## PSN-L Email List Message

Subject: Re: Digest from 11/11/2006 00:01:37
From: ChrisAtUpw@.......
Date: Tue, 14 Nov 2006 13:54:31 EST

```In a message dated 2006/11/14, ivey@.......... writes:

> Why worry about the diagmanetism? A symmetrical arrangement should provide
> balanced forces.

Hi Jack,

Sure, just so long as the ends of the damper and the magnets don't get
close. You don't tend to get problems with Alnico magnets, since the field is
much lower than that provided by NdFeB magnets.

You need to allow for tilt drift with time with a Lehman seismometer
of maybe +/-10mm.

A magnet mounted inside a copper tube will provide some damping OK,
but can you explain how do you plan to adjust / trim this force to be constant
over a movement range of 2 cm?
An alternative is to provide a cylindrical magnet system and a damping
coil with a variable resistance.

The quad bar NdFeB magnet system with a plate moving in between the
poles can be easily set / reset over a very wide range of damping force - the
amount of damping that you want depends on the set period of the pendulum and on
it's mass.

Regards,

Chris Chapman

In a me=
ssage dated 2006/11/14, ivey@.......... writes:

m? A symmetrical arrangement should provide
balanced forces.

Hi Jack,

Sure, just so long as the ends of the d=
amper and the magnets don't get close. You don't tend to get problems with A=
lnico magnets, since the field is much lower than that provided by NdFeB mag=
nets.

You need to allow for tilt drift with t=
ime with a Lehman seismometer of maybe +/-10mm.

A magnet mounted inside a copper tube w=
ill provide some damping OK, but can you explain how do you plan to adjust /=
trim this force to be constant over a movement range of 2 cm?
An alternative is to provide a cylindri=
cal magnet system and a damping coil with a variable resistance.

The quad bar NdFeB magnet system with a=
plate moving in between the poles can be easily set / reset over a very wid=
e range of damping force - the amount of damping that you want depends on th=
e set period of the pendulum and on it's mass.

Regards,

Chris Chapman

```