Eddy Currents

From: DonRath@aol.com
Date: Thu Sep 07 2000 - 11:34:06 PDT

Dear Ivette,

The "mechanical" balances, or "school" balances, that are mentioned in the
Eddy Currents snack are just plain old non-electronic weighing scales.
Typically, you place the object you want to weigh on a pan or platform, and
then slide movable attached weights along calibrated beams until a pointer on
the beams is aligned with a zero mark; the positions of the movable weights
then give the weight of the object being weighed. You can sometimes still
find scales like this in the lobbies of post offices, although many of them
have now been replaced by digital scales. On some of these balances, as the
beam with the weights comes to rest, there may be a metal plate which moves
between two magnets, causing eddy currents in the plate; these eddy currents
in turn cause their own magnetic field, which intereacts with the original
magnetic field to slow down the oscillations and cause the balance to come to
rest faster.

I don't know how high this rates on the "better world" scale, but eddy
currents are used for "magnetic braking" in some "free-fall" amusement park
rides. A copper vane attached to the ride capsule passes between strong
magnets near the bottom of the ride, and the magnetic field produced by the
eddy currents in the copper interacts with the original magnetic fields to
slow the ride down -- it's basically the same idea as described above for the
balance, but on a much larger scale.

Hope this helps.

Don Rathjen
Exploratorium Teacher Institute

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