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

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

When a positively charged rod is brought close to the metal cap, positive charges within the electroscope are repelled. The electrons in the gold leaf and the metal cap are attracted upward, leaving the leaves with a greater positive charge, which causes them to diverge (spread apart) due to the like charges repelling each other.

When the positively charged rod is held close to the electroscope and the metal cap is earthed, charge flows to the ground. This allows the gold leaf to lose its excess positive charge. Consequently, the repulsion between the leaves decreases, and they collapse back together.