Magnetic effect of a current — Describe

Describe how the magnetic effect of a current is used in a relay and a loudspeaker, giving an example application of each.

In a relay, a small current through an electromagnet's coil attracts an iron armature, which closes (or opens) a separate switch contact controlling a different, often higher-current, circuit - used, for example, to safely switch a car's starter motor circuit from a low-current ignition switch. In a loudspeaker, a varying (a.c.) current from an audio signal flows through a coil placed in the field of a permanent magnet; the changing current makes the coil, and the attached cone, vibrate, producing sound waves - used in speakers and headphones to convert electrical signals into sound.