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. The pith-ball electroscope and the gold-leaf electroscope are two classical types of electroscope that are still used to demonstrate electrostatics.
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.
This attraction occurs because of induced polarization of the atoms in the pith ball. The pith is a nonconductor, so the electrons are not free to leave their atoms and move about the ball, but they can move a little within the atoms. If, for example, a positively charged object is brought near the ball, the negative electrons in each atom will be attracted and move slightly toward the side of the atom nearer the object. The positively charged nuclei will move slightly away. Since the negative charges are now nearer the object than the positive charges, their attraction is greater than the repulsion of the positive charges, resulting in a net attractive force. This separation of charge is microscopic, but since there are so many atoms, the tiny forces add up to a large enough force to move a light pith ball.
The pith ball can be charged by touching it to a charged object. Then the ball can be used to distinguish the polarity of charge on other objects, because it will be repelled by objects charged with the same polarity or sign it has, but attracted to charges of the opposite polarity.
Often the electroscope will have a pair of suspended pith balls. This allows one to tell at a glance whether the pith balls are charged. If one of the pith balls is touched to a charged object, charging it, the second one will be attracted and touch it, communicating some of the charge. Now both balls have the same polarity charge, so they repel each other, and hang in an inverted 'V' shape with the balls spread apart. The distance between the balls will give a rough idea of the magnitude of the charge.
The gold-leaf electroscope was developed in 1787 by 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 metal disk or ball is attached to the other end where the charge to be tested is applied. To protect the gold leaves from drafts of air they are enclosed in a glass bottle open at the bottom, mounted over a conductive base. Usually 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.
When the metal disc is touched with a conductive charged object the gold leaves spread apart in a 'V'. This is because the charge on the object is conducted through the disc and metal rod to the leaves. Since they receive the same sign charge they repel each other and thus diverge. If the metal rod is touched with a finger, the charge is transferred through the human body into the earth and the leaves of electroscope close together.
If the electroscope is brought near a charged object, without touching it, the leaves also diverge, because charges in the disk with identical polarity to the charged object are repelled to the leaves. If the electroscope is touched, and hence grounded, in this condition, the leaves close, but spread again if the charged object is moved away. The electroscope becomes charged with polarity opposite to the polarity of the charge in the object. The electroscope was charged by electrostatic induction.