Photograph of a Leyden jar, the simplest and earliest form of capacitor (a device for storing electric charge). It consists of a glass jar with a layer of metal foil on the outside and a similar layer on the inside. Contact with the inner foil is via a central metal rod. The device is usually charged from an electrostatic generator. Now rarely used outside of the classroom, the Leyden jar was invented at the University of Leyden, in Holland, in 1746.