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 Provost
Like 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 phenomenon called precession. A proton-precession magnetometer gets many protons to wobble in sync and then measures the frequency of their wobbles, which is proportional to the intensity of the ambient magnetic field.
The weak strength of Earth’s magnetic field (at least compared to that of an MRI machine) means that protons wobbling under its influence do so at audio frequencies. Get enough moving in unison and the spinning protons will induce a voltage in a nearby pickup coil. Amplify that and pass it through some earphones, and you get an audio tone. So with a suitable circuit, you can, literally, hear protons.
The first step is to make the pickup coils, which is where the bottles of Morton seasoning blend come in. Why Morton seasoning blend? Two reasons. First, this size bottle will allow you to wrap about 500 turns of wire around each one with about 450 grams of 22-gauge wire. Second, the bottle has little shoulders molded at each end, making for excellent coil forms.
Why two bottles and two coils? That’s to quash electromagnetic noise—principally coming from power lines—that invariably gets picked up by the coils. When two counterwound coils are wired in series, such external noise tends to cancel out. Signals from precessing protons in the two coils, though, will reinforce one another.
Don’t try this indoors or anywhere near iron-containing objects.
A proton magnetometer has three modes. The first is for sending DC current through the coils. The second mode disconnects the current source and allows the magnetic field it had created to collapse. The third is listening mode, which connects the coils to a sensitive audio amplifier. By filling each bottle with distilled water and sending a DC current (a few amperes) through these coils, you line up the spins of many protons in the water. Then, after putting your circuit into listening mode, you use the coils to sense the synchronous oscillations of the wobbling protons.
Mumm’s circuit shifts from one mode to another in the simplest way possible: using a three-position switch. One position enables the DC-polarization mode. The next allows the magnetic field built up during polarization to collapse, and the third position is for listening.
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