The diagram shows a gold leaf electroscope - Leaving Cert Physics - Question c - 2005

One use of an electroscope is to measure electric charge or detect the presence of charge.

When a positively charged rod is brought close to the metal cap of the electroscope, the positively charged electrons in the metal are attracted upwards towards the rod. This causes a deficiency of electrons in the metal cap, resulting in a positive charge at the cap. Since the gold leaf also gains a similar positive charge due to induction, both the gold leaf and the metal cap repel each other, leading to the divergence of the leaf.

When the positively charged rod is then earthed, the excess positive charge at the metal cap is neutralized as electrons flow from the ground into the electroscope. This neutralization results in a loss of charge in the gold leaf, leading it to collapse as the repulsive force between the leaf and the metal cap diminishes.