The Caliper

Where IS North?

A new experiment idea for your Magnetic Field Sensor!

It depends. Do you mean geographic north or magnetic north? The geographic (true) north pole is the point at 90° N latitude. It is aligned with the rotational axis of the earth. The magnetic north pole is the point to which a compass needle points. It is currently in northern Canada, but moves at an average rate of 15 km per year. Depending upon your location, the difference between geographic north and magnetic north, called magnetic declination, can range from 0° to 30° or higher.

In this experiment, students will measure the magnetic field of the earth using a Vernier Magnetic Field Sensor. They will then use that data to determine magnetic north. Knowing the direction of true north, they can calculate the magnetic declination at their location.

Figure 1

Make copies of the degree wheel and pointer, and cut them out. There is a black-line master at www.vernier.com/north. Alternatively, you can photocopy Figure 1, but it will work best to enlarge the circle to approximately 15 cm in diameter. Tape the circle to each student group’s work space with 0° aligned with true north. You can locate true north using a GPS or blue prints for your school. If these are not available, you can determine true north by using a compass to find magnetic north, then correct for the magnetic declination. If you don’t know the magnetic declination at your location, it can be calculated at several locations on the internet, including www.ngdc.noaa.gov/seg/potfld/java/GeoMag.shtml.

Figure 2

Keep in mind that readings may fluctuate due to deviation, the influence of the immediate environment upon your compass, by things such as electrical currents, computer monitors, or metal brackets. Try to avoid these influences. Tape a pointer on top of the white dot of the Magnetic Field Sensor and bend it so that it is perpendicular to the sensor (see Figure 2). Students will use this as a guide when positioning the sensor.

Connect the Magnetic Field Sensor (with the switch set to HIGH) to Channel 1 of an interface and set up data collection in Events with Entry mode. If you are using Logger Pro, you may want to use the experiment file found at www.vernier.com/north. Hold the sensor vertically. The top of the sensor should be placed at the center of the degree wheel with the pointer pointing toward 0 (see Figure 2). Do not zero the sensor. Be careful to keep the end of the Magnetic Field Sensor over the dot at the center of the circle. Begin data collection. Collect a data point at 0°. Rotate the sensor 15° clockwise and take a second data point. Continue taking a data point every 15° until 360° has been reached.

Magnetic north is located in the direction where the magnetic field reading is the highest. The difference between this peak location and true north (0°) is the declination for your area. The data in Figure 3 were taken at our office in Beaverton, Oregon. Our actual magnetic declination is 18°.