¿How to tell if a cylindrical magnet is diamterically magnetized?
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They'll all diametrically magnetized, because there are no monopole magnets.
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I thought you meant the other meaning for "diametrically".
You can use another magnet to test it, or if you don't have another magnet, then use the Earth's magnetic field by hanging the magnet from a string with the axes either horizontal or vertical to see which orientation acts like a compass.
That's great. For smaller magnets, another thing you can do is to attach a needle on it so that the magnetic field extends outward which allows it to exert more torque.
Of course if you have two magnets, you can just attach them to each other to see how the fields are oriented, but using the Earth's magnetic field allows you to determine polarity.
@Pat Well, at first I rejected your idea because the magnets are so tiny (5 mm and 8 mm diameter) to hang them with a wire or string. I was thinking about hanging them with a hair or something when I remembered an old trick to check if a dice was loaded.
So I got a tray with some water and made a raft with EVA foam for the magnets. I put the supposedly diametrically magnetized one and yesss! it rotated and ended up oriented in the same direction (I had to make a little mark to the magnet to check this)
This magnet was so tiny (5 mm dia.) that a foam raft of 12x12 mm aprox was enough and the response fast.
For the 8 mm magnet that raft was not enough so I got a plastic bottle cap and put foam in the bottom just to make it as flat as possible. it looked that te 5 mm diameter msgnet hadn't power enough to move it fast.
In this case I could turn the cap but the self orientation didn't occur.
I noticed that it is better to use a tray rather than a glass or cup because at that scale it looks like the water surface is not flat enough so the rafts tend to migrate to the edges. In fact, you have to be very careful while trying to rotate the magnet to check them.
No video of the event 🤣🤣 but I got a couple of pictures at least...