103 replies to this topic

[rookie1ja]

The Magnet - Back to the Logic Puzzles
This logic puzzle was published in Martin Gardner's column in the Scientific American.
You are in a room with no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify which one is a magnet?

This old topic is locked since it was answered many times. You can check solution in the Spoiler below.
Pls visit New Puzzles section to see always fresh brain teasers.

Spoiler for Solution

Magnet - solution
You can hang the iron rods on a string and watch which one turns to the north (or hang just one rod).
Gardner gives one more solution: take one rod and touch with its end the middle of the second rod. If they get closer, then you have a magnet in your hand.
The real magnet will have a magnetic field at its poles, but not at its center. So as previously mentioned, if you take the iron bar and touch its tip to the magnet's center, the iron bar will not be attracted. This is assuming that the magnet's poles are at its ends. If the poles run through the length of the magnet, then it would be much harder to use this method.
In that case, rotate one rod around its axis while holding an end of the other to its middle. If the rotating rod is the magnet, the force will fluctuate as the rod rotates. If the rotating rod is not magnetic, the force is constant (provided you can keep their positions steady).

Spoiler for old wording

This is a logic puzzle published in Martin Gardner's column in the Scientific American.
You are in a room where there are no metal objects except for two iron rods. Only one of them is a magnet.
How can you identify this magnet?

[Jay]

I thought about this all day, and after coming up with a solution I was surprised to come home and read that the provided solution actually seemed much more complicated than the one I came up with, it's usually the other way around.

Alternate solution: Break one or both rods and the one who's pieces attract or repel eachother is your magnet.

[sk8erfou]

accually hanging the rods ona string would not help because they will both point in a directin how are you sopposed to know which direction is north? so the obvious answer would be to take both poles... and with the pole in the right hand touch the top of the pole in the left hand with the top of the pole in the right hand... it will either stick or repel... not with the top of the pole in the right hand touch the botton of the pole in the left hand... if the magnet is in the right hand it will do the same as b4 either stick or repel... but if it in your left hand it will do the oposite

[rojinr]

hay... u can hang both the rods.. and both will orient to some direction.. as u said, we dont know which is north.. but we could do one thing.. we can disturb both the hanging rods.. the one whic returns to a perticular orientation/direction is the magnet.. and the other one is the iron rod.. how does that go????

[Garrek99]

There could be one other way.

Metal rods that are magnetized are vulnerable to impacts because the aligned electrons can be misaligned with a good bang. Of course this method has a 50% chance of messing up the magnet but it should yield a correct answer.
So, if you take a rod and bang it a couple of times and see no change in the intensity of the pull then you are most likely holding an ordinary iron rod.

[rockypg]

It IS possible to find the magnetic rod by suspending them on strings::

Suspend both rods, and like most of you pointed out, both will come to rest at some arbitrary positions. Now disturb both slightly, the magnetic rod will always return to the same orientation.

R

[Incognitum]

Use a diamond file to make some shavings off of one rod. Sprinkle these over each rod, if they stick to only one rod you have shaved the non-magnetic rod, but if they stick to both, then you've shaved the magnetic one.

Also, aren't certain stones affected my magnetism? Bring one of those along for a quick test.

Leave the two rods touching for long enough for the non-magnetized one to become magnetized, then they both are.

Brink a floppy disk and rub one rod over it vigorously. Later at home, of the data has been erased, you know you were using the magnetic rod.

[Garrek99]

You can't use files and stuff because the puzzle says ithat there are no metal objects in the room. (No floppy discs either, probably)

[bigbrainbrad]

thus again a prime example of the theory of magnetivity: all that can be attracted will, those which can not..... haven't met me yet seriously joking! oxymorons are cool.

[colorclown26]

Braces can sure help in this puzzle!!!!!!!!!!