A static device that couples two circuits for the purpose of transferring alternating current (AC) from one circuit to another by electromagnetic induction. The current remains at the same frequency, but may change in voltage, phase, or impedance values.A transformer comprises two or more wire coils, or windings, but no moving parts.The voltage induced in a coil on a transformer is proportional to the number of turns around the core. See also AC, frequency, impedance, inductance, phase, and voltage.