Tug-Of-War: How Can The Strength of Two Magnets be Compared?

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Author: Janice VanCleave


How can the strength of two magnets be compared?


  • compass
  • 2 plastic rulers
  • 2 bar magnets


NOTE: Never touch a compass with a magnet. Touching a compass with a strong magnet can change the polarity of the compass needle, causing the end marked north to become a south pole and all directions to be reversed.

  1. Place the compass on a wooden table. Be sure that no magnets or magnetic materials are nearby.
  2. Turn the compass so that the north pole of the needle points to the N printed on the case.
  3. Lay the first ruler on the west side of the compass, so that it points away from the compass (see diagram).
  4. Place the second ruler on the east side of the compass.
  5. Lay the first magnet on the west side of the compass at the 6-inch (l5-cm) mark of the first ruler. Place the south end of the magnet toward the compass.
  6. Place the second magnet on the east side of the compass at the 6-inch (15-cm) mark of the second ruler. Again, place the south end of the magnet toward the compass.
  7. Observe the position of the compass needle.



There are three possible positions for the needle, depending upon the strength of the magnets: it will continue to point toward the north, N; it will move toward the east, E; or it will move toward the west, W.


A compass has a free-moving, magnetized, needle-shaped magnet. The needle swings into a north-to-south position as its poles line up with the magnetic lines of force around the earth. Placing magnets near the compass causes the needle to move from its natural north-to-south position. The south pole of the magnet attracts the north pole of the compass's magnetized needle, causing it to swing toward the magnet. The two magnets placed on opposite sides of the compass pull on the compass needle, as in a game of tug-of-war. If the magnets are of equal strength and at an equal distance from the compass, their pull on the needle is balanced and the needle continues to point toward the magnetic north pole of the earth. If one of the magnets is stronger than the other, that magnet wins the magnetic tug-of-war and the compass needle swings toward the stronger magnet.


Let's Explore

  1. Would placing the north ends of the magnets toward the compass affect the results? Repeat the experiment, this time placing the north pole of each magnet toward the compass.
  2. Do the magnets have to be placed east and west of the compass? Try placing them to the north and south instead, and repeat the original experiment.

Show Time!

To compare the strengths of two magnets, place the compass and the rulers in the same position as in the original tug-of-war experiment. Place the first magnet on the west-side ruler, with its south pole facing the west side of the compass. Slowly move the magnet across the ruler toward the compass; stop the magnet when the compass needle points northwest, NW. Lay the second magnet on the east-side ruler, with its south pole facing the compass. Slowly move the magnet toward the compass until the compass needle points north, N. The magnet farther from the compass is the stronger magnet. Display diagrams showing the position of magnets of equal strength and magnets of unequal strength.

Check It Out!

A magnetometer is an instrument that measures the strength of magnets by moving the magnets until their magnet forces balance. Magnetometers are used to measure the intensity of the earth's magnetic field at a given spot. Read about these instruments and write a report that includes information such as how magnetometers:

  • detect valuable ore deposits.
  • are used to search for oil.
  • are used by geologists to determine rock structure beneath land masses and the ocean.
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