The basic theory behind a working electroscope depends on the atomic arrangements of elements, the induction of the charge, the internal structure of metal elements and the basic idea of unlike charges which attract each other while like charges repel each other.
The electroscope consists of a metal detector knob on the top, which is attached to a pair of metal leaves hanging from the bottom of the connecting rod. When there is no charge, the metal leaves loosely project downwards. But, when a charged object held close to an electroscope, one of the two things are possible.
- When there is a positive charge, the electrons in the electroscope metal are drawn to it and pushes upwards from the leaves. Which results in leaves obtaining a temporary positive state, and since like charges repel, the leaves become separated. As the positively charged body withdraws from the knob, the electrons return to their original location, and the leaves settle.
- When the charge is negative, the electrons in the electroscope metal repel and shift towards the leaves on the bottom. Thus, resulting in obtaining a transient negative charged state of the leaves, and since like charges repel, the leaves split again. As the negatively charged body withdraws, the electrons return to their original condition, and the leaves recover.
In both cases, the electrons return to their original state as soon as the charged body withdraws.
Moreover, the electroscope cannot evaluate whether the charge is positive or negative, but it only indicates the presence of the charge in a body.