It is an instrument used to detect and test small electric charges. It works on the principle that like charges repel each other.
Take two inflated balloons and hang them form a coat hanger from the same point. They will hang free touching each other.
Now charge the balloons by rubbing them with wool. What happens to the balloons? They repel each other.
Gold-leaf Electroscope :-
A simple electroscope consists of a glass bell jar resting on a circular wooden base. The mouth of the bell jar is closed with an ebonite stopper, having a central hole. A brass rod passes through this hole. There is a circular brass disc at the upper end of the brass rod and two gold leaves cut from gold foil at this lower end. Two strips of tin foil are fixed to the inside of the bell jar from the level of the gold leaves to the wooden base, which is also covered with tin foil. When the electroscope is used, the metal foil is earth-connected. The bell jar protects the gold leaves from outside air currents.
If the brass disc at the top is touched by a charged body, the charges are passed on to the gold leaves. Since like charges repel each other, the gold leaves tend to separate from each other and diverge. The larger the charge on the electroscope the greater is the divergence of the gold leaves shows the positions of the gold leaves in an uncharged electroscope.
Uses of an Electroscope :-
1. To detect whether a body is charged or not, touch the brass disc of the electroscope with the given body. If the gold leaves diverge, the body is charged; if the leaves do not diverge, the given body is not charged.
2. To find the nature of the charge (positive or negative) on a charged body, the electroscope should be charged first. Touch the brass disc of the electroscope with a glass rod rubbed with silk. The leaves become positively charged and diverged. Now touch the disc with the given charged body - if the divergence of the leaves increases, the charge is positive and if the leaves collapse the charge is negative.