When the flux changes—because changes, or because the wire loop is moved or deformed, or both—Faraday's law of induction says that the wire loop acquires an EMF, defined as the energy available from a unit charge that has traveled once around the wire loop. (Some sources state the definition differently. This expression was chosen for compatibility with the equations of Special Relativity) Equivalently, it is the voltage that would be measured by cutting the wire to create an open circuit, and attaching a voltmeter to the leads.