A transformer is an electrical device that allows the voltage in an alternating current electrical circuit to be increased or decreased; maintaining power. transformers are made up of two coils of wire wound around an iron core. the coil that receives the energy is called the primary coil; and the coil that delivers the power is called the secondary coil. the principle of operation of a transformer is based on the law of electromagnetic induction; which states that a changing magnetic field induces an electric current in a nearby conductor. when current flows through the primary coil; a magnetic field is created around the iron core. this changing magnetic field induces an electric current in the secondary coil.