## PSN-L Email List Message

Subject: Re: copper weight
From: ChrisAtUpw@.......
Date: Mon, 27 Jun 2005 22:52:23 EDT

```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.

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:
<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
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wrote:> has anyone tried copper instead of lead for the weight on a=
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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

<=
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
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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
```