90. Magnets and Compasses

Various magnets and compasses are available; the strongest magnets are the little ceramic magnets, these can be handed around to the class.


A piece of lodestone picks up small iron objects, or can be suspended so as to point north.
A large, strong bar magnet repels another held by braces and suspends it several inches in the air.

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 attracts it.
A small electromagnet operated from a battery will pick up nails, etc.

A giant compass needle is useful for demonstrating the earth's magnetic field and for determining the north and south poles of various magnets.