The purpose of a capacitor is to store energy in the electric field between a pair of closely spaced conductors that are called plates, and these capacitors are used in electrical circuits as energy-storage devices. Capacitors can also be used to differentiate between signals of high frequency and low frequency, which makes them particularly useful in electronic filters.

When voltage is applied to the capacitor, electric charges of equal magnitude yet opposite polarity build up on each capacitor plate.

The Leyden jar was the first device invented that could acquire electrical charge and store it until the scientist wanted to use it for a demonstration or an experiment. It was named for the University of Leiden, which was the place where it was first used for research . E. Georg von Kleist developed the Leyden jar in the middle of the 18th century in Germany at the same time that Peter van Musschenbroek developed it in the Netherlands. The scientists worked independently, so credit is given to them both. The first Leyden jars were very basic and consisted of glass bell jars coated with metal foil on the inner and outer surfaces. These jars were filled with either water or beer, and the jar had a stopper with a wire running through it that could be hooked up to an electrostatic generator in order to provide a quick source of electricity.