You are taken to a dark room and given two thin long rods - 30 cm long and 1 cm thick. One of them is a permanent magnet and the other is pure iron which is not magnetized.

Both of them are insulated with glass. They weigh the same. The shapes are same too. How can you tell which one of these rods is the magnet? Remember, you can't see anything, and you cannot use anything other than these two rods in your investigation. You are not allowed to break them or remove the insulation either.



  • $\begingroup$ Why do I get the feeling that the hint is a riddle of its own? $\endgroup$ Nov 10 '14 at 21:46
  • $\begingroup$ The hint gives it ALL away [sarcasm] the hint is a puzzle on it's own. $\endgroup$
    – warspyking
    Nov 10 '14 at 21:47
  • $\begingroup$ @Evision I posted my comment and right after yours appeared! We both had the same comment XD $\endgroup$
    – warspyking
    Nov 10 '14 at 21:48
  • $\begingroup$ Excellent! Smithers, release the hounds. $\endgroup$ Nov 10 '14 at 21:48
  • 1
    $\begingroup$ The hint is encoded in rot13. $\endgroup$
    – NobodyNada
    Nov 11 '14 at 0:58

For the two rods, place the end against the middle of the other and compare the magnetic attraction. The magnetic rod will be the one with its end against the other's middle with the highest pull.

  • $\begingroup$ It would be nice if you would wrap that answer inside a spoiler tag. $\endgroup$ Nov 10 '14 at 21:55
  • 3
    $\begingroup$ That would work well if the poles of the magnet are aligned with the long axis of the rod. I believe, however, that it would be possible to have the rod magnetized with the poles running perpendicular to its axis (so the magnetic lines of force would be disks whose plane was likewise perpendicular to the axis). Such a magnet would likely not be anywhere near as strong as one with poles on the long axis, but I believe it could nonetheless be constructed. $\endgroup$
    – supercat
    Nov 10 '14 at 23:11

Balance each rod in turn at its midpoint on your fingertip. Try this several times for each rod in different orientations. The magnet is the one which will tend to rotate towards the same point (north). You can determine this by feel.




Magnets are more attractive near the poles than in the middle


Balance a pole from the middle on the edge of the other (which is up vertically) and see if it moves, turn the pole 90° clockwise and try again. If it moves it's magnetic, otherwise either try to the other, or automatically assume it.


Scrape the tip of one rod against the edge of the other, repeatedly and in the same direction. If there is now repulsion between the two rods for some orientation of ends, then the rod you scraped with is the magnet.

  • $\begingroup$ I'm not sure this works, because they're covered by an unbreakable insulation. $\endgroup$ Nov 11 '14 at 14:11
  • $\begingroup$ @EnvisionAndDevelop Magnetic fields permeate glass. $\endgroup$ Nov 11 '14 at 16:20
  • $\begingroup$ "Scrape the tip of one rod": this can't be done if there is glass in the way. $\endgroup$ Nov 11 '14 at 16:35
  • $\begingroup$ @EnvisionAndDevelop you don't need contact between the metals for this. Scraping the glass tip against the glass sides will work too. $\endgroup$ Nov 11 '14 at 16:36
  • $\begingroup$ this is the one i would have tried, although im not sure some magnetization would not occur rubbing the un-magnetized rod against a magnet $\endgroup$
    – Ewan
    Apr 29 '15 at 9:10

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