How did a Leyden jar store electricity?

The Leyden jar functions as an early form of a capacitor, a device designed to store static electricity and electrical energy.12

In its original design, the jar consisted of a glass vial partially filled with water, with a conducting wire or nail passing through a cork to make contact with the water. To charge the device, the exposed end of the wire was brought into contact with a friction-based electrostatic machine. The glass acted as an insulator (or dielectric), allowing opposite charges to be held on the inner and outer surfaces of the container.132

Later, the design was improved by coating the inner and outer surfaces of the glass jar with metal foil. This configuration creates two conductors of extended area separated by a thin insulating layer, which significantly increases the device's capacitance—the amount of charge it can store at a given potential difference. When the inner and outer coatings are connected to a circuit, the device stores energy by maintaining these separated, opposite charges.132