A gold-leaf electroscope has a vertical rod R attached to a flat metal plate P. Gold is a malleable metal which can be hammered into extremely thin and light sheets. A light gold leaf G is attached to the lower end of the rod.
If the electroscope is positively charged by touching the plate with a positively charged glass rod, G will be repelled from R, because both now carry a positive charge.
You can now experiment as follows. Bring a positively charged glass rod close to P. The leaf G diverges further from R. We now know that this is because the metal (of which P, R and G are all composed) contains electrons, which are negatively charged particles that can move about more or less freely inside the metal. The approach of the positively charged glass rod to P attracts electrons towards P, thus increasing the excess positive charge on G and the bottom end of R. G therefore moves away from R.
If on the other hand you were to approach P with a negatively charged plastic rod, electrons would be repelled from P down towards the bottom of the rod, thus reducing the excess positive charge there. G therefore approaches R.
Now try another experiment. Start with the electroscope uncharged, with the gold leaf hanging limply down. (This can be achieved by touching P briefly with your finger.) Approach P with a negatively charged plastic rod, but don’t touch. The gold leaf diverges from R. Now, briefly touch P with a finger of your free hand. Negatively charged electrons run down through your body to ground (or earth). Don’t worry – you won’t feel a thing. The gold leaf collapses, though by this time the electroscope bears a positive charge, because it has lost some electrons through your body. Now remove the plastic rod. The gold leaf diverges again. By means of the negatively charged plastic rod and some deft work with your finger, you have induced a positive charge on the electroscope. You can verify this by approaching P alternately with a plastic (negative) or glass (positive) rod, and watch what happens to the gold leaf.