A capacitor can quickly store a small amount of electrical energy, at which point it is charged. There are two surfaces inside the capacitor, separated by insulating material. When the capacitor is charged, one surface is positively charged and the other is negatively charged. When a circuit is closed between its terminals, the capacitor releases its charge. At this point, it is discharged.

A typical capacitor stores the charge on thin sheets of foil, with sheets of insulation between them. These are rolled together to form a protective canister. A capacitor’s capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) that appears between the plates. In general, as the capacitance and voltage rating of the capacitor increase, the physical size of the capacitor increases. Some DVOMs can measure the capacitance of a capacitor. That way they can be tested, if they are suspected of being faulty.