Whenever you get an MRI scan, the machine exploits a phenomenon known 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 subject, and these oscillations might 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, usually used to measure Earth’s magnetic subject.
Proton-precession magnetometers have been round for many years and have been as soon as usually 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 lately constructed his circuit and might 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 subject very precisely.
The frequency of the sign emitted by protons precessing in Earth’s magnetic subject lies within the audio vary, so with a pair of headphones and two amplifier built-in circuits [middle right], you may detect a sign from water in seasoning bottles wrapped in coils [bottom left and right]. A MOSFET [middle left] permits for speedy management of the coils. The amplification circuitry is powered by a 9-volt battery, whereas a 36-volt battery fees 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 known as spin, akin to classical angular momentum. In a magnetic subject, protons wobble like spinning tops, with their spin axes tracing out a cone—a phenomenon known 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 subject.
The weak power of Earth’s magnetic field (at the very least in comparison with that of an MRI machine) signifies that protons wobbling below 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 move it by way of some earphones, and also you get an audio tone. So with an appropriate circuit, you may, 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 mean you can wrap about 500 turns of wire round every one with about 450 grams of 22-gauge wire. Second, the bottle has little shoulders molded at every finish, making for glorious coil kinds.
Why two bottles and two coils? That’s to quash electromagnetic noise—principally coming from energy strains—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 that 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 subject 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 utilize the coils to sense the synchronous oscillations of the wobbling protons.
Mumm’s circuit shifts from one mode to a different within the easiest method attainable: utilizing a three-position swap. One place allows the DC-polarization mode. The subsequent permits the magnetic subject constructed up throughout polarization to break down, and the third place is for listening.
Avoiding Damaging Sparks
The second mode might sound simple to realize—simply disconnect the coils, proper? However if you happen to 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 movement. 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 shortly however with out inflicting harm.
To choose up a robust sign, the listening circuit should even be tuned to resonate on the anticipated frequency of proton precession, which is able to rely upon Earth’s magnetic subject at your location. You possibly can work out roughly what that’s utilizing a web based geomagnetic-field calculator. You’ll get the sphere power, and you then’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 might 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 carry the speaker close to the pickup coils. This can create magnetic suggestions and the circuit will oscillate on it’s personal—loudly! You merely must measure the frequency of this tone, after which modify the tank capacitor to carry this self-oscillation to the frequency you need to tune to.
My preliminary try and take heed to protons met with blended success: Generally I heard tones, typically not. What helped to get this gizmo working persistently was realizing that proton magnetometers don’t tolerate giant gradients within the magnetic subject. So don’t do that indoors or wherever close to iron-containing objects: water pipes, automobiles, and even the bottom. A large-open area exterior is greatest, 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 significantly better.
After figuring this stuff 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 known as Spectrum Lab, I confirmed that the frequency of those tones matches the magnetic subject at my location to about 1 p.c. Whereas it’s not a sensible subject instrument, a proton-precession magnetometer of any form for lower than US $100 is nothing to sneer at.
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