Mapping: How is the Needle of a Compass Affected by Magnetic Fields?

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Author: Janice VanCleave


How is the needle of a compass affected by magnetic fields?


  • craft stick
  • 4 teaspoons (20 ml) plaster of paris
  • 2 teaspoons (10 ml) tap water
  • paper cup
  • 1/2 teaspoon (2.5 ml) iron filings (available in a hobby shop)
  • bar magnet
  • sheet of paper
  • compass
  • marking
  • pen ruler

NOTE: Mix the plaster in a throwaway container. Do not wash the container or the craft stick in the sink, because the plaster can clog the drain.


  1. Use the craft stick to mix the plaster of paris and the water in the paper cup.
  2. Pour the iron filings into the plaster mixture. Stir well.
  3. Set the paper cup on top of the north-pole end of the magnet.
  4. Allow the plaster to harden (about 15 to 20 minutes). Then remove the magnet.
  5. Place the sheet of paper on a wooden table.
  6. Turn the cup upside down on top of the paper. Keep the magnet and any magnetic materials away from the cup.
  7. Mapping

  8. Place the compass on the cup's upturned bottom.
  9. Starting with the tip of the marking pen about 1 inch (2.5 cm) above the rim of the cup, draw a thick line on the cup that extends onto the paper about 1 inch (2.5v cm) from the cup.
  10. Making sure the line on the cup is aligned with the one on the paper, rotate the cup one quarter turn to the right.
  11. Wait until the compass needle stops moving, then note the direction that the compass needle is pointing.
  12. Continue to turn the cup, one quarter turn at a time, and note the direction of the compass needle. Do this until a complete rotation has been made and the lines are again aligned.
  13. Mapping


The needle on the compass points in a different direction after each quarter turn of the cup.


The needle of a compass is a magnet that lines up with the earth's magnetic field (an invisible pattern of magnetism around a magnet). The ends of the needle point toward the magnetic north and south poles of the earth. The iron filings in the plaster became magnetized when placed near the magnet, and lined up with the magnet's magnetic field. When the plaster of paris hardened, the tiny iron particles were locked in place, causing the hardened plaster to act like a magnet with magnetic north and south poles. Rotating the cup rotated the "poles" of the iron particles. As a result, the compass needle rotated to line up with the magnetic field of the iron particles, instead of with the magnetic field of the earth.

The earth's magnetic field is currently pointing in a different direction than in times past. Evidence for this shift is found in magnetic rock. It is believed that grains of magnetic material in rock formed from magma and that melted rock lined up with the earth's magnetic field. When the liquid cooled and became a solid, the magnetic grains were locked in place, creating a "map" that pointed in the direction of the earth's magnetic poles. This magnetic map indicates that the magnetic poles of the earth have moved to different places over time.

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