Magnetometer

Magnetometer is a free, simple, and ad-free app that measures the ambient magnetic field strength in microTeslas (µT) along the X, Y, and Z axes using the device's built-in magnetic field sensor.

The axes are defined from the center of the device as follows:
X: towards the right edge of the device, parallel to the screen
Y: towards the top edge of the device, parallel to the screen
Z: upwards from the device, perpendicular to the screen

Note: requires a device with a magnetic field sensor and support of the Android Sensors API