Leyden Jar, an early and simple form of capacitor (electric condenser), a device for storing electricity. It was invented about 1745 by E. G. von Kleist of Prussia and, independently, by Pieter van Musschenbroek of the University of Leiden in Holland. The first detailed experiments with it were done by Musschenbroek. The jar is used today only for physics laboratory demonstrations. It consists of a glass container covered with tinfoil inside and out for about one-half of its height. A brass rod passes through a cork or wood stopper and is connected to the inner tinfoil by a chain.

The jar is charged by connecting a brass ball at the top of the rod to a body with an electric charge. At the same time the outer tinfoil is grounded (connected to the ground), usually by being held in the hand. The inner tinfoil receives an electric charge which induces an opposite charge on the outer tinfoil. The outer charge permits a greater charge on the inner tinfoil, which then induces a stronger charge on the outer tinfoil. Thus the charges can be built up until the jar reaches the limit of its capacity.

The glass keeps the charges apart. But when a conductor is run from the brass rod to the outer tinfoil, the jar will discharge with a spark, producing an oscillating current.