About ten days ago, Prof. Erhard Wielandt, who has pioneered the 
development of force-feedback seismometers, updated his online paper titled 
"Seismic Sensors and Their Calibration", which may be found at

http://www.geophys.uni-stuttgart.de/seismometry/man_html/index.html

This an excellent overview of the issues involved in the design of feedback 
seismometers.

He has also revised and expanded his article "Seismometry" at

http://www.geophys.uni-stuttgart.de/seismometry/hbk_html/index.html


Enjoy.

Brett Nordgren
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