D.I.Y. magnetometer... measure Earth's magnetic field.

[Maxmars]

These things always fascinate me... never fail to educate me...

I never imagined a scientific use for two empty bottles of Morton seasoning blend...

and as the article author opines...,
 

While it’s not a practical field instrument, a proton-precession magnetometer of any kind for less than US $100 is nothing to sneer at.

Check it out:  Listen to Protons for Less Than $100 - Build a DIY magnetometer with a couple of seasoning bottles

When you get an MRI scan, the machine exploits a phenomenon called nuclear magnetic resonance (NMR). Certain kinds of atomic nuclei—including those of the hydrogen atoms in a water molecule—can be made to oscillate in a magnetic field, and these oscillations can be detected with coils of wire. MRI scanners employ intense magnetic fields that create resonances at tens to hundreds of megahertz. However, another NMR-based instrument involves much lower-frequency oscillations: a proton-precession magnetometer, often used to measure Earth’s magnetic field.

Proton-precession magnetometers have been around for decades and were once often used in archaeology and mineral exploration. High-end models can cost thousands of dollars. Then, in 2022 a German engineer named Alexander Mumm devised a very simple circuit for a stripped-down one. I recently built his circuit and can attest that with less than half a kilogram of 22-gauge magnet wire; two common integrated circuits; a metal-oxide-semiconductor field-effect transistor, or MOSFET; a handful of discrete components; and two empty 113-gram bottles of Morton seasoning blend, it’s possible to measure Earth’s magnetic field very accurately....

[Maxmars]

Interestingly, as in, speaking of the Earth's magnetic "map":