While you get an MRI scan, the machine exploits a phenomenon referred to as nuclear magnetic resonance (NMR). Sure sorts of atomic nuclei—together with these of the hydrogen atoms in a water molecule—can be made to oscillate in a magnetic discipline, and these oscillations may be detected with coils of wire. MRI scanners make use of intense magnetic fields that create resonances at tens to hundreds of megahertz. Nonetheless, one other NMR-based instrument entails a lot lower-frequency oscillations: a proton-precession magnetometer, typically used to measure Earth’s magnetic discipline.
Proton-precession magnetometers have been round for many years and had been as soon as typically utilized in archaeology and mineral exploration. Excessive-end fashions can price hundreds of {dollars}. Then, in 2022 a German engineer named Alexander Mumm devised a very simple circuit for a stripped-down one. I just lately constructed his circuit and may attest that with lower than half a kilogram of 22-gauge magnet wire; two frequent integrated circuits; a metal-oxide-semiconductor field-effect transistor, or MOSFET; a handful of discrete elements; and two empty 113-gram bottles of Morton seasoning blend, it’s attainable to measure Earth’s magnetic discipline very precisely.
The frequency of the sign emitted by protons precessing in Earth’s magnetic discipline lies within the audio vary, so with a pair of headphones and two amplifier built-in circuits [middle right], you’ll be able to detect a sign from water in seasoning bottles wrapped in coils [bottom left and right]. A MOSFET [middle left] permits for fast management of the coils. The amplification circuitry is powered by a 9-volt battery, whereas a 36-volt battery costs the coils.James Provost
Like an MRI scanner, a proton-precession magnetometer measures the oscillations of hydrogen nuclei—that’s, protons. Like different subatomic particles, protons possess a quantum property referred to as spin, akin to classical angular momentum. In a magnetic discipline, protons wobble like spinning tops, with their spin axes tracing out a cone—a phenomenon referred to as precession. A proton-precession magnetometer will get many protons to wobble in sync after which measures the frequency of their wobbles, which is proportional to the depth of the ambient magnetic discipline.
The weak power of Earth’s magnetic field (no less than in comparison with that of an MRI machine) signifies that protons wobbling beneath its affect accomplish that at audio frequencies. Get sufficient transferring in unison and the spinning protons will induce a voltage in a close-by pickup coil. Amplify that and go it by way of some earphones, and also you get an audio tone. So with an acceptable circuit, you’ll be able to, actually, hear protons.
Step one is to make the pickup coils, which is the place the bottles of Morton seasoning mix are available. Why Morton seasoning mix? Two causes. First, this measurement bottle will can help you wrap about 500 turns of wire round each with about 450 grams of 22-gauge wire. Second, the bottle has little shoulders molded at every finish, making for wonderful coil kinds.
Why two bottles and two coils? That’s to quash electromagnetic noise—principally coming from energy traces—that invariably will get picked up by the coils. When two counterwound coils are wired in collection, such exterior noise tends to cancel out. Alerts from precessing protons within the two coils, although, will reinforce each other.
Don’t do this indoors or wherever close to iron-containing objects.
A proton magnetometer has three modes. The primary is for sending DC present by way of the coils. The second mode disconnects the present supply and permits the magnetic discipline it had created to break down. The third is listening mode, which connects the coils to a delicate audio amplifier. By filling every bottle with distilled water and sending a DC present (a number of amperes) by way of these coils, you line up the spins of many protons within the water. Then, after placing your circuit into listening mode, you employ the coils to sense the synchronous oscillations of the wobbling protons.
Mumm’s circuit shifts from one mode to a different within the easiest way attainable: utilizing a three-position swap. One place permits the DC-polarization mode. The subsequent permits the magnetic discipline constructed up throughout polarization to break down, and the third place is for listening.
Avoiding Damaging Sparks
The second mode might sound straightforward to attain—simply disconnect the coils, proper? However should you do this, the same principle that makes spark plugs spark will put a dangerous high voltage throughout the swap contacts because the magnetic fields across the coils collapse.
The proton-precession magnetometer is primarily only a multistage analog amplifier.James Provost
To keep away from that, Mumm’s circuit employs a MOSFET, wired to work like a high-power Zener diode, utilized in many power-regulation circuits to permit solely present above a specified threshold voltage to circulation. This limits the voltage that develops throughout the coils when the present is lower off by simply sufficient in order that the magnetometer can shift from polarizing to listening mode rapidly however with out inflicting injury.
To select up a robust sign, the listening circuit should even be tuned to resonate on the anticipated frequency of proton precession, which is able to depend upon Earth’s magnetic discipline at your location. You’ll be able to work out roughly what that’s utilizing a web based geomagnetic-field calculator. You’ll get the sector power, and then you definately’ll multiply that by the gyromagnetic ratio of protons (42.577 MHz per tesla). For me, that labored out to about 2 kilohertz. Estimating the inductance of the coils from their diameter and variety of turns, I then chosen a capacitor of appropriate worth in parallel with the coils to make a tank circuit that resonates at that frequency.
You would tune your tank circuit utilizing a frequency generator and oscilloscope. Or, as Mumm suggests, connect a small speaker to the output of the circuit. Then convey the speaker close to the pickup coils. It will create magnetic suggestions and the circuit will oscillate on it’s personal—loudly! You merely must measure the frequency of this tone, after which alter the tank capacitor to convey this self-oscillation to the frequency you wish to tune to.
My preliminary try and take heed to protons met with combined success: Typically I heard tones, typically not. What helped to get this gizmo working constantly was realizing that proton magnetometers don’t tolerate giant gradients within the magnetic discipline. So don’t do this indoors or wherever close to iron-containing objects: water pipes, vehicles, and even the bottom. A large-open area exterior is finest, with the coils raised off the bottom. The second factor that helped was to use extra oomph in polarization mode. Whereas a 12-volt battery works okay, 36 V does a lot better.
After figuring these items out, I can now hear protons simply. These tones are clearly the sounds of protons, as a result of they go away if I drain the water within the bottles. And, utilizing free audio-analyzer software program referred to as Spectrum Lab, I confirmed that the frequency of those tones matches the magnetic discipline at my location to about 1 %. Whereas it’s not a sensible discipline instrument, a proton-precession magnetometer of any type for lower than US $100 is nothing to sneer at.
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