It is a device used to detect the presence of charge on a body.
Pith ball electroscope
When a charged body is brought near the
electroscope the light weight pith ball is first attracted to the rod.
Once it has touched the rod it gets repelled. When the pith ball comes
in contact with the rod some of the charge from the rod gets transferred
to the pith ball. As a result, it acquires similar charge. Because of
the similar charge it then gets repelled. Attraction precedes repulsion
Gold leaf electroscope
The gold leaf electroscope is a more
sensitive instrument. It consists of a draught proof metal case with
glass sides housing the gold leaves. The gold leaves 'L' and 'M' are
attached at one end of a vertical metal stem S. At the top the stem has a
metal place 'C'. The stem is carefully insulated by plastic material
from the surrounding metal case. The case is earthed to protect the
leaves from electrical disturbances outside.