Question

Why is an gold leaf electroscope provided with a metal foil which is earthed

Answer 1

Two strips of tin foil are fixed to the inside of the bell jar from the level of the gold leaves to the wooden base, which is also covered with tin foil. When the electroscope is used, the metal foil is earth-connected. The bell jar protects thegold leaves from outside air currents.

Golden leaf electorscope is eathed as
Any object that has been earthen will have electrical potential of earth that is assumend to be 0V.

So when a negatively charged rod is bought near the cap of the gold leaf it will reduce the overall electrical potential of the cap,i.e, make it < 0.But since the cap is earthed it’s potential should be zero.

Positive charge will flow from the gold leaf to the cap,so as to compensate for the drop in potential of the cap, or , we could say, negative charge would flow from the cap to the leaf ,trying to make the potential of cap to zero.
In both sense the leaf has an induced negative charge