The gold leaf electroscope is 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. 13.8). The rod passes through an insulator that
keeps the rod in place and also retains the charges. Charges
can move freely from the disk to the leaves through the rod. A
thin aluminum foil is attached on the lower portion of the
inside of the jar. Usually, the aluminum 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.13.9-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.13.9-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.13.10-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 aluminum
foil by a conducting wire (Fig. 13.10-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. Similarly
electroscope can be charged negatively with the help of a
positively charged rod. Can you explain this with the help
of a diagram?
Electroscope can also be charged by the process of
conduction. Touch a negatively charged rod with the disk of a
neutral electroscope. Negative charge from the rod will
transfer to the electroscope and will cause its leaves to
For the detection of type of charge on a body, electroscope is
first charged either positively or negatively. Suppose the
electroscope is positively charged as explained before. Now
in order to detect the type of charge on a body, bring the
charged body near the disk of the positively charged
electroscope. If the divergence of the leaves increases, the
body carries positive charge (Fig. 13.11-a).
On the other hand if the divergence decreases, the body has negative charge (Fig. 13.11-b).
Electroscope can also be used to distinguish between
insulators and conductors. Touch the disk of a charged
electroscope with material 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
of the leaves, it will show that the body under test is an