Wednesday, February 5, 2014

Fun With Magnets!

Spotted this on YouTube today and found it fascinating.  What's it have to do with maps, mapping, etc?

What's being demonstrated here is the concept of a moving magnet establishing an electromagnetic field in a conductive copper tube.  That's why the magnet is repelled from the sides of the tube as it passes through it, and why the magnet can 'push' the tube when it is laid on its side.  Yet the demonstrator can gently place the magnet on the tube if he moves it slow enough.  

Faster movement of the magnet produces stronger eddy currents in the copper tube, creating a greater repellent force.  Slower movement results in a smaller eddy current and a lower repellent force.  This is precisely what's happening with induction dampening as applied in compasses like the USGI lensatic compass and the Brunton-style pocket transits.  The north-seeking magnet sits in a copper cup.  As it swings to align with the earth's magnetic field an eddy current is created in the copper cup that 'repels' (dampens) the movement of the magnet in the horizontal plane.  As the swing of the magnet decreases the eddy current decreases and eventually cancels itself out as the needle stabilizes. Simple and effective.

Plus, the video is just neat to watch!

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