electroscope

An electroscope is a device that detects and qualitatively measures electric charge. It works on the principle that like charges repel; when a conductor inside the instrument is charged, parts such as leaves or a suspended ball experience electrostatic forces and move apart.

The simplest forms are the leaf-type and the pith-ball electroscope. A gold-leaf or foil-leaf electroscope consists of a metal rod attached to a pair of thin leaves enclosed in a supporting case. A pith-ball electroscope uses a lightweight conducting ball suspended from a rod. When the electrode is charged by contact or induction, the leaves or ball deflect due to repulsion or attraction, indicating the presence of charge and, qualitatively, its magnitude.

Operation is typically by transferring charge to the instrument through contact with a charged object or by induction from nearby charges. After charging, the leaves or ball deflect; grounding the instrument removes the charge and the leaves return to the resting position. The device provides a simple, visual indication of electrostatic charge and can be used to compare relative amounts of charge or to demonstrate concepts such as conduction, insulation, and grounding.

Sensitivity and response depend on factors such as leaf length, hinge friction, atmospheric humidity, and the construction of the instrument. While historically important for studying electrostatics, modern measurements of charge and potential are usually performed with calibrated electrometers and other electronic instruments. Electroscopes remain common in education for illustrating basic electrostatic principles.