In a message dated 04/10/02, eianni2@........... writes:

>     I have been using "oil" damping on my Lehman home built seismograph. I 
> would like to replace the "oil" damping with "resistor damping" across the 
> coil. Can anyone tell me the ohmic value it should be, or what the 
> 

Hi Ed,

       You have not given us enough information about your system. It is 
possible to use electromagnetic damping if you have a very tightly coupled 
coil. For instance, you can probably do it if you have a 'speaker magnet / 
coil' type setup. You can also wind a special damping coil and mount it in 
between a pair of large NdBFe wing magnets or similar. These magnets also 
work OK with copper or Al damping plates. Incidentally, this sort of setup 
can give strong signals. 
       If you have a 'traditional' coil and one of the red U Alnico magnets, 
I would doubt if you could get enough coupling to damp the arm effectively. 
It also depends on the what weight you are using. The heavier the weight, the 
more force is needed to damp it. You might be successful with one of the 
large WWII magnetron magnets. 
       To check, I suggest that you remove the oil damping, short out the 
coil and move the arm maybe 1/2" and release it. If it moves much beyond the 
zero position or oscillates, you can't do it with your setup in 'passive' 
mode. If you have a separate distance transducer, it is possible to feed an 
amplified signal into a coil damping system.
       To give a rough idea of passive damping, if you use a couple of the 
1"x 1/2" x 1/4" #33 magnets, http://www.wondermagnet.com/dev/magnet33.html , 
mount them on a 1/4" iron backing plate and put this 1/16" from a 3" disk of 
1/16" soft Al, it will critically damp a 1/2 lb 1 second pendulum OK.
       Hope that this is of some help.

       Regards,

       Chris Chapman 

       
In a message dated 04/10/02, eianni2@........... writes:



    I have been using "oil" damping on my Lehman home built seismograph. I would like to replace the "oil" damping with "resistor damping" across the coil. Can anyone tell me the ohmic value it should be, or what the procedure is to calculate it?



Hi Ed,



       You have not given us enough information about your system. It is possible to use electromagnetic damping if you have a very tightly coupled coil. For instance, you can probably do it if you have a 'speaker magnet / coil' type setup. You can also wind a special damping coil and mount it in between a pair of large NdBFe wing magnets or similar. These magnets also work OK with copper or Al damping plates. Incidentally, this sort of setup can give strong signals. 

       If you have a 'traditional' coil and one of the red U Alnico magnets, I would doubt if you could get enough coupling to damp the arm effectively. It also depends on the what weight you are using. The heavier the weight, the more force is needed to damp it. You might be successful with one of the large WWII magnetron magnets. 

       To check, I suggest that you remove the oil damping, short out the coil and move the arm maybe 1/2" and release it. If it moves much beyond the zero position or oscillates, you can't do it with your setup in 'passive' mode. If you have a separate distance transducer, it is possible to feed an amplified signal into a coil damping system.

       To give a rough idea of passive damping, if you use a couple of the 1"x 1/2" x 1/4" #33 magnets, http://www.wondermagnet.com/dev/magnet33.html , mount them on a 1/4" iron backing plate and put this 1/16" from a 3" disk of 1/16" soft Al, it will critically damp a 1/2 lb 1 second pendulum OK.

       Hope that this is of some help.



       Regards,



       Chris Chapman