13.3 ELECTROSCOPE
The gold leaf electroscope is a sensitive instrument for detecting charges. It consists of a brass rod with a brass disk at the top and two thin leaves of gold foil hanging at the bottom (Fig.13.6). The rod passes through an insulator that keeps the rod in place. Charges can move freely from the disk to the leaves through the rod. A thin aluminium foil is attached on the lower portion of the inside of the jar. Usually, the aluminium foil is grounded by connecting a copper wire. This protects the leaves from the external electrical disturbances.
Detecting the Presence of Charge
In order to detect the presence of charge on anybody, bring the body near the disk of an uncharged electroscope. If the body is neutral there will be no deflection of the leaves (Fig.13.7- a). But if the body is positively or negatively charged, the leaves of the electroscope diverge. For example, if the body is negatively charged then due to electrostatic induction, positive charge will appear on the disk while negative charge will appear on the leaves (Fig.13.7- b). The leaves of electroscope repel each other and diverge because each leave gets similar charge. The divergence of leaves will depend on the amount of charge.
Charging the Electroscope by Electrostatic Induction
Electroscope can be charged by the process of electrostatic induction. In order to produce positive charge on the electroscope, bring a negatively charged body near the disk of the electroscope (Fig.13.8- a). Positive charge will appear on the disk of the electroscope while negative charges will shift to the leaves. Now connect the disk of electroscope to the earthed aluminium foil by a conducting wire (Fig.13.8- b). Charge of the leaves will flow to the Earth through the wire. Now if we first break the Earth connection and then remove the rod, the electroscope will be left with positive charge(Fig.13.8- c).