text: Capacitor, device for storing an electrical charge, sometimes called a condenser. In its simplest form a capacitor consists of two metal plates separated by a non-conducting layer called the dielectric. The dielectric may be air, plastic, waxed paper, or another substance such as the mineral mica. When one plate of a capacitor is charged using a battery or other source of direct current, the other plate becomes charged with the opposite sign; that is, positive if the original charge is negative, and negative if the original charge is positive\nThe electrical size of a capacitor is its capacitance, that is the amount of electric charge it can hold per unit potential difference across its plates—C = Q/V. The SI unit of capacitance is the farad (F). Because this is such a large unit, capacitors commonly have their size expressed in µF (1 microfarad = 10-6 F) or pF (1 picofarad = 10-9 F).