Flywheel/Encoder patterns

[Joao Carlos Silva]

On Tue, 10 Dec 1996, Chris Howard wrote:

> At 15:33 12/8/96 -0800, you wrote:
> 
> >	Pardon my ignorance, but what effect does a magnet have on a non
> >metallic element like aluminum or copper.
> >	How's it work?
> >	And what kind of drag can you expect from such a device?
> >
> 
> I'll presume you know that aluminium and copper are metals and you in fact
> meant non-ferrous elements. :)
> 
> Here's what I remember from physics lectures:
> 
> Faraday's law states that a changing magnetic field will induce an emf in a
> conductor. If the conductor forms a circuit, a current will flow. This is
> how a generator works.
> 
>                 |  ________
>                 | | Magnet
>                 | |________
>                 |                        (ascii amateur)
>  ---------------|
>                 |
>                 |
>                 |
> 
> Consider a metal disk which is rotating on a shaft with a magnet at right
> angles to the disk on one side. The disk will experience a changing
> magnetic field and hence an emf will be induced in it. There are of course
> many paths for current to flow within the disk as it is a conductor.
> Circular currents flow in the disk and, due to the finite resistance of the
> metal, energy is dissipated as heat. The kinetic energy of the disk is
> changed to heat energy and the disk slows down.

I belive also that these currents are called by the English as Eddy 
currents and by the French as Focou currents (or something like that).
If the disk is ferromagnetic the slowing down is much faster.

As usual... hope this help (also hope that i'm not wrong)

Joao Carlos Silva
deec273 at tom.fe.up.pt