Leyden jar. Historical artwork of a man (at left) holding a Leyden jar, the earliest type of capacitor. A capacitor is a device for storing electric charge and the jar is storing electricity generated by the spherical electrostatic generator at right. The Leyden jar contains water and a brass rod. The man holding it acts as an earthed conductor. Now rarely used outside of the classroom, the Leyden jar was invented at the University of Leyden in the Netherlands in 1746.