Leyden Jar

The Leyden jar was invented in 1745 by Pieter van Musschenbroek. It was the first capacitor that was used as a devide for storing electric charge and to conduct early experiments in electricity. A typical design consists of a top electrode electrically connected usually by a chain to a metal foil which coats part of the inner surface of a glass jar. An electrostatic generator charges the jar and is connected to the inner electrode while the outer plate is grounded. Wrapped around the outside of the jar is a conducting foil which matches the internal coated area. The jar has equal but opposite charges in the inner and outer surfaces of the jar. Originally, the device was just a glass bottle that was partially filled with water and had a metal wire passing through a cork closing it. However, it was soon discovered by Watson in 1746 that it was better to coat the exterior of the jar with metal foil. This was better because it left the impure water inside acting as a conductor, connected by a chain to a sphere (external terminal). Initially it was believed that the charge was stored in water and after further investigation by Benjamin Franklin it was concluded that the charge was stored in the glass—not the water. Today we know that the the charge is actually stored in a thin layer along the facing surfaces which touch the glass also known as the dielectric.