In a message dated 2008/06/14, paleoartifact@......... writes:

> The "key" or strongest dampening area is simply the joints (edges) between 
> the mounted magnets as it is here where the "magnetic arching crossover" 
> field is the strongest, and hence the eddy current.  Another way of putting it is 
> that its not the center/broad area of the individual magnets where the very 
> strongest eddy current is induced in your aluminum/copper; its the area 
> immediately near the (crossover) joints thereof.  
> 
> I put up another brief web page for this subject, see:
> 
> http://seismometer.googlepages.com/magnetdampening

Hi Meredith,

       If you put your magnets on the backing plate in even numbers, you get 
a slightly higher internal field, but also a much reduced stray field from the 
backing plates. 

       An odd number of magnets has to return the field of one magnet between 
the backing plates around the edges of the magnets, or through the mounting 
bolts, rather than directly between the closer magnet surfaces, when they are 
mounted in pairs.

       It is the length of the N/S edge junctions which provides most of the 
damping. If you use rectangular magnets, mount the longest edges next to one 
another. The greatest damping is when the plate moves perpendicular to this 
direction. The damping plate should overlap the magnets by about half this length 
to get the greatest effect.

       I prefer to use a Copper damping plate. Aluminum is slightly 
paramagnetic and you can get appreciable forces on the damping plate if it's edge moves 
close to the edges of the magnets. This can cause the system to be slightly 
unstable in operation and make the arm 'skate off' to the stops beyond a 
moderate deflection.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/06/14, paleoartifact@......... writes:



The "key" or strongest dampenin=
g area is simply the joints (edges) between the mounted magnets as it is her=
e where the "magnetic arching crossover" field is the strongest, and hence t=
he eddy current.  Another way of putting it is that its not the center/=
broad area of the individual magnets where the very strongest eddy current i=
s induced in your aluminum/copper; its the area immediately near the (crosso=
ver) joints thereof.  



I put up another brief web page for this subject, see:



http://seismo=
meter.googlepages.com/magnetdampening




Hi Meredith,



       If you put your magnets on the backing=20=
plate in even numbers, you get a slightly higher internal field, but also a=20=
much reduced stray field from the backing plates. 



       An odd number of magnets has to return=20=
the field of one magnet between the backing plates around the edges of the m=
agnets, or through the mounting bolts, rather than directly between the clos=
er magnet surfaces, when they are mounted in pairs.



       It is the length of the N/S edge juncti=
ons which provides most of the damping. If you use rectangular magnets, moun=
t the longest edges next to one another. The greatest damping is when the pl=
ate moves perpendicular to this direction. The damping plate should overlap=20=
the magnets by about half this length to get the greatest effect.



       I prefer to use a Copper damping plate.=
 Aluminum is slightly paramagnetic and you can get appreciable forces on the=
 damping plate if it's edge moves close to the edges of the magnets. This ca=
n cause the system to be slightly unstable in operation and make the arm 'sk=
ate off' to the stops beyond a moderate deflection.



       Regards,



       Chris Chapman