The primary is fed with a varying alternating electric current that creates a varying magnetic field of voltage around the conductor. According to the principle of mutual inductance, which is a special case of electromagnetic induction applied to two coupled conductors, the secondary, which is placed in this varying magnetic field, will develop a potential difference called an electromotive force (EMF). If the ends of the secondary are connected together to form an electrical circuit this EMF will cause a current power fed into the primary is delivered to the secondary.