object due to the Coulomb electrostatic force. The electric potential or voltage of an object equals its charge divided by its capacitance, so electroscopes can be regarded as crude voltmeters. The accumulation of enough charge to detect with an electroscope requires hundreds or thousands of volts, so electroscopes are only used with high voltage sources such as static electricity and electrostatic machines.

Gold leaf electroscope:

The gold-leaf electroscope was developed in 1787 by British clergyman and physicist Abraham Bennet,[2] as a more sensitive instrument than pith ball or straw blade electroscopes then in use.[4] It consists of a vertical metal rod, usually brass,[citation needed] 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.[4] 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.[citation needed] 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.[citation needed] 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.[4] 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.[citation needed]
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.