What is the primary function of a transformer?

A TRANSFORMER increases ('step-up') or decreases ('step-down') AC VOLTAGE using electromagnetic induction between two coils (primary and secondary windings). ASVAB Electronics Information tests components. The voltage change depends on the ratio of coil turns: more turns on the secondary = step-up (higher voltage); fewer = step-down (lower voltage). Transformers work only with AC (changing current creates the changing magnetic field needed). Used in power distribution (high voltage for transmission, stepped down for homes) and adapters. Knowing the transformer changes AC voltage is commonly tested.