“A magnetometer, sometimes also called Gaussmeter, measures the strength of the magnetic field. It is an essential tool to test the strength of permanent magnets and electromagnets and to understand the field shape of nontrivial magnet configurations. If it is sensitive enough it can also detect if iron objects got magnetized. Time-varying fields from motors and transformers can be detected if the probe is fast enough.
Mobile phones usually contain a 3-axis magnetometer but they have been optimized for the weak earth magnetic field of ~1 Gauss = 0.1 mT and saturate at fields of a few mT. The location of the sensor on the phone is not obvious, and it is not possible to place the sensor inside narrow apertures such as the bore of an electromagnet. Moreover, you might not want to bring your smartphone close to strong magnets.
Here I describe how to make a simple portable magnetometer with common components: a linear hall sensor, an Arduino, a display and a push-button. The total cost is less than 5EUR, and the sensitivity of ~0.01mT on a range of -100 to +100mT is better than what you might naively expect. To get accurate absolute readings, you’ll need to calibrate it: I describe how to do that with a home-made long solenoid.”