Orient yourself with a magnetometer!

This activity allows students to understand how a compass works and to use the smartphone's magnetometer to orient themselves. It develops the understanding of the earth's magnetic field and its practical applications.

For over a thousand years, the magnetic compass has been humanity's most reliable navigation tool. Chinese navigators first discovered that a magnetized needle, free to rotate, always points roughly toward the north. This simple observation, based on the interaction between a small magnet and the Earth's vast magnetic field, enabled global maritime exploration, from the Polynesian voyagers to Columbus and Magellan. Today, every smartphone contains a digital magnetometer that performs the same function with far greater precision, measuring the Earth's magnetic field along three perpendicular axes with sensitivity better than 0.1 microtesla. By analyzing the horizontal component of this field, the phone can determine the direction of magnetic north to within a few degrees. This experiment guides students through the process of using the raw magnetometer data to find north, understand the difference between magnetic and geographic north, and appreciate the physics of the Earth's magnetic field.

Learning objectives:

The student uses the FizziQ magnetometer to determine the direction of magnetic north. By measuring the horizontal component of the earth's magnetic field in different orientations, the student identifies the direction where this component is maximum corresponding to magnetic north and then compares his result with the indication of a traditional compass.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

30

What students will do :

- Determine the direction of magnetic north using the smartphone magnetometer
- Identify the horizontal component of the Earth's magnetic field and understand its role in orientation
- Compare the magnetometer-based direction with a traditional compass reading
- Understand the concept of magnetic declination (difference between magnetic and geographic north)
- Explore how magnetic interference from nearby objects affects the readings

Scientific concepts:

- Earth's magnetic field
- Orientation
- Magnetic and geographic north
- Magnetometer
- Horizontal component of the field

Sensors:

- Magnetometer (3-axis magnetic field sensor)

What is required:

- Smartphone with the FizziQ application
- A classic compass for comparison (optional)
- A space without magnetic disturbances
- FizziQ experience notebook

Experimental procedure:

Find an outdoor location away from buildings, vehicles, and metal structures. These create magnetic interference that distorts compass readings.
Open FizziQ and select the Magnetometer sensor. Choose the horizontal component (Bx or By, depending on phone orientation).
Hold the smartphone horizontally at waist height. Slowly rotate your body and the phone through a complete 360° turn, observing how the horizontal field component changes.
Identify the orientation where the horizontal magnetic field is maximum. This direction points toward magnetic north.
Mark this direction with a physical reference (a stick, a line on the ground, or a landmark).
If you have a traditional compass, compare its north indication with your magnetometer-based direction. They should agree within ±5°.
Record the maximum horizontal field value (typically 15-25 µT in mid-latitudes).
Now investigate magnetic interference: bring a metal object (keys, scissors, or a magnet) close to the phone and observe how the readings change.
Walk around the outdoor area and note any locations where the magnetic field direction shifts unexpectedly. These indicate underground metal or magnetic anomalies.
Look up the magnetic declination for your location (the angle between magnetic north and geographic north). At your latitude, this may be 0-5° or more.
If visible, identify geographic north using the sun's position or a known landmark, and verify the declination angle.
Discuss why magnetic north shifts over time and what causes the Earth's magnetic field.

Expected results:

The horizontal magnetic field component should vary smoothly as the phone rotates, reaching a maximum when pointing toward magnetic north and a minimum when pointing south. The maximum value is typically 15-25 µT in mid-latitude locations. The magnetometer-based north direction should agree with a traditional compass within ±5°, assuming no nearby magnetic interference. Near buildings, cars, or metal structures, the readings may be distorted by 10-30° or more. The magnetic declination varies by location and changes slowly over time; in Western Europe it is currently about 0-3° east.

Scientific questions:

- Why does the horizontal component of the magnetic field reach its maximum when the phone points north?
- What causes the difference between magnetic north and geographic north (magnetic declination)?
- Why can magnetic interference from buildings and vehicles affect compass readings?
- How has the position of the magnetic north pole changed over historical time?
- What generates the Earth's magnetic field, and why does it slowly change?
- How do modern ships and aircraft compensate for magnetic declination in their navigation systems?

Scientific explanations:

The Earth's magnetic field, generated by the movements of the Earth's outer core, acts as a huge magnetic dipole, creating a field of approximately 25-65 µT at the surface. This field is oriented approximately toward the geographic poles, but with a significant offset: the magnetic north pole is currently in the Canadian Arctic.

The Earth's magnetic field has both a horizontal and a vertical component. The horizontal component, parallel to the earth's surface, is the one that allows orientation: it points towards magnetic north.

The smartphone's magnetometer measures the magnetic field along three orthogonal axes (X, Y, Z) with a sensitivity of approximately 0.1 µT. To determine the direction of magnetic north, simply rotate the smartphone horizontally and identify the orientation where the Y component of the field is maximum (assuming the phone's Y axis is pointing toward the front of the device).

This is precisely the principle used by traditional compasses, where a magnetic needle naturally aligns with the lines of the Earth's magnetic field. Potential difficulties include: 1) Local magnetic disturbances from metallic objects or electronic devices; 2) The calibration of the magnetometer which can drift over time; 3) The limited accuracy of the sensor (generally ±2-3°).

This simple experiment allows us to understand the fundamental principle of orientation by magnetism, a technique which revolutionized navigation long before the invention of GPS. It also provides an opportunity to discuss the difference between magnetic north and true north, as well as magnetic declination which varies depending on position on the globe.

Extension activities:

Frequently asked questions:

Q: The magnetometer shows north in a direction that seems wrong. What could cause this?
R: Nearby metal objects or electronic devices can strongly distort the magnetic field. Move at least 5 meters away from buildings, cars, and power lines. Also check that your phone case does not contain a magnet.

Q: How do I know which axis measures the horizontal component?
R: Hold the phone horizontally and rotate it. The axis that shows the largest variation during rotation is measuring the horizontal component. In FizziQ, the compass mode may directly indicate the heading.

Q: The readings are very noisy and unstable. How can I improve them?
R: Hold the phone steady and wait several seconds for the reading to stabilize. Some phones require a calibration gesture (figure-8 motion) before the magnetometer gives accurate readings. Check FizziQ documentation for calibration instructions.

Q: Why is the maximum horizontal field value different from the total Earth's field?
R: The Earth's field has both horizontal and vertical components. At mid-latitudes, the field dips into the ground at about 60-70°, so the horizontal component is only about 40-50% of the total field strength.