What Is the Leyden Jar?

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  • Written By: Geisha A. Legazpi
  • Edited By: Shereen Skola
  • Last Modified Date: 12 September 2019
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The Leyden jar is a device that was used in the 1700s for electrostatic energy storage. Electrostatic particles are electrically charged particles that may remain relatively stationary in conductors or insulators. Sometimes called “Leiden jar,” the Leyden jar used a glass jar that was and still is a popular insulating device. As in the capacitors, the glass jar acted as a dielectric between two conductors, and was made up of an inner and outer conductive foil. The thin foil was easily formed to assume the shape of the inside and the outside of the glass jar.

Early capacitors used two conductive plates separated by an insulator referred to as a dielectric. The bigger the surface area of the capacitor plates, the larger was the capacitance value. It is best to limit the potential difference between the plates of a capacitor. As the voltage across the two plates increases, there is a point where there will be arcing across the dielectric that will degrade the performance of the capacitor. In general, capacitors are used in power and signal filter circuits.


Usually, the outer foil is in contact with the person performing a Leyden jar experiment. The inner foil is usually attached to an insulated inner conductive electrode that protrudes at the top of the Leyden jar. A chain at the bottom of this inner electrode usually connects it to the inner foil. In early experiments, water was poured inside the jar to connect the electrode to the inner foil.

The Kleistian jar was developed earlier than the Leyden jar. It was much like the Leyden jar, but without the outer foil. The principle involved in retaining electrical charge was that electrically charged conductors, insulated from the earth, retain electrical charge. If the electrical potential in these conductors is high enough to ionize the surrounding air molecules, a corona discharge results. It can be seen in high-voltage distribution equipment and other high-voltage devices.

Franklin bells are made up of a center bell hanging from an insulating thread tied to the center of a conducting horizontal bar. Two side bells hanging from conductive chains are tied to the ends of the horizontal bar. As it is, when an electrical charge is made available at the center bell, there is a need for a way to transfer the charge to the side bells.

Additional bobs tied to insulating threads on each side of the center bell provide the needed charge transfer. When the bob has a neutral charge, it gets attracted to the center bell. If the voltage at the center bell and at the bob is the same, there is repulsion and the bob is driven to the side bell. As it connects to the side bell, the charge of the bob is neutralized. As long as the Leyden jar is providing charges to the center bell, the bobs keep swinging, ringing the Franklin bells continuously.


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