What Is a Leyden Jar?

Invented independently by Ewald Georg von Kleist and Pieter van Musschenbroek in 1745–46, the Leyden jar was the first device capable of storing significant electrical charge. It consists of a glass jar lined inside and outside with metal foil (originally water inside and a hand outside). A metal rod through the lid connects to the inner foil. When charged by a friction generator, the two foil layers act as the plates of a capacitor with glass as the dielectric. Leyden jars could store enough charge to deliver a painful shock and were used in early experiments by Benjamin Franklin to demonstrate the electrical nature of lightning.