In a message dated 03/10/02, richg_1998@......... writes:

> I am building a Lehman and I wonder if the coil on the boom is better than 
> the
> magnet on the boom or is it not an issue? From what I have seen, building
> seismometers seems to be an art form.

Hi Richard,

       Building seismometers may well be an art form. Getting them working 
well maybe where the application of some science / technology is desirable. 
Seismometers are designed to have the ability to detect extremely small 
motions and forces. Otherwise they are of little use as seismometers. If you 
are not in a very isolated and quiet location, putting a powerful U or bar 
magnet on the arm will pick up small changes in the local magnetic field from 
a wide variety of sources, a visit from the garbage truck, moving the car, 
switching on the TV, magnetic storms, down to the steel in your belt buckle. 
If your intention is to monitor the garbage truck, etc., that is fine. If 
your desire is to only detect seismic vibrations, it is not so good. 

       It is magnetic fields which extend out from the apparatus which enable 
it to interact strongly with outside events. If you enclose the magnet in a 
screen, the interactions will be reduced. They will not be eliminated, since 
you have to use ferromagnetic materials to make the screen. This is done in 
some small modern seismometers, but they usually have a magnetic screen 
provided. Putting an effective magnetic screen around a 3 ft Lehman might be 
more difficult. 

       If you have any doubts, why not download the PSN archives and do a 
keyword search? 

       Regards,

       Chris Chapman
In a message dated 03/10/02, richg_1998@......... writes:



I am building a Lehman and I wonder if the coil on the boom is better than the

magnet on the boom or is it not an issue? From what I have seen, building

seismometers seems to be an art form.




Hi Richard,



       Building seismometers may well be an art form. Getting them working well maybe where the application of some science / technology is desirable. Seismometers are designed to have the ability to detect extremely small motions and forces. Otherwise they are of little use as seismometers. If you are not in a very isolated and quiet location, putting a powerful U or bar magnet on the arm will pick up small changes in the local magnetic field from a wide variety of sources, a visit from the garbage truck, moving the car, switching on the TV, magnetic storms, down to the steel in your belt buckle. If your intention is to monitor the garbage truck, etc., that is fine. If your desire is to only detect seismic vibrations, it is not so good. 



       It is magnetic fields which extend out from the apparatus which enable it to interact strongly with outside events. If you enclose the magnet in a screen, the interactions will be reduced. They will not be eliminated, since you have to use ferromagnetic materials to make the screen. This is done in some small modern seismometers, but they usually have a magnetic screen provided. Putting an effective magnetic screen around a 3 ft Lehman might be more difficult. 



       If you have any doubts, why not download the PSN archives and do a keyword search? 



       Regards,



       Chris Chapman