Listen to the Earth: DIY Proton Magnetometer Brings Subatomic Sounds to Life
For less than $100, a resourceful engineer has demonstrated that it’s possible to detect the faint “hum” of protons aligning with Earth’s magnetic field, opening up a new realm of accessible scientific exploration.
The technology behind Magnetic Resonance Imaging (MRI) relies on the principle of nuclear magnetic resonance (NMR), where atomic nuclei – particularly those of hydrogen atoms in water – oscillate within a magnetic field. These oscillations are detectable with specialized equipment. While MRI scanners utilize powerful magnetic fields generating resonances in the megahertz range, a far simpler application of NMR allows us to eavesdrop on the subtle magnetic signals of our planet. This is achieved through the proton-precession magnetometer, traditionally used in fields like archaeology and mineral exploration.
For decades, these instruments have been costly, often running into the thousands of dollars. However, in 2022, German engineer Alexander Mumm unveiled a remarkably streamlined and affordable design. One builder recently replicated Mumm’s circuit and confirmed its effectiveness, requiring less than half a kilogram of 22-gauge magnet wire, two integrated circuits, a metal-oxide-semiconductor field-effect transistor (MOSFET), a handful of discrete components, and, surprisingly, two empty 113-gram bottles of Morton seasoning blend.
The key to this low-cost marvel lies in its ability to detect the audio-frequency signals emitted by protons precessing – wobbling – in Earth’s magnetic field. By wrapping coils of wire around the seasoning bottles filled with water, and amplifying the resulting signal, one can literally hear the protons. A MOSFET controls the coils, while a 9-volt battery powers the amplification circuitry and a 36-volt battery charges the coils.
Like an MRI scanner, a proton-precession magnetometer measures the oscillations of hydrogen nuclei, or protons. These protons possess a quantum property called spin, which can be visualized as angular momentum. When placed in a magnetic field, protons don’t simply align; they wobble like spinning tops, tracing a cone – a phenomenon known as precession. The magnetometer synchronizes the wobble of numerous protons and measures the frequency, which is directly proportional to the strength of the surrounding magnetic field.
Because Earth’s magnetic field is relatively weak compared to that of an MRI machine, the proton wobble occurs at audio frequencies. When enough protons move in unison, they induce a voltage in a nearby pickup coil. Amplifying this voltage and channeling it through headphones produces an audible tone. “With a suitable circuit, you can, literally, hear protons,” one builder explained.
The Morton seasoning bottles aren’t merely a quirky choice; they serve a crucial function. Their size allows for approximately 500 turns of 22-gauge wire to be wrapped around each bottle using about 450 grams of wire. Furthermore, the molded shoulders at each end of the bottle provide an ideal form for creating the coils.
The use of two counterwound coils wired in series is also deliberate. This configuration effectively cancels out external electromagnetic noise – primarily from power lines – while simultaneously reinforcing the signals from the precessing protons.
However, successful operation requires careful consideration of the surrounding environment. “Don’t try this indoors or anywhere near iron-containing objects,” cautions the builder, noting that water pipes, cars, and even the ground can introduce disruptive magnetic gradients. An open space, with the coils elevated, is optimal.
Mumm’s circuit operates in three distinct modes. The first applies a DC current to the coils, aligning the protons in the water. The second mode allows the magnetic field to collapse, and the third is “listening mode,” connecting the coils to a sensitive audio amplifier. A three-position switch seamlessly transitions between these modes.
A critical design element is the inclusion of a MOSFET, functioning as a high-power Zener diode, to prevent damaging voltage spikes during the field collapse. Without this component, disconnecting the coils would generate a spark, similar to a spark plug, potentially damaging the switch.
To maximize signal strength, the listening circuit must be tuned to resonate at the expected frequency of proton precession, which varies depending on location. This frequency can be calculated using an online geomagnetic field calculator and the gyromagnetic ratio of protons (42.577 MHz per tesla). The builder calculated a resonance frequency of approximately 2 kilohertz. A tank circuit, consisting of a capacitor and the coils, is then constructed to achieve this resonance.
Tuning the tank circuit can be accomplished using a frequency generator and oscilloscope. Alternatively, Mumm suggests a simpler method: attaching a small speaker to the circuit’s output and bringing it near the pickup coils. This creates magnetic feedback, causing the circuit to oscillate audibly. By adjusting the capacitor, the oscillation frequency can be precisely tuned.
Initial attempts weren’t always successful, but two key factors proved crucial. First, minimizing magnetic field gradients by conducting the experiment in an open space, away from metallic objects. Second, increasing the polarization current from 12 volts to 36 volts significantly improved signal strength.
“After figuring these things out, I can now hear protons easily,” the builder reported. The authenticity of the signal was confirmed by draining the water from the bottles, which eliminated the tone. Further validation came from using audio-analyzer software, Spectrum Lab, to verify that the frequency of the tone matched the expected magnetic field strength at the builder’s location to within 1 percent.
While not intended as a practical field instrument, a functional proton-precession magnetometer built for under $100 represents a significant achievement in accessible scientific experimentation.
