From Academic Kids
Electrostatic induction is a method by which an electrically charged object can be used to create an electrical charge in a second object, without contact between the two objects.
The effect can be demonstrated using an electroscope, which is an instrument for detecting electric charge. The electroscope is first discharged, and a charged object is then brought close to the instrument's terminal. This causes a redistribution of the charges inside the electroscope, so that the electroscope's terminal gains a net charge of opposite polarity to that of the object. At this moment, the electroscope has not acquired a net charge: the charge within it has merely been moved around. An electrical contact is then briefly made, e.g. by using the experimenter's finger, between the electroscope terminal and earth. This causes charge to flow from earth to the terminal, attracted by the local surplus of charge at the terminal. The electroscope now contains a net charge opposite in polarity, and slightly weaker than, that of the charged object. When the electrical contact to earth is broken, e.g. by lifting the finger, the extra charge that has just flowed into the electroscope cannot escape, and the instrument retains a net charge.