Figure 1. The Earth's magnetic field. click for copyright

The first compasses were just lodestones — a naturally occurring magnetic ore — on small sticks placed in a bowl of water. This simple device pointed to the pole star and so was used, therefore, for navigation by early mariners. Shortly after the first compasses were used, people discovered that an iron or steel needle that had been touched or rubbed with a lodestone would also align with the pole star.

The Earth's magnetic field has a shape like a strong bar magnet placed near the center of the Earth with its S pole near the north geographic pole and its N pole near the south geographic pole. The Earth's magnetic field, which is three-dimensional, is inclined at about 11 degrees from its axis of rotation. A compass is just a magnet held on top of a pivot so the magnet can rotate freely. A compass points in a direction that lies along the magnetic field at the point. There is also a component of the magnetic field perpendicular to the surface of the Earth.

• Gather all necessary materials.
• Pass out worksheets to students.
• Read through the steps of the activity and, using information from the "Introduction/Motivation" section, have a few relevant questions ready for students. (See assessment section below for suggestions.)

1. Make an X in the center of the outside bottom of the plastic bowl using electrical tape. Use the tape to determine the four cardinal directions.
2. Fill the bowl with enough water (about ½ way) so the compass "needle" will be able to move freely.
3. Magnetize the needle (a pin) by stroking it two dozen times with the magnet.
4. Stick the compass needle (pin) through the center of a packing peanut (the best way to do this is to crush the peanut into a small ball, and stick the pin or needle through the middle. It is okay if there is only a small bit of the needle covered by the peanut since the peanut is only there to be used as a floatation device for the needle). In some circumstances, more than one packing peanut may be needed.

Figure 2. A simple compass for measuring Earth's magnetic field in a horizontal plane. click for copyright

5. Place the needle/peanut assembly into the bowl of water. What happens? (The magnetized needle should rotate to be oriented north/south. Verify orientation with a real compass.) Have students record answers on their worksheets.
6. Have students walk around the room with the compass they made. What happens to the compass needle as you move about the room? (Answer: it should always rotate so that it is pointed north/south.) Have students record answers on their worksheets.
7. Hold the compass flat near the top of any iron or steel object in the room (the most common steel objects in a classroom are filing cabinets and garbage cans, but any object than contains steel — small refrigerators or radiators, for example — will work). What happens when the compass is near the topmost part of the object? (Answer: the needle should point at the object.) Move the compass down to the bottom of the steel object, still holding it flat. What happens? (Answer: the needle should flip around 180 degrees.) Record on worksheets.
8. Now have students create a formation. Have students form a large "X" with groups of students pointing toward the four cardinal directions. Use the compass to resolve any disagreement among the students.
9. Remove the needle and packing peanut assembly from the water.

Discussion Questions: Solicit, integrate and summarize student responses.

• What if you knew you needed to head north to get to a relative's house, but you do not know which way is north? How would you proceed? (Possible answers: Look at where you are going, use a map, use a compass.)
• How does a compass locate directions? (Answer: A compass indicates the magnetic field on the surface of the Earth and determines the direction.)

Worksheet: A worksheet entitled, Find Your Own Direction, is provided to allow students to record their observations during the activity — beginning when the compass is placed in the bowl of water.

Formation: Have students form a large "X" with groups of students pointing toward the four cardinal directions. Use the compass to resolve any directional disagreement among the students.