Listen to Protons for Less Than $100

Summary

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. The frequency of the signal emitted by protons precessing in Earth’s magnetic field lies in the audio range, so with a pair of headphones and two amplifier integrated circuits [middle right], you can detect a signal from water in seasoning bottles wrapped in coils [bottom left and right]. A MOSFET [middle left] allows for rapid control of the coils. The amplification circuitry is powered by a 9-volt battery, while a 36-volt battery charges the coils.James ProvostLike an MRI scanner, a proton-precession magnetometer measures the oscillations of hydrogen nuclei—that is, protons. Like other subatomic particles, protons possess a quantum property called spin, akin to classical angular momentum. In a magnetic field, protons wobble like spinning tops, with their spin axes tracing out a cone—a phenom...