Re: falling magnet demo

[Bob Muir <muirrob@hamlet.uncg.edu>]

On Mon, 10 Mar 1997, Roger A. Pruitt wrote:

> We found that a single, thin-walled copper pipe did not produce large enough
> eddy currents to slow the magnet appreciably. We thought about having a
> gunsmith bore out a large copper bar, but first tried nesting copper pipes
> of increasing diameters. This worked extremely well, and we have been using
> a 2.5 meter pipe of this construction successfully for the past seven years. 
>
> Roger
 
We use plain copper (electrical) conduit (not especially thick walled!)
about 1.5 m long.  It works great with a "cow" magnet.  I just tried it
with a different bar magnet which is pretty strong, but its diameter is
considerably less than that of the tube.  There was a noticable, but not
dramatic lower rate of fall that outside the tube.  Cow magnets are pretty
strong.  

We have cut 10cm long longitudinal slots in another copper tube to
interrupt the eddy currents. It works!  You can hear (and sort of see) 
the cow magnet accelerate in the regions of the slots and slow at the
places between the slots where the tube is whole (needed to preserve the
shape of the tube). 

bob 
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