text: what's capacitor?   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 the 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).