Electrostatic Proof Plane

By touching the proof plane to a surface, it will acquire the same charge distribution as the surface. By measuring the charge on the proof plane, the charge density on the surface can be determined. The greater the charge on the proof plane, the greater the charge density on the surface where the proof plane made contact.

Ground the proof plane and check that it has zero charge with an electroscope.  Rub the flat surface of the proof plane on the surface of the unknown charged object.  Again, bring the proof plane to an electroscope to indicate the charge on the unknown object.