Electroscope

An electroscope is an instrument which is used to detect the presence of charge and to determine the nature of the charge in a body. There are two types of the electroscope. They are:

1. Gold leaf electroscope
2. Pith-ball electroscope

Gold Leaf Electroscope (GLE)

A gold leaf electroscope is a sensitive device used to detect the presence of charge in a body. A gold-leaf electroscope consists of a brass rod R to which two thin gold leaves are attached to one end and metal disc D at another end. The leaves are parallel to one another as shown in the figure. The rod is fixed in a glass bell jar J so that the gold leaves lie inside it. and the disc lies outside the jar. The bell jar is placed on the wooden base B. The lower walls and the bottom of the bell jar are covered wit tin foil. In other to keep the air inside the gold leaf electroscope dry, a small packet containing CaCl₂ is placed inside it.

Application of Gold Leaf Electroscope

1. To detect the presence of charge on the body
   To test whether a body is charged or not, the body is brought near the disc. If the leaves diverge the body is charged one.
2. To know the nature of charge on the body
   The nature of the charge on a body is examined by charging gold leaf electroscope with known charge.

Charging a gold leaf electroscope by Induction

A gold leaf electroscope is charged negatively or positively by induction

To charge a GLE negatively

• Let a positively charged glass rod is brought near the disc of an uncharged GLE. Then, by induction, the disc acquires a negative charge and leaves acquire a positive charge. As the two leaves acquire the same charge on the inner surface of the tin foils and the positive charge on the outer surface. Now Tin foil is earthed leaving negative charge inside and removing the positive charge.
• The disc of the GLE is earthed. Then the free positive charge on the gold leaves flows the earth through the earthing and the leaves collapse.
• The disc has the bound negative charge due to the positive charge on the glass rod. There is no charge on the leaves and hence the leaves collapse.
• Now remove the glass rod Away from GLEso that bounded negative charge are free and distributes equally on The rod, on the disc, and on the gold leave. Hence, GLE is negatively charged.

To charge a GLE positively

• Let a negatively charged ebonite rod is brought near the disc of an uncharged GLE. Then, by induction, the disc acquires the positive charge and leaves acquire a negative charge. As the two leaves acquire the same charge on the inner surface of the tin foils and the negative charge on the outer surface. Now Tin foil is earthed leaving positive charge inside and removing the negative charge.
• The disc of the GLE is earthed. Then the free negative charge on the gold leaves flows the earth through the earthing and the leaves collapse.
• The disc has the bound positive charge due to the negative charge on the ebonite rod. There is no charge on the leaves and hence the leaves collapse.
• Now remove the ebonite rod Away from GLEso that bounded positive charge is free and distributes equally on The rod, on the disc, and on the gold leave. Hence, GLE is positively charged.

Pith Ball Electroscope

It consists of a small pith ball suspended with a silk thread from an insulated stand as shown in the figure.

Application of Pith Ball Electroscope

1. To detect the presence of charge on the body
   To test whether a body is charged or not, bring the body near the uncharged pith ball. If pith ball does not move from the equilibrium, the body does not carry charge but if pith moves towards or backwards the body then it posses charge
2. To know the nature of charge on the body
   The nature of the charge on a body is examined by charging pith ball with known charge. Suppose, pith ball is charged positively and the charged body under test is brought near to pith ball. If there is repulsion then the body is positively charged otherwise it is negatively charged.