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.
Avoiding Damaging Sparks
The second mode might seem easy to achieve—just disconnect the coils, right? But if you do that, the same principle that makes spark plugs spark will put a damaging high voltage across the switch contacts as the magnetic fields around the coils collapse.
The proton-precession magnetometer is primarily just a multistage analog amplifier.James Provost
To avoid that, Mumm’s circuit employs a MOSFET, wired to work like a high-power Zener diode, used in many power-regulation circuits to allow only current above a specified threshold voltage to flow. This limits the voltage that develops across the coils when the current is cut off by just enough so that the magnetometer can shift from polarizing to listening mode quickly but without causing damage.
To pick up a strong signal, the listening circuit must also be tuned to resonate at the expected frequency of proton precession, which will depend on Earth’s magnetic field at your location. You can work out approximately what that is using an online geomagnetic-field calculator. You’ll get the field strength, and then you’ll multiply that by the gyromagnetic ratio of protons (42.577 MHz per tesla). For me, that worked out to about 2 kilohertz. Estimating the inductance of the coils from their diameter and number of turns, I then selected a capacitor of suitable value in parallel with the coils to make a tank circuit that resonates at that frequency.
You could tune your tank circuit using a frequency generator and oscilloscope. Or, as Mumm suggests, attach a small speaker to the output of the circuit. Then bring the speaker near the pickup coils. This will create magnetic feedback and the circuit will oscillate on it’s own—loudly! You merely need to measure the frequency of this tone, and then adjust the tank capacitor to bring this self-oscillation to the frequency you want to tune to.
My initial attempt to listen to protons met with mixed success: Sometimes I heard tones, sometimes not. What helped to get this gizmo working consistently was realizing that proton magnetometers don’t tolerate large gradients in the magnetic field. So don’t try this indoors or anywhere near iron-containing objects: water pipes, cars, or even the ground. A wide-open space outside is best, with the coils raised off the ground. The second thing that helped was to apply more oomph in polarization mode. While a 12-volt battery works okay, 36 V does much better.
After figuring these things out, I can now hear protons easily. These tones are clearly the sounds of protons, because they go away if I drain the water in the bottles. And, using free audio-analyzer software called Spectrum Lab, I confirmed that the frequency of these tones matches the magnetic field at my location to about 1 percent. 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.
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