3D compass on the cheap

In Same, between the storms, we unearthed and further developed an old magnetism activity, now doable with only a pair of ring or disk magnets, a thread, and a pair of chair legs.

Magnet on a string: instant compass

A magnet free to turn will align itself with the earth’s magnetic field.  This phenomenal phenomenon was first shown to me outside a compass case by my mentor professor in year three studying physics at university. I was amazed and distraught: my past teachers had failed me.  Something so simple, so fundamental, yet my teachers had never thought it worth conveying. I, a physics student, was left thinking there was something special and mysterious going on inside a compass.

To see this effect, you can float a cork skewered through with a magnetized needle in a bowl of water, as seen in many a science books from the 50s, but you can also just hang a magnet by a thread, stripping away any pretense of complexity.  It swings to rest in one position, and more importantly, will not stay in any other position.


Creep another magnet up underneath it and you’ll find you have full control of the hanging one, turning it forth and back with the flick of the one in your hand.  This shows a magnetic force field, that is, force over a distance without direct contact, normally the stuff of sci-fi or mystical levels of martial arts.


Take the other magnet away and the hanging one comes back peacefully to point north again, aligned with biggest field around, that is, the one from the earth.

What I learned much later is that the effect is not just two dimensional, north-south and east-west.  Rather, the earth’s field, like all magnetic fields, bursts like a bouquet of flowers out of one pole, and each stem gracefully bending around to stab back into the earth at the other pole.  Thus every place on earth has an up-and-down component to the field.

You can purchase the commercial version of this consisting of a tiny magnet on a gimbal. This is very nice and can be used to explore the shape of a field near a larger magnet.  But with just a thread and two magnets, you can isolate and explore the three dimensions of your local earth’s magnetic field.  Here near the equator, we’ve got little vertical component, but  with this arrangement, we were able to see it, we think.

  • First you need two ceramic ring magnets.  I think strong and small works better.
  • Stretch a thread between the legs of a chair, or anything that you can move around, like the mouth of a bucket or a sturdy box.
  • With the thread in the vertical position, sandwich it between the two magnets.  Mark the north side of the magnet pair, and note the north-south direction. (These ceramic magnets generally have the poles on the faces.)


  • Now tilt the chair so that the thread is horizontal, pointing exactly north-south.  Tweak the magnets on the thread until they are suspended exactly horizontally.


  • Finally, tilt the chair 90 degrees the other way so the thread is still horizontal, pointing exactly east-west.  Balanced and supported near their center of mass, the magnets should tilt at the angle of the local magnetic field.


Above 40 degrees latitude, a significant tilt should be apparent, north pole down in the northern hemisphere, south pole down in the southern. (Don’t forget that snarl of history and linguistics, that the north geographic pole of the earth holds a south magnetic pole.) Near the equator, the magnets will rotate to hang nearly vertical.

To confirm, turn the entire arrangement 180 degrees. If your setup is good enough, you should see the same angle.

Now close your eyes and imagine it was 2000 years ago, you had just found these interesting rocks that stick to each other, and when you dangle one from a string, it always points the same away. What in the world could be going on here?


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