A man is shown standing in a wooded area, in front of a stone wall, facing toward the camera. To the left of him, on a rock, are a selection of compasses. Further to the left, another scene is shown, of two compasses. One has a brass-colored metal ring around it, and a timer above it reads 00:04:19. A timer above the other reads 01:47:02.

A New Kind Of Inductively-damped Compass

January 11, 2026 by Aaron Beckendorf 39 Comments

At some point during our primary school careers, most of us probably constructed a simple compass, often by floating a magnetized needle on a cork in a cup of water. The water in such a configuration not only lets the needle spin without friction, but also dampens out (so to speak) the needle’s tendency to swing back and forth across the north-south line. Liquid-filled compasses use the same principle, but even well-made compasses can develop bubbles when exposed to temperature or pressure variations. Rather than accept this unsightly state of affairs, [The Map Reading Company] designed a new kind of liquid-free, inductively-damped compass.

It’s hard to design a compass that settles quickly, even if it uses a strong magnet, because the Earth’s own magnetic field is just so weak, and the stronger the internal magnet is, the more likely it is to be thrown off by nearby magnetic objects. As a result, they tend to swing, overshoot, and oscillate around their final orientation for some time. Most compasses use liquid to damp this, but a few, mostly military compasses, use a conductive baseplate instead: as the magnet moves, it induces eddy currents in the baseplate, which create a weak magnetic field opposing its motion, slowing the magnet down. Inductively-damped compasses don’t get bubbles, but they don’t let you see a map through the baseplate. [The Map Reading Company] dealt with this by making the baseplate transparent and surrounding the compass needle with a ring of high-conductivity copper alloy. This gave him a clear baseplate compass for easy map reading which would never develop bubbles. It’s a simple hack, and should be easy to replicate, but it still seems to be a new design. In fact, [The Map Reading Company] is releasing most of the design to the public domain. Anyone can build this design.

If this prompts your interest in compasses, check out the Earth inductor compass. We’ve also seen a visualization of the eddy currents that damp these oscillations, and even seen them used to drive a bike.

Thanks to [Mel] for the tip!

Levitating Magnet In A Spherical Copper Cage

October 13, 2024 by Danie Conradie 21 Comments

Lenz’s Law is one of those physics tricks that look like magic if you don’t understand what’s happening. [Seth Robinson] was inspired by the way eddy currents cause a cylindrical neodymium magnet to levitate inside a rotating copper tube, so he cast a spherical copper cage to levitate a magnetic sphere.

Metal casting is an art form that might seem simple at first, but is very easy to screw up. Fortunately [Seth] has significant experience in the field, especially lost-PLA metal casting. While the act of casting is quick, the vast majority of the work is in the preparation process. Video after the break.

[Seth] started by designing and 3D printing a truncated icosahedron (basically a low-poly sphere) in two interlocking halves and adding large sprues to each halve. Over a week, the PLA forms were repeatedly coated in layers of ceramic slurry and silica sand, creating a thick shell around them. The ceramic forms were then heated to melt and pour out the PLA and fired at 870°C/1600°F to achieve full hardness.

With the molds prepared, the molten copper is poured into them and allowed to cool. To avoid damaging the soft copper parts when breaking away the mold, [Seth] uses a sandblaster to cut it away sections. The quality of the cast parts is so good that 3D-printed layer lines are visible in the copper, but hours of cleanup and polishing are still required to turn them into shiny parts. Even without the physics trick, it’s a work of art. A 3d printed plug with a brass shaft was added on each side, allowing the assembly to spin on a 3D-printed stand.

[Seth] placed a 2″ N52 neodymium spherical magnet inside, and when spun at the right speed, the magnet levitated without touching the sides. Unfortunately, this effect doesn’t come across super clearly on video, but we have no doubt it would make for a fascinating display piece and conversation starter.

Using and abusing eddy currents makes for some very interesting projects, including hoverboards and magnetic torque transfer on a bicycle.

Continue reading “Levitating Magnet In A Spherical Copper Cage” →

Tech In Plain Sight: Speedometers

August 22, 2024 by Al Williams 29 Comments

In a modern car, your speedometer might look analog, but it is almost certainly digital and driven by the computer that has to monitor all sorts of things anyway. But how did they work before your car was a rolling computer complex? The electronic speedometer has been around for well over a century and, when you think about it, qualifies as a technlogical marvel.

If you already know how they work, this isn’t a fair question. But if you don’t, think about this. Your dashboard has a cable running into it. The inner part of the cable spins at some rate, which is related to either the car’s transmission or a wheel sensor. How do you make a needle deflect based on the speed?

