Direct current (DC) is an electric current that flows in one direction, unlike alternating current (AC) which changes direction periodically. DC can flow through a conductor such as a wire, semiconductors, insulators, or even through a vacuum as in electron or ion beams. DC is generated in a number of ways, such as through an AC generator equipped with a device called a "commutator," a device called a "rectifier" that converts AC to DC, or batteries that provide DC generated from a chemical reaction inside the battery. DC is defined as the "unidirectional" flow of current, where current only flows in one direction, and voltage and current can vary over time as long as the direction of flow does not change. Direct current is used in any electronic device with a battery for a power source, and it is also used to charge batteries, so rechargeable devices like laptops and cell phones come with an AC adapter that converts alternating current to direct current.