Transformer, illustration. A transformer is an electrical device that transfers electrical energy between two or more circuits through electromagnetic induction. Two wire coils are wound on a soft iron core. The primary coil (left) is connected to a source of alternating current, which induces an alternating current in the secondary coil (right). The secondary coil voltage and the primary coil voltage are in the same ratio as the number of turns of the secondary and primary coils. This is a step-up transformer, where the secondary coil has more turns, and therefore greater voltage, than the primary coil.