ian wrote:
> has anyone tried copper instead of lead for the weight on a Lehman?  The 
> idea being that it could also be used for magnetic damping.

Either can be used for magnetic damping.  Lead's advantage is its 
density at 1.27 times copper's.  It is also easier to cast into a 
compact lump that is the right shape to be attached to a pendulum.

The higher conductivity of copper (13 times that of lead) increases 
its damping effect for a given magnetic field strength, but lead is no 
insulator.  A good compromise is to use a block of lead (for mass) 
attached to a thin sheet of copper or aluminum (1.6 times higher 
resistivity than copper).  The thinness of the sheet allows you to get 
a pair of magnets closer together on opposite sides of it, increasing 
the magnetic field strength.

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