Your smartphone contains a tiny but powerful sensor that most people never think about: the magnetometer. This unassuming component is what makes your compass app work, helps your maps know which direction you're facing, and can even detect hidden magnets or metal objects nearby.
In this guide, we'll explore how your phone's magnetometer works, where manufacturers typically place it, and how you can use this sensor for practical applications and fun experiments. Whether you want to understand the technology behind your compass app or explore magnetic fields in your environment, this beginner-friendly guide has you covered.
What You'll Learn
- How Hall effect sensors detect magnetic fields
- Where phone manufacturers place magnetometers
- How your compass app converts raw data into directions
- Practical experiments you can try at home
- Common sources of interference and accuracy issues
What Is a Magnetometer?
A magnetometer is an instrument that measures magnetic field strength and direction. In smartphones, this sensor is incredibly small, typically just a few millimeters across, yet it can detect the Earth's magnetic field with enough precision to tell you which way is north.
The Hall Effect Explained
Most phone magnetometers use a principle called the Hall effect, discovered by physicist Edwin Hall in 1879. When an electrical current flows through a conductor and a magnetic field is applied perpendicular to that current, a voltage appears across the conductor. This voltage is proportional to the strength of the magnetic field.
Current Flows Through Sensor
A small electric current passes through a thin semiconductor material inside the sensor chip.
Magnetic Field Deflects Electrons
When a magnetic field is present, it pushes the moving electrons to one side of the conductor, creating a charge imbalance.
Voltage Is Measured
The resulting voltage difference (Hall voltage) is measured by the sensor's electronics. Stronger magnetic fields produce larger voltages.
Three-Axis Measurement
Modern phone magnetometers contain three separate Hall effect sensors oriented at right angles to each other (X, Y, and Z axes). This allows the phone to measure magnetic field strength in all three dimensions, which is essential for determining compass heading regardless of how you hold the device.
Measurement Units: Magnetometers typically measure in microteslas (uT) or milligauss (mG). The Earth's magnetic field ranges from about 25 to 65 uT depending on your location, with stronger fields near the poles and weaker fields near the equator.
Where Is the Magnetometer Located?
Phone manufacturers place the magnetometer in different locations depending on the device design. However, there are some common patterns you can rely on.
iPhone
Apple typically places the magnetometer near the top of the device, often close to the front-facing camera and earpiece speaker. This location minimizes interference from the battery and other internal components.
Android Phones
Android manufacturers vary more in placement. Samsung often positions it near the top edge, while other brands may place it along the side or back. Check your device specifications or use a sensor app to find the exact location.
Finding Your Magnetometer's Position
You can locate your phone's magnetometer using a simple experiment:
- Download a magnetometer app that shows real-time readings
- Hold a small magnet (like a refrigerator magnet) about 5 cm from your phone
- Slowly move the magnet around the edges of your phone
- Watch for the highest readings - that's where the sensor is located
Caution with Strong Magnets
Avoid using very strong magnets (like neodymium magnets) close to your phone. While they won't damage the magnetometer itself, they can potentially affect other components like the compass calibration data or, in older devices, the magnetic stripe on the speaker.
How Phone Compasses Work
Your phone's compass app does more than just read the magnetometer. It combines data from multiple sensors and applies complex calculations to show you an accurate heading.
From Raw Data to Direction
1. Raw Magnetometer Reading
The magnetometer outputs three values representing magnetic field strength along the X, Y, and Z axes. These raw values include both the Earth's magnetic field and any local interference.
2. Calibration Correction
The phone applies stored calibration data to correct for hard iron distortions (permanent magnetic fields from phone components) and soft iron distortions (materials that warp the Earth's field).
3. Tilt Compensation
Using the accelerometer, the phone determines its orientation relative to gravity. This allows it to calculate the horizontal component of the magnetic field even when the phone is tilted.
4. Heading Calculation
The phone uses trigonometry (specifically, the arctangent function) to convert the horizontal magnetic field components into a compass heading in degrees from magnetic north.
Magnetic North vs True North
There's an important distinction between magnetic north (where your compass points) and true north (the geographic North Pole). The difference, called magnetic declination, varies by location and changes over time. Most smartphone compass apps automatically correct for this using your GPS location and a database of declination values.
Fun Experiments to Try
Your phone's magnetometer opens up a world of exploration. Here are some practical experiments you can try at home or around your neighborhood.
