Public Seismic Network Mailing List (PSN-L)
From: Charles R. Patton <patton@..........>
Subject: Re: seismometer/gravimeter
Date: Tue, 29 Apr 1997 10
Roger, you wrote:
A magnet is adjustably mounted nearby so that it very nearly balances the weight of the free end and relatively heavy of the filament, greatly magnifying the force of gravity. The lens is actually the tip of the filament fused into a glass bead. This free end swings up and down past a light emitter and detector behind little slits in aluminum foil. Therefore only a few microns of vertical motion are easily registered with a circuit ....
I followed this and subsequent posts with great interest. I especially liked the micromanometer. I do have one comment about the above though. When Vince Migliore was publishing GeoMonitor, he had several contributions from Ray Cole and Ken Cornell concerning magnetic field strength detectors. This was in conjunction with experiments to monitor electromagnetic fields as possible pre-cursors to seismic activity. One of the last versions of a detector was a long arm, off-center, on an edge pivot whose weight was counter-balanced by opposed magnets. Ken added a coil/oscillator structure which, in addition to providing a method of position measurement, caused the arm to continually rock, eliminating the hysteresis problem. Several people built variations of this instrument and saw changes at different times. My point is that by using a magnetic field as a very good spring, you also open yourself up to measuring variations in the local geomagnetic field also as that would change the effective "rate" of the "spring". So did you magnetically shield or otherwise do some tricks to try to null or eliminate this potential source of error?
Charles R. Patton, Editor, Geo-Monitor