[R-390] Gear chemistry, magnetic effects on non-ferrous metals

[Tisha Hayes]

The dragging effect that you felt when moving a strong magnet over the gear
is generated by eddy currents being induced in the metal. It is described
with Lenz's law and works just as well with aluminum that has no ferrous
content.

It has a few really neat (and common) applications, the magnetic drag is
also used in vending machines to detect fake coins as a good coin has a
very specific amount of drag that is the result of the eddy currents as it
moves past a magnet. Too fast or too slow and the coin is rejected.

I used a plastic pipe with a magnet dropped through the center as a great
teaching tool of the effect. The magnet takes so many milliseconds to go
from end to end and I set up a set of photodetectors at each end with a
signal generator and pulse counter to show the time for the magnet to pass
through the pipe Then I slipped an aluminum doughnut outside of the plastic
tube and dropped the magnet through the center. It was very apparent in how
much the eddy currents slowed down the magnet on its fall through the pipe.

*Ms. Tisha Hayes*
*"*There are many who live in the mountains and behave as if they were in **the
town; they are wasting their time.*
* It is possible to be a solitary in one's mind while living in a crowd;
and it is possible for those who are **solitary to live in the crowd of
their own thoughts.*"*
**-Amma Syncletica of Alexandria**