A transformer uses electromagnetic induction to change alternating voltage levels.
It normally has two coils, called the primary and secondary windings, wrapped around a magnetic core. Alternating current in the primary creates a changing magnetic flux; that changing flux induces a voltage in the secondary. The voltage ratio is approximately equal to the ratio of secondary turns to primary turns.
A step-up transformer raises voltage and lowers current for approximately conserved power, apart from losses. A step-down transformer lowers voltage and raises current. Transformers require changing magnetic flux, so ordinary transformers do not operate directly from steady direct current.
The modern transformer developed during the late nineteenth-century expansion of alternating-current power systems. Practical cores often use laminated steel to reduce eddy-current losses. Transformers are used in power transmission, chargers, audio equipment, and electrical isolation.