Electroscope – It is an instrument used for detecting and testing the nature of charge on a body.
It consists of a brass rod which has a brass disc connected to its upper end and two very thin gold leaves attached to its lower end. By the help of a cork this assembly is mounted inside a glass jar with the brass disc projected outside as shown in fig. 15.3.A thin aluminum foil is pasted on the lower portion of the inner surface of the jar. Usually, this foil is grounded by means of a copper wire. When any charge is transferred to electroscope, it spreads in the discs and the leaves. Due to the force of repulsion between similar charges on the leaves, they diverge. The divergence of the leaves depends upon the quantity of charge on the leaves.
In order to detect the presence of charge on a body, bring it near the disc of an uncharged electroscope. If the body is charged, the leaves of the electroscope involved diverge otherwise they would remain in their normal position (fig. 15.4).
fig. 15.4: if an object is uncharged, the leaves remain in normal position
The divergence in the leaves of the electroscope due to the presence of a charged body is due to electrostatic induction suppose the body which is brought near the electroscope has negative charge. Due to induction, positive charges will appear on the disc and negative charges on the leaves. As like charges repel each other, therefore, due to the force of repulsion between the negative charges present on the two leaves, will diverge (fig. 15.5). In order to detect the kind of charge on a charged body the electroscope will have to be first charged either with positive charge or with negative charge. In order to charge the-electroscope positively, touch its disc with a positively charged body. If it is to be negatively charged touch its disc with a negatively charged body. In this process a portion of the charge present on the body is transferred to the disc and the leaves of the electroscope. After charging the electroscope, bring the body under test near the disc of the electroscope, if the divergence of leaves increases the body has the same kind of charge as on the electroscope. On the other hand if the divergence decreases, the charge on the body will be opposite to the kind present on the electroscope.
Fig. 15.5: the leaves diverge due to the pressure of similar charge.
Electroscope-can also be used to distinguish between insulators and conductors. Touch the disc of a charged-electroscope with the body under test. If the leaves collapse from their diverged position the body would be a good conductor. If there is no change in the divergence it will show that the body is an insulator. However, a gradual collapse will indicate that the body is not a good conductor.