It was fun to visualize magnetic field with an array of retired Android phones. It was, admittedly, a gross underutilization of hardware power. It was more a technical exercise than anything else. There are much cheaper ways to visualize magnetic fields. I learned from iron filings in school way back when, but that got extremely messy. Ferrofluid in a vial of mineral oil is usually much neater, unless the vial leaks. I decided to try another option: magnetic field viewing films.

Generally speaking, such films are a very thin layer of oil sandwiched between top and bottom sheets, suspending magnetic particles within. The cheap ones look like they just use fine iron particles, and we see the field among slightly different shades of gray caused by different orientation of uniformly colored particles. I decided to pay a few extra bucks and go a little fancier. Films like the unit I bought (*) have much higher visual contrast.

As far as I can tell, the particles used in these films present different colors depending on orientation of magnetic field. When the field is perpendicular to the film, as when one of the magnet poles, the film shows green. When the field is parallel, we see yellow. Orientation between those two extremes show different colors within that spectrum. When there's no magnetic field nearby, we see a muddy yellow-green.

Playing with a Hall switch earlier, I established that this hard drive magnet has one pole on one half and another pole on the other half. Putting the same magnet under this viewing film confirms those findings, and it also confirms this film doesn't differentiate between north or south poles: they both show as green.

This was the simplest scenario: a small disc salvaged from an iPad cover shows a single pole on the flat face.

Similarly simple is the magnet I salvaged from a Microsoft Arc Touch Mouse.

This unexpected complex field was generated by a magnet I salvaged from a food thermometer. I doubt this pattern was intentional, as it does nothing to enhance its mission of keeping the food thermometer stuck to the side of my refrigerator. I assume this complex pattern was an accidental result of whatever-was-cheapest magnetization process.

The flat shape of this film was a hinderance when viewing the magnetic field of this evaporator fan rotor, getting in the way of the rotor shaft. The rotor is magnetized so each quarter is a magnetic pole. It's difficult to interpret from a static picture, but moving the rotor around under the film and seeing it move interactively makes the relationship more visible. It is also higher resolution and responds faster than an array of phones.

This disc was one of two that came from a 1998 Toyota Camry's stock tape deck head unit. I don't know exactly where it came from because I didn't perform this particular teardown.

https://twitter.com/Regorlas/status/1222730540014456832

We can see it laid out on the tabletop in this picture, bottom row center next to the white nylon gears.

And finally, the motor that set me on this quest for magnetic viewing film: the rotor from a broken microwave turntable motor. Actually looking at the plastic hub color, I think this is the broken rotor that got replaced by the teardown rotor sometime later.

And since I'm on the topic, I dug up its corresponding coil from that turntable motor teardown. Curious if I would see a magnetic field, I connected it to my benchtop DC power supply and slowly increased the voltage. I could only get up to 36V DC and didn't notice anything. This coil was designed for 120V AC, so I wasn't terribly surprised. I'll have to return to this experiment when I can safely apply higher voltage to this coil.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.