Measuring the Earth's Magnetic Field

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Author: Jerry Silver

The Idea

The Earth has a magnetic field that goes from the South Pole to the North Pole. The magnetic South Pole is actually close to the geographic North Pole. We can measure how strong the horizontal component of the Earth's magnetic field is by comparing its effect to that of a magnetic field produced by the current flowing in a coil of wire.

What You Need

  • insulated wire several meters in length
  • compass, preferably one mounted on a low-friction pivot
  • ruler
  • protractor
  • cylindrical shape to wrap the coil (The diameter of the shape depends on the length of the compass needle. The diameter of the coil needs to be larger than the length of the compass needle.)
  • ring stand or other support to hold the coil of wire
  • DC power supply capable of current in the range of 1.0 amp or higher
  • DC-ammeter or a multimeter configured as an ammeter in the 0–10 A range
  • room with nonferrous tables and free of stray magnetic fields


Measuring the earth's magnetic field.

Measuring the earth's magnetic field.

The Earth's magnetic field is:

Measuring the earth's magnetic field.

(1.26 x 10–6 is the same as 0.0000126 and you can multiply inches by 0.00254 to get meters.)

  1. Set up the compass. Make sure it is freespinning and pointing to the north. Metal desks containing iron or steel may interfere with this. Also, motors or loudspeakers may have significant magnetic fields that could affect the outcome of this measurement.
  2. Form a coil of 15 turns using the cylindrical shape to form the coil. (For a small hand-held compass, a 1½ inch diameter pipe is a good form. For the pivot type compass, a soup can or coffee can is more appropriate.) After the coil is formed, withdraw the object used to wind the coil. Leave some wire at the start and end of the coil to allow it to be connected into a circuit.
  3. Support the coil using a ring stand or other support. The coil is oriented vertically with the plane of the coil facing east and west. The compass should be contained inside the plane of the coil, as shown in Figure 117-1. A top view of this is shown in Figure 117-2 for clarity. Notice the ends of the compass points to the turns of the coil.
  4. Make sure the DC power supply is turned off and the ammeter is set to read currents in the range of 1–10 amps.
  5. After stripping the insulation from the ends of the coil, attach one end to the positive terminal of the ammeter and the other end to the negative terminal of the DC power supply. You can use jumper wires or attach the coil directly. Refer to Figures 117-1 and 117-2 for the appropriate connections.
  6. Complete the electrical circuit by connecting the negative terminal of the ammeter to the positive terminal of the DC power supply.
  7. Place the protractor so the zero degree line ins aligned with the direction the compass exposed only to the Earth's magnetic field.
  8. Slowly and carefully turn on the DC power supply. Increase the current reading on the ammeter until the compass needle deflects 45 degrees from its starting position.
  9. At this point, the horizontal component of the Earth's magnetic field is balanced by and equal to the magnetic field of the coil.
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