Electrometers and Electroscopes

Electric charge is measured using a device called an electrometer, which consists of a metal knob connected via a conducting shaft to a flat, vertical metal plate. A very light gold leaf, hinged at the top, is attached to the plate. Both the plate and the gold leaf are enclosed in a glass vessel to protect the delicate leaf from air currents. When charge is deposited on the knob, some fraction is conducted to the plate and the gold leaf, which consequently repel one another, causing the leaf to pull away from the plate. The angular deflection of the gold leaf with respect to the plate is proportional to the charge deposited on the knob. An electrometer can be calibrated in such a manner that the angular deflection of the gold leaf can be used to calculate the absolute magnitude of the charge deposited on the knob.

An electroscope is a somewhat cruder charge measuring device than an electrometer, and consists of two gold leaves, hinged at the top, in place of the metal plate and the single leaf. When the knob is charged, the two leaves also become charged and repel one another, which causes them to move apart. The mutual deflection of the leaves can be used as a rough measure of the amount of electric charge deposited on the knob.