Take heed to Protons With a Selfmade Magnetometer

Once 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 a whole lot of megahertz. Nevertheless, one other NMR-based instrument includes a lot lower-frequency oscillations: a proton-precession magnetometer, typically used to measure Earth’s magnetic subject.

Proton-precession magnetometers have been round for many years and have been as soon as typically utilized in archaeology and mineral exploration. Excessive-end fashions can value hundreds of {dollars}. Then, in 2022 a German engineer named Alexander Mumm devised a quite 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 built-in circuits; a metal-oxide-semiconductor field-effect transistor, or MOSFET; a handful of discrete parts; and two empty 113-gram bottles of Morton seasoning mix, it’s doable 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 possibly can 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 expenses 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 energy of Earth’s magnetic subject (at the very least in comparison with that of an MRI machine) signifies that protons wobbling below its affect achieve this 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 means of some earphones, and also you get an audio tone. So with an appropriate circuit, you possibly can, 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 let 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 varieties.

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. Indicators 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 means 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 (just a few amperes) by means 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 doable: utilizing a three-position change. One place allows the DC-polarization mode. The following permits the magnetic subject constructed up throughout polarization to break down, and the third place is for listening.

Avoiding Damaging Sparks

The second mode may appear straightforward to realize—simply disconnect the coils, proper? However for those who do this, the identical precept that makes spark plugs spark will put a harmful excessive voltage throughout the change 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 stream. This limits the voltage that develops throughout the coils when the present is reduce off by simply sufficient in order that the magnetometer can shift from polarizing to listening mode shortly however with out inflicting injury.

To select up a powerful sign, the listening circuit should even be tuned to resonate on the anticipated frequency of proton precession, which can rely upon Earth’s magnetic subject at your location. You possibly can work out roughly what that’s utilizing an internet geomagnetic-field calculator. You’ll get the sector energy, 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 can 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 deliver the speaker close to the pickup coils. This may 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 deliver this self-oscillation to the frequency you wish to tune to.

My preliminary try and hearken to protons met with combined success: Generally I heard tones, typically not. What helped to get this gizmo working persistently was realizing that proton magnetometers don’t tolerate massive gradients within the magnetic subject. So don’t do this indoors or wherever close to iron-containing objects: water pipes, automobiles, and even the bottom. A large-open house 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 known as Spectrum Lab, I confirmed that the frequency of those tones matches the magnetic subject at my location to about 1 %. Whereas it’s not a sensible subject instrument, a proton-precession magnetometer of any sort for lower than US $100 is nothing to sneer at.