When a charged material (a polycarbonate rod for example) is brought near the top of an electroscope, the charge travels down the metal part (electrode) and charges the gold leaf. In the case of the two leaf electroscope, both leaves become equally charged and repel each other. In the case of the single leaf, it repels against the metal sheet which also carries the same charge.
Charge can be transferred to the electroscope by ?wiping? a charged rod or balloon on the top plate. The electroscope is actually measuring the potential difference between the earth and the plate.
Because of the cost of real gold leaf, schools tend to use ?Dutch Metal?, an alloy of 80% copper and 20% zinc. It can be purchased in leaves by the book from many suppliers.
If care is taken over the electroscope, the leaf will not have to be replaced for a long time, a small amount of sticky tape is usually enough to secure a small strip when needs be.