A magnetometer measures magnetic field strength and direction. It can detect Earth's field at 50 microtesla, a welding arc at several millitesla, or a nuclear magnetic resonance signal at femtotesla levels, a span of more than 15 orders of magnitude across different sensor technologies.
This article covers how the four main magnetometer types work, the key parameters for selection, and where they are used across navigation, industrial, and scientific applications.
A magnetometer is a sensor that measures the strength, direction, or both properties of a magnetic field at a point in space, producing an output in units of tesla (T), gauss (G), or amperes per metre (A/m).
The choice of magnetometer technology depends almost entirely on the sensitivity range needed: Hall effect sensors handle millitesla fields in industrial equipment, while fluxgate sensors read Earth-level microtesla fields for navigation, and SQUID sensors detect femtotesla biomedical signals.
What a Magnetometer Measures and Why It Matters
Every current-carrying conductor and every permanent magnet produces a magnetic field. A magnetometer quantifies that field, giving engineers a way to detect position, orientation, material properties, and current flow without physical contact.
The SI unit of magnetic flux density is the tesla (T). Earth's magnetic field is approximately 25 to 65 microtesla at the surface. A rare earth permanent magnet produces 1 to 1.4 tesla; an MRI scanner operates at 1.5 to 7 tesla.
A SQUID magnetometer resolves signals as small as 1 femtotesla, the level of magnetic fields from electrical activity in the human brain.
Unlike a Hall effect proximity sensor that simply detects the presence or absence of a magnet, a full magnetometer provides a calibrated vector measurement on one, two, or three orthogonal axes.
This vector output is what makes magnetometers useful for compass heading, current measurement, and non-destructive testing.
The four main magnetometer technologies each occupy a different sensitivity band and cost tier. Understanding which band matches your application is the first step in selecting the right sensor for the job.
Magnetometer Working Principle: Four Core Technologies
Hall Effect Magnetometer
A current-carrying semiconductor in a magnetic field develops a transverse voltage proportional to the field strength. Simple, low cost, and robust. Range: 1 uT to several tesla. Used in current sensing, position detection, and motor control.
Fluxgate Magnetometer
A soft ferromagnetic core is driven into saturation by an AC excitation coil; an external DC field creates second-harmonic distortion in the sense coil output proportional to that field. Range: 0.1 nT to 1 mT. Excellent for compass and geophysical survey applications.
AMR and GMR Magnetometers
Anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) sensors change their electrical resistance in proportion to an applied magnetic field. Arranged in Wheatstone bridge configurations for temperature compensation. Range: 1 nT to 1 mT. Used in smartphone compasses, hard drive read heads, and position encoders.
SQUID Magnetometer
A superconducting loop with Josephson junctions produces quantum interference sensitive to changes smaller than one flux quantum. Requires cryogenic cooling. Range: 1 fT to 100 uT. Used in MEG brain imaging and geophysical surveys.
Hall Effect: The Working Principle Behind the Most Common Magneto-meter
When current I flows through a conductor of thickness t in the presence of a magnetic field B perpendicular to the current, charge carriers experience a Lorentz force that pushes them sideways. This sideways displacement of charge creates a transverse voltage called the Hall voltage.
The key insight is that semiconductor materials with low carrier concentration (n) produce much larger Hall voltages than metals for the same current and field. This is why all practical Hall magnetometers use silicon or III-V semiconductor materials rather than metal conductors.
The Hall voltage for Earth's field is only in the microvolt range, so every Hall magnetometer IC includes an on-chip chopper-stabilized amplifier that converts this signal to a usable output while rejecting amplifier offset and 1/f noise.
Magnetometer Comparison: Sensitivity, Range, and Cost
| Type | Sensitivity Range | Resolution | Operating Temp | Relative Cost | Primary Use Case |
|---|---|---|---|---|---|
| Hall Effect | 1 uT to 10 T | ~1 uT | minus 40 to 150°C | Very low (under $2) | Current sensing, position, motor control |
| AMR | 100 nT to 1 mT | ~1 nT | minus 40 to 85°C | Low ($2 to $10) | Smartphone compass, angle sensing |
| GMR | 1 nT to 100 mT | ~0.1 nT | minus 40 to 150°C | Low to medium | Hard drive read heads, current sensing |
| Fluxgate | 0.1 nT to 1 mT | ~0.1 nT | minus 55 to 85°C | Medium ($50 to $500) | Navigation, geophysics, UXO detection |
| Proton Precession | 10 nT to 100 uT | ~0.1 nT absolute | 0 to 50°C | High ($1,000+) | Geomagnetic surveys, mineral exploration |
| SQUID | 1 fT to 100 uT | ~1 fT | Near absolute zero | Very high ($100,000+) | MEG brain imaging, NDE, research |
Magnetometer Hall Voltage and Field Strength Calculator
Magnetometer Applications Across Industries
Choosing the Right Magnetometer for Your Application
✅ Correct Approach
- Define the minimum and maximum field strength your application needs to measure before selecting a type
- Use Hall effect or GMR for strong fields (above 1 uT) in industrial, automotive, and consumer applications
- Use fluxgate when you need sub-nanotesla resolution at Earth-field levels for navigation or survey work
- Account for temperature drift: all magnetometers shift sensitivity with temperature, requiring either compensation or calibration
- For 3-axis heading, always perform hard-iron calibration (sphere fitting) after installation to remove local magnetic offsets
❌ Common Mistakes
- Selecting a Hall sensor for a navigation application where the Earth-field resolution is below its noise floor
- Mounting a compass magnetometer near ferrous metal or PCB power planes without accounting for hard-iron distortion
- Ignoring stray fields from nearby motors, relays, or power cables that saturate the sensor
- Confusing flux density (tesla) with field intensity (A/m): different materials and standards use both, and mixing them up causes calibration errors
- Assuming a magnetometer can replace a gyroscope: a magnetometer gives heading, not rotation speed
Watch: How Magnetometers Work
Questions Engineers Ask
External References
What We Learn Today
- A magnetometer measures magnetic field strength and direction in units of tesla (T) or gauss (G)
- Four main types: Hall effect (strong fields, low cost), fluxgate (Earth-field navigation), AMR/GMR (compass and position), SQUID (femtotesla biomedical and research)
- Hall voltage formula: V_H = (I × B) / (n × e × t); semiconductors give larger Hall voltage than metals because of lower carrier concentration
- Fluxgate magnetometers detect field by measuring second-harmonic distortion in a saturated ferromagnetic core; resolution down to 0.1 nanotesla
- SQUID sensors require cryogenic cooling and resolve fields as small as 1 femtotesla, used in MEG brain imaging
- Always perform hard-iron calibration after installing a compass magnetometer to remove local magnetic offsets from the enclosure
- A magnetometer gives heading and field strength; it cannot measure angular rotation rate; that requires a gyroscope