1. A capacitor in which an electrolyte serves as a plate; the other plate is wound aluminum foil.

A thin layer of oxidation on the foil is the dielectric.

2. A capacitor having an electrolyte between two plates.

A thin layer of oxide is deposited on only the positive plate. The oxide acts as the dielectric for the capacitor.

Electrolytic capacitors are polarized and so they must be connected in correct polarity to prevent a breakdown; so, although electrolytic capacitors have comparatively high values of capacitance for their sizes, they also have a high leakage of current and therefore must often have their polarities checked.