Electrostatic pendulum with its base. The electrostatic pendulum is a brass support at whose curved top is suspended a pith ball at the end of a cotton thread. The pendulum is a sensitive indicator of the electrostatic state of other bodies through the behavior of the pith ball. If the pith ball is brought near a charged body, it polarizes its dipoles so as to be attracted to the charged body. If, and when, it touches the charged body it becomes charged with the same polarity and flies away. It can be discharged by touching a grounded metal conductor.
