CoachA compass is a small, light magnet needle balanced on a fine pivot so it can swing freely. Left in peace, the needle obeys the Earth-magnet: its marked end settles pointing north. Under the needle, a card shows the four cardinal directions: North, East, South, West.
示例
Example 25.7 (Make one yourself)
Stroke a steel sewing needle with one pole of a magnet, twenty times, always the same way, lifting the magnet away between strokes: the needle becomes a small magnet itself. Lay it gently on a slice of cork floating in a bowl of water — and watch the cork slowly turn until the needle points north–south. You have built the sailor’s compass of a thousand years ago.