Capacitance  --  The ability of an electrical component (or components) to store electric charges. Charges present in a conductor attract opposite charges to nearby but not electrically connected conductors. A signal may therefore cross between one component and another, between which there is no direct path. Capacitors are used in systems for coupling, smoothing, and tuning purposes. In lines, capacitance is the reason for loss of signal; this varies with distance between plates or components. Parallel conductors in a signal line may take on the properties of a capacitor, thereby attenuating high frequencies. Stray capacitances from the mains are the cause of the ac Hum, which is transferred unintentionally from component to component.