Charging a Gold Leaf Electroscope
A gold leaf electroscope can be charged either by induction or by contact.
Charging by induction involves bring a charged rod near to the metal cap. The charged rad attracts charge of opposite polarity to the cap and repels charge of the same polarity to the leaf and stem. The cap is then earthed and the charged rod withdrawn leaving the charge on leaf and stem to redistribute itself over the cap, stem and leaf.
When a gold leaf oscilloscope is charged by contact, as below, a charged rod actually makes contact with the cap, allowing the charge to pass onto the cap and down to the stem and leaf. The oscilloscope is then withdrawn, leaving behind a net charge of the same polarity as the charged rod.