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A Bicycle Powered By A Different Kind Of Eddy

May 16, 2023 by Ryan Flowers 31 Comments

When you think of a bicycle and an Eddy, you’d be forgiven for thinking first of Eddy Merckx, one of the most successful competitive cyclists to ever live. But this bicycle, modified by [Tom Stanton] as shown in the video below the break, has been modified by ditching its direct drive gearing in favor of using the friction-like eddy currents between magnets and copper to transfer power to the wheel.

Before even beginning to construct a mechanism for powering the bicycle, [Tom] had to figure out the basics: what kind of materials could be used for a metal disk? The answer, after much testing, turned out to be copper. What kind of magnets work best, and in what formation? Expensive high grade, aligned North to South pole for added eddy-dragging goodness. Would the mechanism work with any efficiency?

The end result is interesting to watch, and it’s not exactly as you’d have expected. Yes, eddy currents drive the copper hub, but at a 100 RPM difference. Where does all of that energy go? Hint: not to the wheel, and certainly not into propelling the bicycle. All in all it’s a fantastic experiment with unpredictable results.

If bicycle based bumbling about bakes your biscuits, you might appreciate this tennis-ball-enhanced ride too.

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Progressive Or Thrash? How Metal Detectors Discriminate

November 26, 2019 by Kristina Panos 37 Comments

Metal detecting is a fun pastime, even when all you can find is a little bit of peace and a whole lot of pop tabs. [Huygens Optics] has a VLF-based metal detector that offers much more feedback than just a beep or no beep. This thing is fancy enough to discriminate between types of metal and report back a numerical ID value from a corresponding range of conductivity.

Most pop tabs rated an ID of 76 or 77, so [Huygens Optics] started ignoring these until the day he found a platinum wedding band without looking at the ID readout. Turns out, the ring registered in the throwaway range. Now thoroughly intrigued by the detector’s ID system, [Huygens Optics] set up a test rig with an oscilloscope to see for himself how the thing was telling different metals apart. His valuable and sweeping video walk-through is hiding after the break.

A Very Low-Frequency (VLF) detector uses two coils, one to emit and one to receive. They are overlapped just enough so that the reception coil can’t see the emission coil’s magnetic field. This frees up the reception coil’s magnetic field to be interrupted only by third-party metal, i.e. hidden treasures in the ground.

Once [Huygens Optics] determined which coil was which, he started passing metal objects near the reception coil to see what happened on the ‘scope. Depending on the material type and the size and shape of the object, the waveform it produced showed a shift in phase from the emission coil’s waveform. This is pretty much directly translated to the ID readout — the higher the phase shift value, the higher the ID value.

We’ve picked up DIY metal detectors of all sizes over the years, but this one is the ATtiny-ist.

Continue reading “Progressive Or Thrash? How Metal Detectors Discriminate” →

Visualizing Eddy Currents

February 19, 2019 by Al Williams 18 Comments

If [Electroboom] gives up making videos and decides to become a lounge lizard in the Poconos, we hope he adopts the stage name Eddy Currents. However, he is talking about eddy currents in his recent video post that you can see below.

We know he doesn’t really think he can get the magnet to slow down with one sheet of aluminum foil and that he stages at least most of his little electric accidents, but we still enjoy watching it. Meanwhile, he also has a good explanation of why a copper pipe will slow down a magnet and how eddy current affects transformer efficiency.

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Can You 3D-Print A Stator For A Brushless DC Motor?

December 18, 2018 by Dan Maloney 33 Comments

Betteridge’s Law holds that any headline that ends in a question mark can be answered with a “No.” We’re not sure that [Mr. Betteridge] was exactly correct, though, since 3D-printed stators can work successfully for BLDC motors, for certain values of success.

It’s not that [GreatScott!] isn’t aware that 3D-printed motors are a thing; after all, the video below mentions the giant Halbach array motor we featured some time ago. But part of advancing the state of the art is to replicate someone else’s results, so that’s essentially what [Scott!] attempted to do here. It also builds on his recent experiments with rewinding commercial BLDCs to turn them into generators. His first step is to recreate the stator of his motor as a printable part. It’s easy enough to recreate the stator’s shape, and even to print it using Proto-pasta iron-infused PLA filament. But that doesn’t come close to replicating the magnetic properties of a proper stator laminated from stamped iron pieces. Motors using the printed stators worked, but they were very low torque, refusing to turn with even minimal loading. There were thermal issues, too, which might have been mitigated by a fan.

So not a stunning success, but still an interesting experiment. And seeing the layers in the printed stators gives us an idea: perhaps a dual-extruder printer could alternate between plain PLA and the magnetic stuff, in an attempt to replicate the laminations of a standard stator. This might help limit eddy currents and manage heating a bit better. Continue reading “Can You 3D-Print A Stator For A Brushless DC Motor?” →