Mechanical Electrometer

A mechanical electrometer (also known as an electrostatic voltmeter) is a device that measures electrical charge or potential difference by using the forces in an electrostatic field. The device consists of a metal plate connected to a solid metal rod and a rotating metal needle. When the metal plate becomes electrically charged, the charge is distributed along the surface of the rod and needle. Because of the repulsive force between like charges, the rotating needle is pushed away from the solid rod. The degree to which the needle is displaced from the vertical rod provides a mechanical indication of the amount of charge present. This displacement is shown on a scale that is typically calibrated in kilovolts (kV).