Magnetic treasure hunt

This activity allows students to understand how metal detectors work and explore the magnetic properties of different materials.

For centuries, prospectors and treasure hunters relied on dowsing rods and intuition to find buried metals. Today, metal detectors use sophisticated electromagnetic principles to locate hidden objects with remarkable precision. At the heart of every metal detector is a sensor that responds to disturbances in the local magnetic field caused by ferromagnetic materials. Your smartphone contains exactly such a sensor: a three-axis magnetometer originally designed for compass applications, but sensitive enough to detect a set of keys hidden under a thin layer of sand. Ferromagnetic metals like iron, nickel, and cobalt create their own local magnetic fields that add to or distort the Earth's background field of approximately 25-65 microtesla. Even small objects can produce measurable disturbances of several microtesla at close range. This experiment transforms the smartphone into a metal detector, challenging students to locate hidden metallic objects by systematically scanning an area and interpreting the magnetometer readings, just as real treasure hunters and archaeologists do in the field.

Activity overview:

The student uses the magnetometer on his smartphone to detect metallic objects hidden under a blanket or in sand. He methodically explores the area by observing variations in the magnetic field and analyzes how these variations differ depending on the size and type of metal of the hidden object.

Level:

Middle school

FizziQ

Author:

Duration (minutes) :

35

What students will do :

- Use the smartphone magnetometer to detect hidden ferromagnetic objects
- Map the magnetic field variations across a search area and identify anomalies
- Understand the difference between ferromagnetic and non-ferromagnetic materials
- Investigate how the size and distance of a metallic object affect the magnetic signal
- Connect the experiment to real-world applications of magnetic detection technology

Scientific concepts:

- Magnetic field
- Magnetic properties of metals
- Magnetic detection
- Disturbances in the Earth's magnetic field
- Ferromagnetism and diamagnetism

Sensors:

- Magnetometer (3-axis magnetic field sensor)

Material needed:

- Smartphone with the FizziQ application
- Various metal objects of different sizes and compositions
- A cover or sandbox to hide objects
- FizziQ experience notebook

Experimental procedure:

Open FizziQ and select the Magnetometer sensor. Choose the absolute magnetic field (total magnitude) for the easiest detection.
Before hiding any objects, take a baseline reading of the local magnetic field. Walk around the search area and note the background value (typically 25-65 µT). Record this as your reference.
Ask a partner to hide 3-5 metallic objects of different types and sizes (iron nail, steel scissors, aluminium can, copper coin, small magnet) under a blanket or in a sandbox.
Stand at one edge of the search area. Hold the smartphone at a constant height of about 5-10 cm above the surface.
Move the smartphone slowly and systematically across the area in parallel lines, like mowing a lawn. Watch the magnetometer reading for any increase above the baseline.
When you detect an anomaly (a reading significantly higher than baseline), mark the location and record the peak magnetic field value.
After scanning the entire area, reveal the hidden objects and check which ones you successfully located.
Note which objects were detected and which were missed. Ferromagnetic objects (iron, steel, nickel) should produce strong signals, while non-ferromagnetic metals (aluminium, copper) may be undetectable.
For each detected object, measure the detection distance: at what height above the object does the anomaly become undetectable?
Create a table recording: object type, material, approximate mass, peak field value, and detection distance.
Repeat with the objects at different depths (under 1, 2, or 3 layers of fabric or centimeters of sand) and observe how the signal decreases.
Discuss why some metals are magnetic and others are not, and how real metal detectors overcome this limitation.

Expected results:

Ferromagnetic objects (iron nails, steel tools, nickel coins) should produce clear magnetic anomalies of 5-50 µT above the baseline, easily detectable at distances of 5-15 cm. Larger objects and stronger magnets produce larger anomalies detectable at greater distances. Non-ferromagnetic metals (aluminium, copper, brass) will produce little or no detectable signal with a magnetometer, as they do not create static magnetic disturbances. The detection distance decreases rapidly with depth, following approximately an inverse cube law (the field from a magnetic dipole decreases as 1/r³). Students should find that a small iron nail is detectable at about 5 cm, while a larger steel tool may be detected at 15-20 cm.

Scientific questions:

- Why can the magnetometer detect iron and steel objects but not aluminium or copper ones?
- How does the detection distance depend on the size of the hidden object? Why?
- What is the difference between ferromagnetic, paramagnetic, and diamagnetic materials?
- How do professional metal detectors work differently from a magnetometer?
- Could you use this technique to find buried archaeological artifacts?
- Why does the magnetic signal decrease so rapidly with distance from the object?

Scientific explanations:

A smartphone's magnetometer measures the magnetic field in three spatial directions with a sensitivity of approximately 0.1 μT. Ferromagnetic materials (containing iron, nickel or cobalt) significantly disrupt the Earth's magnetic field (by approximately 25-65 μT) by creating their own local magnetic fields.

This property allows their detection even under a layer of sand or earth. The disturbance depends on the mass of the metal, its composition and its distance from the sensor.

Objects made of non-ferrous metals (aluminum, copper) are less detectable because they are not magnetizable. This technique is used professionally for the detection of underwater wrecks, landmines or archaeological objects.

The main limitation is the detection depth which decreases with object size in an approximate relationship of 10 times the object diameter. The magnetometer on smartphones is sometimes automatically calibrated by the operating system, which can make measurements difficult when moving the device slowly.

To get around this problem, FizziQ offers direct access to raw sensor data.

Extension activities:

Frequently asked questions:

Q: The magnetometer readings keep changing even without any hidden objects. Why?
R: The Earth's magnetic field is constant at a given location, but your smartphone sensor is sensitive to nearby sources of magnetic interference: tables with metal frames, electrical wiring in floors and walls, and even the phone's own internal components. Choose a location away from large metal structures.

Q: I cannot detect any of the hidden objects. What is wrong?
R: Ensure you are using ferromagnetic objects (iron, steel, nickel). Aluminium cans and copper coins are not ferromagnetic and will not be detected. Also keep the phone very close to the surface (5 cm or less) and move slowly.

Q: Why does a permanent magnet produce a much stronger signal than a steel nail?
R: A permanent magnet has a large, aligned magnetic dipole moment, creating a strong external field. A steel nail only becomes temporarily magnetized in the Earth's field and produces a weaker disturbance. However, even this weak disturbance is detectable at short range.

Q: Can I use this to find pipes or cables in walls?
R: Yes, many smartphone apps use the magnetometer to locate steel pipes, nails, and reinforcing bars in walls. However, the limited range (a few centimeters) means it only works for objects close to the surface.