What Is a Magnetometer? How Your iPhone Detects Magnetic Fields

Every iPhone made in the last decade contains a tiny but remarkable sensor called a magnetometer. Most people know it only as the sensor that powers the Compass app. But this unassuming chip can do far more — from detecting hidden metals in walls to measuring electromagnetic interference from appliances to finding lost objects in grass.

In this guide, we'll explain exactly how magnetometers work, what makes the iPhone's sensor special, and the practical applications that go far beyond pointing north.

The Science: How Magnetometers Measure Magnetic Fields

A magnetometer measures the strength and direction of magnetic fields. The Earth itself generates a magnetic field — roughly 25 to 65 microteslas (uT) at the surface, depending on your location. This is the field that makes compass needles point north.

Modern smartphone magnetometers use a technology called 'Hall effect sensing' (or in newer devices, 'magnetoresistive sensing'). When a magnetic field passes through a semiconductor material, it causes a measurable voltage change proportional to the field strength. By combining three sensors oriented along the X, Y, and Z axes, the chip can determine both the strength and direction of the magnetic field in three-dimensional space.

The result is surprisingly precise. iPhone magnetometers can detect field changes as small as 0.1 microtesla — sensitive enough to detect a small magnet from several inches away, or a cluster of nails behind drywall.

What Can a Magnetometer Detect?

Understanding what creates magnetic fields helps you understand what a magnetometer can — and cannot — find. There are three main sources of detectable magnetic fields.

• Permanent magnets: Refrigerator magnets, magnetic phone mounts, speakers, hard drives, and electric motors all contain permanent magnets that produce strong, static magnetic fields
• Ferrous metals: Iron, steel, and nickel are 'ferromagnetic' — they distort the Earth's magnetic field even without being magnetized themselves. This is how you detect nails, screws, rebar, steel pipes, and iron objects
• Electrical current: Any wire carrying electrical current generates a magnetic field proportional to the current. AC wiring in walls, power cables, and operating appliances all produce detectable fields

What a Magnetometer Cannot Detect

It's equally important to understand the limitations. Magnetometers respond to magnetic fields, not all materials or energy sources.

• Non-ferrous metals: Copper, aluminum, brass, gold, and silver produce very weak magnetic signatures and are difficult to detect without industrial equipment
• Plastic, wood, and glass: These materials are magnetically invisible
• Non-energized wires: A wire with no current flowing produces no magnetic field (though the copper itself may have a very faint residual signature)
• Deep objects: Magnetic field strength drops with the cube of distance — an object 6 inches deep produces roughly 1/8th the signal of one at the surface

Understanding Microtesla Readings

Flux displays magnetic field strength in microteslas (uT), the standard SI unit for magnetic flux density. Here's a practical reference guide for interpreting readings.

• 25-65 uT: Earth's background magnetic field (varies by location and orientation)
• 70-100 uT: You're near a small ferrous metal object (screw, nail, small tool)
• 100-200 uT: Significant metal nearby (steel pipe, cluster of fasteners, large tool)
• 200-500 uT: Strong source — large metal object, electric motor, or power transformer
• 500+ uT: Very strong source — large magnet, industrial equipment, or you're touching the object

The 3-Axis Breakdown: X, Y, and Z

Flux provides a full 3-axis breakdown of the magnetic field, showing the X, Y, and Z components separately. This isn't just a technical curiosity — it's genuinely useful for locating objects.

When you're scanning a wall, watch which axis shows the strongest change. If the X-axis (horizontal) spikes, the metal is to your left or right. If the Z-axis (perpendicular to the phone) spikes, the metal is directly behind the phone. By watching all three axes, you can triangulate the position of hidden objects with surprising accuracy.

The total field strength (displayed as the main number) is calculated as the magnitude of the 3D vector: sqrt(x² + y² + z²). This is always positive and represents the overall field intensity regardless of direction.

Practical Applications Beyond Metal Detection

Once you have a magnetometer app on your phone, creative applications abound. Teachers use them to demonstrate electromagnetic principles in physics classes. Hobbyists use them to check if antiques contain real iron or steel versus aluminum or plastic reproductions. Audiophiles use them to check speaker magnet polarity. Even geologists use phone magnetometers for preliminary field surveys of magnetic rock formations.

One popular use is checking for magnetic interference near sensitive electronics. If your monitor displays strange colors, your speakers hum, or your compass gives inaccurate readings, a magnetic field scan can identify the source. Move your phone slowly around the area and look for abnormal readings — the source of interference will produce a localized spike.

Tips for Getting the Most Accurate Readings

1. Remove magnetic accessories: MagSafe cases, magnetic car mounts, and metal phone cases all interfere with readings. Remove them before scanning.
2. Calibrate by rotating: If readings seem unstable, slowly rotate your phone in a figure-8 pattern. This helps the magnetometer recalibrate its internal offsets.
3. Note your baseline: Before scanning, hold your phone away from any metal and note the reading. This is your local baseline — deviations from this number indicate nearby magnetic sources.
4. Move slowly: The sensor samples many times per second, but rapid movement can cause you to miss narrow field spikes. Slow, deliberate scanning produces the best results.
5. Keep consistent distance: When comparing readings, maintain the same distance from the surface you're scanning. Even small changes in distance significantly affect field strength.