Transformers are often used in electrical circuits to change the voltage of electricity flowing through a circuit. Transformers can be used either to increase the voltage called stepping up or decrease the voltage step down. When a current flows through a wire, it makes a magnetic field around the wire. Also, if a wire is in a magnetic field that is changing, a current will flow in the wire. In a voltage transformer, a wire brings a current in one side. That current makes a magnetic field, which in turn produces a current in a wire on the other side of the transformer. The second current flows out of the transformer. Transformers are passive devices – they don’t add power. A high voltage and low current exits the transformer carrying almost the same amount of power along the transmission lines that the incoming low voltage and high current did. Transformers are very efficient.