20. Compliance, Reluctance, Coercion

This pretty demo uses a "permalloy" rod, a soft iron rod, a hammer, and the compass dip needle (which is shown here).

Using the dip needle find the direction of the earth's magnetic field in the class room (plunging about 60° earthward to the magnetic north). Arrange the soft iron rod perpendicular to the earth's field, and strike its end several times with a hammer. This insures that it is demagnetized, which is demonstrated by showing that either end of the rod will attract either end of the compass needle (by magnetic polarization). Now align the rod with the earth's field and strike it several times to shake its domains around and magnetize it. That the rod is magnetized is demonstrated by showing that the north end of the rod repels the north end of the compass needle and the south end of the rod repels the south end of the compass needle. This rod retains its magnetization, no matter how it is oriented in the earth's field.

However, the permalloy rod is so compliant that if held along the earth's field its north end will repel the north end of the compass needle, and now if it is smoothly reversed without any hammer strikes, the other end will repel the north end of the compass needle! Its domains line up with the earth's field without any impact blows.