In a message dated 28/06/2005, jpopelish@........ writes:

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. 

Hi John,
 
    You can use a flat horizontal 1/4" thick Cu plate  for the mass and the 
damping plate quite OK. Trying to use sheet lead for  damping is likely to be 
less successful - the resistivity is a bit too high. You  need to keep the 
edges of the Cu plate well away from the edges of the magnetic  field - to avoid 
small magnetic forces.
    Brass is another good material for making mass  weights, but it is not 
much use for damping. Brass is about 8.6 gm / ml, Cu  is 8.9, Lead is 11.3.  



The  higher conductivity of copper (13 times that of lead) increases its 
damping  effect for a given magnetic field strength. 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.


If you use a N + S pair of rectangular 1" x  1/2" x 1/4" NdFeB magnets on two 
opposed 1/4" thick soft iron backing  plates, you should get ample damping. 
You hold the iron plates apart at a set  distance using 1/4" mild steel bolts 
and nuts. I use 3.5" long by 2" wide by  1/4" bright rolled mild steel strip. 
The longer the set period of the Lehman,  the less damping you need.
 
      Regards,
 
    Chris Chapman





In a message dated 28/06/2005, jpopelish@........ writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>ian=20
  wrote:
> has anyone tried copper instead of lead for the weight on a=
=20
  Lehman?  The 
> idea being that it could also be used for magne=
tic=20
  damping.

Either can be used for magnetic damping.  Lead's=20
  advantage is its 
density at 1.27 times=20
copper's. 

Hi John,
 
    You can use a flat horizontal 1/4" thick Cu pla=
te=20
for the mass and the damping plate quite OK. Trying to use sheet lead f=
or=20
damping is likely to be less successful - the resistivity is a bit too high.=
 You=20
need to keep the edges of the Cu plate well away from the edges of the magne=
tic=20
field - to avoid small magnetic forces.
    Brass is another good material for making mass=20
weights, but it is not much use for damping. Brass is about 8.6 gm / ml=
, Cu=20
is 8.9, Lead is 11.3.=20
 
<=
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  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>The=20
  higher conductivity of copper (13 times that of lead) increases its dampin=
g=20
  effect for a given magnetic field strength. A good compromise is to use a=20
  block of lead (for mass) attached to a thin sheet of copper or aluminum (1=
..6=20
  times higher resistivity than copper).  The thinness of the sheet all=
ows=20
  you to get a pair of magnets closer together on opposite sides of it,=20
  increasing the magnetic field strength.


    If you use a N + S pair of rectangular 1"=20=
x=20
1/2" x 1/4" NdFeB magnets on two opposed 1/4" thick soft iron backing=20
plates, you should get ample damping. You hold the iron plates apart at a se=
t=20
distance using 1/4" mild steel bolts and nuts. I use 3.5" long by 2" wide by=
=20
1/4" bright rolled mild steel strip. The longer the set period of the Lehman=
,=20
the less damping you need.
 
      Regards,
 
    Chris Chapman