Detect Hidden Magnets
Use a magnetometer app to find hidden magnets in everyday objects. Check the clasps on bags and purses, magnetic cabinet closures, and the backs of kitchen knife holders. You'll be surprised how many magnets surround us.
Find Metal Studs in Walls
While a proper stud finder works better, your magnetometer can detect the screws and nails in wall studs. Move your phone slowly along the wall about 2 cm from the surface and watch for spikes in the magnetic field reading. Mark the spots where you see consistent readings.
Map Your Local Magnetic Environment
Walk around your home or office while recording magnetometer readings. You'll discover that certain areas have stronger or distorted magnetic fields due to electrical wiring, appliances, and structural steel. This can help you understand why your compass occasionally gives odd readings in certain locations.
Measure Power Line Fields
Stand beneath overhead power lines (at a safe distance) and observe how the magnetic field fluctuates. The alternating current creates a pulsing magnetic field at 50 Hz or 60 Hz depending on your country's electrical grid. You may see the total field strength varying as the current oscillates.
Pro Tip: For the best readings, hold your phone flat and away from your body. Metal jewelry, belt buckles, and even the iron in your blood can affect readings at very close range.
Apps That Use Magnetometer Data
Numerous apps take advantage of your phone's magnetometer for various purposes. Here are some common categories and examples.
Compass Apps
- Built-in iOS and Android compass
- Commander Compass (hiking-focused)
- Digital Compass for navigation
Metal Detectors
- Metal Detector apps
- Stud finder applications
- EMF meter tools
Science and Logging
- Physics Toolbox Sensor Suite
- Sensor Kinetics
- Phyphox (physics experiments)
Navigation and AR
- Google Maps (direction facing)
- Augmented reality games
- Star-gazing applications
Limitations and Accuracy Considerations
While phone magnetometers are useful tools, they have limitations you should understand for reliable results.
Calibration Requirements
Phone magnetometers need regular calibration. If your compass seems inaccurate, try the "figure-8" calibration motion: hold your phone and rotate it in a figure-8 pattern, tilting it in all directions. This allows the phone to sample the magnetic field from multiple orientations and calculate better correction factors.
Sources of Interference
| Interference Source | Effect | Solution |
|---|---|---|
| Phone cases with magnets | Constant offset in readings | Remove case or recalibrate with it on |
| Nearby electronics | Fluctuating interference | Move away from laptops, speakers, TVs |
| Metal structures | Field distortion and false readings | Step away from cars, buildings, fences |
| Power lines and wiring | 50/60 Hz oscillating interference | Increase distance or average readings over time |
Accuracy Expectations
Consumer-grade phone magnetometers typically achieve accuracy of plus or minus 2-5 degrees under ideal conditions. In environments with significant magnetic interference, accuracy can degrade to plus or minus 10-20 degrees or worse. For critical navigation, always use GPS and multiple references rather than relying solely on the magnetic compass.
NullField Lab and EMF Research
NullField Lab is a research tool that uses your phone's magnetometer in a unique way: to detect and compensate for electromagnetic field (EMF) interference from power grid infrastructure.
How NullField Lab Uses the Magnetometer
The electrical grid operates at 50 Hz (in most of Europe, Asia, and Australia) or 60 Hz (in the Americas). These alternating currents create subtle oscillating magnetic fields that permeate our environment. While these fields are too weak to notice consciously, NullField Lab can detect them using your phone's magnetometer.
Detection Process
NullField Lab samples the magnetometer at 60 Hz and uses a sliding window Discrete Fourier Transform (DFT) to identify the exact grid frequency with 0.01 Hz precision. This allows the app to track real-time variations in grid frequency, which typically fluctuates by plus or minus 0.2 Hz depending on load conditions.
Research Applications
Researchers and experimenters use NullField Lab to explore the relationship between environmental EMF and personal experiences. The app provides tools for:
- Monitoring local grid frequency in real-time
- Visualizing magnetometer vector alignment
- Exploring audio-based EMF compensation techniques
- Logging environmental magnetic field data for personal research
The magnetometer's ability to detect these subtle environmental fields makes it a valuable tool for anyone interested in understanding the electromagnetic environment around them.
See your phone's magnetometer in action detecting grid frequency variations.
Disclaimer: This article is for educational purposes only. NullField Lab is a research tool for personal experimentation, not a medical device. Magnetometer accuracy varies between devices and is affected by environmental conditions. For critical navigation or measurement applications, use professional-grade equipment.
