Listen to Protons With a Homemade Magnetometer

When you have an MRI, the machine uses a phenomenon called nuclear magnetic resonance (NMR). Certain types of atomic nuclei, notably those of the hydrogen atoms of a water molecule, cand be made to oscillate in a magnetic field, and these oscillations can be detected with spools of thread. MRI scanners use intense magnetic fields that create resonances at tens to hundreds of megahertz. However, another NMR-based instrument involves oscillations at much lower frequencies: a proton precession magnetometeroften used to measure the Earth’s magnetic field.

Proton precession magnetometers have been around for decades and were once often used in archeology and mineral exploration. High-end models can cost thousands of dollars. Then, in 2022, a German engineer named Alexander Mumm designed a very simple circuit for a simplified circuit. I recently built his circuit and can attest to this with less than half a kilo of 22 gauge magnetic 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’s seasoning mix, it is possible to measure the Earth’s magnetic field with high precision.

The frequency of the signal emitted by the protons precessing in the Earth’s magnetic field is in the audio range, therefore 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 quick control of the coils. The amplifier circuit 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, i.e. protons. Like other subatomic particles, protons possess a quantum property called spin, similar to classical angular momentum. In a magnetic field, protons wobble like tops, their axes of rotation tracing a cone, a phenomenon called precession. A proton precession magnetometer oscillates many protons in a synchronized manner, then measures the frequency of their oscillations, which is proportional to the strength of the ambient magnetic field.

The low strength of the Earth’s magnetic field (at least compared to that of an MRI machine) means that protons flickering under its influence do so at audio frequencies. Move in enough unison and the spinning protons will induce a voltage in a nearby pickup coil. Amplify that and put it through headphones, and you get an audio tone. So with the right circuit you can literally hear protons.

The first step is to make the pickup reels, which is where the Morton seasoning mix bottles come in. Why Morton Seasoning Blend? Two reasons. Firstly, this size of bottle will allow you to wind around 500 turns of wire around each with around 450 grams of 22 gauge wire. Secondly, the bottle has small shoulders molded into each end, giving it excellent spool shapes.

Why two bottles and two coils? This is to eliminate electromagnetic noise, mainly from power lines, which is invariably picked up by the coils. When two counterwound coils are wired in series, this external noise tends to cancel out. However, the signals from the precessing protons in the two coils will reinforce each other.

Do not try this indoors or near objects containing iron.

A proton magnetometer has three modes. The first is to send direct current through the coils. The second mode disconnects the current source and allows the magnetic field it had created to collapse. The third is the listen mode, which connects the coils to a sensitive audio amplifier. By filling each bottle with distilled water and sending a direct current (a few amps) through these coils, you align the spins of many protons in the water. Then, after putting your circuit into listening mode, you use the coils to detect the synchronous oscillations of the flickering protons.

Mumm’s circuit switches from one mode to another in the simplest way possible: using a three-position switch. One position activates DC bias mode. The next position allows the magnetic field created during polarization to collapse, and the third position is for listening.

Avoiding damaging sparks

The second mode may 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 when the magnetic fields around the coils collapse.

The proton precession magnetometer is primarily just a multi-stage analog amplifier.James Provost

To avoid this, Mumm’s circuit uses a MOSFET, wired to operate as a high-power Zener diode, used in many power regulation circuits to allow only current to flow above a specified threshold voltage. This limits the voltage that develops across the coils when the current is turned off just enough so that the magnetometer can switch from bias mode to listen 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 the Earth’s magnetic field at your location. You can determine approximately what this is using an online geomagnetic field calculator. You will get the field strength and then multiply it by the gyromagnetic ratio of the protons (42.577 MHz per tesla). For me, that was about 2 kilohertz. By estimating the inductance of the coils from their diameter and their number of turns, I then selected a capacitor of appropriate value in parallel with the coils to create a tank circuit which resonates at this frequency.

You can adjust your tank circuit using a frequency generator and an oscilloscope. Or, as Mumm suggests, attach a small speaker at the circuit output. SO move the speaker closer to the pickup coils. This will create a magnetic feedback and the circuit will oscillate on its own—strong ! You just need to measure frequency of this tone, then adjust the tank capacitor to bring this self-osblinking at the frequency you want tune in.

My first attempt at listening to protons met with mixed success: sometimes I heard tones, sometimes not. What helped make this gadget work consistently was realizing that proton magnetometers do not tolerate large gradients in the magnetic field. So don’t try this inside or near objects containing iron: water pipes, cars, or even the ground. It is best to have a large outdoor space, with the coils raised off the ground. The second thing that helped was applying more punch in bias mode. If a 12 volt battery works well, a 36 volt battery does much better.

After understanding these things, I can now hear protons easily. These sounds are clearly proton sounds, because they disappear if I empty the water from the bottles. And, using free audio analysis software called Spectrum Lab, I confirmed that the frequency of these tones corresponds to about 1% of the magnetic field of my location. Although not a practical field instrument, any proton precession magnetometer for less than US$100 is nothing to sneeze at.