Capacitors are passive devices that are used in almost all electrical circuits for rectification, coupling and tuning. Also known as condensers, a capacitor is simply two electrical conductors separated by an insulating layer called a dielectric. The conductors are usually thin layers of aluminum foil, while the dielectric can be made up of many materials including paper, mylar, polypropylene, ceramic, mica, and even air. Electrolytic capacitors have a dielectric of aluminum oxide which is formed through the application of voltage after the capacitor is assembled. Characteristics of different capacitors are determined by not only the material used for the conductors and dielectric, but also by the thickness and physical spacing of the components.