A magnetometer is a device that measures the strength and direction of magnetic fields. Its uses span an enormous range, from the compass app on your phone to spacecraft orbiting Saturn. Because different materials and processes create distinct magnetic signatures, magnetometers have become essential tools in navigation, medicine, defense, geology, archaeology, and dozens of industrial applications.
Navigation in Your Smartphone
Nearly every modern cellphone contains a tiny magnetometer. It detects the direction of Earth’s magnetic field, which is what makes your phone’s compass and turn-by-turn navigation work. Without it, your mapping app would know your location from GPS but wouldn’t know which way you’re facing. The sensor is a small chip, typically just a few millimeters across, built using the same micro-manufacturing techniques as other phone components like accelerometers and gyroscopes.
Mapping What Lies Underground
Geologists fly magnetometers over large areas of land, a technique called aeromagnetic surveying, to peer beneath the surface without drilling a single hole. Different rock types have different magnetic properties: iron-rich igneous rocks produce strong signals, while sedimentary layers tend to be magnetically quiet. By mapping these variations from the air, geologists can identify the boundaries of underground basins, locate faults and folds in rock layers, and pinpoint areas likely to contain ore deposits or oil-bearing structures.
This approach dates back decades. In fact, the fluxgate magnetometer, one of the most widely used types, was originally developed in the 1930s by a geophysicist at Gulf Oil specifically for finding ore deposits. The same basic principle, detecting small disturbances in Earth’s background magnetic field, now drives applications far beyond mining.
Archaeology Without Excavation
Archaeologists use magnetometers to locate buried structures before ever picking up a trowel. Fired bricks, kilns, hearths, pottery, and iron artifacts all disturb the local magnetic field in detectable ways. A research team working in Fayoum, Egypt, for example, used magnetic surveys to identify possible buried archaeological targets by spotting high positive magnetic anomalies against the surrounding geology. Some anomalies turned out to be related to mudstones containing iron minerals rather than human-made structures, which is why archaeologists typically combine magnetic data with other techniques like ground-penetrating radar to narrow down what’s worth excavating.
Detecting Submarines and Shipwrecks
A submarine is essentially a large mass of steel moving through the ocean, and steel distorts Earth’s magnetic field in a way that a sensitive magnetometer can pick up. Military forces have exploited this since World War II, when both Japanese and American anti-submarine units deployed magnetic anomaly detectors (MADs) mounted on aircraft or towed behind ships to find shallow submerged enemy submarines. The technology remains in use today.
The same principle works for locating shipwrecks on the ocean floor. Interestingly, submarines sometimes operate near known wrecks deliberately, using the magnetic clutter of sunken ships to confuse MAD systems trying to track them.
Brain Surgery Planning
One of the most precise medical applications of magnetometers is magnetoencephalography, or MEG. Your brain’s neurons produce faint magnetic fields when they fire, and MEG uses extremely sensitive sensors to detect those fields from outside your skull. The result is a functional map of brain activity that shows not just where something is happening but exactly when it activates, with millisecond-level timing that other brain imaging techniques can’t match.
Neurologists and neurosurgeons rely on MEG for two main purposes. The first is locating the exact origin point of epilepsy-related seizures so surgeons know precisely what tissue to target. The second is mapping the brain’s sensory, motor, language, and vision areas before removing a tumor, so the surgical team can avoid damaging critical functions. Providers often combine MEG with MRI to overlay functional activity onto a detailed anatomical image of the brain, creating what’s called magnetic source imaging.
The sensors that make this possible are called SQUIDs (superconducting quantum interference devices), and they are the most sensitive magnetometers in existence. A typical SQUID sensor can detect magnetic fields as faint as 2 to 3 femtoteslas per unit bandwidth. For perspective, Earth’s magnetic field is roughly 50 microteslas, meaning SQUIDs can pick up signals about ten billion times weaker than the field that moves a compass needle. That sensitivity is what allows them to register the tiny magnetic whispers of individual brain regions.
Exploring Other Planets
Spacecraft carry magnetometers to study the magnetic environments of other worlds. NASA’s Cassini orbiter, for instance, spent over a decade measuring Saturn’s magnetic field from different locations around the planet. Because Saturn’s magnetic field is generated by electrical currents swirling in metallic liquid hydrogen deep inside the planet, mapping the field from orbit gave scientists a way to study the planet’s otherwise unreachable interior.
Cassini’s magnetometer also tracked the boundaries where Saturn’s magnetosphere meets the solar wind, revealing how the planet’s magnetic bubble constantly changes in size and shape. It even made the first measurements of the magnetic environment around Saturn’s moon Titan, showing how Titan interacts with the outer fringes of Saturn’s magnetosphere and, during parts of its orbit, with the solar wind directly.
Motors, Vehicles, and Industrial Systems
Hall effect sensors, a common type of magnetometer, are embedded throughout modern vehicles and industrial equipment. Inside brushless electric motors, Hall sensors detect the position of the spinning rotor so the motor controller knows exactly when to energize each coil. This is what keeps the motor turning smoothly and efficiently. These motors show up in car sliding doors, power windows, windshield wipers, seat adjustment systems, engine cooling fans, and pumps.
Hall sensors are especially important at low speeds and at startup. At higher speeds, a motor’s own electrical signals can indicate rotor position, but at zero or low speed those signals are too faint to read. Hall sensors provide reliable position data regardless of speed, which is critical for any system that needs strong torque from a standstill, like an electric vehicle pulling away from a stop.
How Different Sensor Types Compare
Not all magnetometers work the same way, and the choice of sensor depends entirely on what you need to measure.
- Hall effect sensors are the workhorses: cheap, small, and robust enough for phones and motors. They measure magnetic fields by detecting the voltage that builds up when a current-carrying conductor sits in a magnetic field. Their range tops out around 5 Tesla, which covers most industrial and consumer needs.
- Fluxgate magnetometers are more sensitive than Hall sensors and have been the standard for geophysical surveys and military submarine detection since the 1930s. They work by measuring how an external magnetic field distorts the magnetization cycle of a small core of magnetic material.
- SQUID sensors operate at the extreme end of sensitivity, detecting fields in the femtotesla range. They require cooling to superconducting temperatures, which makes them expensive and bulky, but nothing else can match their precision. They’re the backbone of every MEG brain-imaging system in clinical use.
- NMR magnetometers use the same nuclear magnetic resonance principle behind MRI machines. They measure how atomic nuclei in a sample respond to a radiofrequency pulse in the presence of a magnetic field. Their resonant frequency is directly proportional to field strength, making them exceptionally accurate for calibrating other instruments.
The range of sensitivities across these types spans many orders of magnitude, which is why magnetometers can serve purposes as different as guiding your morning commute and mapping the interior of a gas giant a billion kilometers away.