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New type of compass perhaps?

  1. Jun 3, 2012 #1
    Hello everyone.

    As a youngster I was asked to float a sewing needle on top of a full glass of water after I had magnetised it with a permanent magnet. The glass was so full the water was convex, slowly edging the point of the need toward the center of the glass. Noticing the capillary action on either side of the needle, I kwew I could float it. Pushing it ever so gently from the back end it set adrift.

    The idea was to have the needle act as it were a compass needle and it was sucessful in doing so. Just about an hour ago I setup this same procedure but with an 1/32" soft iron rod x 12" long and floated it on boat shaped styrofoam attached to each end ballancing the rod above the waters surface. As expected this also pointed North/South.

    If I now take a glass rod, which I don't have, and induce static charge in that rod, place it on a non-static floatation device as above, would it tend to point North/South as well?

    I've been playing with a rotating permanent magnet rotor, situated 1/2 way inside this coil. When this "compass needle", the rotor, reaches a certain rpm, it start to migrate from its starting point to approx. 90 degrees, at high rpm. When the rotor reaches that point it starts to slow down, reverse rotational direction and return to its starting point to repeat the process over again. Seeing that this was not what I was looking for in regards to fashioning a new type of compass (hobby). I played with diodes to clean up The DC signal which greatly increased rpm. To find that now the rotor rotates 360 degrees and then returns as before. This is much more toward the intended design.

    My Question is: Is the rotor behaving as the two examples above? Is it possible that the rotor is able to be influenced by Geomagnetic fields? I understand that Earths magnetic field is much too weak to be used as anything else other than a point of reference. The end result to my R+D would be to fashion a device that would be much more sensitive to Earths field.
  2. jcsd
  3. Jun 4, 2012 #2


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    Well, I think adding the rotor just confuses things. The problem is that now you also have angular momentum to contend with, instead of just the magnetic field. And spinning objects frequently behave in rather counterintuitive ways.

    Imagine, for example, that I have a spinning flywheel with a rod through it, and I let go of the rod on one side of the flywheel, only holding the other side enough that the flywheel doesn't drop from my hands entirely. What is the preferred motion?

    Many people seem to think that what the flywheel will do is spin in more or less the same direction, but, as it loses energy, will point more towards the ground. This is not what happens. Instead, the flywheel starts rotating around the single hand holding the one side of the flywheel I'm still holding. This motion can be easily understood by noting the torques on the system, but it complicates things when trying to build a compass.

    If you want a simple, clean measurement of the Earth's magnetic field, it's often better to simply use a static magnet. Another option would be to use a magnetometer:

    Also, bear in mind that the existence of other electronic devices nearby can screw up any measurement of the Earth's magnetic field.
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