A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors — the transformer's coils or "windings". Except for air-core transformers, the conductors are commonly wound around a single iron-rich core, or around separate but magnetically-coupled cores. A varying current in the input or "primary" winding creates a varying magnetic field in the core(s) of the transformer. This varying magnetic field induces a varying electromotive force or "voltage" in the output or "secondary" winding. If a load is connected to the secondary, an electric current will flow in the secondary winding and electrical energy will flow from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding is in proportion to the primary voltage, and is given by the ratio of the number of turns in the secondary to the number of turns in the primary.