Capacitors such as a Leyden jar consist of layers made of an electrically conducting material (such as metal foil) separated by layers of a nonconducting material (glass in the case of the Leyden jar, but it can also be wax, mica, oil, paper, tantalum, plastic, ceramic material, or even air). If an electrical voltage is applied to the layers of a capacitor, the plates will become charged, one positively and one negatively. If the externally applied voltage is then removed, the plates of the capacitor remain charged, and the presence of the electric charge induces an electrical potential between the plates. Today’s capacitors are used for a wide variety of purposes in electric power systems, radio receivers, computers, and nearly every other electrical device. They range in size from the size of a refrigerator to the microscopic capacitors built into integrated circuits. The capacity of the device for storing electric charge (called its capacitance) can be changed by changing the area of the plates, by increasing or decreasing their separation, or by using different kinds of materials for the non-conducting layers.
The unit of capacitance was known as the "Jar" until 1888, and later the SI unit of "Coulomb" was assigned in 1922.