A transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another. A varying current in any one coil of the transformer produces a varying magnetic flux in the transformer's core, which induces a varying electromotive force across any other coils wound around the same core. Transformers are commonly used in our daily life, and provide the voltage we need, for example, from 220 volt to 110 volt. Such emerging work is achieved by varying \_\_\_\_.