Purpose: Demonstrate flipping of magnetic domains in iron.
Description: A soft iron wire is positioned in the core of a large coil. The coil is attached across the microphone input of an audio amplifier, which in turn is connected to a loudspeaker. Hold the iron core fixed as you bring a small magnet near one end of the wire. Magnetic domains flip into alignment with the applied magnetic field as the magnet comes closer and the field increases. When a domain flips, it emits a short electromagnetic pulse which is picked up by the coil as a spike and heard as a click on the loudspeaker. To make the experiment more convincing, do it with a piece of copper wire and without any wire at all in the coil.
