electroscope n : measuring instrument that detects electric charge; two gold leaves diverge owing to repulsion of charges with like sign
An electroscope is a scientific instrument that is used to detect the presence and magnitude of electric charge on a body.
Two classical types of electroscopes are:
- Pith-ball electroscope.
- Gold-leaf electroscope.
Pith-ball electroscopeA pith-ball electroscope, invented by John Canton, comprises a light pith ball suspended by a silk thread from the hook of an insulated stand. In order to test the presence and magnitude of a charge on an object, the object is brought near to the pith ball without coming into physical contact with the object. Two balls are commonly used, and become separated when charged.
If the pith ball is attracted to the object, the inference is that the object's charge is of opposite polarity or sign to that of the pith ball, while if the pith ball is repelled, the two objects have charge of the same sign, though not necessarily the same magnitude.
Gold-leaf electroscopeThe gold-leaf electroscope was developed at the end of the eighteenth century, by Abraham Bennet, as a more sensitive instrument than similar electroscopes using pith balls or straw blades then in use. It comprises an insulated metal rod, usually brass, at one end of which are fitted two pieces of thin gold leaf and with a metal disc at 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 rods at the sides of the foil sections, to impede excessive divergence, capture charge leaking through the air that could accumulate in the glass walls, and increase the sensitivity of the instrument. As of 2007, aluminum foil is also used instead of gold.
When the metal disc is touched with a conductive charged object the leaves of the electroscope diverge. This is because the charge on the object is conducted through the disc and metal rod to the leaves. As 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.
- [Anon.] (2001) "Electroscope", Encyclopaedia Britannica
electroscope in Italian: Elettroscopio
electroscope in Dutch: Elektroscoop
electroscope in Slovenian: Elektroskop
electroscope in Chinese: 验电器