The Leyden (or Leiden) jar is a device used to store electrical energy. Unlike a battery, though, it does not use a chemical reaction to make and store this energy. It is actually the earliest example of a capacitor, which works by forcing two plates of conductive material to retain equal but opposite charges. Ewald Georg von Kleist first developed this type of capacitor in Germany in 1744. The Dutch scientist Pieter van Musschenbroek in Leiden (or Leyden), unaware of von Kliest's work, also created one about two years later. The most commonly used name refers to the second inventor and his home city.
In the Leyden jar, one conductive plate is placed inside of a container made of an insulating material (such as glass or ceramic), called a dielectric. The other conductive plate is on the outside of the dielectric, an arrangement which keeps the electrons from moving from one plate to the other. When one plate is "filled" (charged) with extra electrons, they cannot move to the other plate, but are close enough to "push" (repel) the same number of electrons out of the other plate through the dielectric, giving it an equal but opposite (positive) charge. The greater this charge imbalance becomes, the greater the "pressure," which can be relieved by connecting the two plates with another conductor. This "flow" of charge (current) is usually much quicker than that across a battery because the entire closed capacitor circuit has less resistance to the flow.