EM field — invisible layer

experiment 02 · magnetometer

the field
bends around everything.

Earth's magnetic field is ~50 µT — steady, invisible, everywhere. Your phone's magnetometer reads it 40 times a second. Metal objects, power lines, and electrical equipment warp this field. Your phone sees the warp.

hidden camera detectors cost $30. Your phone's magnetometer does the same thing for free. Sweep it slowly near a wall — a spike above 200 µT means metal or a powered device behind the plaster. Hotel rooms. Airbnb. Dressing rooms. This is how it works.

—

µT · total field magnitude

waiting

—X µT

—Y µT

—Z µT

—

anomaly log

no anomalies detected yet

▶ how does this work?

The Magnetometer API reads the raw magnetic field in microtesla on all three axes. Earth's background is 25–65 µT depending on location. Objects containing ferromagnetic material (steel, iron) or powered electronics distort this field at close range.

A sustained reading above 100 µT while stationary suggests a nearby anomaly. Above 200 µT is significant. A fridge motor reads ~500 µT at 5 cm. A transformerless power brick reads ~300 µT at 10 cm.

On iOS, permission must be requested via a user gesture — the "start sensor" button handles this.