Northwestern Physics Demos                                 Go back to Magnetism
DCS# 5K20.25NWU# E-60 Location:  under catwalk
title: Magnetic Tube Drop
aka: Eddie Current Braking / Cow Magnet Drop

An aluminium tube down which blue cylinders can be dropped.  The non-magnetic one falls freely and takes little time to fall.   The other is a pile of ring magnets (a cow magnet:  used to trap ferrous materials in the first stomach of a cow to prevent ingestion injuries.) with a strong magnetic field that produces eddie currents in the alumium tube and has a very long time of fall.  Small rare earth magnets will also work well.  
   There is a magnetic indicator material attached to the tube to show the passage of the magnet.  
    The velcro on the end of the tube is for attachment to a scale to show the different weights of the device as the magnet descends and then falls out.
   A camera pointed down the tube will show the magnet being repelled from the walls.

    Why do eddie currents produce such a slow terminal velocity?
     What would happen if the aluminum tube had less resistance to current flow?
pix of set upset up