This is a device for storing electric charge. Simple capacitors usually consist of two plates made of an electrically conducting material (e.g. a metal) separated by a nonconducting material (e.g. glass, mica, oil, or air). It is also called Condenser.

If an electric potential is applied to the capacitor plates, the plates will become charged, one positively and one negatively. If the externally applied voltage is then removed, the capacitor plates remain charged, and the electric charge induces an electric potential between two plates. This phenomenon is called electrostatic induction.

The capacity of the device for storing electric charge (i.e. its capacitance) can be increased by increasing the area of the plates, by decreasing their separation, or by varying the substance used as an insulator. The dielectric constant is a measure of the increase in capacitance due to a particular insulator used to separate the plates.

The leyden jar, a form of capacitor invented at the University of Leiden in the 18th century, consists of a narrow-necked glass jar coated on part of its inner and outer surfaces with conductive metal foil.